Shallow fin keel ?

Discussion in 'Sailboats' started by frank smith, Jan 25, 2011.

  1. frank smith
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    frank smith Senior Member

    It will have a single mast with about 375 sq ft of sail this net a SA/D of a little over 17.
    this is low I think and the aspect ratio may have to be lowered to increase SA with the shallow keel .The rig will be the Hasler type , and I am taking straight out of the book. It seam to be the most straight forward way to deal with it .

    I have been thinking about that bow and reserve buoyancy , you may be right that it need to be tweaked out a bit. Currently there is a little bump in the diagonals that I dont like. But that may be the program , and a model will tell me more, I hope .
    F

    J
     
  2. sorenfdk
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    sorenfdk Yacht Designer

    If you have Principles of Yacht Design, then read it! You'll find everything you need to know there!
     
  3. PAR
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    PAR Yacht Designer/Builder

    Frank if considering this rig, you'd be wise to sail one along side a conventional rigged sloop. Some people swear by their choices, but their justifications are so cleverly biased that you think it's truth, when in fact it's just an absurd bias toward their choice justifications. There's a number of very good reasons the rig isn't main stream.
     
  4. frank smith
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    frank smith Senior Member

    I have read all the pros and cons about the junk rig. It has definite negatives
    , as all rigs do ,but the ability to handle the sail form the relative safety of
    an enclosed space is a definite plus. They also look cool.
     
  5. gonzo
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    gonzo Senior Member

    How can you handle it form an enclosed space compared to other rigs? For example compare it to a furling rig, jib and main.
     
  6. PAR
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    PAR Yacht Designer/Builder

    Frank, you can handle pretty much any rig from a pilothouse. Running rigging is user specific and generally doesn't have much limitation, only the owner's experience and inventiveness. This rig has more then just a little wrong with it, which is why it's not particularly popular.
     
  7. frank smith
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    frank smith Senior Member

    What about cost ?
     
  8. frank smith
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    frank smith Senior Member

    To follow up . After looking at many possibilities , including NACA foils , I have concluded that for the purpose of this design a simple flat keel will do .
    It would have a parabolic LE and a sloped TE .
    It could be of welded steel plate, this would add to ballast , and be simpler than a glass composite . Cast lead might be the simplest and cost about the same because of the lower volume needed. It would also put the weight
    where it will do the most good and allow a lower ballast to disp. ratio.

    What do you think?

    F
     

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  9. gonzo
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    gonzo Senior Member

    What is the purpose of the design? If going well to windward and having minimal resistance downwind is the purpose, then that poorly performing foil does not fulfill it.
     
  10. frank smith
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    frank smith Senior Member

    It is not a foil. The purpose is to provide lateral resistance . I am not stuck on it, but it is the first go round . Stuck on stability figures, and that that will take some time. When that is done I will get back to the the keel. I am looking for simplicity here , and do expect to make changes , but I have to start somewhere. Maybe a steel plate with lead ballast bolted to the bottom , why not ?
     
  11. gggGuest
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    gggGuest ...

    Bethwaite is indeed excellent, but surely completely at the other end of the spectrum from the design challenge here.

    Ultimately all sail boat hulls/keels are foils. When not blowing dead downwind a sailcraft is extracting energy from the difference in speed of the wind and the water, and the efficiency of both the foils in the air and foils in the water are equally important to how well the craft will sail. So called "Leeway" is an indication of how efficiently the under water foils are working: even if you consider a merchant ship with no separate keel the hull is still acting as a foil. Lateral resistance is provided by the foil effect, even if it does it very inefficiently.

    So for any given craft you really should consider efficiency of the underwater structure as an integral part of the design. Efficiency may not be an especially important design factor for the intended purpose of the boat of course, nothing wrong with that, but there is little point in having a super high efficient keel with an inefficient rig and vice versa.

    So it does all come back to the purpose of the design. If you want it to be able to claw to windward in big breeze and surf off a lee shore you need pretty damn high efficiency. If on the other hand you have no intention of being foolish enough to get into that sort of situation in the first place and will have a big iron donkey to get you out of trouble if you should get badly caught out by something weird in the weather then you don't, and other factors become more important.
     
  12. frank smith
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    frank smith Senior Member

    Gonzo , would this be better ?
     

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  13. gonzo
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    gonzo Senior Member

    Probably. A flat plate is a foil also, just a very poor one.
     
  14. viking north
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    viking north VINLAND

    Long Keel Design

    Frank, as you probably know i've been playing around with the design of the full keel for my build. I have done alot of research and asked alot of questions on the forum. Your presentation on post #23 is as good as it gets for a keel thats too long to facilitate a foil due to extreme cross sectional dimensions even for a minimal low aspect foil that will give you any great amount of lift. Your next best is what your drawing looks like and i classify as a quazi foil, I.E a long keel having foil features built into it. That being a low aspect foil's leading edge(parabolic) a parallel mid section and a foils tapered trailing edge. Blend the root into the hull with plenty of taper say a 6in. radius minimum if possible. Keep the cross sectional taper to a minimum, the straighter the better. Keep your leading edge angle less than 40 deg. if possible. On my build my cross sectional shape is a 50/50 combination of a taper section and a straight section. I had to do this to blend into the existing keel and to facilitate my parallel pairs of keel bolts. To take advantage of the ability to "heave to" i have included some forefoot area. To incorporate better handling (easier tacking)I have also broken the full keel into a long keel and skeg. Included is a sample drawing,(still under review to attain a 25% taper and a 75% straight section) and a couple of photos of construction of the male mold to build the new keels shell. The idea here is not so much to create lift as much as to use every trick to reduce drag although from all my research i'm inclinded to believe there is some lift and something i'm anxious to find out once launched ??? :) -- Geo.

    A yacht is not defined by the vessel but by the care and love of her owner.
     

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    Last edited: May 14, 2011

  15. TeddyDiver
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    TeddyDiver Gollywobbler

    The only usefull aspect having foil shaped long keel is to have more volume for the tankage in the keel. However it's better to be floodable to keep the Cog constant.
     
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