Keel design

Discussion in 'Sailboats' started by Michail, Oct 9, 2011.

  1. Michail
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    Michail Junior Member

    I am building a small sailboat based loosely on an exisiting design, and need advice on daggerboard keel.

    The original design uses only plywood and unidirectional glass fibers.

    Compared to the original design, I have to increase both the depth of the keel by about 15 % and the weight of the keel bulb by about 25 %.
    Depth: 160 cm.
    Length: 45 cm.
    Width: 8 cm.
    Bulb weight: 150 kg.

    I am thinking of using two stainless steel plates bolted together with plywood boards inbetween. From what I have seen, stainless steel and epoxy do not guarantee a strong bond, so in addition to epoxy bolts should be used.

    Is this a good idea or not?
     

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  2. messabout
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    messabout Senior Member

    You can probably make a pretty substantial fin with the method your drawing shows. Just eyeballing the section shape, I'd say that the camber is too far forward and the leading edge is too blunt, but strength and stiffness will probably be very good
     
  3. PAR
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    PAR Yacht Designer/Builder

    Agreed, that shape looks pretty draggy . . .
     
  4. John Perry
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    John Perry Senior Member

    How one goes about making something like this will probably depend on what equipment you have available and what kind of materials you are already familiar with. To me, your construction looks substantial but also quite a lot of work and quite expensive in materials.

    Years ago I made a keel a bit like this for my own boat, albeit somewhat thinner section and only about half the ballast weight. I simply included sufficient unidirectional glass in the outer sheath to carry the bending moment. Since you propose to sheath the whole thing in glass anyway, is it not easier to simply make the glass thick enough to carry the bending moment and forget about all the custom metalwork? A bit more glass, but little extra work compared with just applying the glass sheathing which you are going to do anyway ....

    A variation might be to make the keel initially as a forward and aft part both individually sheathed, then push them together before the resin cures, then lay more glass over the whole. That way you could get a sort of shear web in the middle, but whether that is really necessary is a bit hard to say without a proper analysis.

    As for the exact cross section, I assume this is only a rough sketch. The shape sketched might not be optimal but it looks to be better than the flat plate keels seen on some boats, including some fairly high performance ones.

    One more thought - if you really want some metal in your keel, how about a length of stainless steel round bar down the leading edge. My keel got a dent in it where I sailed into an underwater sewer pipe (unused pipe I think). When I repaired it I added a half round metal section at the leading edge, but it would have been better to have mounlded in a substantial bar down the leading edge from new.
     
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  5. michael pierzga
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    michael pierzga Senior Member

    I don't like stainless steel...hard to bond to it, paint never adheres well and its prone to corrosion. Why not epoxy and glass fiber over the ply core...cheap and strong.

    Also remember to address the Compression load caused by the bolts clamping the lead ballast and steel arrangment. Compression tubes are logical.
     
  6. Michail
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    Michail Junior Member

    The shape was a rough drawing. I am going to use a profile something like 63-018 as defined by Delftship. It looks thick, but the original design has a section which is about 40 cm long and 7 cm thick, i.e. about 18 % thickness, so the proportions are the same.

    Since I am not an engineer, I would dare to reduce its thickness by much, and since I am lenghtening the keel by 15 %, the flexing force would be squared, i.e. 32 % + extra 25 % for heavier keel bulb, so the resulting increase in load on the keel shold be about + 65 %. That is why I would like to introduce steel into the equation. Besides, it is heavy so it will give a natural ballast which may allow to reduce the weight of the bulb keel. (the plates as in my drawing

    Stainless steel is relatively cheap (6 USD/kg) and easy to work with here.

    Unidirectional glass seems not to be readily available. What I have seen is a very thick 2200 g/m2 unidirectional glass for piping. But I am not sure how whether this will work.
     
  7. michael pierzga
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    michael pierzga Senior Member

    It might be the absoulte cheapest to use a simple welded steel keel construction. Properly prepared and painted steel is very durable. additional Fairing could be foam covered with glass.
     
  8. Michail
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    Michail Junior Member

    Stainless steel is relatively cheap (6 USD/kg) and there are several cheap stainless steel welders. I was not able to find uniderection glass, only a very heavy 2200 g/m2 which is used for heavy piping. I suppose that it is not very appropriate for the purpose.

    The outlines of my drawing were not exact, but the overall proportions of length and width of the section are correct. I am thinking to use a Delftship foil of 63-018.

    The original design has a keel 1.35 m deep, 40 cm. wide and 7 cm thick, sheathed in fiberglass. the 40 to 7 cm gives a ratio of 18 %, so the proportions are the same.

