Twin Keels

Discussion in 'Sailboats' started by DomesticPirate, Mar 9, 2009.

  1. DomesticPirate
    Joined: Mar 2009
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    DomesticPirate New Member

    I'm trying to find information on twin keel boats, pros and cons. I'm trying to design a hull and would like to incorperate them into the design, unless there's something I've overlooked. Any thoughts?
     
  2. yipster
    Joined: Oct 2002
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    yipster designer

    type twin keels in the search field under the search button and you get 129 results
    thats a lot but i recall many good conversations worth checking
     
  3. DLM84
    Joined: Feb 2009
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    DLM84 Junior Member

    what is the big deal with twin keels?? twin rudder I can under stand but why twin keel?
     
  4. lewisboats
    Joined: Oct 2002
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    lewisboats Obsessed Member

    Twin keels or (if I am correct) Bilge keels are very useful in shallow areas with a larger tide. It allows the boat to sit on the bottom upright and safe during low tides and saves on dock fees. Very popular in GB.

    Steve
     
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  5. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    As Steve has mentioned, bilge keels are handy to those in areas with large tidal ranges, permitting the boat to take to ground bolt upright. The extra drag of the second fin usually is a detriment to performance, but not a substantial one if well designed. Bilge keels can also afford slightly less draft on the same hull form, because of the additional lateral area the second appendage offers.

    As a whole, the bilge keel arrangement is argument with limited returns, some design briefs require this compromise, while most don't.
     
  6. DLM84
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    DLM84 Junior Member

    So the boat is suppose to sit on the 2 keels like little legs and I suppose the rudder makes the 3rd leg? Would mean they all would have to be very beefy or *Snap* *Uhh Ohhh*
    So why not one of the lifting keel designs? I have seen some cruisers that did it quite well. Of course that means they needed twin rudders on boats W/O lift out rudders but that is an advantage when keeled over into the wind it seems like to me.

    Thanks,
    Darren M.

    P.S. I just boat out on lake stockton, cruisers are to much money for me now but maybe when I retire (in about 40 years or so) I could get one.
     
  7. sharpii2
    Joined: May 2004
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    sharpii2 Senior Member

    Hi, Everyone.

    Here might be an example of the usefulness of twin keels

    I have been working on a small, sea going sharpie for some time. Until now, it has been nothing more than a few vague sketches and a page full of calculations. Now I have decided to push on and complete the design.

    It was to be the smallest, simplest boat I would ever try to cross an ocean in.

    On top of that, it was to be as easy to build out of plywood as I could make it. It is entirely cylindrically developed so that all the vertical and athwartship frames could have straight sections. Such developemnt should also make the side an bottom panels easier to lay out.

    I intended this boat to be driven by a free standing rig which is one reason for drawing this boat with a deep, constant dead rise. This way (I hope) there would be less of a snap roll which could be deadly on a free standing mast. Another reason for the 'V' bottom is so that two long keels could be tucked under the boat without increasing the draft at all.

    With the twin, long keels, the boat could stand on its own when the tide runs out and maybe even be stored on land without needing a cradle. Both attributes could come in handy as the boat is used for its primary purpose, nearly penyless cruising.

    The mast is nearly at the center length of the hull, but would be off set as not to impede on the minimal cabin. The sail rig on it would be very similar to a Chinese lug, as to make shortening up sail easy. Being that the boat is quite narrow and has quite sharp ends, working on deck is going to be impractical, as well as working far aft or far forward. So having almost all the sail area directly accessable from the middle of the boat would be good for safety as well as convenience.

    The interior arrangement is nothing more than a cabin sole with integral water tanks and stowage below and other stowage lockers and a galley bay on each side. This boat is really a small boat that will have to carry a lot of stuff. The 'bunk' would be an excercise mat or a self inflating air mattress which would be rolled up and shoved out of the way when not used.

    The twin keels would be made of concrete and scrapped re-bar, not needing any foundry work to make. Each keel would weigh approx 300 lbs. That, along with the easily reefed sail and the heavy stores stowed under the cabin sole, I hope, will allow me to boost the sail area from 141 sf, on an earlier version, to about 190 sf on this one.

