Lifting Foil Design

Hi there,
I am just looking for a bit of help with designing a lifting foil for a 50ft catamaran. The goal is to produce a foil that can produce 2 tonnes of lift at the leeward bow. This is not a foil for lateral resistance (the boat already has dagger boards) but will contribute some as it will be a J foil. The foil geometry has already been worked out and I am now trying to deal with the structure. The guy who did the geometry should hopefully be able to give me the maximum load on the foil which I can use for design. I am designing the foil to have unis top and bottom with a shear web in the centre. I am and Engineer and have used FEA before but never for composites . Is this something I can work out by hand? I am not sure that I can just use good old beam theory for this one as it is a composite curved beam which probably makes beam theory not very accurate. I have come across an example in a book 'stress without tears' where an aeroplane wing spar is analysed using a simplified method whereby the foil was assumed to be a cantilever beam where only the web takes all the shear and the flanges deals with the tension/compression, this is for plywood however. Is this approach an over simplification?
Any help on the topic would be fantastic!
Cheers!

 

Yes, this is an over-simplification. You can't use beam theory because the failure mode for a composite J foil is likely to be through-thickness failure at the root of the curve, and beam theory won't even consider this, let alone actually calculate it. Make an FE model, but you need to fully understand what you are modelling and how it will respond to get accurate results. Making models in FE is really easy, making models in FE that produce accurate results requires an understanding of the method and the structure being modelled. A lot of our work is remodelling structures that have already been modelled by supposed experts who did not capture the structural response because they did not understand it. Garbage in, garbage out.

 

Thanks for that, seems I may have to give FEA a crack after all! Hopefully I can find stuff in the help files for composite beams

 

Be careful, a J foil is not a composite beam, it's a 3 dimensional structure loaded in 6 degrees of freedom. You need to create a solid model of the foil, with enough discretisation to capture the through thickness effects in each layer and the change in radius in the corner. You also need to incorporate the effects of structural deformation on loading, and consider dynamics.