righting moment of a keel

Discussion in 'Stability' started by rapscallion, Feb 7, 2007.

  1. rapscallion
    Joined: Oct 2006
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    Location: Wisconsin

    rapscallion Senior Member

    I'm considering changing my fixed keel boat into a retracting keel boat. Currently the boat has a 1500lb trapezoid keel (5 ft draft). The keel I was considering would also have a 5 foot draft, but all of the weight would be in a 1050lb bulb. when comparing the righting moments, do I just think of the keel as a lever and just work out the vectors?
  2. Crag Cay
    Joined: May 2006
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    Location: UK

    Crag Cay Senior Member

    YEs it's a lever and you work out the moments. As long as they are similar it shouldn't have any knock on effects on sizing the rigging etc.

    HOWEVER: It's a massive job and one that will destroy the resale value of your boat. I presume that in order for you to consider this option, you are being coerced by a swarm of heavily armed aliens who have set you this test to prove the superiority of the human race. If so - good luck.

    If not, we need to talk!
  3. Crag Cay
    Joined: May 2006
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    Location: UK

    Crag Cay Senior Member

    Hi there, and thanks for the email.

    There are some realities of lifting keels you need to consider. Some only lift to aid recovery on ramps and improve trailerability. Once they are locked down to sail, they offer no advantages over a fixed keel boat. The lifting process is often quite involved and sometimes requires a lifting arm, tackle and bracket to be installed on the cabin top etc.

    In shallow water, you only get a real advantage if you have a system that allows the keel to kick up or can be lifted easily under way should you strike bottom. Although the process of lifting the keel with this sort of system is easy, the mechanism to achieve it with a heavy ballasted keel is very complex and costly.

    Both systems involve some form of box in the interior of the boat which will completely dominate (and ruin) the accommodation in your Laser 28. Also it not an easy structure to engineer as is must be close fitting enough for the fin to not slop, but still allow unimpeded lifting and must be strong enough to withstand the bending moments when heeling and the massive shock loads should you run aground.

    The Laser 28 is a one-design. Their value is intrinsically linked to them being one-design. You're the only person who wants one with a lifting keel. Oh, and that other bloke who's offering you 5000 bucks for it.

    For the money and work this project would entail, you are better off spending it on doing up your current boat and getting it on the market in the spring. Then look to invest the sale money in a lift keel boat. I've seen Melges 24's go for not much more than 20000 dollars in good nick. There must be tired examples for less.

    But if you want true shallow water performance, you need a multihull.
  4. Richard Larsson
    Joined: Sep 2007
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    Location: Sweden

    Richard Larsson New Member

    I agree with Crag, it´s very expensive to do a liftingkeel. But if you want to have some information you should look up the keel i´ve made to my boat.

    With this keel you can choose between a deep of 2,4m and a deep of 1,1m. It’s a huge different. The keelblade is telescopic, so it’s placed in a waterproof box under the table. Some other constructions have to be big inside this is not.

    When you go aground the keelblade and the bulb rotate back and the damages is minimal. It’s a big ball bearing that let the upper part of the blade rotate. Even in waves the boat rotate when the keel is hanging down. The pitching resistance is minimized with this quality.

    The construction allows you to carry a heavy bulb at a large deep for maximum performance. The blade is built with stainless steel which is a very strong material. The material make it possible to combine a correct profile with a strong mechanical structure.

    I lift it with a tackle and a winch from 2,4m to 1,1 m in 30sec.

    The rudderfeeling is better, it’s quicker. Even the autopilot has an easier work to do. With the original keel the autopilot couldn’t steer accurate in downwind or sidewind. Now it works much better.

    You get more information at www keeltech se

    Joined: Sep 2007
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    Location: LOS ANGELES

    RAY TOSTADO Junior Member

    If the boat you plan to work with is a true design, not just a lucky floater, the keel form has been intergrated into the overall performance and safety of the boat.

    Keels also add directional stability when so intended. They also are a part of the roll calculations as how they add lateral resistence, by their surface area, in contest to the forces of the sail plan when at optimum speeds.

    Reducing the surface plane requires a recalculation of the ballast factor as righting moment in a forward motion. The use of dagger keels with bulbs is a trade off for reducing wetted surface and drag. It is an on going arguement with the less drag supporters winning.

    Lots of neat one-designs might be converted to faster=fun types by adding bulb keels.

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