bilge keels

Discussion in 'Boat Design' started by David Dewberry, Mar 24, 2006.

  1. David Dewberry
    Joined: Mar 2006
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    David Dewberry Junior Member

    I have a 40' long by 12' beam headboat that rolls. I'm thinking about bilge keels and wanted to see what kind of success anyone has had with bilge keels or if I would be just wasting my money and time.


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  2. Guillermo
    Joined: Mar 2005
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    Guillermo Ingeniero Naval

    I have added bilge keels to my motorsailer. They are long and narrow, much in the way they do for fishing boats, as I fiberglassed them to hull, not bolted.
    Roll reduction is poor at anchor and let's say 20% when on way. Shorter and wider ones should produce more dumping.
    You may want to have a look at Michael Kasten's 'Roll reduction strategies' page:

    You must take into account that bilge keels do not affect stability, only dump more or less the rolling movement. So, if what you have is a tender boat, bilge keels will not solve the problem. If she's tender, you'd probably have to ballast the boat rather than adding bilge keels.
  3. mmd
    Joined: Mar 2002
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    mmd Senior Member

    At first blush it looks like you are suffering from a high vertical centre of gravity. With a relatively narrow boat (Length / Beam ratio is 3.33; the lobsterboats around here - similar hull shape and often used for passenger cruises for deep-sea fishing or whale-watching - are normally betwen 2.90 - 3.00) combined with the high deck placement, when you have a load of folks on board your boat is gonna roll quite a bit. Most boats have the deck installed a foot or two below the sheer.

    The addition of roll-suppression devices on a boat with minimal stability can lead to false confidence - the initial rolling is reduced, making you think that the boat is safe, but the underlying instability remains. If the roll supression system is compromised, the instability reasserts itself and you are suddenly in very big trouble. For example, if you install "flopper-stoppers" the rolling will be reduced, but if everyone is on one side of the boat watching someone fight "the big one" and the boat takes a roll far enough to pull the flopper-stopper "bird" out of the water on the opposite side, there is no mechanism to retard the boat's desire to continue the roll to the side that the customers are on, possibly with disasterous results. Therefore, the addition of bilge keels, flopper-stoppers, etc., should be delayed until you evaluate the initial stability of the boat. Does Florida have state or Coast Guard regulations regarding minimum stability for commercial passenger excursion boats? If so, have your boat evaluated, and note that the current USCG value for the "normal" weight for a person on board (140 lbs) is woefully inadequate; it should more reasonably be at least 175 lbs. If no state or USCG stability evaluation service is available, hire a naval architect to perform an inclining experiment and subsequent stability analysis on your boat. From this you will be able to have hard data on the safety of your boat and definitive advice on how to improve it. It won't be cheap, but it is a business expense that should have some tax advantages, and it should allow you to negotiate a better insurance rate for coverage when you can show professional stability evaluation documentation.

    I apologise for being so depressing, but there are too many tragic incidents where well-intentioned boat operators modify boats for public useage and unknowingly make structural changes that compromise the safety of their boats and jeopardize the live of their customers. Granted, I don't know the details of your boat in detail so may be grossly in error, but your description of the rolling problem combined with the photos you've shown gives me the impression that you have an issue with initial stability with your boat. As you are involved in commerial passenger operations with it, I urge you to seek professional opinions on the safety of your vessel.

    The likely solution, if I am correct, will probably be the addition of ballast low in the vessel's bilges and/or a limit on the number of passengers and vcrew on board.
  4. David Dewberry
    Joined: Mar 2006
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    David Dewberry Junior Member

    mmd, thanks for your comments. I'm not having a stability problem though. The boat has a lot of ballast and has gone through the necessary test with Coast Guard and has COI and stability letter for 33.

    Roll may have been the wrong word. I think rock is better. Once it starts to rock, it takes several seconds to stop... like a weeble wobble.

    I use a device called "Rocker Stoppers" that look like mushrooms on a rope that hang in the water at anchor and work kind of like a sea anchor. They do help, but are a pain to put in the water every time we anchor and can get caught in fishing lines. The sharp rock back and forth is what I hope to dampen.

    Also, The boat rocks more with a lite load than with say 20 to 30 passengers.

    Thanks... any more ideas and comments are appreciated.
  5. Guillermo
    Joined: Mar 2005
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    Guillermo Ingeniero Naval

    I seems you've got resonancy rolling with beam seas (Quite common). Yes, you may use bilge keels to dampen that, although they will not work for all waves periods. Short and wide ones, firmly fixed to the hull, work better to that end. If you can do the thing without bolting through, the better.
    Notice that bilge keels increase friction resistance, and if boat speed is higher than hull's, they may not be recommendable at all, specially if they are wide . And in many cases they perform an annoying tendence to 'bump' whith passing waves.
    Although rocker-stoppers in their various forms are a pain to settle, they work much better than bilge keels if mounted on poles. I use them often when at anchor, quite happily.
    Nothing's perfect...:rolleyes:

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    FAST FRED Senior Member

    I have a Uniflite 50 Navy utility boat that was converted for lobstering.

    The lobster co installed bilge keels about 8 inches wude and 30 ft long on both bilge turns.

    They do not seem to harm the speed , but we run displacement 8k not Navy flank of 12.

    They do dampen the rolling in most sea states while anchored,or stopped, at least we have never experienced a periodic roll as a sail boat does running downwind , while anchored in cross chop.

    HOW much they do ? has not been established till I can find another Navy 50 to be alongside in rough conditions.

    These are about 1/2 inch thick GRP and simply glassed on. This is strong enough as we have taken the ground many times and the vessel rests on her keel and one bilge keel.

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