Extending a 19 foot Center console

Discussion in 'Boat Design' started by johnnythefish, Jun 28, 2016.

  1. johnnythefish
    Joined: May 2016
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    johnnythefish Junior Member

    Having gone full circle in my thought process I am now wondering whether extending my 19 foot Center console (8 foot beam), 12 degree constant deadrise planing hull) would help reduce the "hump" between displacement and planning.

    My thought process is as follows - I don't think it would be too big a job (I built the boat) to extend the hull by 2 feet at the transom. This would then make the boat 21 foot and long enough to consider an Armstrong type bracket.

    This would extend the water line and raise hull speed and it would also give me 4 more feet in the cockpit (as I would be doing away with the motor well).

    Keeping the console and t top where they are would also keep the weight fairly balanced (a thumb suck guess) and might also mean the boat squats less when everyone stands in the back whilst trolling.

    I am only going to do this if it will realistically improve my trolling speed fuel efficiency at 8 knots and I am not risking ruining a perfectly good boat?

    Any thoughts
     
  2. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    So the reason for doing this is to troll at 8 knots without wallowing in a trough ? I'm tipping the difference may not be as great as you hope for. You are still well past "hull speed". And you are right to fear unwanted ill effects from extending it out that far, especially running down-sea, if it is already a little dodgy in that department, it will get worse. Get those clowns to stand further forward, and gun the engine when you get a strike !
     
  3. johnnythefish
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    johnnythefish Junior Member

    I do think a longer boat would ride better and 4 extra feet would be nice - I guess the question is if I extended the hull accurately without introducing rocker and keeping the deadrise the same what could go wrong!
     
  4. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    A humpless transition to full plane mode, is hull shape derived and no additional length will make this happen, unless the hull has these shapes. You're hull speed is about 6.4 MPH (S/L 1.35) on an assumed 17' LWL, so increasing to 19' LWL will make your hull (displacement) speed just 6.7 MPH and hardly worth the bother. Even if the LWL was 21', displacement speed would still only be increased to 7.1 MPH.
     
  5. johnnythefish
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    johnnythefish Junior Member

    Ok - thanks - that's pretty clear.

    Now on a more hypothetical basis what would I have to do to that hull to make a smoother transition?

    Let's say I added 4 feet for example of variable deadrise towards the transom?

    Or put big chine flats?

    Or made the rear taper to a narrower transom?
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    The only way a 20 foot boat would be operating well at 8 knots is to have a semi-displacement type of hull, which your planing hull is not. There is basically little that can be done to keep it running level and not use much fuel.
     

  7. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Without a really clear idea of what your boat is shaped like, suggestion would simply be guessing. Making a monohedron a warped bottom is going in the opposite direction.

    I'm also not sure what you're attempting to get from this boat. If it's shaped like most powerboats, it'll just suck at displacement and semi-displacement speeds. It's the nature of things. If you want better low speed handling, you'll need to reshape the hull aft and this will kill it's full plane mode abilities. The contrary is also true, if you want to make the transition zone to full plane mode smoother and with less "hump", you can kiss of any low speed handling attributes. As far as making the transition to full plane better, well this could get quite complex, requiring a whole new bottom.

    As a rule, warped bottoms get up on plane quickly, though usually have a fair bit of bow rise associated with the transition zone. Monohedrons tend to be sluggish in transition, comparatively, but usually rise up uniformly (if it's a modern design) and maintain this trim level, pretty much. Big chine flats aren't easy to design well and if they are can help a monohedron get up on plane quicker, but they can limit top speed potential.
     
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