power cat design

Discussion in 'Boat Design' started by discoveryse97, Apr 18, 2014.

  1. Richard Woods
    Joined: Jun 2006
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    Richard Woods Woods Designs

    If you hadn't mentioned the Mercs I would have suggested my Skoota 28 or 36 as suitable boats

    See here http://sailingcatamarans.com/index.php/designs/6-powercats/264-skoota-28

    I agree the 1000 mile range (I assume meaning with built in tanks) is not realistic. At 7mpg, like my Skoota, you'd need 150gals, roughly 0.5T, or 15% of all up weight.

    But more relevant is - "why 1000 miles"? it won't get you across an ocean, and is un-necessary for coastal cruising. You'd be carrying a lot of extra weight, thus use more fuel, even if you bought fuel cheap somewhere and stored it - it's not really cost effective.

    I also agree that waterjets are in-efficient at that speed.

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     
  2. discoveryse97
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    discoveryse97 Junior Member

    i am looking at a minimum 100 gallons fuel if i use i tri hull design.

    richard woods... i truly like the Scylla - 10.65m performance cruiser design. i could also use other motors. like the compact subaru turbo motors. or for a larger boat use v8 diesels. jet drives arnt a must just nice as i have them allready
     
  3. discoveryse97
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    discoveryse97 Junior Member

    looked at the wrong boats. lol. i like the skoota 28 alot. and could easly use different motors
     
  4. discoveryse97
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    discoveryse97 Junior Member

    as far as range like i said input is always welcome. even if i got around 5-700 i could make it work
     
  5. Mr Efficiency
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    Mr Efficiency Senior Member

    If you insist on jets, your only chance is a full planing design, but 40 feet would be too big for your engines. Maybe high 20's or low 30's feet maximum, with a cruise speed low 20's knots, still burning fuel like it is going out of fashion. I would not like to quantify the losses through using a jet drive at those speeds, but would be at least 15-20% greater than a sterndrive or shaft drive. Maybe quite a bit more, depending on the type of jet unit you are using, some are designed for lower boat speeds, most are not. And your range will be something like 200-250 miles, if you require substantially more than that, you face an uphill job imo.
     
  6. discoveryse97
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    discoveryse97 Junior Member

    ok ty. then no jet drives. like i said this is a project that will take a long time.you are right aswell the jet drives are from a pair of boats that had 454s in them
     
  7. discoveryse97
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    discoveryse97 Junior Member

    also the skoota speed is impressive for so little power.
     
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    As a general rule, the attraction of using second-hand car diesels as a cheaper entree to boating is largely illusory.
     
  9. Stumble
    Joined: Oct 2008
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    Stumble Senior Member

    Discovery,

    I think you need to define exactly what you want the boat to do, not what engine you want to use. Once you know what you want to do and how much you are willing to spend to do it, then people can start making suggestions on what may or may not be possible, realistic, and cost effective.

    Defining the engine and drive system before defining the usage is doing it backwards.
     
  10. Mr Efficiency
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    Mr Efficiency Senior Member

    That is a sail boat, not at all suited to your ideas.
     
  11. Navygate

    Navygate Previous Member

    This appears a design disaster!
    Cheap motors, but fuel costs are no object.
    This thread could go on for months and get nowhere.
    Hire a Naval Architect or learn about the design spiral,
    or both.
     
  12. Richard Woods
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    Richard Woods Woods Designs

    Thank you for the compliments on my designs. The Skoota is proving a popular boat in the PNW, with a number building.

    Last year we only used the 3gal tanks that came with the engines, but this spring I fitted our "big" tanks, 16gal each. That gives us about 200 mile range. Since last refuelling (in Port Ludlow, Wa) we have motored maybe 150 miles (currently we are in the BC Gulf Islands) and the tanks say 1/4 full. We also carry about 30gals in loose cans. So our total range is about 400 miles, maybe a bit more, at 12 knots. At 6 knots we do around 9mpg so over 500 miles

    I don't like using big tanks because of the ethanol problems - although we do have water separators in our fuel lines. I actually drew 30gal tanks and there is plenty of room for them in the aft lockers, and still leave room for "stuff". Each locker is 6ft x 2ft x 18in - real coffin berths. With those tanks you'd easily get your required range

    You are welcome to visit us any time, either in BC or back in Port Townsend, where we will once again be exhibiting our Skoota in the Wooden Boat Festival in September

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     
  13. groper
    Joined: Jun 2011
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    groper Senior Member

    The Cat i designed sounds like what your looking for...

    [​IMG]

    Powered by twin 60-90hp 4strk outboard motors.
    Efficient cruise @ 16kts ~ 4mpg, fast cruise @ 20+ kts.
    650Litres fuel internal tanks, with plenty of space to carry extra or build in more tankage.
    35ft LOA.
    Maximum displacement 4 tonnes.
    All flat panel construction - tortured composite sandwich panels or plywood.

    [​IMG]

    [​IMG]

    [​IMG]
     
  14. kerosene
    Joined: Jul 2006
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    kerosene Senior Member

    This.
    300hp out of mecedes 300d might be doable for drag racer but that kind of service is nothing like engine's life as a boat propulsion unit. Typically a car engine tuned for boat use will make substantially less power than the automotive counter part.
    Car uses max power only for short occasions, 20 seconds at a time already being rare. Boat needs to be able to do full power hours on end. That never happens to a car engine.

    I would think 100hp+ in constant duty is probably optimistic from said engine.

    The rest of the soecs like the piercing bow for the right looks tell me this is a dreamer project.
     

  15. FAST FRED
    Joined: Oct 2002
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    FAST FRED Senior Member

    Looking for diesel engines , for service life , 3CI ( cubic inches) per 1 long term HP seems to work fine.

    The nicest cruising is done when you figure the HP required at cruise and match that HP to an engine at low RPM , 1500 down to 1200 is usually nice and quiet and vibration free.

    A Sport fish with a scale can tow you with a GPS to figure the power required at different speeds.
     
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