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Question about foils


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Newly acquired nacra, foils have lots of little chips in the trailing edges.  I'm cleaning them up and repairing but the question is how to finish the trailing edge, I seem to recall that the correct shape is a slightly assymetrical squared -off shape rather than sharp.  Thoughts?  Thanks.

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That’s what I understood makes for silent/no vibration merging of the flow as the two tiny vortices overlap.

Sharp is quiet but tough to maintain. 

On bigger foils I like to fully tape the trailing edge for a proper structural and waterproof closure which inevitably makes for a somewhat fat exit then I add a sacrificial, thickened resin, knife edge (read 1/32” rad) trailing edge.

 

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When I raced H16s many years ago the fast guys all had squared-off trailing edges on their rudders. Maybe about 1/16" or so across, maybe a bit less. No hum at all, the water broke clean off the trailing edge. You don't want a round trailing edge, it will hum and slow you down a tiny bit. If you're not racing the humming is still irritating.

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On 4/27/2019 at 9:53 PM, boardhead said:

That’s what I understood makes for silent/no vibration merging of the flow as the two tiny vortices overlap.

Sharp is quiet but tough to maintain. 

On bigger foils I like to fully tape the trailing edge for a proper structural and waterproof closure which inevitably makes for a somewhat fat exit then I add a sacrificial, thickened resin, knife edge (read 1/32” rad) trailing edge.

 

Can you describe this a bit more? I might try on my recreational foiler. We're starting to find its limits in speed and handling, and I think drag on the rudders is a big factor.

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On 4/27/2019 at 8:27 PM, mundt said:

Newly acquired nacra, foils have lots of little chips in the trailing edges.  I'm cleaning them up and repairing but the question is how to finish the trailing edge, I seem to recall that the correct shape is a slightly assymetrical squared -off shape rather than sharp.  Thoughts?  Thanks.

Tricks of the trade:

1) Use toughened epoxy mixed THICKER than peanut butter consistency to fill the nicks

2) Bad nicks need to be filled with carbon, see Goodall website for a youtube video on the process

3) Older Nacra boards are gelcoat at the trailing edge so chip in about 3 seconds when stared at wrong. I don't bother re-filling with gelcoat, its easier just to fill with epoxy and call it a day.

4) When using the epoxy, I take clear packaging tape, rip a small piece off and put that down against the tape then wrap that around the board trailing edge. If you can then get some wood across both sides can clamp you'll have a perfect repair with very little sanding. Use less epoxy than you think.

Boards make noise for more than just the trailing edge, yes thinner is better but its hard to get them thin enough and keep them in one piece. Specifically the area at the back of the board with them all the way down I wouldn't worry too much about, better to be too thick there. The other reason for board noise/vibration is vertical scratches, those need to be removed and that is A LOT of work to clean up.

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Go sailing. I just ground into a stress crack on a rudder built in the same manner, right where it leaves the casting. The underlying laminate was intact. Sure I reinforced it with carbon as that+paint is smarter than gelcoat, but it was fine as-is and I suspect your boards are too.

Does anyone have a good paint that can take flex in very highly loaded areas (t-foils on rudders)? I'm thinking Durepox as that is what the Moth guys use but something compatible with the Awlgrip/craft system would be nice.

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thanks, I agree that if they aint broke yet the core is probably pretty strong but it's still tempting to add carbon wherever one sees signs of stress, including my frontal lobes...

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1 hour ago, samc99us said:

Does anyone have a good paint that can take flex in very highly loaded areas (t-foils on rudders)? I'm thinking Durepox as that is what the Moth guys use but something compatible with the Awlgrip/craft system would be nice.

Far cheaper and with all the benefits of flexible paint, think of what your car paint sprayer uses on plastic bumpers and trims, do you ever see cracks or paint peeling, nope. I've used 2K Acrylic automotive paint now for over 10 years and the benefits are huge, never discolours, never needs polishing like gelcoat, quite tough ( not as good as gelcoat ), easy to repair, flexible, bonds really well and if you use 2K easy sand primer it all even get easier and less expensive to build and repair.

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6 hours ago, samc99us said:

I'm thinking Durepox as that is what the Moth guys use but something compatible with the Awlgrip/craft system would be nice.

I know Lars has gone through a variety of products and Durepox was one of them (he finished my T rudders with it and they were NICE).  I'd use that since you probably want to wet sand it instead of a glossy finish.

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On 5/1/2019 at 3:33 PM, samc99us said:

Don't use carpet in the trunks...yes I know its easier said than done, 4 hours of filing later...

I def don't have carpet. Foil trunks have a big HDPE piece with the foil profile cut through. Minimal slop to allow for rake adj (top HDPE piece slides fwd and back) but tight otherwise. We're in Miami and launch from a beach so there's always some sand. 

Foils are bottom insert, which adds to the complications.

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[Corsair F-31, aftermarket CCI/Phil's foils high-aspect dagger, rest of the boat mostly stock]

I've got some slop between the daggerboard case and the foil.  I was thinking of procuring a big 4x8' sheet of thin HDPE and fully lining the slot, then if needed, shim it from behind to both tighten things up (stop side/side clunk) and get smoother up/down (cocked board binds slightly glass-on-glass when under load).

Any advice/guidance/reminders?  Does anything stick to HDPE if one wanted to bond to it?

Cheers,
ben

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