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Thread: Centerboards: Plywood, shaped foils or built from strips?

  1. #1
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    Default Centerboards: Plywood, shaped foils or built from strips?

    Interesting note from Gougeon: "In a foil shape, there is simply too much endgrain exposed; it became laden with moisture and thus lost its stiffness and failed. A new board was built of mahogany strips (as described in our publication 000-448 Rudder Blades and Centerboards) and is working fine after eighteen years."
    “Eventually, all things merge into one, and a river runs through it. The river was cut by the world's great flood and runs over rocks from the basement of time. On some of those rocks are timeless raindrops. Under the rocks are the words, and some of the words are theirs."

  2. #2
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    Default Re: Centerboards: Plywood, shaped foils or built from strips?

    I think the problem is that half the wood is goin the wrong way, not the exposed endgrain.
    If one has the time and the materials, strip plank is far superior to ply though.

  3. #3
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    Default Re: Centerboards: Plywood, shaped foils or built from strips?

    Laminated wood strips every time.Long ago there was a special plywood for centreboards and rudders called Anti Fracture Board and it worked brilliantly because it has a lot of the plies oriented in the right direction.Ordinary plywood tends to break at the keel line or the bottom of the rudder stock.

  4. #4
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    Default Re: Centerboards: Plywood, shaped foils or built from strips?

    I have seen scores of plywood centre & dagger boards break over the years, Half the grain goes the wrong way & they are weak. Capsize & stand on it, Snap! Run aground hard same thing.

  5. #5
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    Default Re: Centerboards: Plywood, shaped foils or built from strips?

    All this is probably true, but in this day and technology, and especially as might be advocated by Gougeon, why wouldn't one glass the board like crazy complete with barrier coat and maybe graphite against the abrasion? At this point the orientation of the ply fibers might become secondary to the tensile strength of the glass. Besides, if we are talking centerboards over dagger-boards, the thing will go down at an angle and the fibers will likely be 30/60 degrees rather than 0/90.

    am I thinking right??
    Ken

  6. #6
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    Default Re: Centerboards: Plywood, shaped foils or built from strips?

    If you build enough strength into the fibre layer,you might as well go for a foam core since it will be lighter than plywood.If you are planning a light cladding stick with solid wood for a core.

  7. #7
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    Default Re: Centerboards: Plywood, shaped foils or built from strips?

    I specify plywood for the centreboard in several of my designs, but put a hardwood leading edge and tip in the structure. I do use a fairly thick foil though, for example the "offcentreboard" in my own Long Steps is made up of two layers of 9mm, and two layers of 6mm plywood, lead in a pocket in the centre layers, plus the hardwood tip and leading edge. All glassed over. There is no exposed end grain, and no shortage of strength.

    John Welsford
    An expert is but a beginner with experience.

  8. #8
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    Default Re: Centerboards: Plywood, shaped foils or built from strips?

    Quote Originally Posted by rbgarr View Post
    Interesting note from Gougeon: "In a foil shape, there is simply too much endgrain exposed; it became laden with moisture and thus lost its stiffness and failed. A new board was built of mahogany strips (as described in our publication 000-448 Rudder Blades and Centerboards) and is working fine after eighteen years."
    A properly laid up plywood foil is just as good as a solid wood foil if the bending moment of is adequate. That means that the foil is thick enough for the length of the board. I started with racing Windmills in 1966 that specifies a 3/4" ply that had 44" exposure of 16" top chord and 5 1/2" chord at the bottom. Such a DB would be anathema to the responders of this post as 3/4" ply is simply not strong enough and I did see several broken boards, mostly from standing on them to right a capsized boat. With a crew weight of, say, 200 lbs on the capsized board, some failures are assured in rough conditions which is the Windmills forte. A daggerboard, strip laminated from 3/4" pine, spruce, or similar (the max allowed) has seen no failure that I am aware of. I fit an epoxy encapsulated synthetic line to the nose and foot which is pretty much indestructible. This nose survives slamming it with a 3# hammer on a bench with no damage and has been used on larger boats up to a 46' power catamaran. Sheathing a foil makes good sense in any size.

  9. #9
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    Default Re: Centerboards: Plywood, shaped foils or built from strips?

    Quote Originally Posted by John Meachen View Post
    If you build enough strength into the fibre layer,you might as well go for a foam core since it will be lighter than plywood.If you are planning a light cladding stick with solid wood for a core.
    You might well be right, John, but this being the Wooden Boat Forum.........

  10. #10
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    Default Re: Centerboards: Plywood, shaped foils or built from strips?

    Quote Originally Posted by Mike-in-Suffolk View Post
    You might well be right, John, but this being the Wooden Boat Forum.........
    But very few of us use only wood.

    I think of wood as a building material that’s easy to work, available, affordable, sometimes attractive and has useful engineering properties. A lot of our discussion in the forum is about combining wood with other materials - metal fasteners and connectives, adhesives and coatings, other natural and man made fibers. If we analyze the material properties that a design situation needs and what various materials offer, we can make a better product.

    Foils have their own issues. The bending loads are often greater in one direction than others, so a material whose strength is equal in all directions, like plywood, is inappropriate. They’re subject to local impacts on the edges and should be a firm material there. They’re constantly immersed, often in a confined trunk, so they need to be either very well encapsulated if of a material that swells when wet or of a material that doesn’t swell. THey’re in a location where weight can aid the boat’s stability.

    Materials other than wood can have a place in foils. Boards cut from galvanized, aluminum or stainless plate can be practical. Structural foams are light weight, easily shaped and stable if wetted. Trailing edges should be thin and purely wood foils can be fragile there. I’ve used glass composite such as G-10 and pultruded glass sail batten stock, inlaid into a kerf in the foam here. If exposed as the foam is shaped, the sheathing can bond to it nicely.
    Sheathings such as fiberglass composite offer structural value, impact resistance and encapsulation. Cloth or biaxial reinforcement on the bias improves torsional stiffness and can cure humming in faster craft.
    Reinforcing fibers such as glass or carbon can serve other structural needs . Longitudinal strips of unidirectional material can supply needed longitudinal strength.

    Keep an open mind.

  11. #11
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    Default Re: Centerboards: Plywood, shaped foils or built from strips?

    Making nicely shaped wood centerboard and rudder is one of the most rewarding boat projects. You can make something beautiful that improves the way your boat sails. They aren’t very expensive either.

    The reason for laminating the wood is to scramble the grain direction and prevent warping. It as also sometimes a nice idea to alternate heavier, tougher wood ( like mahogany) with lighter soft woods. It is also a way to build thicker foils out of thin stock, just set the fence to the blank thickness you require. There are dozens of techniques for gluing up laminates that are straight. A thickness planer is also very convenient.
    It’s a good idea to glass blades. Soft wood fails on the compression face, and fiberglass helps resist compression checks.
    Another good stunt is to inset a strip of G10 or equivalent tough material on centerline at the leading and trailing edge. This toughens thing enormously and makes it possible to make nice thin trailing edges (less than 1/8”), which makes them less likely to sing or vibrate.
    It is also worthwhile to perfect the fit in the boat. UHMW is your friend.
    SHC

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