Estimating Weight and Analyzing Hydrostatics

- in other words: Will it Float?

by Dave Tiessen, Mechanical Team Lead

The image below is rendered from the 3D computer model we created of our transatlantic sailboat (sailbot) hull earlier this year.  Having a 3D model of our boat is great for helping us design and build the boat, but it has a lot more value than just that.  It’s also very useful in facilitating a number of important analyses of our hull design.  These include analyses of weight, weight distribution, and hydrostatics – all of which we will delve into in today’s blog post – as well as stability and sailing characteristics, which we will save for a future post.

Figure 1 - Render from 3D CAD model of 2014 UBC SailBot hull

Figure 1 – Render from 3D CAD model of 2014 UBC SailBot hull

Estimating Weight

As with most engineering projects involving “things that go”, weight is an important parameter for SailBot.  Amongst other things, the boat’s weight and weight distribution will affect the level and attitude it floats it, how it responds to waves, how it performs, and how much reserve it has in case of damage or heavy weather.

Estimating weight is an integral process of designing the boat.  In the first stages of design, a desired length and rough shape of the hull are decided.  Then an estimate of how much that hull will weigh can be made given its size and the materials it’s to be composed of.  After that, all of the main components are listed and given an estimated weight.  The total weight is tallied and a percentage, or margin, added for minor components and things we’ve missed.  Next the weight estimate is compared to the estimated volume of the hull that will be underwater when floating as intended.  This volume is called “displacement”.  It’s how much water the boat displaces as it floats.  For everything to work out, the weight of water displaced has to equal the weight of the boat.

Usually, on the first try, the weight and displacement don’t agree, so the dimensions of the hull are adjusted and the weight of the new hull estimated.  This process is carried out iteratively until finally the displacement and weight agree.  A spreadsheet with some formulas to estimate volume and weight based on hull dimensions and shape facilitates this process.

Since SailBot is meant to take part in a competition, there are some additional constraints on the initial design process.  The rules of the Microtransat competition dictate that the waterline length of the hull cannot be over 4 metres.  Fortunately, many of the required components in an autonomous sailboat are relatively light, so we were able to design the 2014 SailBot with a relatively low displacement for its length.  That means that the hull can be narrow and shallow in the water, giving us the speedy shape we were after.

In the early stages of the weight and volume estimation cycle, the estimates are necessarily rough.  As the cycle continues and the hull shape becomes better and better refined and defined, so do the estimates become better and better.  The estimates of the weight of the hull are based on the weight per area of the materials used to build it.  Now that the final shape of the hull is defined and precisely captured with a 3D CAD model, we can easily find the exact areas of the hull surfaces and the interior partitions.  Furthermore, at this point in the design, most of the main internal components – electronics, servos, batteries and so on – have been specified, so we know their exact weights.  This refinement of information contributing to the weight estimate allows us to make a fairly accurate assessment at this point.

The latest estimate of the completed weight of the 2014 Microtransat SailBot is 126 kg.  This weight does not include the lead bulb that will be incorporated into the bottom of the keel.  The keel bulb is a very important component contributing to the stability of the hull.  The heavier the bulb, the less tippy the hull is.  Originally, the bulb was planned to be 75 kg.  But as the hydrostatic analysis presented below shows, that weight would make the SailBot about 10 kg too heavy.  With a revised keel bulb weight of 65 kg, the keel bulb is still 1/3 of the total boat weight.  Furthermore, SailBot’s keel is particularly deep, giving the bulb a long level arm which makes it more effective at keeping the boat upright.  Stability analysis showed that even with this reduction to keel bulb weight, the SailBot will still be very stable.  We will talk more about stability soon in an upcoming blog post.

Weight Distribution

Now we have a pretty good idea how much our boat will weigh.  But total weight is not the whole story.  The distribution of weight is also important.  As you can imagine, if the boat weighs as much as the water it should displace but all of that weight is concentrated at the very back, the boat will not float on a level trim as intended.  Earlier weight estimates could overlook distribution, because the final locations of most things were undecided:  they could be moved to achieve a favourable distribution.  Now that we are in the later stages of design, it’s important to take into account the location of each item of mass in order to determine the centre of gravity of the boat.  The centre of gravity is an imaginary point at which we can pretend all of the mass of the boat is concentrated.  It’s a useful concept when we want to investigate how applying forces to the hull will affect it.

In boat design, the centre of gravity is usually measured in three components:

  • Longitudinal Centre of Gravity (LCG)
  • Transverse Centre of Gravity (TCG), and
  • Vertical Centre of Gravity (VCG)

Of the three, TCG is perhaps the most critical, but also the easiest to deal with.  If the TCG is in the wrong place, the boat will lean to one side, or heel.  However, it’s easy to know where the TCG has to be.  Because SailBot, like most boats, is symmetrical from side to side, we know that the TCG needs to be along the centreline of the boat.  Many components, such as the hull itself, will naturally be distributed such that the boat is balanced.  As we design the location of other components, we can take care to make sure we have them balanced in order to maintain the TCG on the centreline of the boat.

Figure 2 - Defining longitudinal and vertical centres of gravity: LCG and VCG

Figure 2 – Defining longitudinal and vertical centres of gravity: LCG and VCG

LCG is not so simple.  The shapes of the forward and aft sections of the hull of SailBot, and of most boats,  are not the same.  So we can’t just assume LCG should be in the middle.  To know where we want it, we need to get into hydrostatics, which we’ll talk about soon.  But for now, we want to know where it is for the current design.  The boat is much less sensitive to variation in the position of LCG than of TCG.  If you think of pressing down on the middle of a long board floating in the water, you can imagine that it would be easy to make it roll quite a bit by moving your finger a bit to one side.  It would take a lot more force, though, to raise one end of the board by moving your finger a bit towards the other end.  However, it is still important to get the LCG in the right spot if we want to achieve our desired trim.