    Steering would be done from inside the cabin with a simple yoke set up, or from the rear hatch (not shown) with a push-pull stick. The hatches haven't been drawn because I haven't figured out where they are going to be yet.

    So that's where it is so far.

    There is a lot more to work out. An auxiliary propulsion system has not yet been devised.

    Here, I hope I have shown some of the reason someone might opt for twin keels, even though, from the pure performance standpoint, they are clearly inferior.

    I have designed a single, long keel for this same boat, which would quite convincingly make it more seaworthy, as it's center of mass would be higher up, if the boat ever capsized. But the single keel increase the level draft (beaching and grounding) from less than 2 ft to almost 3 ft.

    From what I have read, twin keels tend to make a boat more directionally stable, which, IMHO, is a must for any low cost, long distance cruiser.

    I think way too much design emphasis has been put on performance, these days, and not enough on other factors which improve useability and cost effectiveness. If a boat doesn't need a cradle, for instance, on land storage could be that much cheaper and on land accessibility would be greatly improved.

    Having an un-stayed mast would cut down the set up time and cost to such an extent that the boat could be primarily kept on land and only launched when needed. That may come out cheaper than slip/mouring rent.

    I have toyed with the idea of larger versions of this design. A 37.5 by 7.5 0ne could neatly store in a shipping container, with only its mast removed.
     
  8. sharpii2
    Joined: May 2004
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    Location: Michigan, USA

    sharpii2 Senior Member

    Hi, again.

    I'm now going to try to add the attachments which were supposed to go with my last post.
     

    Attached Files:

  9. Brent Swain
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    Brent Swain Member

    I decided on twin keels 25 years ago on my current boat, and it was one of the best decisions I made. It is part of the reason I haven't paid to tie to a dock in 25 years. I can spend a winter in the South Pacific for less than most people pay for moorage in a year.
    In the 80's, most of my boats were being built with single keels. Now most are twin keelers, and of those who went for single keels, over 90% wish they had twin keels. The huge rise in moorage rates and the ever increaseing waiting lists for marinas will continue and twin keelers will consequently become ever more common.
    I have no complaints about performance . Two trips from Hawai to BC , the first 1,000 miles hard on the wind , in 23 days, in a heavily loaded 31 foot twin keeler is not all that bad time.
    Twin keels have the advantage over centreboards of having no moving parts.
    Not much tidal range in misouri.
    Brent
     
  10. Tanton
    Joined: Nov 2003
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    Tanton Senior Member

    Twin Keelers.

    I started my interest in twin keels over twenty years ago with the 38' steel Pen-Gwyn. Depth due to the local condition in that part of Wales,
    I settled on a minimum draft of 3'-6'. With apprehension about her sailing ability. P-G is a heavy double ender for cruising. Her performance is similar to other vessels. Most of the P-G were built with 48' or even of 54" of draft.
     
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  11. sharpii2
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    Location: Michigan, USA

    sharpii2 Senior Member

    Hi, DLM.

    The way to protect the rudder is to make a sturdy skeg deeper than it.

    Often, though, the Center of Gravity of the boat is ahead of the keels.

    What happens then is the boat pitches forward and the bow becomes the third leg, lifting the rudder clear from harm. I saw such a design in the '70's.
     
  12. Brent Swain
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    Brent Swain Member

    On my 31 the centre of gravity is about 2/3rds the way back along the bottom, on the 36 it is halfway along.
    Twin keels eliminate the troublesome underwater moving parts of centreboard designs. They also roll a lot less.
    Brent
     
  13. timothy22
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    timothy22 Junior Member

  14. Boston

    Boston Previous Member

    dam
    now I have to go read over a 129 posts
    sounds like a great topic
    thanks
    B
     

  15. Manie B
    Joined: Sep 2006
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    Manie B Senior Member

    Good reading guys thanks
    the Bray research is very interesting
     
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