Lifting keels in motor boats ?

Discussion in 'Boat Design' started by rustybarge, Jul 4, 2014.

  1. rustybarge
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    rustybarge Cheetah 25' Powercat.

    Hi all,

    A few years ago I was sitting waiting for ferry in the Isle of Wight on the Solent, just a few miles off the coast of England, to return back to the mainland. It was a choppy blustery day with a steep waves. A long line of boats were coming into the marina in a beam sea, sailboats, motor boats of all sorts from trawlers and planing craft of all sorts sizes and shapes.

    One thing I noticed is that the sailboats hardly rolled at all, the motor boats experienced extreme rolling motions.

    The design concept:

    Why not use a lifting keel to give adjustable roll stabilisation in a motor boat?

    Fully down at 6' draft it would give max roll resistance in a beam sea, half way in a quartering sea, and fully up going downwind.........
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    You don't think the sails were a major factor there ?
     
  3. rustybarge
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    rustybarge Cheetah 25' Powercat.

    Most of the sailboats were motoring with the sails down as they were very close to the entrance to the marina, but of course the sails would damp roll considerably.

    Fixed ballast in a motorboat can cause a snappy roll if it is heavy enough to work properly in a beam sea, so the lifting keel idea would make it adjustable.

    ..rather like the concept of spume(?) water tanks on the wheel house roof to help roll damping, but under the centre of gravity rather than above it.
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    Planing boats ( not cats) are typically stabilised in beam seas by dynamic effects, and don't require drag from appendages to add to the fuel bill, they would not be suitable candidates.
     
  5. rustybarge
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    rustybarge Cheetah 25' Powercat.

    I think there are two hull forms that could benefit, full displacement where added weight is a bonus to seakeeping, and ultra slim dipl. Hulls which can operate at speeds higher than the hull speed.

    Perhaps The lifting keels could possibly be like lee boards on a Dutch barge to save the loss of accommodation inside the hull with a centrally located lifting keel?
     
  6. rustybarge
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    rustybarge Cheetah 25' Powercat.

    A vertical lifting keel like this would work well, with a massive lead bulb keel hanging on the end to give maximum stability.

    [​IMG]
     
  7. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Even with the sails doused, the rig and mast offer a lot of roll dampening. Power craft roll characteristics are more shape related than anything else. A deep fin with a hunk of lead on it isn't going to help much, at the small angles of heel seen with the typical roll rates, experienced by powerboats. In very rough conditions, with steep, short seas, yes, you'd gain some benefit, but you also have a draft, weight and drag penalty to pay.

    Simply put, the presence of a fixed or retractable ballast bulb doesn't do much, until it's canted over quite a bit, so it can act as leverage. Most power boaters aren't willing to motor at a 15 - 20 degree angle, to quell a rolling issue, which is what is necessary for a ballast bulb to be effective. It's mere presence below the keel isn't enough, it has to be working on the CG, which requires a fair bit of heel.
     
  8. Tad
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    Tad Boat Designer

    Avard Fuller was using deep dagger boards to damp roll in his light-displacement powerboats starting the 1960's. It worked for him. The main benefit (IMO) is no awkward deck work with paravanes.

    JimHawkins1.JPG
     
  9. rustybarge
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    rustybarge Cheetah 25' Powercat.

    The idea of a bulb keel was to lessen the wave energy in a beam sea from acting on a conventional lifting keel plate which is at 90* to the waves. The bulb itself is round and offers little lateral surface area, and the the connection leg is very slim and would have very little drag in any direction.

    Of course under normal motoring conditions it could be raised into a recess in the hull bottom.....
     
  10. rustybarge
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    rustybarge Cheetah 25' Powercat.

    Damn, somebody's thought of it already.

    Just wondering if that's just a centreboard, or is it a ballasted keel? 10' of draft is an awful lot!

    Love the concept.
     
  11. NavalSArtichoke
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    NavalSArtichoke Senior Member

    One type of passive anti-roll tank is the Flume tank, with 'Flume' being a trademark. There are other types of tanks, like Frahm tanks and U-tanks, which perform essentially the same function:

    http://www.neely-chaulk.com/narciki/Anti-roll_tank
     
  12. rustybarge
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    rustybarge Cheetah 25' Powercat.

    Thanks.

    I read that there can be some safety issues with out of phase wave patterns, maybe the reason they never became popular.

    Also seems to be a very complex solution with baffles, pipes, valves, chambers and pumps!
     
  13. DCockey
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    DCockey Senior Member

    Two different phenomena are being confused.

    Appendages such as keels, centerboards/daggerboards and bilge keels can dampen rolling by transferring energy into the water as the boat rolls. This is a hydrodynamic phenomena and is independent of the mass of the appendages. Note that this is a dynamic effect only and does not affect static stability.

    Lowering the center of gravity will increase static stability but will also decrease the roll period. The boat might roll less but it generally roll quicker (everything else being equal). Generally too quick a roll is considered very undersirable. (Added) However, if the center of gravity is lowered by a large mass hung under the boat the moment of inertia will be increased which would tend to increase the roll period.
     
    Last edited: Jul 5, 2014
  14. d1970
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    d1970 Junior Member

    Interested to see the mechanism,and its durability.
     

  15. Mr Efficiency
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    Mr Efficiency Senior Member

    DCockey is right of course, roll damping, and stabilising by lowering the COG are two different issues.
     
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