Figure 3 - Differences in effect of transverse and longitudinal offsets of weight

Figure 3 – Differences in effect of transverse and longitudinal offsets of weight

VCG makes no difference to heel or trim in static conditions.  It is very important, though, when it comes to stability.  The lower the VCG, the more stable the boat.  Without the keel, SailBot’s VCG is about 12 cm above the waterline.  That’s not bad for a boat with an 80 cm beam.  But when the deep, heavy keel is added, SailBot’s VCG goes down to about 50 cm below the waterline.  That makes the boat very stable and resistant to heeling – a good thing in a sailboat.

So how do we determine these centres of gravity?  We’ll work with LCG as an example.  First we choose a convenient place to measure from.  In this case, the transom or back end of the boat.  Then we measure the distance from the transom forward to each component.  We multiply that distance by the weight of the component.  That gives us a measure of the torque that would be required to hold that component up if it was at the end of a stick of that length.  We then sum up all the torques, or moments, for all of the components.   Now we have a measure of how much torque, or twisting force, you would have to apply to hold the boat up by its transom.  If we divide that by the total weight of the boat, we are left with the distance forward from the transom that that total weight would have to act in order to produce the same torque.  So if all that weight was concentrated into one single spot, it would have to be at our calculated distance forward to produce the same effect.  That’s the definition of the LCG.

Figure 4 - The balance of weight multiplied by lever arm (image from wpclipart.com)

Figure 4 – The balance of weight multiplied by lever arm (image from wpclipart.com)

When the latest weight estimate for SailBot was completed, we calculated that the LCG was 1.79 m forward of the transom.  That’s a little behind the middle of the boat, which is what we would expect.

Hydrostatics

In boat design, hydrostatics is principally concerned with the volume of water a hull displaces and with where the geometric centre of the displaced volume lies.  These will change with the shape of the hull, of course, and with its depth in the water.  Changing the attitude of the hull – its trim or heel – will also change its hydrostatic characteristics.

The reason we are interested in the volume of water a hull displaces is obvious: as mentioned earlier, the weight of water displaced must equal the weight of the boat.  If we want our boat to float on its design waterline, we must ensure that the volume displaced with the hull at this depth is the same as the total weight of the boat.  If the volume displaced is not enough, the boat will sink deeper to reach equilibrium.  If the volume is too much, the boat will float too high unless more weight is added.

The reason for our interest in the geometric centre of the displaced volume may not be so obvious.  That point is also known as the centre of buoyancy (CB).  Just as we can imagine that all of the weight of an object is concentrated at its centre of gravity (CG), we can imagine that all of the buoyant force pushing the boat up is concentrated at its centre of buoyancy.  If the CB is vertically aligned with the CG, then we can see that the downward force of gravity can be nicely balanced by the upward force of buoyancy along the same line.  In this case, the boat will float steadily.  However, if the CG and CB are not vertically aligned, the force of gravity and buoyancy will also not be aligned.  The offset will create a torque that will roll the boat until the two forces are aligned.

Figure 5 - Effect of misalignment of centre of gravity (CG) and centre of buoyancy (CB).  Image modified from Harvard Natural Sciences Lecture Demonstrations - Fluid Statics

Figure 5 – Effect of misalignment ofcentre of gravity (CG) and centre of buoyancy (CB). Image modified from Harvard Natural Sciences Lecture Demonstrations – Fluid Statics

As you can see in the diagram above, this torque can work in our favour when the CB shifts to one side because the boat has heeled.  In this case, the torque is known as a righting moment and helps to restore the boat to an even keel.  But, if the CB for the boat floating at its intended waterline doesn’t match up with its CG, the boat will change its heel and trim until the two do line up.  In other words, it won’t float in the intended attitude.

If our hull were rectangular or spherical or some other regular shape, finding its volume would be easy.  But how do we find the volume of such an irregular shape as a boat hull?  Well we break it into smaller problems and use some approximation.  Let’s imagine finding the volume of another irregularly shaped object as an example: a loaf of bread.  If we first slice the loaf of bread, we can measure the area of each slice.  There are many ways to do this, but we can imagine tracing the slice on a piece of graph paper and then counting the squares inside our slice.  If we multiply the area of the slice by its thickness, then we have a measure of its volume.  At the ends of the loaf, where the surface curves in, our estimate won’t be so good, but we can make up for that by taking thinner slices.  Once we’ve found the volume of all of the slices, we can add them up to find the volume of the entire loaf.  Or, we could add the slices up to a certain point to measure the volume of the loaf up to that point.

Figure 6 - Finding volume by slices

Figure 6 – Finding volume by slices

The approach taken to find the volume of our boat is more-or-less the same as that described above.  For the boat, though, we probably want to take horizontal slices on waterlines, rather than transverse slices like a loaf of bread.  We can take very thin slices to make out estimate accurate, and we can add the slices up to any waterline we like to see how much volume we’ve displaced at that waterline.

Now how do we find the centre of buoyancy?  We find it the same way as we found the centre of gravity.  To find the longitudinal centre of buoyancy (LCB), we can use a 3D grid to divide our hull into cubes.  Let’s say that each cube was a cubic centimetre.  Each would then have a weight of one gram if made of water.  So if the cube was instead displacing water, it would have an upward buoyant force of 1 gram.  For each cube, we can multiply that force by its distance forward from the transom to find a torque.  If we add up all the torques from each cube within the hull and divide them by the total displacement of the hull, we are left with a lever arm length: the LCB.

