Why Aren't Bilge Keels More Popular ?

Discussion in 'Boat Design' started by PsiPhi, Jun 12, 2009.

  1. PsiPhi
    Joined: May 2007
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    PsiPhi Newbie

    Just wondering if someone can explain to the uninitiated why Bilge Keels/Fins/Boards aren't popular.

    From what I have read there seems to be two objections.

    1) Reduced stability because the ballast is not so low.
    But flat bottomed boats seem to do reasonable well even when the centreboard is up - or am I being naive?

    2) Increased Wetted Area.
    But if you take a design with an 8 inch fin keel, and replace it with 2x 4 inch bilge keels - doesn't that add up to the same thing ?

    Just wondering.
     
  2. lewisboats
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    lewisboats Obsessed Member

    They are very popular in Europe...especially GB where there is a large tide and they can ground as the tide goes out and lift as it comes in. Other than the objections noted there isn't much wrong with them except that they don't look like racing boats so they can't be any good...same with Leeboards. Just blatant prejudice me thinks.

    Steve
     
  3. PsiPhi
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    PsiPhi Newbie

    Thanks for that, maybe I am on the right track then.
    We have some shallow waters around here, 8 inches in places, which I would like to explore, but still be able to go up the coast.
    I'm thinking about a small catboat like Catfish or the Charles Wittholz design, but they all have deep (to me) keels, 14inches plus, maybe I could 'customize' one of these?
     
  4. Tad
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    Tad Boat Designer

    The effectiveness (lift) and resistance, (drag) of any appendage is dependent on more than just it's surface area. Do a search on aspect ratio.

    So far, the majority of testing has shown that two smaller keels create more drag and less lift than one big deep one. Thus if sailing performance is a criteria, twin keels are not a popular choice.
     
  5. gonzo
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    gonzo Senior Member

    My experience with bilge keels is that they point less and are slower all around. The shallow draft and the boat staying upright at low tide makes up for it in some locations.
     
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  6. FAST FRED
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    FAST FRED Senior Member

    A more modern bilge keel would be two foils sticking down with any ballast mounted at their lower ends.

    Should be quite efficient , and still take the ground .

    Could be built with as much foil area as the current 12 ft deep 33ft racing boats , yet would be a good cruiser.

    Why not ?

    Don't ask me I love the concept (power or sail) of a boat that can take the ground.

    FF
     
  7. Tiny Turnip
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    Tiny Turnip Senior Member

    My 16' Winklebrig is a ballasted trailer sailor, with twin bilge *plates* and a residual long keel. She takes the ground but doesn't lie level, as the plates retract, and can be difficult to put about in a strong blow, but maybe thats because I'm a poor sailor -others seem to manage!
    Thanks to Julian Swindell and Roger Parish for the Photos.
     

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  8. BeauVrolyk
    Joined: Apr 2009
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    BeauVrolyk Sailor

    I have been wondering myself, for an entirely different reason - 'cuz I hate the idea of putting a boat on the shore, if a re-think of the "bilge keel" is called for.

    I would suggest that they be lifting keels (weighted daggerboards?) and that they have bulbs on the bottom. They be set to that when the broad beamy boat I'm imagining (think TP52 or Volvo70) is heeled over to get the fat beam out of the water the keel is then pointed straight down, probably 25 degrees or so. You'd hoist the windward keel up and leave the leeward one deployed. That gets the wetted surface down.

    Of course, it would actually be better to have internal ballast that you could move to windward and daggerboards that pull all the way up so you all can stick your boats on the mud.

    B
     
  9. Milan
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    Milan Senior Member

    There are such designs. Erik La Rouge is one of the designers.
    http://www.lerouge-yachts.com/
     

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  10. peterAustralia
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    peterAustralia Senior Member

    Lynx 14 has dual weighted dagger boards.

    Each board is light enough so that it can be lifted up by one person. So that may be an option. Assume you are interested in a small monohull trailer-sailer here.

