Semi displacement powercat

Discussion in 'Boat Design' started by Sailcy, Mar 23, 2016.

  1. Sailcy
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    Sailcy Junior Member

    "Admittedly there is an amorphous hybrid called a semi displacement craft which attempts to bridge the gap. But generally speaking the planing and displacement vessels are quite distinct in their performance, purpose and area of operation"

    http://www.webring.org/l/rd?ring=mu...catamarans.com/news/2006/04/catcomparison.asp

    As mr Tennant call the semi displacement cat- amorphous years ago.
    Has any progress been made till today to establish a clear definition of what exactly the characteristics clarify Powercat as semi displacement?
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    What utility would such a definition have for you, assuming your interest is not entirely academic ? Planing cats typically have a smaller length to beam (demi-hull) ratio, compared to displacement-type cats, and fully submerged transoms. To qualify as semi-displacement, the ratio would be greater, and presumably the transom submersion less, than the planing craft. The only advantage that occurs to me, over a displacement cat, for an intermediate type, is that pitching is likely to be less dramatic.
     
  3. Alik
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    Alik Senior Member

    Many high-speed catamarans are 'semi-displacement', even if by Froude number they operate in planing mode. The reason is that they do not produce enough lift on bottom to be considered planing craft.
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    Presumably to qualify as "planing" or "semi-planing" there would need to be some ability to decrease wetted hull surface area at speed, however small.
     
  5. Sailcy
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    Sailcy Junior Member

    What utility would such a definition have for you, assuming your interest is not entirely academic ?
    Currently I'm in a process of building a semidisplacement Powercat. But unlike the original purchased design I am making quite an experiment by placing the hulls much closer to each other resulting a total trailerable beam 2.55m. The boat is 8.5m long, bridge deck clearance is 450mm. The hulls are asymmetrical, which I hope will help to reduce bow wave interference. I believe to clearly understand the subject I'm dealing with will be benefiting.

    ...they do not produce enough lift on bottom to be considered planing craft.
    Thanks Alik! That actually was my guess, - the proportion between hydrodynamic lift and hydrostatic buoyancy. As the planning vessel considered as approx 2/3 of the weight supported by a lift, what figures apply to semidisplacement, 50/50? And what are the main weak points regarding seakeeping abilities of semi design?
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    Your bridge deck clearance seems very high for a 2.55 metre beam. The key factors in determining dynamic lift are speed, weight, and shape and area of the bottom surfaces. It is impossible to estimate what the % will be without that data.
     
  7. jamez
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    jamez Senior Member

    What is the original design you are modifying?
     
  8. Sailcy
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    Sailcy Junior Member

    Your bridge deck clearance seems very high for a 2.55 metre beam.

    Actually, I didn't change anything apart of the boat width (and the layout obviously) original was 4.2m which is quite good I believe for sea keeping ability. 450mm is the original bridgedeck clearance. I even made a provision for another 100mm to minimise slamming if needed.

    The boat is Skoota 28 of Richard Woods design.
    http://www.sailingcatamarans.com/index.php/designs/6-powercats/264-skoota-28
     
  9. Mr Efficiency
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    Mr Efficiency Senior Member

    Richard Woods regularly contributes here, best to ask him his opinion.
     
  10. Mani Kandasa
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    Mani Kandasa Junior Member

  11. Richard Woods
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    Richard Woods Woods Designs

    Sailcy posted several similar questions on my own forum pages. I answered him as below

    "I think you are a bit tied up with the semantics. I am sure you read my Skoota article here

    http://sailingcatamarans.com/index.php/ ... catamarans

    In brief:

    I always try to design "efficient" boats. Ones that need either low engine power or small sail areas for good performance. Others may fit huge rigs or big engines onto slow hulls to increase performance. I prefer to refine the hulls to offer less resistance. And I try for mid teen speeds in my faster boats (cats/tris power/sail) as too much speed is wearing, uncomfortable and requires more skill than many have to remain safe.

