Is plywood suitable material for centerboard?

Hello all,

I'm in an early development stage of a ~5m length, skin-on-frame folding monohull sailboat design (some people here might remember me considering multihull, but I fallen back to monohull due to financial and design complexity reasons).

Main characteristics are as follows, but subject to change:
Length: ~5m
Width: 1.3m
Sail plan: bermuda sloop, 12-13m^2
Weight: 60-80kg (still hard to predict)

Building method - skin on frame construction, stringers are made from aluminum tubes (2-3cm diameter), ribs - from 2-3cm thick marine plywood. Not sure about the mast yet, but probably 2 or 3 piece aluminum, 10cm diameter. Sails will be from Tyvek, with battens (I made some Tyvek sails already).

Now I'm deciding on centerboard construction. I decided on NACA 0012 airfoil shape, having red that this is most suitable shape for boat of this size. Chord length is decided by maximum thickness - it shouldn't exceed thickness of marine plywood, which I'm thinking to use for this centerboard. If plywood is 3cm thick, chord length is 25 centimeters. For planform, I think elliptical, since I also red it's most efficient - and the length of centerboard (from the point where it exits the hull) will be something around 60-80cm. Also, for the trailing edge, I'll shorten it by 1cm and round-fill, so it won't go any lower than 3.1mm thick.

So let's say 25cm chord, maximum thickness - 3cm, length 70cm. I think I will achieve that shape with some planes, chisels and rasps, after which I'll cover it with few layers of waterproof varnish.

My question is - will it hold, and for how many years? I red that centerboards are subject to extreme forces, especially twisting. Marine plywood is a sturdy material, but it ain't steel or composites. So I'm not sure. I thought maybe some of you might suggest if this is safe enough, or maybe I should use thicker plywood or go for NACA 0015 or higher profile.
I don't think I can get marine plywood which is thicker than 3cm, but I can always layer it with epoxy (not preferable, though).

About reinforcing - glassing is out of the question (mainly, budget, secondly, I already tried it - don't want to fail so miserably ever again), and reinforcing with metal - well, I'm not sure what tools will be needed...

Any insight into this centerboard deal would be greatly appreciated. Thank you for reading.

 

Well, the suitability depends merely on you and your needs.

Pros of plywood:

- cheap
- easy to work with
- good control over the section shape (due to the multiple layers of veneer that become visible when you shape the blade)
- quite durable if you use marine grade plywood and seal it with epoxy (probably with some glass fibre sheathing as well)

Cons:

- not very stiff: almost half of the fibres of the wood run "the wrong way"
- not very strong: see above, therefor no real thin sections possible
- not very light (compared to foam/carbon construction)

If you are willing to compromise on performance, then there is nothing wrong with plywood. But there are better (but more expensive!) materials if you want it light, stiff, strong and thin at the same time.

 

Structurally it should be OK. But it doesn't have nearly enough area for the Sail Area (SA) you envision. The absolute minimum board area should be 2.5% of your SA, or: 12sm * 0.025 or 0.30sm. What you propose is just two thirds of that: 0.80m * 0.25m or 0.20sm. And that is assuming that the area mentioned is all going to be outside the case.

 

Well, weight isn't an issue. I'm not building a racer. It has to be strong and cheap. Are there alternatives to plywood that would cost about the same?

sharpii2, thanks... I never knew of such minimum. I'll adjust that area.

 

Yes, make a blank of several strips of solid wood (cedar is frequently used for that). Glue them together with epoxy so they do not warp, then shape your blade from this.

You might also want to read this, where the construction of such a wooden foil is dealt with in detail.

 

The basic problem with plywood in a board, foil or blade is "rolling shear" within the veneers. The constant bending and torquing of the oriented fibers within the sheet, causes a shearing failure in time. If you do elect to use plywood, use thin layers of plywood, with lots of veneers. On a 30 mm thick board, consider 3 layers of 9 mm plywood (5 veneers each), glued together, with a light sheathing for protection. Lastly, orient the 3 layers, at 12 degrees to each other, to take best advantage of the 0/90 fiber orientation. Simply put, the center 9 mm piece, should orient with it's grain vertical, while the one side is canted 12 degrees forward and the other side 12 degrees aft. This sandwich is slightly stronger and this strength is free, just by orienting the grain as mentioned. Personally, I'd use a strip planked board, but in a small, lightly loaded boat like yours, the plywood will work fine. Also consider a flat sided foil, instead of a NACA section. They're much easier to shape and for the speeds you'll move at, just about as efficient too, for much less work. More importantly, they're generally stronger, as you don't remove nearly as much material in shaping.

 

We once had a very popular one design racing class here in the states. It was called the Windmill Class. The class rules required that the dagger boards were to be no more than 3/4 inch (20 mm) thick. Most of them were made of 3/4 ply. The rules allowed the leading and trailing edges to be flat tapered only one and one half inches in front and back.

These 16 foot boats were very good to windward and the board was adequate. It was possible to break a board when grounding hard but the 3/4 ply was usually no problem. It was permissible to use solid lumber which was usually made up from several strips of 3/4 wood. The more competitive sailors usually did it that way but the one and a half inch taper was still required. No NASA sections allowed. I confess to having cheated for a while with a fancy NASA profiled board and it was not a damned bit better than the legal board. In fact it was quicker to stall that the legal one.

