Easter came and, as already announced in winter, the idea was to build a hull for the Yamaha 15A. The hull had to satisfy the following conditions: simple and quick construction, maximize flying height on foils, enough volume to sustain two people and lowest possible weight.
Many construction techniques and hull types were evaluated. After lots of discussion we decided to build an internal structure made of wood, glue blocks of foam around it and cover the whole hull in glass fiber. First the shape of the hull was modeled in CAD in order to have all dimensions of the internal structure. The idea of this structure is to absorb all loads from foils, motor and crew, it is composed of two I-shaped girders made of white fir (flanges 28x9mm, web 15×80), traversal structure is composed of 15mm RINA certified marine plywood. The boxes for the foils are 8mm plywood. The hull is 3m in length, maximum beam is 1m and at stern it is 150mm high.
The dimensions of the two girders were quickly verified with an Ansys FEM simulation. Mechanical properties of fir were taken from chapter 4 of Dietenberger, et al. Wood handbook : wood as an engineering material. Madison, WI : USDA Forest Service, Forest Products Laboratory, General technical report FPL, 1999. As expected we had an ample safety margin.
We passed a pleasant morning in the local do it yourself shop selecting our two girder webs and flanges, we discovered that typical construction fir is far from being as straight as we expected, and lots of them had 3 or more knots near the edges. At the end we managed to get 2 almost knot-free exemplars with grains as straight and continuous as possible.
The girders were glued together using epoxy resin. While they were curing the bulkheads and parts for the foil boxes were roughly cut out and the foam (PS15, 15kg/m^3) was glued into blocks.
After curing, a static load test was performed. The results were very satisfactory but to give the final touch, the webs of the girders were covered in 280g/m^2 glass fiber fabric with 45° orientation. Curing was done under about 0.8bar vacuum and 40°C. This was not really necessary but over excitement got the better of us.
In the meantime all parts for the traversal structure and foil boxes were CNC milled. Next followed the assembly of the structure. The pictures are really quite self explaining. Once everything was glued, aligned and where necessary clamped the whole structure went into the oven.
Notice the reinforcements in the corners with triangular stock and the extra plate where the motor will be attached. Weight at this point was 10kg. After curing a quick static loading test was performed. The structure proved to be very stiff, the maximum deformation was under 5mm with a 170kg load. A dynamic load test with the same weight (us jumping on it) confirmed that the stiffness of the structure should be sufficient.
To avoid local problems around the foil boxes and at critical joints some glass fiber reinforcements were applied. While the structure was curing in the oven the CNC hot wire cutter was put to work.
After a few hours the foam was inserted into the structure. It was glued into place with a very micro balloon rich resin. Once again everything was put into the oven.
The only part which remained to be glued was the bow, this last joint was done with 5 minute epoxy.
The foam increased the weight to 16kg.
Now came the time to make the big block of foam we had lying before us look like a boat. Milling gigantic templates was out of question, it would have been over the top. We simply used some strips of wood as guide for the hot wire cutting. With some measuring and a little bit of guessing we pinned the strips to the foam with nails. The only real constraints were that the strip had to be perpendicular at stern and tangent to the curve formed by the two front traversal parts of the structure. For the bow thinner strips were used to allow curves with smaller radius. Cutting the V form of the underside involved quite some planning. As can bee seen from the pictures the strip was put off center and is never parallel to any part of the hull. When we removed the excess pieces we were surprised by how symmetric the result was!
What was missing from this project until now? Exactly! Sanding… time to catch up on that. A few gaps were filled and all the corners were rounded.
Before starting with the glass fiber skin that would cover the whole boat a quick weight check was in order: the scale stopped at 14kg.
We were very excited by the fact that we could put the glass fiber roll on the object to be covered and cut the required pieces directly. This probably is the advantage of building large things!
First some unidirectional material was applied on the deck between the two girders. The fiber is oriented perpendicular to the direction of motion. The idea of this reinforcement was to distribute the weight of the crew better and avoid subjecting the underlying foam core to excessive compression forces. We bought the UD material from an other team. It should be around 500g/m^2. The entire deck was then covered in 280g/m^2 glass fiber cloth. To avoid the hull sticking everywhere when turned around, we applied a layer of plastic film normally used by florists. We hoped to get a nice almost smooth surface with this measure. As it turned out later the finish was indeed good, this because the florist-film proved to be very difficult to remove and we left it on the laminate.
After covering the entire hull with the same glass fiber cloth, this took quite some time, we built our largest vacuum bag ever and with two pumps managed to get 0.2bar. The oven was not quite long enough, with some spare foam we managed to extend it.
Seven hours later we returned to the shop and had a look at our hull. The vacuum had made the skin adhere nicely everywhere and, as mentioned before, the florist-film gave the deck a nice finish. Weight of the finished hull is 21kg.
We dashed down to the yacht club, attached the motor and bought our craft in contact with water for the first time. The calculations performed in CAD proved to be accurate: the boat seemed to float fine in all expected operating conditions.
When the initial respect for the boat had dissipated we floored it. The goal of the day was to find out how fast we would be moving along at full throttle. We accelerated very quickly to about 15 knots and on a patch of flat water reached an excellent 17.0 knots!
Handling of the boat is quite tricky, especially in turns the hull drifts a lot. Throttle response is instant because of the very low weight (about 240kg for boat and crew).
Since the hull is designed to fly the difficult high speed handling is of no concern, it provided a few interesting situations during testing though! At high speed every minimal wave is felt and a 15cm wave results in the crew taking a shower. Weight distribution is also very important especially during turns. In slow displacement mode weight is needed at the bow while on plane it is shifted as far to stern as possible.
A note regarding our power plant: once warm the Yamaha 15A performed flawlessly the whole time proving the effectiveness of the work done last winter (see Xmas project 2009).
After about half an hour of testing we headed for the beach. After the madness of the last days we had time to relax and start looking at the future. We had reached every goal of the design and the build process went quicker than expected. The design was finalized in the final hours of Thursday, Friday we went shopping and had the boat in the water on the following Tuesday at 5 P.M.
Before summer some minor finishing work will be performed on the hull and the 15A will receive a new fuel filter assembly and a kill switch with bracelet. A prop with higher pitch could be an option.
Here is some video footage of our Easter Project 2010: