Outrigger Cabin Cruiser Design

Discussion in 'Powerboats' started by GalenP, Dec 31, 2012.

  1. GalenP
    Joined: Dec 2012
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    Location: Washington State

    GalenP Junior Member

    How Much Power For 10-12 Cruising, 15-17 Top? Consider free Yanmar Diesel?

    I am living in Port Townsend now. A good place to build this boat.

    For my design (power outrigger cabin cruiser) I have always intended to use outboard power. But I have a free, nearly new Yanmar diesel available, so I need some advice.

    My desire is 10-12 knots cruising speeds, but capable of 15-17 for passage crossing.

    The boat is drawn at 33'. It has a slender waterline length/beam ratio of about 11:1. It is a displacement hull. Weight will be around 2500-3000 lbs dry, maybe 3500-4000 when cruising. It will be trailerable.

    Based on other boats, I would guess I will need a 40-50hp outboard to achieve my desired speeds.

    But I have available for free a nearly new Yanmar 27hp marine diesel. It weights about 300lbs sans shaft/prop, etc. A 50hp outboard is about 230lbs.

    So although I think the outboard is a better fit for this lightweight multihull, I am considering the diesel because it's free, would be more fuel efficient, and is potentially very reliable and long lasting.

    First Question:
    How much HP do you think it will take to achieve my desired speeds with this boat?

    Second Question:
    What speeds do you think I could expect from the 27hp Yanmar?

    Thanks
     

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  2. Mr Efficiency
    Joined: Oct 2010
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    Location: Australia

    Mr Efficiency Senior Member

    Sounds optimistic to be expecting 15 to 17 knots top speed, and having a 50% difference between cruise and max implies you will have a lot of spare revs to play with at your cruise speed, which is not likely either with an engine at the low end of the possible power options. 27hp is still 27 hp, petrol or diesel.
     
  3. GalenP
    Joined: Dec 2012
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    Location: Washington State

    GalenP Junior Member

    Thank you. A number of similar sized and weight catamarans achieve the 15-17 knot top speed with 40hp or less. So I think it is possible. But probably not with a 27hp motor.
     
  4. Mr Efficiency
    Joined: Oct 2010
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    Location: Australia

    Mr Efficiency Senior Member

    You have in your favour a low profile, low frontal area boat with good aerodynamic penetration, I am very sceptical about the speed figures quoted for displacement cats with broad, high structures, in strong headwinds.
     
  5. SpiritWolf15x
    Joined: Apr 2010
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    Location: Vancouver, Canada

    SpiritWolf15x Senior Member

    Neat idea, nice blueprints but this whole idea looks like it's be very prone to tipping over... Especially going hard to port.
     
  6. Mr Efficiency
    Joined: Oct 2010
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    Location: Australia

    Mr Efficiency Senior Member

    I was more concerned about exposure to breaking seas on the "wrong" side. Handle accordingly.
     

  7. GalenP
    Joined: Dec 2012
    Posts: 25
    Likes: 1, Points: 3, Legacy Rep: 13
    Location: Washington State

    GalenP Junior Member

    In sailing versions of this style of boat they have proven very difficult to capsize. So without sails this will be very stable. The outrigger will also have a water ballast tank for rougher conditions.
     
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