Of course doing all of this by hand would be very tedious.  For SailBot we used a specialized software called GHS.  It computes displacement and CB using a process analogous to what we’ve discussed above, but with very thin slices and very small cubes so the accuracy is high.  GHS can actually do many more sophisticated analyses than these, but finding displacement and CB are a good start.  Once we’ve found these we can find a number of other interesting parameters as well.  Let’s start with a few of the key characteristics determined in our GHS analysis of the SailBot hull.  These characteristics were measured with the boat at a baseline draft of 1.38 m.  That means the lowest point of the hull is 1.38 m below the surface.  This analysis discounts the boats keel, which of course would make it much deeper than that.  We’ll go through the characteristics below to see what they can tell us.

Table 1 - Hydrostatic characteristics of UBC 2014 SailBot hull at a baseline draft of 1.38 m

Table 1 – Hydrostatic characteristics of UBC 2014 SailBot hull at a baseline draft of 1.38 m

The displacement volume of 0.188 cubic metres or 188 litres is actually quite low for a hull as long as SailBot.  When floating as designed, this means that the boat is narrow and shallow – helping to make it fast.  It also hints at the fact that most of the hull’s volume is out of the water, giving it plenty of reserve buoyancy, particularly in the overhanging bow.  That’s important for seaworthiness in rough weather, and to prevent the bow from digging in when the boat is sailing with a strong tailwind.

In fresh water, the displacement weight of the boat would be about 188 kg, because one litre of water is about one kilogram in weight.  But since SailBot will sail in salt water, which is slightly heavier, its displacement weight at the design waterline is 192.8 kg.  If we subtract the 126 kg determined in the latest weight estimate, that leaves about 66 kg for the keel bulb, as mentioned earlier.

The LCB of 1.80 m is very satisfying.  You’ll notice that it’s almost exactly the same as what we found for LCG in the weight estimate.  As we’ve discussed, that’s exactly what we want.  This result means that we don’t need to change the planned layout of our current design significantly to achieve the right weight distribution.  Our design will allow the heavy main battery to be slid forward or aft on its mountings to adjust for any small discrepancies in the final positions of LCG and LCB.

As mentioned earlier, the maximum waterline length allowed in the Microtransat competition is 4 m.  We’ve designed SailBot with a 3.9 m LWL to have a safe margin below the limit.  The maximum breadth of 0.8 m shows that SailBot is a relatively long and narrow hull with a length to beam ratio of about five to one.  That fact is further apparent in the prismatic coefficient.

Prismatic coefficient (Cp) is a commonly use metric for quantifying the shape of a boat hull. It compares the displacement volume of the actual hull to the displacement of a prism of the same length that has a constant cross-sectional area equal to that of the maximum underwater section of the hull.  The way to imagine this prism is to think of the largest underwater cross-section of the actual hull – usually around the middle of the boat.  Now extrude this cross-section out in both directions to the bow and the stern of the boat with no tapering.  Essentially what the prismatic coefficient tells us is how much the ends of the boat taper compared to its middle.  Boats with fine ends have a low Cp.  Boats will blunt ends have a higher Cp.  SailBot’s prismatic coefficient of 0.606 is at the high end for sailboats, which are usually in the range from 0.5 to 0.6.  However, that finding is misleading because SailBot also has a higher length to breadth ratio than most sailboats.  A very long, narrow hull like an outrigger canoe will have a high Cp simply because the cross-section of the hull doesn’t change much along most of its length, though it may have very fine ends.

Wetted surface is just what is sounds like – the surface area of the hull that is underwater and “wet” when floating.  This number is important in terms of drag.  A high wetted surface will lead to high friction drag – important when sailing in light wind conditions.  At higher speeds, however, other sources of drag become more important.  The fin keel design of SailBot means that its wetted surface will be much less than that of an equivalent boat with a traditional full keel design.

Having determined wetted surface and volume displacement, we can now computer two final metrics of interest.  These are sail area/wetted surface and sail area/(volume displacement)2/3 – two common metrics used for sizing yacht sails.  The currently planned sail for the 2014 SailBot is a windsurfing rig with an area of 5.9 m2.  Computing the metrics with this sail area and the data from Table 1 above gives the following results.

Table 2 - Comparison of sail size metrics for SailBot to typical values for sailboats.  Typical values are taken from Principles of Yacht Design by Lars Larsson and Rolf Eliasson.

Table 2 – Comparison of sail size metrics for SailBot to typical values for sailboats.  Typical values are taken from Principles of Yacht Design by Lars Larsson and Rolf Eliasson.

These results show that SailBot is squarely in the middle of typical ranges for sailboats in terms of sail area to hull size.  However, SailBot has a much deeper keel for its hull size than typical sailboats.  That means the boat could handle a larger sail in normal conditions.  However, since SailBot must cross the notoriously rough North Atlantic without maintenance, it’s best to keep the sail area conservative.  Stay tuned for the upcoming stability blog post for more about the benefits of SailBot’s deep keel.

Story of IRSR 2014

by UBC SailBot IRSR 2014 team: Serena Ramley, Kurtis Harms, Josh Andrews, Arek Sredzki, Jian Lik Ng, Tu Anh Le, Tobias Kreykenbohm, Jamie Lee, Bryan Luu, Kristoffer Vik Hansen,  Daniel Kim, and Youssef Basha.
 

After several weeks of thorough testing, both on land and on the water with the help of UBC Sailing Club and Hollyburn Sailing Club, the UBC SailBot competition team headed to the International Robotic Sailing Regatta 2014, hosted by ASME (American Society of Mechanical Engineers) and California Maritime Academy. The venue was California Maritime Academy in Vallejo, and this is the story of IRSR 2014.

Saturday June 7 – Practice Day 1

Just like IRSR 2013, we were the first team to arrive, making sure we didn’t waste any valuable practice time. For the first practice day we assembled our racing sailboat setup and organised all our tools and equipment for the following competition week in California.

