Why do modern powerboats have little or no keels ?

Discussion in 'Boat Design' started by cyclops2, Nov 25, 2011.

  1. cyclops2
    Joined: Sep 2010
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    cyclops2 Senior Member

    Are they a power eater ? Serve no real purpose ? Cost too much money to build in ? No one sees them. so delete them.

    Why no more ?
     
  2. eyschulman
    Joined: Jul 2011
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    eyschulman Senior Member

    Many modern boats have keels and they are usually associated with a trawler-displasment-or lobster(down east)type boat and are very popular in some areas. Now the high speed and offshore sport fisherman type rarely have keels. Things that protrude from bottom that have wetted area and cause resistance slow a boat. It's the old trade off between advantages of keel and disadvantages which way you go depends on what you expect and what you want the boat to do.
     
  3. PAR
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    PAR Yacht Designer/Builder

    Modern full plane powerboats don't really need keels (skegs actually). Most of these boats are outboard or outdrive propelled, so the leg provides sufficient lateral stability.

    Displacement and semi-displacement craft are a different animal altogether and need a skeg for directional stability.

    This is the "Cliff's Notes" version as the subject is considerably more complex then this, but suffice it to say, typical production craft, with their steeper deadrise and thrust vectored propulsion don't need them, while slower moving, typically straight shaft equipped boats can benefit from them.
     
  4. cyclops2
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    cyclops2 Senior Member

    Aaahaaa

    Cliffs notes was EXCELLENT !! :):):)

    Your description of the times when a " keel " or skeg is needed, fits my extremely windy & choppy wave area on weekends. Also the docking conditions I have.

    See everyone. Designing a boat for " Crappy Water " conditions is not too difficult. It just takes very experienced & talented people. :)

    A VERY RARE combination .

    Thank you both.

    Rich

    ( The 21' x 4' vertical bow x 23 degree constant deadrise hull. D I Y. )
     
  5. messabout
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    messabout Senior Member

    Vertical (plumb) bows force the vee'd section skin to have wicked twist at the bow, often producing some peculiar geometry near the forefoot. twenty three degrees is a lot of deadrise so your boat is apparently designed for very lumpy water.

    Is this an in service boat or one that is merely on the drawing board?
     
  6. michael pierzga
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    michael pierzga Senior Member



    Hell yes skegs have a purpose. They protect the vessels running gear. Without a skeg to shield the prop in a grounding the P bracket tears out of the hull and she sinks.
     
  7. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    If the bow is fine the twist can be manageable, though it is a common problem with plumb or nearly so stem profiles.

    A skeg's primary purpose isn't to protect the running gear, though it is an added benefit if you can warrant the drag they produce.
     
  8. cyclops2
    Joined: Sep 2010
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    cyclops2 Senior Member

    How quickly we foget the odd ball.

    Do you remember this old wood design that I have cut the bottom " V " to a constant 23 degrees of deadrise ? The warped / flattened transom frames of the plans does not exist anymore. The bow section frames are followed as designed by Charles Mower.
    The boat is supposed to look old. But be VERY insensitive to wind, waves or chop from any direction. Or a combination of all at once on a holiday week end.
    The boat will have the use of the full time automotive power steering system.
    A concern that just popped into my head is how much rudder area is required to EASILY force the boat to turn rapidly as a car in a emergency & cause the boat to lean into the turn as speed increases to 40 mph. I have read that deep, narrow rudders cause that desirable condition. Spade type ??
    I have been only in 1 boat in my life that could have the rudder whipped into a full turn at 45 mph and lean so far over that you did not feel any side shift at all. ...Just a massive downward force into the seat cushion & a uncontrollable collapsing of my lungs as I was forced to exhale.
    That is the depth and amount of rudder area I am looking for.

    Rich
     

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  9. cyclops2
    Joined: Sep 2010
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    cyclops2 Senior Member

    I am either going to build a good handeling boat or a dog.

    I have tried to use Peters advice as I can.
     

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