More Stuff Done Wrong

I see this article on how to build boards is making the rounds on the internets.  People, don’t do a foil that way. The writer either has never pushed his boats or does not do math, or both.

kohlrudder-daggerboard-construction-2-300x230 Makes a nice sculpture but not for real boards.

 Stressform, which he uses to to form the boards, is a fine way to make many things including masts and hulls.  But not boards.  As you all know, a board is a heavily loaded beam.   With a huge point load usually about half way up it.  Typically deflection calculations govern on board design.  The board in the above link has thin plywood skins.  The skin of any board can do the majority of the beam work as it is farthest from the centroid.  But plywood has only about 1/8th the modulus of carbon fiber.  So it will be about 8 times more bendy.  Carbon fiber will have between 10 to 20 times the bending strength also.  Carbon fiber should be used on all boards with an aspect ratio of 3 or more, which are most of mine.

In this how-to, there is a lumber spar down the center.  And he adds carbon to it.  But it is at the centroid of the foil.  That is where material contributes the least to structure of any place on the foil.  So they put the carbon fiber where it is least useful.  Stuff done wrong.  The spar is supposed to also provide shear transfer between the two sides.  It does, but only for about 10 times the skin  thickness either side of the spar.  So most the plywood is also wasted as a beam in this scheme.  This why I have core everywhere in a board.  It transfers all the loads from one side to the other.

Finally, remember that point load  halfway up the board?  It can be several thousand pounds load on one point.  Compression strength perpendicular to grain of the plywood will be between 100 psi to 300 psi, depending.  Carbon fiber will be thousands of psi.  The board failures that I have seen have been compression side buckling.  This looks like buckling waiting to happen.

I have a new 14 page chapter on how to build foils in both the new CM Construction Manual and in the Composite Construction Manual.  I will put those online next week and make available here.  Just trying to do an intervention everybody, before its too late.  I already am too late on one guy.  He bought Kohler plans to an imaginary cat that was a textbook example of Things Done Wrong.  More on that soon.

14 thoughts on “More Stuff Done Wrong”

  1. I have just built a 3.2 meter dagger board using the hollow ply filled with two pack foam this method is super quick and strong and proven with no failures to date.

  2. Hi All,
    I have built a foil to Bernd’s plans and I can say it is very stiff and rock solid. In fact I have done a lot of material strength tests during the construction of my cat. I have tested balsa core, foam core, polycore, ply core and Hoop pine core. I haven’t tested Cedar or Pallonia (Kirri).
    I have in fact driven my Mitsubishi Pajero 4wd over it. I have placed one end on three paving bricks and driven up it as you would a ramp. I’m convinced I have made the right choice. By the way there are many examples on the internet showing broken foam foils. Regards Craig

    1. they are beautiful boards and I’m sure you love them. the plywood still has about 10% the strength of carbon in bending strength and probably in perpendicular compression. failure would probably be in that mode. a badly done foam foil proves nothing unless you know the laminate and the loads. a board badly done surviving only shows that it has not been pushed. The deflection would be some 10 more also. it is a beautiful board I agree.

  3. they didnt even put a rocker in it. its as good as a piece of ployowd the same shape. also the part about using lots of resin is also bull. Not pros.

  4. I would use it in a jam situation or as spare method when resources are limited. It would not be the first time when I took big risks. I do a lot of real life testing when possible.
    One of my friends used mild steel for a rudder with zincs to make it from Pago to New Zealand (a few thousand miles with major storms).

    1. I have this belief that if you do everything right, the ocean might let you pass. If you do stuff wrong, you might have to pay for it.

  5. Kurt… all of that is probably right, but things ought to be engineered for the plannes use. Otherwise it is only overkill.

    I never read in Mr. Bernd Kohler plans advertising anything even close to 22 knots. He also advises putting epoxy and glass over the plywood sides.

    I think that in real life, and for the boats he designs, the “indeterminate foil section” and scantlings are well within the lift needs and safety limits. and he definitely has a cue for down-to-earth approaches.

    Kind regards, Stefano

  6. The wood/carbon spar is at the heighest load area at the 30 % of the chord. This is the reason aircrafts have ther the main spar to.
    All the aircraft designers are wrong?
    I use this system now for 30 years on almost all of my designs without any failure.

    1. I learned long ago just because someone famous does something doesn’t mean it is always right. Do you dispute the compression strength of plywood? Do you dispute the mass of material being at the centroid where it is doing the least work? Do you dispute the strength and modulus of the plywood compared to carbon? Do you dispute the minimal shear transfer from the two sides? I strive to get as much structurally rational as possible. If one starts with bad engineering model, it does not get better. And that is just the structure. To be optimum a foil must have an accurate section of that model. A fold up probably won’t give you an accurate foil section. Those look like the nose is too sharp, leading to narrow bucket.

    2. it did just occur to me that possibly by using an indeterminate foil section you are not getting the high lift loads that could break a foil. For example a 22″ chord with 72″ span, a typical tri foil, at 22 knots and 4 degrees angle of attack, generates 5000 lbs of lift. Run those numbers with your strength properties and section properties and see how that looks. Not good.

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