1. Can anyone estimate the size of the shark that did this ?
2. Which species.
3. How much polyfiller will it need.
4. Will brown paper be strong enough with pva as a skin ?
5. has anyone got the naca end profiles to allow cutting of a fresh foam core to enable a replacement to be made.
6. What's the best epoxy for the layup?
7. Are there any tricks for the bearing/pivot block for a foam centreboard ?
8. Should I use carbon tows or rovings in the leading and trailing edges ?
9. Should I just make one out of marine ply ?
10. Why is my wife looking at me in an odd way since I bought this thing ?
To summarise ... HELP !
John For several years I have used a NACA 63006 section.The ordinates can be found at http://www.pdas.com/profiles.htm#63-006.It needs some care when tacking but compensates in straight line performance.If you get really keen you can download Profili or some of the useful programs at http://www.suhail.ch/soft.htm . Like Mike,I avoid Epiglass and have a preference for SP-now known as Gurit.I use a 25mm dia Tufnol bush,as long as the case is wide, as a pivot and the lack of compressability allows the board to pivot while applying enough force to seal the rubber washers on the outside of the case. Marine ply should be avoided.The only ply fit for foils was anti-fracture board from Thames marine ply-note the use of the past tense. Carbon will do no harm but may do less good than you anticipate.A little flexibility in a centreboard is sometimes useful. Good luck with the project.
G-Whizz Thanks John,
One Idea I'm toying with is laminating one from 4mm birch ply with several layers of glass cloth on the bias in the centre and single layers between each ply. Then glass cloth the whole thing. ( Extra on the leading edge). The glass cloth in the centre should allow a nice thin trailing edge which will be completely waterproof if chipped. Is this a bad Idea ?
How was the original centreboard constructed ? Was the one in the photo, which looks like rotten plywood, an original ?
John I would avoid the use of the hybrid birch ply and glass construction for a number of reasons.The grain orientation in the ply will not br totally favourable and the material itself is one of the less durable types of wood.The Thames ply referred to in my earlier post was made of a timber that was rated as durable and which had the overwhelming majority of the veneers aligned to cope with the loadings.
The use of layers of glass cloth between layers of ply will not accomplish too much until the outer surface of the board is approached,the additional strength near the centre is too close to the neutral axis to enhance the strength.It will also be very challenging to cope with the different hardness of the varying materials,unless you intend to use 5 axis machining and carbide cutters.
Depending on your experience and access to materials,either a laminated wood core or structural foam would be good choices.
The use of a glass strip along the trailing edge will help to preserve the most vulnerable part of the foil.My choice would be to cut a deep groove and insert a strip of cured epoxy/glass.
I just checked the suhail link and it worked,I used Sea Monkey as the browser.Several interesting programs there and I would particularly like to learn to use the Lisa FEA freeware to model a Javelin rig.
I have no idea what type of centreboard would have been originally fitted.For what its worth when we had 121 it had a GRP board,not the lightest but durable.
G-Whizz Thanks again John, When is your CNC machine free ? Are there any current trends for the outline of the board? Or is the one in the photo fairly typical ? I have no idea if it is even the correct length or chord. Is there a dimensioned drawing anywhere ?
Mike The rules control the size only by stating that the board must retract fully inside the case and the max width is 27mm.
Myself and the Earls redesigned our boards about four years ago with Milanes. We opted for high aspect or long and thin.
In theory at least, it is supposed to give us an edge upwind when fully up to speed. The down side is that it is a lot easier to stall coming out of tacks and gybes.
Short and fat is good for low speed and long and thin is good for high speed. Take your pick
We're in the middle of an emergency re-fit on our boat and I thought I'd take a shot or two of the board for your interest.
Click on the image for a bigger version.
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G-Whizz That trailing edge looks sharp enough to shave on! What timber is the core made of ? Sitka ?
I've just stripped the rudder on mine and it looks like cedar much redder in tone than your centreboard.
Mike Its very sharp and not very crew trapeze hook proof when you capsize! Milanes uses yellow and red ceder I believe and then its encased in epoxy glass.
G-Whizz Ok here's a tricky one ... where can you buy cam cleats with 41mm fixing centres ?
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