Schionning 38' motorsailer Catamaran:14kts on twin 75hp.

Discussion in 'Motorsailers' started by rustybarge, Jun 5, 2014.

  1. rustybarge
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    rustybarge Cheetah 25' Powercat.

    Hi All,

    Just thought I would post the latest offering from Schionning Catamarans, a 38' motor sailer with twin 75 hp diesels to give a 14kts cruise!

    What do you think?

    http://www.spiriteddesigns.com.au/crossover

    [​IMG]

    Flattened aft planing section......

    [​IMG]


    [​IMG]
     
  2. Richard Woods
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    Richard Woods Woods Designs

    Fast motor sailor catamarans are not a new idea. Pat Patterson launched his Star Twins 43 in 1992 and then sailed it almost round the world - much of it in the Roaring Forties. It cruises at 15 knots with twin 100hp Yanmars. I've spent several weeks on it, it's huge, comfortable boat.

    In 1987 we built a modified Banshee, extended to 37ft. With twin 90hp it would tow a waterskier at 22 knots. It did 16knots with one 90hp. I sailed it at 19 knots

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     
  3. rustybarge
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    rustybarge Cheetah 25' Powercat.

    Did you incorporate an aft flat planing section like on the schionning?
    or is it necessary at all on a displ. Design sailboat hull, because the hull is still not planing at 15kts surely?
     
  4. Richard Woods
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    Richard Woods Woods Designs

    The Banshee was a standard hull because that is what the owner wanted, or rather he didn't want to pay for modifications. The Star Twins has a very flat run aft

    if I was to draw a motorsailer catamaran I would certainly have a deep flat aft section. In fact I'd probably have tilting aft sections, something like a very large trim tab. We did some test tanking on a model hull like that when I was a design student.

    Richard Woods
     
  5. rustybarge
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    rustybarge Cheetah 25' Powercat.

    I have some doubts about the design brief for a motor sailer cat like the schionning. The sailing hull form is specifically designed for sailing speeds of about 8 kts average in choppy conditions; that's what most cruisers report cruising at for comfort reasons, although a cat could go faster it would experience slamming in short seas.

    So if you add a flat aft section that will make the hull even more susceptible to slamming, and a cruise speed of 14kts under power seems like an academic exercise that would be unobtainable under normal operating conditions.

    My power cat has slim hulls with a deep vee section, and is designed to cruise at 15kts in all conditions, unlike the motorsailer with its broad round bilge shape.

    ...so am I right in saying you could only cruise at 8 kts unless the conditions were very calm with this hull design?
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    Why are you going to get slamming at more than 8 knots ?
     
  7. rustybarge
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    rustybarge Cheetah 25' Powercat.

    Here's a 11mtr powercat, obviously a much narrower beam.
    Slim hulls with fine bow entry and a deep vee to cut through steep short waves found in coastal areas in the UK.........

    [​IMG]

    [​IMG]
     
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    But the boat in question will have a good clearance under it, I don't see why it will be slamming
     
  9. rustybarge
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    rustybarge Cheetah 25' Powercat.

    The motorcat hulls appear to be very flat broad section hulls in comparison to a powercat, and look to be about 4' wide mid section; at planing speed surely these would slam in a short steep sea?
     
  10. Mr Efficiency
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    Mr Efficiency Senior Member

    L/B ratio given as 13:1, so under 3 feet, I can't see the shape so well.
     
  11. rustybarge
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    rustybarge Cheetah 25' Powercat.

    Would a cat sailboat hull profile make a good planing hull form? The hull seems to have lots of rocker, causing instability at planing speed. The bow section has lots of buoyancy to help the bow lift up over the wave face, that would cause excessive pitching at 15kts.

    Would this hull with the flattened aft section plane with very high bow trim angle?
     
  12. Mr Efficiency
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    Mr Efficiency Senior Member

    With that L/B ratio it will slip through the water at the 14 knots OK, but you can see it is a compromise job between the two propulsive modes, and looks more tilted to the sail side of the equation really. Hard to see it not pitching pretty good at some headings under power. Is it possible to design a boat where the line of propulsion is so different in each mode, and still have a satisfactory result ?
     
  13. rustybarge
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    rustybarge Cheetah 25' Powercat.

    Another way of looking at this compromise design is that The boat would only use the 15kts planing capability in good conditions to run for cover to avoid bad weather. This makes good practical sense, who wants to go a fast speed in rough conditions for hours at a time even if you have a suitable boat?

    At the normal average sailing speed of around 8kts it can still motor upwind in rough conditions while remaining comfortable.

    ...so you are paying the extra cost of twin 75 hp diesels weighing at least 500kg so that you can plane at 15kts in good seas.......it could make sense.
     
  14. rustybarge
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    rustybarge Cheetah 25' Powercat.

    Just got an email Update: the complete kit for the hull/superstructure/furniture inc all materials and glues in container ready for shipping is Aus$108k, or about £60k, plus shipping.

    6 weeks assembly from the precut parts and moulded hull bottom and topsides.....not sure if that's for one person or more.

    Seems like good value.
     

  15. philSweet
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    philSweet Senior Member

    What strikes me as off is the "berths- three queen cabins" and displacement - <13000 pounds. That just doesn't jibe with me. I think cruising weight would be at least as heavy as my Catalina 38 at 17 or 18k. In which case it will be an 8 knot boat anyway. If you only allow 500# of stuff to come aboard, what do you need three cabins for?

    I helped out on an Alliaura 50 that they did a nice job of squeezing 3 queen cabins into, but it weighed 56000 pounds. Never got it over 5 knots, though. It also had twin 75's.
     
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