19ft P-Cat powered by 18hp Nissan outboard.

Discussion in 'Multihulls' started by Roy Achoy, Jun 8, 2008.

  1. Roy Achoy
    Joined: Jun 2008
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    Roy Achoy Junior Member

    I have a P-Cat that I want to make into a Costal fishing boat. Any Input?
     
  2. garydierking
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    garydierking Senior Member

    Outboard P-Cat

    18 hp is probably more than you need, but if you can mount it far enough forward it should be okay. A critical factor is the shape of the outboard's lower unit. When not mounted on a traditional transom, they can ventilate horribly and cost you power and speed.
    Have a look at my solution; it made an astonishing difference.
    http://homepages.paradise.net.nz/garyd/outboard.html

    Gary
     
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  3. Doug Lord

    Doug Lord Guest

    Roy, I have been considering a power experiment on my 21' tri-I was thinking 8hp -16hp starting with 8 on one side. It will be fairly light-200lb.
    Good Luck with your boat!
    ---------------------
    Gary, absolutely brilliant-can I borrow it if I go ahead with my experiment?!!
     
  4. garydierking
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    garydierking Senior Member

    Please do try it. It's even better when you've tried it without the fairing first. Some outboards have a fairly decent shape but the smaller they are, the less attention they give to streamlining them.

    Gary
     
  5. Roy Achoy
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    Roy Achoy Junior Member

    I had decided to use the Nissan 18hp due to the light weight 69lbs vs 90lbs for the Johnrude 9.9hp.

    My first thought was to mount a pair of them one on each hull. But I am thinkt that mounting it on the centerline and using a 5hp for trolling would be better in terms of range vs weight.

    I want to put a steering consle on the bridge deck simalar to on used on a 17ft boston whaler. Not sure thought If I want it that narrow or going with a much wider structure that would offer more protecton from spray and wind.


    The area where the centerboards are located I plant to make into insulated fish holds/ Coolers.

    Gas tanks will be the 6 gal portable type I have figuer out where they will go but I will need to keep the weght in the centers of the hulls for trim.

    My rough guess is that with the 18hp I can get about 12 knots and burn about 1 gallon per hour. or 60 miles per gas tank.

    My use will be San Pedro to Catalina in the early am. Then trolling the waters south of Catalina for Marlin & Tuna during the day. Run back to San Pedro at night after the wind dies down.

    I would like to have any input?
     
  6. Chris Ostlind

    Chris Ostlind Previous Member

    Roy,

    I grew-up in Redondo and lived in San Pedro for 7 years, making many trips across that very piece of the channel.

    I should think that you'll get all the power you need from a 5-9hp outboard. Your boat is already very light, you won't be able to add too much in the way of additional displacement weight, especially aft, or you'll start to really sink the hulls and the fastest you might want to go on that particular hull set would not exceed 12 knots. It's not the same as sailing those hulls as you will get quite a bit of splash pounding from the low beam height.

    Now, if you were to raise the freeboard, you'll get a better ride, a more comfortable boat and have more discrepancy as to how you move about the fishing platform. These boats are not made for a lot of weight aft when they are not being sailed.

    The smaller, suggested engine package will give you much greater range from the same fuel capacity, it will virtually sip gas and when you want to wick it up, you'll have the thrust you need at hand. Yamaha 9.9HT's are intensely powerful for a boat this light and slender of hull.

    The worst conditions will be in the afternoons when the wind is piping-up out of the western end of the channel and there will be some degree of sea swell being generated. The rest, as you indicate, will be calmer and far more friendly to your fishing interests.

    Gary's suggestion as to the fairing is wonderful, though having hard deck in place will blunt some of the need if the engine is boxed nicely. I'm guessing you will steer with the rudders and not the engine.

    Chris
     
  7. Richard Woods
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    Richard Woods Woods Designs

    Gary is right about the problems caused by ventilation caused by an unfaired outboard leg. And also right that a 18hp is probably un-necessarily large for a P-Cat.

    Over 20 years ago I did some experiments with our 35ft Banshee sailing catamaran (a performance cruiser with full bridgedeck cabin). 45ft mast, weight about 3.5T

    With a 4hp outboard it would do 4 knots, with a 9.9hp 6 knots. With a 50hp 10 knots, with a 75 hp 11 knots. These engines were all centrally mounted in the standard nacelle.

    It was very clear that the bigger engines were doing little except suck air and depress the sterns.

    We then moved the engine to a transom. With a single 90hp transom mounted engine we did 16 knots. With twin 90hps we did 22 knots towing a water skier.

    Sailing, the same boat did 18 knots. So maybe the perfect motor sailer??? A sistership is currently halfway across the Pacific having left the UK last summer, on a trip round the world.

    On my 20ft trailable powercat Skoota I am fitting a central 15hp outboard and expect to do 12 knots.

    I have sketched a 34ft powercat cruiser as a low wake, low fuel consumption boat. Based on similar non-planing designs I expect to do 15 knots with twin 25hp engines. I also hope to better 12mpg at cruising speed.

    You can see more on my website

    www.sailingcatamarans.com

    Best wishes

    Richard Woods of Woods Designs
     
  8. Roy Achoy
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    Roy Achoy Junior Member

    The reason I choose the Nissan 18hp was that it weighs 22 lbs less than the Johnrude 9.9. Having a bigger motor I felt I would be able to run it at a lower RPM to reach the cruising speed there by saving fuel.

    The low bridgedeck will mean it will be a wet boat that is why I am think about putting a larger center console sort of like a fly bridge style mounted on the hulls rather than a narrow console like a walk around.
     
  9. Roy Achoy
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    Roy Achoy Junior Member

    Richard
    What do you atribute the differance in speed by mounting the motor on the hull rather than on the centerline to?
     
  10. Richard Woods
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    Richard Woods Woods Designs

  11. Roy Achoy
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    Roy Achoy Junior Member

  12. Roy Achoy
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    Roy Achoy Junior Member

    I have a chance to buy Two johnson 6hp Sailmaster longshafts. The weight of both motors is about 40lbs more than the single Nissan 18hp.
    I could mount them on the stern of each hull.

    Would this be better than the 18hp mounted on the centerline?
     
  13. Richard Woods
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    Richard Woods Woods Designs

    Far superior manouverability, redundancy (sounds like you are getting old engines) so safer. More complex steering arrangements. Two fuel tanks to stow. Transom reinforcing required. Wrong props?

    But on balance I'd say yes

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     
  14. Roy Achoy
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    Roy Achoy Junior Member


  15. Roy Achoy
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    Roy Achoy Junior Member

    I was able to speak at legnth with Owen Minney, he built the P-Cats.
    He has converted over 20 to use outboard motors.
    His latest version is powered by a 50hp Honda mounted on the centerline at the rear of the bridgedeck. He is getting over 30mph in testing at Lake Tahoe.
    He did add a Vee shaped deflector under the bridgedeck in front of the motor leg.
    He said the key to making the P-Cat work was to keep it light and to keep the weight in the center of the boat.
     
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