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landmarine 02-02-2010 04:00 PM

25 foot trailer yacht - Rudder Questions
I have a GRP 25 foot trailer yacht that I'm wanting to improve the rudder on. Basically the foam GRP one I have weighs almost 10kg and is not optimised in shape and its leading edge is 40mm aft of the gudgeon pivot line - It feels like I'm dragging 2 buffalos!!

I intend to construct a new one out of Divinycell (HD) / e-glass / epoxy resin (resin infusion) - and wanted a few tips if possible on the following subjects?:

1) Is the green code Divinycell suitable? blade is approx 1300mm x 360mm - and will need two 20mm thick sheets laminated together with e-glass in between for datum line and strength at the edges
2) What would you recommend in terms of laminate cloths? Someone suggested two layers of uni-glass or carbon followed by a single tri-axial sheet??
3) What additional thickness allowance do I need to make for the 3? skins (what cloth weight should I use)
4) I'm getting assistance from a boat builder but just wanted a 2nd opinion on things - The trailing edge is the difficult one as we need to have a thickness of no more than 1-2mm here. We are using a modified NACA 0012 section with hollowed out trailing edges (concave on exit straights)
5) any other pointers you think might be helpful please??:) :)

Any assistance appreciated thanks

sailingdog 02-02-2010 04:21 PM

What weight is the green code Divinylcell foam. I'd point out that you may want to weight the rudder so that it is neutrally buoyant or a bit heavier than that it doesn't tend to float up.

I wouldn't use uni-glass in this application, since the loads are going to be multidirectional. Triaxial roving would be a good material to use...with some regular cloth or CSM over it to protect it from the fairing/sanding process.

I'd also recommend putting in a layer or two of kevlar, preferably under CSM/cloth but over the triaxial roving. The reason I'd add the kevlar is that it is highly abrasion and puncture resistant.

I wouldn't use Carbon fiber in a rudder, since the carbon fiber can cause galvanic issues with metal hardware, which the fiberglass and kevlar will not.

Hope that helps.

landmarine 02-02-2010 04:31 PM

thanks for your prompt reply. As we are racers we wanted the extra buoyancy in the stern - The rudder is a swinging one located in a box - I calculate when fully immersed in seawater the buoyancy will be around 7.5kg - we reckon we can hold it down with the tie-down rope (multi-purchase if need be). Thanks for the good advice:o

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