Moral: Don't sweat the board, just make it big enough to do the job. A too small board that is beautifully shaped will not be as efficient as a slab sided board that is big enough. I confess that if I was building a board It would not be slab sided.....It would also be pretty big no matter how much I hate the added wet surface. "Holding on'" makes progress to weather. Going a little faster while skidding to leeward does not make good as much distance.

 

Alright, if you say that NACA is more effort than profit, I'll stick to the slab, and just round edges a bit. In that case, is one-layer 3cm plywood good enough, or do I still need to do these 3 layers with 12 degrees? I tried to plot it on paper, and I saw that it will cost considerably more. Also, there is this epoxy thing...
I can't find anywhere to buy wooden strips in my country, only plywood and planks - there are many planks with 3cm thickness and up, only that the maximum width is no more than 30cm, which is not a lot. I'll keep looking, but if I have to stick with plywood - PAR, are these 3 layers worth all the trouble?
And out of pure interest, why exactly 12 degrees? Seems an odd number - not the usual 45...

 

In the USA, most construction lumber is 1 1/2" thick. Best centerboards and rudders are made by cutting lumber in 1" slices from the construction lumber. These strips are glued after flipping the individual pieces end for end (for stability) so that the resulting board is 1" thick. This plank is then planed down to 3/4" and shaped. This makes a stable and strong board with all the fibers running in the best direction for strength in a foil.

You probably have something similar to work with.

I sailed a Windmill with a plywood centerboard for several years but did see a couple that broke off when being stood on to recover from a capsize.

 

The 3 piece arrangement is stronger, will flex and bend less and is more durable, but no you don't "have" to use it. You can pick up 38x89 (2x4) or 38x140 (2x6) stock at reasonable prices. These can be ripped to 30 mm and edge glued as I and Tom have described. This is the preferred wooden board making method for several reasons, the biggest being longitudinal strength and stiffness, which is what you want in a board. Being strip planked with opposing grains orientations (note Tom's comments) makes the board more dimensionally stable.

The reason you use 12 degrees (11.25 degrees is exact) is it keeps the grain orientation vertical, but subdivides the 0/90 orientation along the grain paths, making them wider.

Below is shown a 3 layer sandwich of a 0/90 orientation. The one on the left has a 30 degree separation and is ideal for making the panel uniformly stiff. The three layers evenly subdivide the 0/90 orientation, spreading the fibers evenly. In a board, this isn't what you want. What you need is longitudinal stiffness and better cross grain support, to help prevent rolling shear. The 12 degree separation, shown on the right improves the 0 and the 90 degree orientation, by spreading out their load paths with supporting fibers that also cross the others in a sympathetic angle. In other words the 12 degree angled pieces share a significant percentage of the vertical or horizontal fiber loads, but because they cross over this fibers also improve stiff in the same direction. Simply put, failure will be along load paths, which follow grain orientation lines, so if the lines are sympathetically arranged, you get a stiffer, stronger panel, just because of fiber orientation.

 

For me it is difficult to understand how will do boat with such design properties, but I think that available righting moment will be relatively low due to small boat width, and so will be centerboard loading. Plywood centerboard of your size can sustain such loads and the main problem is to protect it from water.
If you are interested in other cheap method of building strong centerboards from Al sheets and tubes check this link. Pictures are self-explaining:
http://gik.fordak.ru/index.php?topic=113.0

 

Okay, I see. I will definitely look for these wooden strips. They got to be sold somewhere. If I go with 2 layer strips, with end-to-end flipping, like tom28571 suggested, then 30mm is enough? There may be capsizes, so I wouldn't want to break it if I stood on it. But since you suggested to dismiss NACA and go with slab, I guess thicker centerboard will add much more forward resistance.

Nnnnnnnn, I still haven't made proper righting moment calculations, so I may make boat wider - problem is, that going with skin-on-frame, and making skin from PVC material, there is only as wide as boat can be before the perimeter of hull in widest point exceeds 1.5m, which is the width of PVC roll, and then I'll have to use several strips, which will complicate matters a lot. Already it's exceeding a bit, so probably I'll have to scale boat down.

What about mast diameter, why highlight it? Is it too thin? It will be shrouded, of course, and will stand up to 6 meters high, maybe less.

 

13 sq. meters for 1.3 meter width is too much, or you need hiking wings/trapeze. On the other hand I doubt that skin-on-frame construction will survive torsion loads during hiking. Assemble your parents folding kayak and try it.

100 mm is overkill for your mast, IMO. In attachment is example of mast cross-section which is used on catamarans with approximately the same sail area and displacement.

 

Is there some approximate length/width proportion that I could use as an initial reference?
I remember that kayak... Well, it was extremely unstable, but that's kayak. Sitting on the side was possible - but then again, it's Russian construction - these things are tanks, they can survive anything.

Okay, at least there is one thing I overshot on the good side I'll stick to some 7cm diameter, then.

 

Laukejas,
good to see you back here. You ask good questions!

I like your direction now. The big folding trimaran for under $400 was just too much to accomplish.

I saw the big sail area and was concerned too. We should get that right before you cut the daggerboard since the two should be proportional. It's OK to limit boat width based on your material width. Wide skin on frame boats are difficult to build. The narrower boat will be easier to push so you will need less sail, it's just a mater of getting it right. The thing to do is to calculate the righting moment from the sail and then look at how far off the center of the boat your weight would have to be to counter (equal) that force. What NNNNN is saying is that to counter 12sqm of bermuda rig your weight would need to be way outside the boat in the 15mph wind boats are commonly designed to. My suggestion would be to design the boat to use only the main sail for 15mph and above, and just use the jib in light wind.