DSC_7808

The UBC SailBot competition team at California Maritime Academy

Already the first afternoon we were able to put our robotic sailboat entry, Thunderbird 2013 (TB2013), in the ocean, with winds reaching a steady 5-8 knots. Getting TB2013 on the water the first day is very critical, as it allows us to check that the whole system is up and running, just like it had been for the past weeks in Vancouver.

Unfortunately, within 10 minutes of being on the water, the halyard on the main sail snapped. With the sun setting, we dedicated the rest of the night to repair the boat and fix up details to make Sunday the best possible testing before the competition.

Jamie and Tobias carrying the boat back for repairs on the halyard

Jamie and Tobias carrying the boat back for repairs on the halyard

Sunday June 8 – Practice Day 2

Although the forecast predicted 15 knots throughout the week, we wanted to be as prepared as possible. We even brought our largest rig, which is designed for winds less than 5 knots. It was definitely worthwhile as the wind during practice day 2 was under 5 knots, a rare low this close to San Francisco.

By Sunday evening everything was ready for competition. However, just before finishing up for the day the wind sensor, one of our most delicate components, broke while rigging the boat on land. Everyone quickly mobilized, and using the amazing knowledge and testing experience we have internally in the competition team, it did not take long before the wind sensor was as good as new.

Jamie and Josh discussing what can be done.

Jamie and Josh discussing what can be done.

Tu Anh connecting the magnetic encoder for the wind sensor.

Tu Anh connecting the magnetic encoder for the wind sensor.

DSC_7606

Tobias re-calibrating the wind sensor the next morning.

Monday June 9 – Competition Day 1

The first event, Fleet Race #1, relies on manual control to weave through the course. The wind was very weak at 4-6 knots and the current very strong – no issue for Kurtis, Arek and Josh, our computer scientists and sailors. Their strategy of sailing TB2013 as quickly as possible further offshore and into more windy areas worked perfectly as TB2013 crossed the finish line shortly after. A well deserved win for UBC in Fleet Race #1.

Start of first fleet race! From left to right: Virginia Tech’s Orca, US Naval Academy’s Sea Quester, Memorial University’s Trixie, UBC’s Thunderbird 2013, and Aber’s Kitty.

Start of first fleet race! From left to right: Virginia Tech’s Orca, US Naval Academy’s Sea Quester, Memorial University’s Trixie, UBC’s Thunderbird 2013, and Aber’s Kitty.

Thunderbird 2013 sailing in to victory in the first fleet race.

Thunderbird 2013 sailing in to victory in the first fleet race.

Later in the afternoon, the Navigation Challenge and Stationkeeping Challenge began.

The Navigation Challenge tests the boat’s navigational accuracy. The goal of this challenge is to round one windward mark and then thread the needle through a three metre-wide gate. The wind direction kept changing in the navigation test, but Thunderbird 2013 re-calculated its route and scored 10 out of 10 points.

We were very happy with the Hemisphere GPS for its unwavering accuracy during the Navigation Challenge.

We were very happy with the Hemisphere GPS for its unwavering accuracy during the Navigation Challenge.

The goal of the Stationkeeping Challenge is to keep the boat in a 40m x 40m box for at least five minutes. The boat must then exit the box as quickly as possible. For this challenge, Bryan implemented a new “failsafe” strategy to help ensure that the boat wouldn’t leave the boundaries when faced with changing currents and winds. The code also included a 2-second buffer for when exiting the box. With these two strategies together, the boat exited the box at :02 and :01 consistently.

Tuesday June 10 – Competition Day 2

Before Fleet Race #2 this morning, winds were predicted to be around 10 knots and rising to 15 knots. This is just on the safety boundary between our medium and small rig. After evaluating speed versus maneuverability, we decided that control was more important and used the smallest rig. Even though the US Naval Academy boat had a head start and a slightly larger rig this day, Thunderbird 2013 quickly caught up. We knew we made the right decision about rigs as soon as we rounded the first buoy – Kurtis was able to control the boat with ease, but unfortunately, the USNA boat got caught as it went around the buoy. Once again we finished the challenge well before everyone else, this time faster than the previous day´s manual fleet race, down to 14 minutes from 19. And with this we came first in Fleet Race.

Kurtis using manual RC on the Thunderbird 2013

Kurtis using manual RC on the Thunderbird 2013

Being that TB2013 had performed so well on Stationkeeping the day before, we decided not to try this challenge again. We instead worked on maintaining and preparing the boat for the Presentation Challenge the following day.

Due to the stronger winds this day, the other teams had troubles doing well on Stationkeeping, and the UBC team ended up with the quickest exit for Stationkeeping overall. With this we got another 10 points in the competition, totalling 30 points after Day 2.

Wednesday June 11 – Competition Day 3

Day 3 was devoted to the Presentation Challenge. With a 20 minute long presentation followed by 10 minutes of questions, the team had to explain and reason every part of our project and our team to a panel of judges with expertise in mechatronics, naval architecture, and systems engineering. The judges seemed to enjoy our presentation, and awarded us 10 points for it.

This year, the International Robotic Sailing Regatta was co-hosted by ASME OMAE (International Conference on Ocean, Offshore and Arctic Engineering). In addition to our presentation, we also showcased our boat to the conference attendees at the Palace Hotel in downtown San Francisco.

We especially enjoyed meeting Erik Berzins at the OMAE conference. Erik is a UBC graduate and one of the original founders of the International Robotic Sailing Regatta. His talk, “AC Unleashed – Secrets of the Cup”, was very popular among both IRSR attendees and ASME OMAE attendees.

Kristoffer explaining our project to interested ASME OMAE attendees.

Kristoffer explaining our project to interested ASME OMAE attendees.

