Is there a simple formula for a lamination schedule for foam core centerboard?

I have spent hours digging around dozens of websites and found some suggestions that seemed close but nothing right on the money.

Background: I want to update the 1977 designed poured foam centerboard for a Stiletto 27 which are fat and short. I have hand shaped a 4'X8'x2" sheet of 10lb foam into three 8ft long boards each 18 inches, 16 inches and 14 inches wide using a NACA 0070 template. Now I need to laminate them and looking for a ballpark schedule. I plan on using the vacuum bag technique with epoxy but not opposed to composite materials.

Worst case scenario I plan on simply adding 2oz layers alternating with a strip of 6oz mat on the chord line until it is heavy enough to not need a downhaul on the board.

Any thoughts?

Thanks in advance

Brad in Jax
2x Stiletto 27's (one for sale soon)

I've vacuum bagged stuff myself-mostly large, remote controlled jet wings. These are the 30 pound, 200 mph, 80 inch wingspan jobs, so flutter and strength are a big hairy deal. I don't think you'll get a lot of feedback as not too many people seem to do this. At least that are on these forums.

Please post your progress as I'll be doing the same, but with some rudders for a H21.

I'd first consider your layup directions-you want to alternate cloth on the bias because it will give you better torsional rigidity-you don't want them to flutter. You also have the opportunity to incorporate unidirectional carbon fiber on each side to give a more rigid shape against lateral loads, like an I-beam. I would epoxy this down on the cores first, because it's generally thicker and you can hide it with your glass cloth. If you don't do it all in one layup, then be sure to have peek ply on top to pull out any amine blush and give the next layer tooth. A single layup will be stronger, just messier. On hollow wings we used to shoot for something around a 18-20 ounce, combined layup, not including carbon reinforcements. On such a large dagger with a bunch more abuse, I'd bet twice that is about right, but don't know for sure. 40 ounces seems like a lot...Just guessing.

And post cure, for sure. Simple hot box with lights made out of foam board works wonders.

Run some small scale tests; that's what I'd do, then share. Once I get my equipment running in about a month (after other projects), I'll do the same.

Good luck!

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Chuck C
NACRA 500 Mk2
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Here's a lead: https://www.boatdesign.net/threads/daggerboard-cores.43739/

Looks like 50-60 oz, with additional reinforcements is correct. I'd build it like a wing, with a very strong full-depth spar right at the CP/ thickest point of the foil. Basically, tying the two skins together as they will be stressed. Fiberglass does way better under tension.

Looks way more complicated based on those threads, but possible. Good luck!

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Chuck C
NACRA 500 Mk2
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My post got dumped...

Charles came to about the numbers I did, using my personal rules of thumb...50oz of high modulus carbon. NACA 0070 is probably too thin, especially at the top of the trunk to handle the loads. High aspect ratio F18 boards are about 30oz of carbon (mostly uni, high modulus is better, ) on each side of an epoxy foam core and they are 12% thick.

IM7 uni carbon from these guys is about the best bang for the buck: https://www.sollercomposites.com/UNI.html

Skins, I would want minimum of 20oz of fabric on the bias, again, carbon is preferred, glass can work. By bias I mean 45/45 to the leading edge.

Your core is a bigger problem. In high load, high performance applications I use Rohacell HF71 or similar (Divinycell H100) for a shear web with carbon on both sides. In this application you need extremely high compressive load strength at the bearing. This is generally done with a titanium insert for a big boy boat, or a core of high density vertical grain balsa. You could also get away with a few other options (basically solid carbon all the way through).

Edit: If you shaped with 10lb density PVC foam or closed cell foam you might be okay. If you used 10lb density hardware store pink/blue foam, not so much.

The reality is you need to hire an engineer. I am one and qualified to do this work but my consult might be above your project budget. The original designer may have some cheaper options for you.



Edited by samc99us on Nov 13, 2019 - 04:48 PM.

I have some 7" wide unidirectional carbon fiber for your project. Pete 909-800-5237

I did a box approximation, and got 60oz per side (.094 inches) which is surprisingly close to samc99us answer. I then tried to simulate it with FEA solidworks, and the program crashed.