    I am not an engineer, so I am worried about reducing the thickness of keel.

    Since I am going to lengthen the keel by about 15 % and add about 25 % extra weight as compared to the original design, I would have to increase the strength of the keel by the order 1.15 x 1.15 x 1.25 = +65 %, and that is why I decided to introduce steel into equation.

    (I have to do this because I have increased the dimensions of the boat by 11.4% lengthwise and 3.5 % in width and depth, i.e. an increase of about 20 % displacement, and this with slightly slimmer outlines which require better stability, plus I am using the single-skin fiberglass, which would give another 20 % - 25 % extra weight, so to compensate for this I am increasing the depth and weight of the keel)

    What I am specifically worried is about is:

    - Corrosion and stainless steel bolts underwater. The sheathing of the keel will be done with vynilester resin, so delamination and possible skin rupture can not be excluded. The sheathing is intended mainly as a protection from water.
    - Epoxy bond. Especially the outside “cheeks” of 12 mm. plywood would be held only by epoxy and by the vinilester sheathing. Is it OK, or would it be better to use extra fasteners?

    There are a few collateral questions about guides. From what I have seen, it would be a good idea to put four rollers in the upper part for smooth gliding. If I have two steel plates, I can easily insert rollers on bolts from hard plastic (called here “Technil”). That is also one of the reasons why I am thinking about steel.
    - For the size of the keel (150 kg. bulb + 50 kg. keel) I am talking about, would it be a good idea to add rollers as guides, or one can do without them (i.e. allow the keel to glide naturally in the fiberglass sheath? The fiberglass sheath I plan to do is about 12 mm. thick, reinforced externally by plywood plates (probably 15 mm) which would transmit the forces to two bulkheads).

    I plan to construct the keel after I finish the fiberglassing the mold (which is approaching completion), but since I want to integrate the keel well/sheath with the body, I have to decide now on the well dimensions. And these depend on both the keel profile and the guides… For the time being I am thinking of using a roughly rectangular shape and only in the exit point to reduce it to the profile shape...
     

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  9. Perm Stress
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    Perm Stress Senior Member

    Stainless will not live long when deprived of oxygen and subjected to moisture inherent in wood/glass submerged fin.
    One more imprtant culprit is wildly different stiffness of steel as compared to wood -in really loaded structure like keeel any bond betwwen them, included bolts, will soon become loose.
    Instead of unidirectionnal you can use woven cloth or sathin (do not recall a proper name, it looks like jeans cloth). Stifness wil be about half of unidirectional, ~triple the amount for same strength.
    And incomparably more homogeneus structure against steel/wood one.
     
  10. TeddyDiver
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    TeddyDiver Gollywobbler

    Twill..
    BR Teddy
     
  11. Michail
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    Michail Junior Member

    Did you mean standard steel, i.e. not stainless?
     
  12. michael pierzga
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    michael pierzga Senior Member

    Yup, regular steel. Many steel keels around although I have never seen one so small. The boat Im sailing now has an aluminum keel with lead shot added down low for righting moment ballast. Personally for a small craft I would build a keel blade Glass epoxy covered core like plywood. Its so easy.... Your blade is so small why make things complicated . Anything metal will require extra detailing , sandblasting and epoxy primer coating is not cheap
     
  13. Michail
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    Michail Junior Member

    I have three major problems with plywood-fiberglass:
    - I pretend to use vynilester resin and not epoxy for sheathing (due to local costs). If the sheathing is structural (such as is the case with plywood), a delamination issue is probably very dangerous.
    - The original design has already a rather thick profile. Since I am increasing loads, the resulting profile would have to be even more thick. In fact I estimated that I need to have about 45 x 9 cm profile (20 %) to be in the same strength range. I can make the keel longer (i.e. not 45, but 55 cm.) but that would change lateral plan, so I would have to adjust rudders plus the whole structure
    - Doing a keel entirely from steel is probably not too difficult, but I am not sure how a gliding daggerstyle keel would work - the paint would be stripped after a few cycles of hoisting it.
    - Is it possible to do the keel alsmost entirely out of fiberglass? I.e. use a ply wood core of about 30 mm. for shape, and then cover it with 25 mm. of Roving/Mat?
     
  14. michael pierzga
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    michael pierzga Senior Member

    I would think the layup would consist of Uni directional, layers of Biaxial and Mat. This is a common rudder layup and should work on a small craft . You should Ask for specific engineering advice from a pro.
     

  15. Michail
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    Michail Junior Member

    Unidirectional fiber available locally is a 2200 g/m2 which is used for heavy piping. No lightgweight unidirectional is available here.
     
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