This will be part 1 of a 3 part series which talks about the process of designing and building a fiberglass boat. The electronics and a basic system was tested on a small prototype boat and the next step after this was building a full size fiberglass boat.
- Part 1 – designing the hull
- Part 2 – building the plug
- Part 3 – fibreglassing to create the mould and boat
Designing the Hull
There were many things to consider when designing the boat. The main factor that determined its size was that it had to carry two solar panels. Each solar panel is 1.1m long and 0.6m wide, and we also wanted room on the deck to install a wind turbine. This meant that the overall size of deck required was 2.8m x 0.8m. Even with 200w of solar panels this really isn’t a lot of power because 200w is the maximum power output they could potentially produce. Whereas in reality it’s likely you’d have a lot less than that.
So this means that the main factor in our design was creating a boat that was as lightweight and efficient as possible. Let’s also not forget the fact that it’s going to go to the Arctic so it needs to be able to withstand sub zero temperatures.
Now it was pointed out to me that I could have taken a kayak and modified it, and at the time that seemed like a lot of work for something which would need lots of modifications. In hindsight this would have actually been the quicker option, but where’s the fun in that!
The first design choice is whether to go for a mono or multi hull design like a catamaran. Whilst the catamaran is more stable, if it were to capsize it would be a lot harder to upright. Due to this and the simplicity of building we have gone for a mono hull. If the boat was the size of the deck required, then this would have a large surface area and displace a lot of water which also creates more friction. Therefore using the inspiration of an aircraft carrier we’ve designed a large upper deck which tappers down to a narrow hull.
So what should the depth of the boat be, well the Solar Voyager and Sea Charger had designs where the deck was very close to the water but I wanted a deck that was high out of the water. This would reduce the amount of water on the solar panels but also allow a wind turbine to properly operate. A downside of this is that a deeper boat increases weight but also creates a larger exposed surface where the wind is likely to pull the boat off course. However I felt that the turbine definitely needed to be out of the water and overall hoped that the power gains would compensate for this.
|Max Speed||Weight||Draft||DL Ratio|
Now that the boat’s dimensions and shape have been established it’s possible to calculate different performance metrics. The primary one is speed as the max speed is determined by it’s length, which in this case has a LWL (that’s the length of the boat in water) of 2.5m, which for a displacement hull brings the top speed to 3.7 knots. The only way to increase the speed over this is to get the boat planning, but as this is a slow efficient boat, this will not be done.
The DL ratio (displacement length ratio) is another interesting metric as this will tell us how heavy the boat is for its size.
D/L = DLT ÷ (0.01 x LWL)³
DLT = displacement long ton (2,240 pounds)
LWL = length water line (ft)
60kg = 132 pounds
2.5m = 8.2ft
(132/2240)/(0.01*8.2)^3 = 106.877
With a LWL of 2.5m and an estimated displacement of 60kg our DL ratio is 107. This puts the boat in the light category. In our case lighter is better for efficiency however this does mean that the boat is going to rock around a lot in swells.
I read lots about boat design and other metrics such as stability and other ratios but I’m not a boat designer and I think that after absorbing so much information it was actually difficult to create the perfect design, but in part 3 I’ll talk about the reality of the design and some of the downfalls.
With the design in place, I printed a small scale version on a 3D printer, and I could actually see a mistake in the symmetry of the design.
“always print your design and test it”
Therefore I’d recommend printing any designs out as it allows you to see in real life the shape of your model, whereas looking on the computer screen can sometimes be misleading.
When you look at most boats the propellers don’t directly come out of the stern, but they come out just before the stern submerged under water. To actually build this in to an efficient design was going to be pretty hard due to the curves required. Plus, if the motors were to come out directly from the stern if the boat is not submerged enough then the propellers may not always make contact with the water. Therefore the solution has been to have the propellers coming out of pods beneath the boat which ensures that they are always in the water and provide maximum propulsion. As we can 3D print the motor pods we’ll be able to have a streamlined design.
Fibreglass is a common material used to construct boats but as I had no experience in this area I spent weeks researching how boats are made and how fibreglassing is done. One of the big questions was how many layers of fibreglass to use and what kind. This is very important because it directly impacts the boats strength and weight which are both critical for it to make it to the Arctic.
The issue is that most resources around fibreglassing are for full sizes yacht construction or small dinghies. Therefore to create my design I’ve had to use guidance from what other people have built and make estimates. Normally I’d like to calculate everything and take a scientific approach, but sometimes with the complexity of the formulas or the difficulty in what you’re trying to achieve this is not always possible.
Traditional fibreglassing techniques for boat construction include alternating layers of chopped strand matt (CSM) with woven roving. These days epoxy resin is preferred but it is very expensive, so I decided to use polyester resin. These decisions all impact the design as they determine the weight and therefore how much of the boat will sit in the water.
Another fibreglass method uses vacuum forming and whilst this is popular I don’t have the resources for this, so we used the laminating method which is also very common.
Based on what other projects had used I decided to use 6 layers for the mould, alternating 300gsm CSM and 600gsm woven roving. For the boat I used a layer of 300gsm CSM, 600gsm woven roving, and then a layer of 300/600 combination matt (which is basically the CSM and woven roving combined).
The idea is that because it’s already been bounded together you can fibreglass both layers as one at the same time and save resin. Each layer should be 1mm thick which should give us a 4mm thick hull.
We calculated that the weight should be 20.7kg which was actually a pretty accurate estimate as the hull turned out to weigh 20.1kg.
Calculating the displacement is important as we can work out how far the boat sits in the water. Physics tells us that for 1kg of weight pushing downwards on the water then 1 litre of water is displaced which is 10cm³. To work out at what depth the boat sits in the water, if you have a 10kg boat, then 10L of water is displaced. Therefore on your model you need to calculate the volume at different heights from the bottom. At the height where the volume is 10L that is the depth of the boat in the water. You should be able to calculate this in most modelling software but you can also make very rough approximations if you treat the shape of the boat as a square or a triangle.
Method of Construction
The method of creating the fibreglass boat starts with a plug. This is essentially the shape of the boat you want to create which you then fibreglass on top of and create a mould. By flipping the mould, you can then fibreglass on the inside of that to make the actual boat. The advantage of this is that if you use a mould to create the boat, then when you remove it from the mould the outside of the boat will be smooth.
However if you decided to skip the mould step and just fibreglass over a plug, you’d get the same boat however the smooth side would be on the interior. This is less than ideal as the surface wouldn’t be smooth and therefore you’d have to do lots of sanding. After spending countless hours of sanding and sculpting for other parts of the plug I’d recommend avoiding extra sanding at all costs.
“avoid extra sanding at all costs”
I always forget that materials are a lot harder to sand than I expect, so be warned! The main disadvantage to using a mould is the extra time and materials used in creating it. Also the mould has to be thicker than the boat so that it is completely rigid, and normally as moulds are used multiple times this means that they are designed to last.
So now we have the boat designed, it’s time to read about how to create the plug in part 2.