You need the composites simulation suite in Solidworks to simulate this properly or the program will crash generally speaking. There are some other ways to do the modelling but its generally more time than its worth. We use it to evaluate the buckling mode of the core. Reality is its hard to get material data on these things, so you have to fabricate some samples and test.

samc99; To your point - what side load values are we looking for?

I know that the point of failure will be focused on the exit from the board trunk (as expected), but really have no idea of what lateral loads are imparted on a Stilleto 27 versus say my Hobie 21? I'm equally intrigued about what the loadings on rudders would be. Just round numbers would do, because this is backyard engineering and tests would be some kind of destruction test using weights. Very un-scientific. However, gets one much closer to success. Afterall, hard-headed folks like myself are going to do this, eventually and never go into business making this stuff - that's for sure.

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Chuck C
NACRA 500 Mk2
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I got 2000 lbs, assuming that all of the sail force is transmitted to the board, and finding that force using a pressure chart and the known sail area. That seems rather dubious to me. The rudder has a more direct calculation: just take the maximum Cl of the foil section and plug it into the lift formula along with maximum speed.

I redid the force on the board using a right moment method, as detailed in this file https://docs.google.com/f…p0dk5PckhtOFE/edit?pli=1 and got 1000lbs, instead. Nice short basic read on design, but no mention of construction (as usual)

ps i know nothing



Edited by ziper1221 on Nov 15, 2019 - 09:05 PM.

Many thanks for the thoughtful and informed replies. I have taken 3 different approaches to this problem: mathematical, computer modeling and direct application of experience and all come out to a similar result.

I will be laying up the three boards with a light, med and bulletproof schedule and then testing.

Watch for updates

Brad
Jacksonville FL
2x Stiletto 27's (one for sale soon)

Have you calculated how much skin you will need to get the foam neutrally buoyant? And remember, if you use carbon, any glass becomes more or less structurally irrelevant.

ziper1221

Have you calculated how much skin you will need to get the foam neutrally buoyant? And remember, if you use carbon, any glass becomes more or less structurally irrelevant.

Yes, about 45oz is neutral. Going with straight west system epoxy and 8x6oz layers bagged and heat cured with no carbon at this time as these three boards are basically test boards. Once I have a size, shape and schedule I am happy with I will redo in Carbon Fiber.

So is it worth it? Used Centerboard is 1500 (if you can find one) new is 2k. But that will still be the short, fat wide board designed in 1977.

Here are the numbers:

I bought a 4'x8'x2" 10 lb density closed cell foam sheet for $200 which was enough for 3 foam cores. $500 for enough epoxy, 6oz mat and other supplies to finish 3 boards. So my cost per board not including labor is a bit more than $200 a board. I estimate each board at about 10 hours of labor.

$400 a board in round numbers if I paid myself for my time.

Completely worth it.

Brad
Jacksonville, FL

2x Stiletto 27 (one for sale soon)

If by 6 oz mat you mean chopped strand mat, absolutely do not use that. Pick up some unidirectional glass, and use that for the long axis, and then some +-45 for torsional strength.

ditto on not using mat for this application. Mat is for building up bulk, which helps with strength, but uni-directional materials and cloth are the best here; much stronger per unit weight. From experience. I THINK ziper means to lay the 6 ounce cloth at a 45 degree bias across the long axis of the board. That technique is very common to prevent twisting/warp/flutter. Alternate directions with layers. Can't speak to using uni-directional materials in the same manner.

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Chuck C
NACRA 500 Mk2
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ditto on not using mat for this application. Mat is for building up bulk, which helps with strength, but uni-directional materials and cloth are the best here; much stronger per unit weight. From experience. I THINK ziper means to lay the 6 ounce cloth at a 45 degree bias across the long axis of the board. That technique is very common to prevent twisting/warp/flutter. Alternate directions with layers. Can't speak to using uni-directional materials in the same manner.

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Chuck C.
H21SE 408

Nicely explained.

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If by 6 oz mat you mean chopped strand mat, absolutely do not use that. Pick up some unidirectional glass, and use that for the long axis, and then some +-45 for torsional strength.

Great tip! Thank you!

Please keep us informed on your progress. I am building a tip up rudder for my Hobie Holder 20.

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Hobie Holder 20.

COOL BOAT!