    The two main types of boat are displacement and planing. A displacement boat has its speed determined by its length and in general longer boats go faster. That definition is modified by the boats "Slenderness Ratio", basically a measure of how heavy/long/narrow the hull is. Boats with a fine hull go faster than a fat hull. But they are still displacement boats. So a typical 30ft sailing catamaran is a displacement boat when under power as it not sensible to try to exceed say 7knots.

    To be a true planing boat it has to have all three of these: an efficient lifting surface, an angle of attack to generate that lift and sufficient power to get up to planing peed (once there the power can be reduced. Much the same as when you take off in a plane, once at cruising altitude the revs are cut back.)

    An efficient lifting surface has a large surface area and a high aspect ratio. A wide flat boat clearly has both. A long narrow hull has a poor aspect ratio (similar to a low aspect ratio keel). And of course it cannot have a big flat area as that would make it wide and thus no longer "narrow" . So the only way to make it lift is to add extra power and/or increase the trim angle (or angle of attack). But a high angle of attack increases drag, so is inefficient. Thus planing power catamarans can never be efficient when compared to a monohull "speedboat"

    So boats can be divided into displacement boats or planing boats. Clearly there is a cross-over area and that is where the "semi displacement' or "semi planing" boats fit.

    The Skoota 28 goes faster than 7 knots (in fact twice that) so is clearly not a displacement boat. Equally it travels almost level, has low power engines and narrow, inefficient planing hulls. So it cannot be a planing boat.

    Thus I call it semi displacement, but I could say "semi planing" and it would mean the same thing

    Does that make sense?"

    Later, in reply to more questions, I wrote

    "It depends on how different designers define their boats. It's like saying "is your sailing boat a racer, a cruiser, a cruiser/racer or a racer/cruiser.?"

    We have now cruised over 4500 miles from the Pacific NW to the Bahamas on our own Skoota 28. No problems at all. It is designed as a coastal cruiser, RCD Cat B/C. The boats in the videos you have seen are probably Cat A, maybe even commercial. Certainly bigger than the Skoota 28.

    One disadvantage of the small engines is that there isn't the power to drive through big waves at speed. But as you know we crossed the Gulf Stream in a small craft advisory with 6-8ft waves. Again no problems apart from the rain and cold. We cross back to Florida in about 2 weeks

    But I have always said "What is the difference between motoring a 28ft sailing Gypsy catamaran in a gale to motoring a Skoota, why would one be more seaworthy than the other? "

    I followed Gropers build and offered comments years ago. It's not quite the style of boat I have designed. My new Skoota 32 and the Skoota 36 are both liveaboard cruising boats. Gropers is a weekender/fishing boat. He has very much less accommodation than my designs, but a bit more speed. He is cruising at 20 knots with twin 110hp. I expect the Skoota 32 to cruise at 16 knots with twin 60hp

    I don't use the prediction software Groper used (it is a free program written by an Australian hydrodynamicist). Instead I use my own which I developed back in the early 1980's and was based on the Taylor 64 series, plus the tank testing I did when a student, plus some refinements to better suit my type of hull.

    I have tested the result full size on the same 35ft catamaran from 4 hp to 180hp and the tests match the predictions so I am happy with the predictions. Very roughly for every knot faster over 10knots you need an extra 10hp on a 35ft catamaran. So speed costs!

    Obviously the final performance very much depends on the built weight and, in the case of powercats, the correct choice of propellors

    The build of the first Skoota 32 is now underway. Photos will be posted when I get them, maybe next week. Certainly in April."

    Written on board my Skoota 28 in the Abacos, Bahamas. We have now been living on board for over 7 months

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     
  12. Mr Efficiency
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    Mr Efficiency Senior Member

    Narrowing the beam dramatically turns it into a different boat in more ways than one. The idea of 450mm (18") span clearance with 2.55m beam worries me.
     
  13. Sailcy
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    Sailcy Junior Member

  14. Mr Efficiency
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    Mr Efficiency Senior Member

    I'm not seeing the value in this idea, your accomodation is very restricted and the low speed efficiency is reduced by having the hulls so close together. What is the point of it ?
     

  15. Sailcy
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    Sailcy Junior Member

    Trailerable, coastal day charter boat
     
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