During the Summer Project 2010 we were asked by Fabian if we could help him with the construction of a keel for the boat he was building. It is a StarTruc22 designed by Paolo Bua.
As the project started to take shape it soon became clear that we also would design and build the rudder. We started with the design of the keel. The first step was to perform some rough simulations to get an idea of the order of magnitude of the stresses. As expected a spar made of HM carbon fiber would not have difficulties handling the load of a keel bulb weighting around 150Kg.
We decided to use T700SC fiber made by Toray. The tensile modulus can be determined with reasonable accuracy using the fiber volume content. The ultimate load however is not so easily predictable, to determine these numbers some tensile tests were performed.
The load case where the boat is heeled 90 degrees (and the bulb is in the air) was used to design the spar.
For the moulds the original idea was to cut sections using the hot wire CNC cutter and join them on a MDF board. After a test cut it became apparent that this would never work: the foam had residual stresses, by cutting one skin away the panel would distort, the hot wire cut also distorted the part.
A change of plan was in order. The foam blocks were glued on the MDF boards, let cure under vacuum, roughly cut using a hand hot wire cutter and then CNC milled. In the meantime the T700SC was cut.
This solution provided two big challenges: first a hot wire cutter with 2.6 meters span had to be made quickly using available parts. The thermal elongation of the nickel chrome wire was so large that it had to be taught after heating. The cutter needed 150W of power!
Second: our CNC mill has a maximum travel of 1680mm on the x-axis and the moulds were 2560mm long. A system to quickly align and support the moulds was assembled.
The milling time for one mould was only 4 hours! After some sanding the foam was covered with Oracal 651.
The spars were made first, this gave us the chance to test them and verify the accuracy of our FE models. We measured a deflection of about 37.8mm (since the supports were 100mm tall), the calculated value is 40.43mm. As can be seen form the picture the deflection pattern of the simulation is indeed similar in reality.
Satisfied we proceed with the keel skins. To cope with the torsion loads and to best transfer the loads from the keel to the keel box, 10 layers of 310g/m^2 biaxial glass fabric were used. Two external layers of hybrid carbon/glass fabric were also used, these give a nice black color to the keel. The complete layup of a skin is 2 layers hybrid carbon/glass, 2 biaxial glass, spar, 8 biaxial glass.
When the skins had cured it was verified if the two moulds would close, since the closure was not perfect the laminate had to be milled down to the correct thickness near the trailing edge. We found the product name of our overalls quite amusing by the way.
With the skins ready it was time to start thinking about the closing of the two halves. The keel would be screwed to a plate, which would rest on the deck of the boat. To align the screws a small jig was manufactured.
A problem which caused several headaches was the web of the spar. In the end a classic approach was adopted: the two skins were measured at every thickness change, a sandwich of 10mm Airex with four layers of 310g/m^2 biaxial glass fabric on each side was manufactured and the spars were CNC milled form the sandwich.
The nuts attached to the threaded tubes were additionally secured using some basalt fiber tows. Carbon fiber was avoided because of the potential corrosion problems. The central part of the spar, on the top and bottom, was filled with chopped glass fiber rich resin.
Opening the moulds proved not to be so straight forward: Oracal 751 is slightly permeable and on the leading edge the film stuck to the keel. Sadly the moulds had to be destroyed. The quality of the part however met our expectations.
Next the leading was trimmed and the trailing edge was CNC cut. As can be seen from the picture the keel is perfectly straight
The leading edge was nicely closed but we did not trust the microballoons alone, to avoid any chance of the two halves parting company a preimpregnated carbon fiber sleeve was pulled into the keel. To guarantee a good bond the keel was lifted on one side and resin was poured into the D box until it started to drip out on the other side. The keel was levelled again and excess resin slowly trickled out. To be continued...