Thursday June 12 – Another Perfect Storm

The Long Distance Race, a 4.8 mile race, is the IRSR’s most challenging event because it tests both the boat´s durability and the team´s endurance. Like IRSR 2013, the weather was not on our side this year either. 20 knot winds and 25 knot gusts followed soon after starting the Long Distance Race, a real challenge to every sailbot at the competition.

Heavy winds for Thunderbird 2013 during Long Distance Race.

Heavy winds for Thunderbird 2013 during Long Distance Race.

Do not mistake TB2013 for a submarine.

Do not mistake TB2013 for a submarine.

In our presentation the previous day, we talked about how we can add boundaries into our boat so that it can avoid shipping lanes and other no-go zones. This feature served us greatly as the winds and currents tried to drag Thunderbird 2013 into the nearby shipping lane while trying to complete the long distance race autonomously.

After the first lap, the wind overpowered the sheets causing the sailwinch (which controls the sheets) to over-torque and pull itself up from the screws that normally hold it down. This allowed water to leak into the winch cabinet, shorting out the motor’s leads. Fortunately, we were able to sail the boat back to the dock where we were able to carry out some quick repairs. We´ve learned a lot from our Thunderbird SailBots over the past four years, and this is definitely something we will thoroughly test for our future sailboats.

Tu Anh looking over the wiring for the sailwinch.

Tu Anh looking over the wiring for the sailwinch.

In the competition, the clock keeps ticking even during repairs; we knew that we needed to restart the race in order to get a competitive time. Our team quickly repaired the winch and put Thunderbird back on the water, successfully completing the long-distance challenge in 1 hour and 20 minutes. The US Naval Academy was the only team to receive points for the Long Distance Race, completing ¾ of the course in 1 hour and 50 minutes. We all got soaked trying to follow Thunderbird 2013 in our small chase boats, but the exhilaration and joy of completing the challenge was more than worth it!

As a result of our efforts, we scored 10 points on Long Distance Race. With this amazing result we successfully defended our first place from 2013, and again scored an impressive 50 out of 50 points in the competition.

We have a lot of people to thank for this amazing project –  The rest of the team back in Vancouver, our amazing sponsors who follow our project with great interest and passion, the great people at UBC Sailing Club and Hollyburn Sailing Club who helped us with support boats and support boat drivers during our many on-water testing sessions, the teams at IRSR 2014 who always give us many helpful tips and support, and all the supporters who cheered us through this amazing journey.

Thunderbird 2013 is quite the raceboat!

Thunderbird 2013 is quite the raceboat!

Many awards for the UBC SailBot team after IRSR 2014.

Many awards for the UBC SailBot team after IRSR 2014.

Yes, we are pretty happy with our win.

Yes, we are pretty happy with our win.

The IRSR 2014 competitors.

The IRSR 2014 competitors.

UBC SailBot team returning to Vancouver again.

UBC SailBot team returning to Vancouver again.

Electrical Team Progress Update – May 2014

by Ellinor Crux, Electrical Team Lead

For the UBC Sailbot’s transatlantic vessel to be successful in crossing the Atlantic Ocean, an electrical system is needed to bridge the gap of the software systems discussed in previous posts to the new boat hull. Over the past 8 months, the electrical team has been researching and testing electrical components that will potentially be used on the final transatlantic boat. Through this blog post, each component will be explored in detail.

Sensors

The sensors are the figurative eyes and ears for the boat. They will provide all the information the boat will need to navigate safely to it’s destination. Below are some of the sensors we have been working with.

Wind Sensor

The basics of sailing require that you know which way the wind is heading! The wind sensor measures the speed and direction of the wind. This allows the boat to determine the best way to align it’s sails to travel in a desired direction. The wind sensor we will be using is a model CV7 from LCJ Capteurs. This wind sensor is ultrasonic, meaning it measures the velocity of the wind by sensing the speed of ultrasonic sound waves it emits as they travel between transducers. The ultrasonic operation eliminates the need for moving parts, which enhances reliability of the sensor in marine environments over the traditional cup-and-vane wind sensors.

GPS

Knowing where you are is crucial if you want to know where to go. The GPS module will provide location data for the boat, allowing us to track its movements, and estimate its speed and trajectory.The GPS module and antenna we are using are the Flexpak-G2 Novatel OEMStar GPS. We would like to thank Novatel for donating the GPS module and antenna.

Compass and Accelerometer

The compass and accelerometer allow the boat to determine its heading and orientation in space.The accelerometer also can sense the movement up and down waves, which will be important as a safety check, should the boat suddenly accelerate due to a storm or a large wave.

Communication

Satellite

Once we let the boat go, it would be reassuring if we know where it went! That’s where the communications systems come in. The satellite will report the boat’s location and supply the boat with weather data. The weather data will be used as part of the boat’s path-finding algorithm. The location of the boat has to be reported every day, but we want to keep an eye on the fruit of our labours, so we’ll make sure it calls home more often. We are using a Rockblock satellite modem from “Rock Seven”. Currently, we are working on completing a software library for use with the modem.

We would like to thank Rock Seven for supplying us with credits for use on their satellite service.

Automatic Identification System (AIS)

Our boat is not going to be alone out there. We have to make sure it doesn’t run into any other boats! The Automatic Identification System is a module which allows vessel to vessel communication of heading, location, and planned route. This functionality is important for our boat in navigating around other vessels. We are also exploring ways of detecting vessels or obstacles without AIS technology onboard.

Power

The journey across the Atlantic ocean wouldn’t be possible if the electronics stopped running. Being on the water for at least 2 weeks is bound to use up a lot of power! The main task of the Power Systems subteam is to design the voltage supply and its charging circuit in order to power the electrical components on the boat.