    My cousin has a 29ft steel yacht with bilge keels, it goes well but is slow all round (beamy, hard chine and heavy yacht). The draught on his boat is 1m. The bilge keels provide more storage options which is good. He has moved the boat from a berth to tidal mudflats so as to be much closer to home.

    With current market, a second hand bilge keeled yacht could be bought at a modest price.
     
  11. PsiPhi
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    PsiPhi Newbie

    Thanks guys for a lot of good responses.

    Yes, I am thinking of a monohull trailer sailer, maybe a catboat, 14ft~17ft depending on design.

    As performance isn't a great issue, leisurely cruising only intended, then this could still be a good option for my circumstances.

    The prime goal is to reduce the draft whilst keeping space in the cockpit/cabin.
     
  12. FAST FRED
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    FAST FRED Senior Member

    " ... A more modern bilge keel would be two foils sticking down with any ballast mounted at their lower ends.

    ... as much foil area as the current 12 ft deep 33ft racing boats , yet would be a good cruiser...

    There are such designs. Erik La Rouge is one of the designers. "

    Those are still modern conventional keels just mounted as bilge keels.

    I an thinking of two narrow foils PER SIDE , connected at the bottom with any required ballast.

    The foils could be quite efficient if the vessels side slip worked them hard with out stalling. And the ballast would be both end plate for the foils and foot to take the ground.

    FF
     
  13. Fantasie 19
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    Fantasie 19 New Member

    Hi, I have a small triple keel cruiser which I keep on the South coast of England. They are really popular over here as they enable us to access more of our coast line and estuaries which are inaccessable to deeper keeled boats.
    The general consensus is that bilge keelers are slightly slower, make a bit more leeway and don't point quite as well as fin keelers. All these points are regularly hotly debated on uk forums. I keep my nose out as the tend to get a bit us and them. For me the benefits outweigh the downfalls. My reasons for having a small bilge keeler are mainly financial. The boats are generally cheep to buy,deep moorings on the south coast are very expensive so being able to use a drying mooring is an absolute bonus and it can over winter on my front garden.
    I can get to some of the quietest parts of the habours and estuaries which more than makes up for any percieved lack of performance. If I was in a hurry I wouldn't sail:)

    I like the idea about ballast on the end of the bilge plates. My boat has flat bilge plates and centre iron keel with a ballast ratio of about 36%. I have considered making bulbs for the bottom edge and creating more of an aerofoil shape to the plates in the hope of increasing stability and performance.
     
  14. Steve W
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    Steve W Senior Member

    Alan wright in NZ produced a number of designs such as the Tracker 7.7,The Nova 28 and others which could be built with a twin keel or fin keel,(i built a Tracker) very few chose the twin keel option.They were proper cast lead naca foils canted at maybe 10 degrees.
    I think they are just too much of a compromise for most people,for your needs of a small boat and very shallow water i would be looking in other directions such as maybe one of Ray Aldridges Slider catamarans or a Jarcat or maybe a sharpie.One of the problems with the Twin keels is that they have their least draft when they are upright but as soon as they heel over the draft increases which, while better than a central keel, is not ideal for thin water sailing.A friend of mine owned a brand new C&C30 years ago on lake superior,he and his wife chartered a Bolger Black Gauntlet leeboard sharpie and spent a life changing 2 weeks sailing in Everglades national park,after that trip he sold the C&C and built a Bolger long micro and now owns a 28ft Egret sharpie which he cruises extensivly in Florida.
    Steve
     

  15. cthippo
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    cthippo Senior Member

    I know I'm resurrecting a thread that's been dead for a while, but I wanted to follow up on this topic.

    Most of the posts so far have dealt with bilge keels on sailboats, but what about on power boats? I'm looking at it from the perspective of roll resistance and protection if grounded (especially for keel coolers). I understand the increased drag is an issue, but is it significant on an already high displacement design?

    Also, what are the factors that go into the design of a bilge keel? I've seen some that are almost like a fin and others that look more like strakes. How does the shape affect the performance?
     
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