Reasonably seaworthy cruising powercat conversion

Discussion in 'Boat Design' started by svmegatron, Feb 11, 2012.

  1. tom28571
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    tom28571 Senior Member

  2. sabahcat
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    sabahcat Senior Member

    From Malcolm Tennant
     
  3. svmegatron
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    svmegatron Junior Member

    @sabahcat, that's really interesting. I'd be pretty happy staying below 15 knots. It sounds like the problem with converting a macgregor, stiletto, etc. might be that the sterns wouldn't support the weight of twin outboards mounted to the transoms. Maybe there's something out there with more volume aft that would support the weight.
     
  4. sabahcat
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    sabahcat Senior Member

    Or maybe you could make a new "clip on" modifying the last 10 feet of hull.
     
  5. brian eiland
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    brian eiland Senior Member

    Its not necessarily a question of the sterns having enough bouyancy that's causing them to squat. Much of the squating can be related to speed of the water passing under those sterns and its decreasing pressure.

    Lower you speed requirements.
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    Under sail, the line of thrust is high, and that would push the bows down, countering squat. None of that with the power version.
     
  7. Richard Woods
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    Richard Woods Woods Designs

    I have posted this photo before on this forum.

    A Banshee sailing catamaran at 22 knots towing a waterskier using twin 90hp outboards. Note the extreme trim. Not very efficient. It's due in part to a lack of buoyancy aft and also to no lifting surface aft

    This same boat has been sailed at 19 knots

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     

    Attached Files:

  8. Ilan Voyager
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    Ilan Voyager Senior Member

    Richard, you showed the points but it's useless to try to explain that the hulls of a sail catamaran are not very suitable for a motor boat.
    Yes the are many inconveniences:
    1- The Prismatic coefficient is to small.
    2- The position of the center of buoyancy is too forward.
    3- There is a problem of repartition of weights, and not enough volume aft to exploit the potential of the length of the hulls.
    4- there are not spray rails thus the boat will be very wet.
    5- Probably the boat will be too wide, and without the pressure of sails, the boat will have this typical catamaran high frequency roll that make the stomachs so upset as the hulls are not designed to damper this roll.

    Worst the total price of the operation is not very good; in the price of the sail cat there are a mast, rigging, and sail hardware totally useless on a motor boat.
    Placing the outboards on the stern is not the best idea, a part the weight problem, the engines will end drowned if the sea is coming from behind and in a choppy sea the propellers will spend half of the time moving air. So a pair of pods have to be made just aft the rear beam, plus the place for the gas tanks, and the security items required by the use of gas, highly explosive.
    The dagger or centerboards are useless or too big, the rudders also.
    I do not even evoke the problem of making a windshield nor a tower.
    Morality: after massacring a fine sail cat, and after spending a lot money and work you will end with a not-good motor boat with no resale value...
     
  9. Mr Efficiency
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    Mr Efficiency Senior Member

    I'd like to see how such a sail cat could have outboards stuck to the back of it and operate successfully, keeping them in the water, or having insoluble problems with drag, would seem to be a real headache to me.
     
  10. svmegatron
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    svmegatron Junior Member

    The consensus here seems to be "you can try it, but it's not going to do what you want." I'm okay with this.

    Thank you all for saving me from going down a very frustrating and expensive road. I am going to keep saving up, and hope to build one of Mr Woods' Skoota 28s (a boat designed for the EXACT purpose I have in mind :) ) later this year.
     
  11. Mr Efficiency
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    Mr Efficiency Senior Member

    I couldn't find that skoota 28 ?
     
  12. svmegatron
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    svmegatron Junior Member

    It's at the bottom of this page: http://www.sailingcatamarans.com/skootas.htm

    On that page its labeled as a Skoota 26, but I understand he's lengthened the hulls to give it more load carrying capacity (I think ... don't quote me on that. As I've made obvious, I don't know much about boat design!)
     
  13. Mr Efficiency
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    Mr Efficiency Senior Member

    OK, thanks. The 36 footer has some pics on the site.
     
  14. Richard Woods
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    Richard Woods Woods Designs

    The first Skoota 28 starts building tomorrow (Feb 15th) A bit scary as its a new design, being built by a builder I haven't used before, and I am 1500 miles away in Lake Havasu, Az at the HPCC www.sailhavasu.com

    I drew the Skoota 20 first, just to see how my ideas worked in practice. Then I drew the Skoota 24 and Skoota 36. I have been unsure about how big to make the mid range design. I started with a sketch of a 31fter, then went back down to 26ft and ended up at 28ft as a boat that I felt would be the minimum basic live aboard coastal cruiser, yet would still be transportable

    We will be using our Skoota in the Pacific North West, cruising between Seattle and Alaska. However we may truck it to the Great Lakes and do a Great Circle cruise

    I'll keep you all informed

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     
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  15. Milehog
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    Milehog Clever Quip

    Please do, I'm interested.
     
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