Batteries

The batteries will consist of 4 sets 3 battery cells at 3.7 volts, giving us a supply voltage 11.1 V. A DC/DC converter will be used to step down the higher supply voltage to one which can be used by the digital components. The DC/DC converter we will be using is a Texas Instruments 6A 12V SWIFT DC/DC.

Solar Panels

Sooner or later, the batteries will run out of power. We have decided to harness solar energy due the robustness and reliability of the currently available technology. We have decided to use Solbian brand flexible solar panels of the SP series. Not only are Solbian panels manufactured for marine applications, their light and flexible nature give us options for mounting. Currently we are trying to determine the exact size and location of the solar panels so that we can move on to ordering them. Any input from people experienced with the use of solar panels in marine environments would be welcome!

Motors

The motors are the muscles of the boat. There are two motors on the vessel, one servo motor to control the rudder; and a winch to control the sail.

Servo Motor

The rudder motor is a Torxis i00600 servo motor. This motor was chosen for its high torque output which will be necessary to hold the boat on course as winds pick up and the sea gets rough. The motor was also chosen for its compatibility with the power systems design and three position feedback. The three position feedback feature polls the motor constantly checks it position against the last logged position command, adjusting it accordingly if it has been forcibly moved.

Sail Winch

For the sail winch motor we are currently discussing with our industry mentor and the manufacturer over the final designs. Fabrication on our custom winch motor will begin soon.

Redundancy

We can never be sure everything will go exactly as planned; that’s why we have fail-safes! The current design of the electrical system has several redundancies and fail-safes.The overall design can be viewed in the diagram below. Notice that most components are duplicated and segregated into separate boxes. The segregation of components minimizes collateral damage should there be a leak or damage to the vessel which causes some of the electrical systems to fail.

Sailbot layout

Click on image to see full resolution version.

Beyond the prototype – Designing a Route Making Algorithm to Cross the Atlantic Ocean

by UBC SailBot Software Team

In late November we posted about our design of a pathfinding algorithm prototype for navigating around simple obstacles in order to get to a destination. Since then we have transformed this from a prototype into an expandable weather routing algorithm that fits into our sailing logic architecture.

Control System Architecture

Image 1 - Control System Architecture

Image 1 – Control System Architecture

Our goal in designing the software for the transatlantic boat is to have two main systems.  The first of those systems is the low level control system.  Similar to our design for the International Robotic Sailing Regatta competition in Massachusetts last year, we want our transatlantic boat to be an expert on getting from one point on earth to another. What drives this is a robust autonomous control system that takes inputs and uses sailing logic to produce desired output on the rudder and sail position. In prior years, this was the key piece in the puzzle. For crossing the Atlantic Ocean, however, we have the additional challenge of obstacles. This is where the second system comes in to play.

The route making system (system two) is designed to plan a route across the Atlantic Ocean given that there will be obstacles that gets in our way, and that the obstacles change over time.  As we moved from the route making prototype and started developing a design to take into account crossing the Atlantic ocean we decided to give the route making algorithm design a makeover.

Route making architecture makeover

Our first design for a route making system incorporated all data into one three dimensional pathfinding algorithm, the third dimension being time.  Through testing we have realized many limitations to this original design and developed a new design with a more robust architecture.  The main driver for this change is the large scale patterns that weather systems display which are imperceptible at smaller scales.  Because the boat navigates at a small scale, and thus also receives sensor data of its surroundings on a small scale, we decided the boat requires a two layer architecture – one for the entire Atlantic Ocean, and one for the foreseeable future.

A one layer structure – combining both large scale and small scale datasets – could be done if we only looked into the near future, however the algorithm would suffer from not being able to see the entire Atlantic. We illustrate this in Image 2, as seen below, where the goal of our boat is to make it to Target 2. Imagine that we only look east as far as Target 1.  We would not see that there is an obstacle just following Target 1 and in this case after we reached Target 1 we would have to backtrack to make it to Target 2.  If we instead looked east all the way to Target 2 we would realize that the fastest way to get to Target 2 is to avoid the obstacle all together.  This sort of situation is very applicable to some of the situations we encounter when analyzing data gathered on the Atlantic. As a result, we separate the different scale datasets into a two layer system. The small scale layer relies on the large scale layer to know the path that is best suited over the entire Atlantic, and then calculates the best direction for the next 10km.

Image 2 - Large versus small scale sailing scenario

Image 2 – Large versus small scale sailing scenario

We have documented each layer in better detail below.

Large Scale Layer:

Image 3 - Route-making from the Gulf Of Mexico, avoiding areas with large waves.

Image 3 – Route-making from the Gulf Of Mexico, avoiding areas with large waves.

Scale: each cell will be 1×1 degree or ~100km

Inputs: weather from an online repository, a preset target (Ireland), a preset bias route that the boat should ideally travel.

Outputs: a 2D grid of weights

Description: At this layer, we plot the long term trend of the route that the boat should take, taking into consideration large scale weather systems while trying to keep as close as possible to the pre-set ideal route. The output of this layer will be a 2D grid of weights representing the long term route which will be fet to the small scale layer.

Small Scale Layer:

Scale: ~10km

Inputs: the output of the Large scale Layer, AIS data, other small scale obstacle data.

Outputs: a set of coordinates ~10km apart

Description: This layer will plot a route for the short term obstacle avoidance taking into consideration smaller obstacles such as boats. It will receive a 2D grid of weights which will serve as a bias for the long term route from the large scale layer. This way, the small scale layer will plot a route that will resemble the the large scale route unless there is an obstacle preventing it, in which case it will plot the next best route. The output of this layer will be a set of coordinates which represent the path calculated by this layer and which will be fet to the control system.

