Horses on a 20' center console

Discussion in 'Boat Design' started by kdhorton, Sep 14, 2012.

  1. kdhorton
    Joined: Jun 2010
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    kdhorton HM Small Boat Designer

    I am seeing these humongous outboards on every production CC I'm referencing as I'm working on my own and I'm seeing 115+HP outboard beasts on these relatively little boats.

    According to the calcs laid out in Teale's book (not my sole reference but simplest for this calc) using a 20HP engine I could attain ~25 NMPH with this boat at a displacement of ~620lbs.

    Design waterline is going to be slightly adjusted for LWL and displacement when the boat is idling and I've yet to add weight of the controls, live tank when full and the outboard itself, but I can't imagine that even at half a ton (imperial) I'd need to beef up to 150 or 225 HP to run these things! Is there a logical reason for this or is this another case of selling a boat to a car person?
     
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  2. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    A 20' centre console with ~620 lbs displacement ? The crew for a day's fishing will be more than that !
     
  3. kdhorton
    Joined: Jun 2010
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    kdhorton HM Small Boat Designer

    It's not a traditional CC; its more of a flats boat and I'm adding displacement as I add weight. Thats the ply hull, 'glass tape and bottom. My sheet is showing a little over half a ton finished weight before crew, tackle and consumables but even with the average numbers that still doesn't justify a giant 150 hp engine...
     
  4. philSweet
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    philSweet Senior Member

    Why do you want such a big boat? Most 20'ers in the Keys can do 60Kts. If you only need 50Kts, go with a 19'er. 15-16' boat do 25 knots just fine. Usually with 70-90 hp.
    Can be done with a bit less, but you'll have an oddity on your hands. If you want to look at 20 hp designs, look at the boats from the mid fifties. 20hp was a big OB back then.
     

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  5. kdhorton
    Joined: Jun 2010
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    kdhorton HM Small Boat Designer

    Its a sort of present for an old friend who's a big ol boy. Thanks for the references though, I'll look into some of those.
     
  6. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    The power requirement of a planing hull is dependent on the desired cruise speed, more than any other factor. If you want to cruise at 40 knots, expect to install four times as much hp than you'd require at 20 knots, as a rough guide.
     
  7. tom28571
    Joined: Dec 2001
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    tom28571 Senior Member

    Recent boat builders of CC types have been caught in the macho spiral of speed and power. You are right to question this trend. Speed of a planing boat, assuming a good bottom design, is determined by bottom loading in pounds of displacement per square foot of planing area. Most on the market are deep V hulls that, by definition are heavy and thus require big honking motors. With a shallow deadrise in the aft part of the bottom and no distortions of the flat bottom panels, the Crouch formula gives a good prediction of the speed.

    Still, 20hp is not going to give much performance unless the boat is built very light and not loaded with gear. 620# is probably unrealistic to use as a number for looking at power requirements. You can look at the boats in WoodenBoat Magazine's contest for such boats of a couple years ago. The winner of the contest will perform on 25 hp but I know from experience with the boat that it is marginal power. You can certainly get the results you want with 40 to 50hp with reasonable and careful design. If you want to get high speed of 40mph or more, you will need to go to the big motors too.
    http://www.bandbyachtdesigns.com/ec018.htm
     
  8. tunnels

    tunnels Previous Member

    20 hp on a 20 foot boat you must be joking 120 hp even is on the light side!!
    150 AT LEAST !! better a big motor not working hard ,than a small motor slogging its guts out the moment its pushed in gear .

    20 FOOT 20 HP AND 25 NMPH THATS A LAUGHT MUST HAVE BEEN GOING DOWN HILL WITH TAIL WIND!!
     
  9. Submarine Tom

    Submarine Tom Previous Member

    My 25 foot house boat does 5 knots with a 8 hp four stroke (and no wind I might add).
     
  10. jonr
    Joined: Sep 2008
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    jonr Senior Member

    > Horses on a 20' center console

    I've seen them load horses on boats many times, but never one that small :).
     
  11. Wayne Grabow
    Joined: Aug 2003
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    Wayne Grabow Senior Member

    Listen to Tom. By the time you get your 20" boat loaded up you will be at 1200-1500 pounds. You will need at least 40 hp to perform adequately; 75 would be better. 150 is ridiculous.
     
  12. tunnels

    tunnels Previous Member

    get with it mate !!!

    19.6 WITH A 200 IS A GOOD COMBINATION !!That what my friends fishing boat has and its power on tap if you want it ,use half power and get the durability and econamy that goes hand in hand !!
    I have a 14.6 with a 115 yamaha and its just a fishing boat !!!
    Sure its fast at 6400 rpms but spends most of its life cruising at about 3000 rpms or just over . its a big motor doing 1/4 the work and is more ecomomical than the 70 hp i used to have .
    When the weather is great and the waters like i mirror the thrill factor alone at 6400 rpms is well worth living for .
    You underpower and use small motors then its slogging its gut out and over worked and becomes unreliable ,wears out quickly and very uneconomical and your usually wishing there was more when you are last to go places all the time . No it dosent have to be a race just common sense !!:D
     
  13. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Without pictures of your running surface and a realistic full up weight, including crew, full fuel tanks, full live wells and a couple coolers full of beer, any reply as to HP requirements is just folly. Post some pictures so we can see the bottom and a realistic weight estimate and we can get you close with some simple math.

    You've made some contradictory comments about your weight estimates being a little over a half a ton, yet your displacement is a little over a 1/4 ton. Given this discrepancy, photos and weights please . . .
     
  14. Mr Efficiency
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    Mr Efficiency Senior Member

    Over a waterfall ? :p
     

  15. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    If you use the following formula to work out HP requirements for planing monohulls, you won't be far out:

    (waterline length in metres) x (waterline beam in metres) x (hull only draft in metres) x (desired cruise speed in knots, squared), divide result by 10.
     
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