There are a number of challenges going forward that we are focusing on to improve the route making system.  We are actively looking for good data to base our weather routing off of, and with multiple datasets we also need to define how we are going to weight the datasets against each other.  When routing we also need to determine a good resolution for each layer.  This is integral for us to ensure that we can fit the “good” data for the entire time span we are route making over.

Mechanical Team Progress Report

by Dave Tiessen

The Mech team has been working steadily over the past couple of months on construction of our Microtransat boat’s hull.  The hull is being built using the “cold molded” construction method.  This method involves first building a skeletal mould that defines the shape of the hull.  Thin layers of softwood strips are then bent over the mould and laminated together to form a shell that will become the hull core.  Finally, carbon fibre is laminated to either side of the core to make a very stiff, light and strong hull structure.

We’ve made some good progress on the hull mould: read on to see what we’ve been up to.

Workshop

The first step in building a quality boat was to organize our workspace.  The team did a great job of rearranging our work area to accommodate construction of the 5+ metre Microtransat hull – a much larger project than we’ve tackled before.  The diagram below shows our workshop layout.

Figure 1 - UBC Sailbot workshop layout

Figure 1 – UBC Sailbot workshop layout

The lockers at left have been mostly filled with tools, many of which were donated to our team by sponsor Summit Tools. We finally have a collection of tools that gives us the ability to work quickly and efficiently.

The work tables shown at the right side of the diagram were our first construction project.  They were made in a single session from 2 x 4 and ¾” plywood donated by Windsor Plywood.  See the tables in the picture below.  So far they’ve been providing good service and standing up well.  One of our members discovered, however, that it’s really not too hard to saw through the tabletop, screws and all. ;)

Figure 2 - Sailbot work tables

Figure 2 – Sailbot work tables

Strong back

Next we turned our attention to building the “strong back” – essentially a long box beam that forms the base of our boat mould.  In one session, we built the basic structure in two sections, again with material donated by Windsor Plywood.  In the next session, we joined the sections to create one long beam and added diagonal bracing.  The webs of the beam are of ½” plywood 16” deep with flanges of 2 x 3 at top and bottom.  This construction makes for a beam that is extremely stiff.  The process of bending many stringers and veneers over the mould later on creates a strong aggregate reaction force that tends to bend the ends of the strong back upward.  Robust construction is necessary to counter this bending force, keeping the mould free from distortion.

The next step in preparing the strong back was the addition of the transverse members shown in the picture below.  These members were fastened perpendicular to the strong back centreline with precise separation to accurately locate the frames that would be fastened to them later.

Figure 3 - UBC Sailbot Microtransat mould strong back

Figure 3 – UBC Sailbot Microtransat mould strong back

To provide even greater resistance to the forces pulling the ends of the strong back up, heavy weights were added to each end.  These also help to keep the whole mould from shifting after it is aligned.

The final step was levelling the strong back.  With a laser level to create a level reference datum, we precisely measured the distance from this datum to the top of the strong back rails. The structure was then carefully levelled using height adjustment bolts built into the bottom of each leg.  Having the top of the structure very level gives us a good reference to work with when levelling the frames later on.

Hull lines

The team designed the basic shape of the hull last summer, but until early this year the final hull lines had not been developed.  With the aid of Philip Barron, Naval Architect at UBC SailBot sponsor Robert Allan Ltd,  we created a set of hull lines by first building a 3D model of the hull using Rhinoceros 3D (with Orca plugin) and AutoCAD.  These hull lines are sets of 2D lines representing the 3D shape of the hull in a manner similar to topography lines on a map.  The hull lines are taken as a series of slices horizontally (waterlines), vertically (buttock lines) and transversely to the hull axis (stations).  See the image below of the resulting lines.

Figure 4 - Hull lines of UBC Sailbot Microtransat boat

Figure 4 – Hull lines of UBC Sailbot Microtransat boat

Right now we are particularly interested in the stations, which define the shapes of the frames we need to build and attach to our strong back.  Later these frames will play the main role in defining the shape of the hull mould as longitudinal stringers are bent around them.

Frames

The frames are the most important part of the mould for our hull.  On their own, they resemble nothing so much as a stack of over-sized bread slices.  When they are positioned on the strong back, however, the shape of the coming boat hull finally becomes apparent.

Despite their importance in shaping the mould, the frames will not be part of the final hull structure, so they are constructed from relatively cheap, but stable medium density fibreboard (MDF).  The shapes of the frames were first printed full size on sheets of Mylar.  Mylar is more durable and less prone to expanding and contracting with changing humidity than paper, making it more suitable for this job.  With the Mylar taped to the MDF, the shape of each frame was transferred to the material with multiple pinpricks.

An important feature added at this point was a set of cross hairs on each frame.  When the frames are arranged in space as they should be to form the hull shape, the cross-hairs all fall on a straight, level line parallel to the axis of the hull.  These cross hairs were the key to precisely aligning the frames later.

With the frame shapes carefully marked on the MDF, we were ready to cut them out.  Straight lines were cut using a circular saw and a jigsaw was used to cut the curved sections.  The final shaping was done with a surform – a sort of cheese-grater type tool that is ideal for shaping MDF.

Mounting the Frames

Getting the frames perfectly aligned to match their theoretical position in the model is the key to achieving a fair hull with the intended shape.  The transverse members on the strong back ensure that the face of each frame ends up on the correct transverse plane.  The challenge then is to make sure that each frames is aligned in the correct position within its plane.

To align the frames, we first drilled a large hole centred on the cross hairs of each frame.  The cross hairs were then carried through the hole by gluing thread across.  Next we set up a laser oriented parallel to the strong back and boat axis, as seen on the illustration below.  Starting from one end we positioned each frame on its transverse member such that the cross hairs were centred on the laser beam.  Then, using the levelled strong back top as a reference to ensure frames were level, we fastened each frame on.

Figure 5 - Laser-aligned frames

Figure 5 – Laser-aligned frames

At that point we had all the frames fastened in their correct position and orientation.  However, we needed to brace them to make sure they stay that way once we start bending stringers around them.  We cut sections of MDF to form a box between each frame.  First checking that each frame was level from top to bottom, we fastened these box sections in, locking the whole structure in place.  The result is what you see in the picture below.  Notice the alignment holes still visible in each frame.

Figure 6 - UBC Sailbot Microtransat boat mould with frames attached and braced

Figure 6 – UBC Sailbot Microtransat boat mould with frames attached and braced

Next steps

To complete our mould, the final step is adding longitudinal stringers.  The stringers are long, square ¾” x ¾” strips.  We cut these from clear poplar donated by Windsor Plywood using a table-saw borrowed from the Royal Vancouver Yacht Club.  The clear (knot-free) and straight-grained nature of the wood in the stringers is important in ensuring they will bend smoothly over the frames.

Once the stringers are in place, we’ll be ready to start laying on the cedar veneer strips to create the core of our actual hull.  Stay tuned for more pictures soon!

Visiting Zaber Technologies

(from right) Tyler, Tobias, and Rajat visiting the Zaber team

(from right) Tyler, Tobias, and Rajat visiting the Zaber team

by Karry Ocean

Back in November, our electrical lead – Tyler Jones – with electrical team members – Tobias Kreykenbohm and Rajat Dixit – and co-captain – Karry Ocean – visited Zaber Technologies in Vancouver. Zaber produces high quality, reliable and incredibly capable motion control products. When Lana Rupp, a mechanical design engineer at Zaber, very kindly invited us on a tour and expressed interest in our project, we were obviously thrilled.

When we first arrived we were warmly greeted by Zaber employees and a very friendly dog on the first floor. Shortly, we were led up to their facilities on the third floor and into a conference room. Honestly, were quite taken back by the amount of interest our project had gathered at their company! Soon enough we had a full room of engineers all intending to assist us with our rudder motor. I could tell Tobias was beginning to get a bit nervous with so many engineers surrounding him with questions, but he proved capable of holding his own. Eventually, with the help of Zaber’s knowledgeable and passionate engineers, we confirmed some of our specifications and further developed our motor design.

Zaber offered to review our final motor design and even offered to sponsor us the components! The assistance of Zaber has been really incredible and just their overwhelming interest in our project is so appreciated!

DSC_0133

Once business was taken care of, our group was taken around for a tour of Zaber`s facilities. What amazed us the most was how much was taken care of in-house! Component design, testing, programming and even some manufacturing were all performed by the engineers in their building. It was also clear how passionate all the employees were about their work at Zaber.

Zaber Technologies is located at 1777 West 75th Avenue, Vancouver. They plan to expand in the near future, with a possible move to a larger facility. This means they`re looking to hire, and are welcome to applications!

DSC_0131

Crossing the Atlantic – Factors, factors, factors

 by Michael Schnetzler

For the past few months, the various sub-teams at UBC SailBot have been busy getting a better understanding of the problem to be solved. All design concepts have to be able to deal with the harsh weather in the North Atlantic Ocean. Sustained wind speeds of 80 km/h (43 knots) and wave heights of 7m are not uncommon. This blogpost will look at some of the factors of the voyage.

Route Planning

Image: Currents in North Atlantic (Source: Encyclopaedia Britannica, Inc)

Image: Currents in North Atlantic (Source: Encyclopaedia Britannica, Inc)

There are two potential routes that follow the main currents in the North and South Atlantic. The Northern Route travels from the coast of Newfoundland to Ireland and is also the shortest route. The Southern Route travels from France to the Caribbean. Based on the report by the US Naval Academy (see report here: Route Planning for a Micro-Transat Voyage), it is more feasible for us to choose the Northern Route and start from the coast of Newfoundland. The shorter distance and stronger prevailing winds will also decrease our time at sea. The only issue here is the prevalence of bad weather, sea states, and among other things: ice (think Titanic).

Timing

Due to bad weather in the North Atlantic, the optimum departure time is between July and August. This appears to be the ideal time between the prevalence of ice in the North and the hurricane season that peaks in September. The amount of effective hours of sunlight to power the boat is also a factor, as seen on the graph below.

Image: St. John Weather Data comparison (Source: UBC SailBot, Environment Canada). The best time for solar power would be the time of year when total hours of bright sunshine (in orange) is highest, with the cloud opacity lowest (especially the 8-10 tenths).

Image: St. John Weather Data comparison (Source: UBC SailBot, Environment Canada). The best time for solar power would be the time of year when total hours of bright sunshine (in orange) is highest, with the cloud opacity lowest (especially the 8-10 tenths).


Navigation

The simplest part of the crossing is having the on-board computer calculate the shortest route based on GPS coordinates and apparent wind angles. But the boat will also be equipped with the ship tracking Automatic Identification System (AIS) that will allow it to “see” other vessels such as tankers. This is handy when dealing with dense shipping lanes (another reason to choose the Northern Route).

Image: Shipping lanes in Atlantic Ocean (Source: Sea Lane/Wikipedia.org)

Image: Shipping lanes in Atlantic Ocean (Source: Sea Lane/Wikipedia.org)

However, many smaller fishing vessels don’t use this system and are also trolling long nets at various times during the year. The following image outlines fishing zones that will be in season during the crossing. This, together with the fact that there is a lot of floating debris in the ocean, is one of our biggest challenges to overcome before we launch our attempt. We are still considering different concepts in tackling these obstacle challenges. To continue this conversation, please don`t hesitate in contacting us at captain@ubcsailbot.org.

Image: Fishing zones in proposed route (Source: UBC SailBot)

Image: Fishing zones in proposed route (Source: UBC SailBot)