CS Hull Sailing Performance?

Discussion in 'Motorsailers' started by basilroman, Nov 7, 2010.

  1. basilroman
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    basilroman New Member

    I am designing a 38' sailing catamaran.

    I'd like to use a semi-displacement hull along with powerful outboards to allow coastal motoring at ~20 knots when necessary. However, I'd like to keep good sailing performance, especially to windward, and don't mind burning inefficient amounts of fuel to get the 20 knots when I need it.

    I have read a great deal about Malcolm Tennant's CS hull, which makes intuitive sense as a way to provide flat planing surfaces in the rear of the hull without a deep vertical transom.

    Even though I see this hull form on lots of power cats, I don't see it on any sailing cats. I also cannot find anything on the internet on the sailing performance of the CS hull. Does anyone have any experience, or know of any tests?

    It seems like it might hurt tacking ability for a cat since for the same buoyancy, it extends deeper in the aft section. Also, even though there is not a deep transom, it still has a vertical transom that extends some distance below the waterline.

    I would be comparing this to the typical rounded sailing cat hull with significant rocker and flares above the waterline.

    Any input would be greatly appreciated.
     
  2. dcweed
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    dcweed Independent Designer

    Basil,
    As in monohulls, a motorsailing cat will compromise performance in both modes of propulsion. Just the added weight of engines, fuel, and structure will degrade sailing performance measurably. However, all that aside for the moment, at 20 knots under power you will need some lift aft, which the CS model appears to provide if trimmed properly. As you point out, it also will drag the transom at sailing speeds, and it probably won't tack very quickly. A better alternative may be a sailboat like form, but with less rocker and some cleverly designed, very large articulating trim tabs. Think in terms of a variable geometry hull form. Don't forget some carefully sculpted spray strakes forward to keep her dry at speed. And think of budget, since that will need to be significant. Quite the challenge; best of luck to you. Regards, Weed
     
  3. basilroman
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    basilroman New Member

    Yes, I thought through what could be efficient ways to create variable geometry. A big hinged transom step as a trim tab is one good option under power. The problem though would be how to dissipate the volume at the center of the hull as you move to the rear. If you don't carry the volume all the way back to a vertical transom, you either have rocker and dissipate it vertically, or a canoe stern and dissipate it horizontally. If you limit rocker, you are left with a canoe stern, which leads to horrible pitching on a cat while sailing, and the hinged transom step would not help, it would just slam the water all the time.

    Tennant did have a great idea it seems with his CS hull. Dissipate the bulk of the volume in a shorter submerged canoe stern, then dissipate the remainder in a shallow vertical transom at the end. This keeps the horizontal planing surfaces all under water while sailing so they don't slam. You could of course then add a trim tab transom step for some extra pitch adjustment, but keep it retracted while sailing.

    Since this is for short trips, extra weight is mostly engines, not fuel. I am using my current boat, a Seawind 1000 as a comparable. Two 300 HP Yammies weigh about 900lbs more than my two 9.9 Yammies, but put out 60 times more power. With some extra fuel call it an 8% increase in boat weight.

    The information I am missing is any evidence on how the CS hull actually performs in the real world under sail. Does anyone know of any active sailing cats with the CS hull form?
     
  4. cardsinplay
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    cardsinplay da Vinci Group

    Basil.

    If I were you, I'd begin a nice, email conversation with Kurt Hughes about the relative merits of differing hull forms for a boat that size. He designs an awful lot of power cats in all sorts of sizes, as well as some of the nicest sailing cats and the guy just flat knows his stuff.

    http://www.multihulldesigns.com/
     
  5. Steve W
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    Steve W Senior Member

    Malcolm designed some larger motorsailors with the cs hull form and once told me that they reach faster than the typical sailing cat hull form,obviously not upwind though.
    Steve.
     
  6. sabahcat
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    sabahcat Senior Member

    Interesting that he pretty much keeps the sailing cat hull shape without reversing the rocker into a flat
    [​IMG]
     
  7. cardsinplay
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    cardsinplay da Vinci Group


    Yeah, Kurt is very much into semi-displacement solutions for power multihulls and says that he gets the best results from this hull form when the client is looking for good speed with low fuel consumption. I pretty much concur with him in that one can really only go so fast before the ride gets absolutely uncomfortable and serious fuel mileage is always a good thing. Yes, you can go some faster with a flat run aft and bigger engines to get up on plane, but at what fuel expense and what discomfort at sea?

    At slower speeds, the sailing type of hull is still going to give you great fuel use results, while the planing type hull is going to deliver quite a bit less in that area due to drag. So, all-around predictably with good handling, speed and fuel figures look like a very strong design approach.
     
  8. sabahcat
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    sabahcat Senior Member

    Not saying kurt has it wrong but after talking to Malcolm when he was in Australia and from personel obsevation of many sailing cats under power, squatting, along with to much weight in the aft end, seems to be a problem with traditional rounded underwater shapes on multis when they hit 7 to 10 knots of hull speed, under power.

    I am only chasing around 14 max and 10 cruise so I decided to reverse the rocker to a flat section aft and square the edges up a bit as well through that section in an attempt to prevent this happening.

    For illustration purposes I have my cabin stuck on my line drawings, vines on the fence prevent a clear hull photo being shown

    [​IMG]
     
  9. APP
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    APP Junior Member

    Hi,

    Does anyone know where is today the Tennant's CS motorsailer Cat "Cordova"? Is there any other similar boat around of that length?

    Regards
    APP
     
  10. keysdisease
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    keysdisease Senior Member

  11. APP
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    APP Junior Member

  12. keysdisease
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    keysdisease Senior Member

    It is in the spec sheet I linked to:

    24” 3 bladed fixed bronze props

    You will not get the motoring performance you are looking for with a folding / feathering prop.

    Steve


     
  13. brian eiland
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    brian eiland Senior Member

    I would be more concerned with the depth of the bow sections of the CS hull form to resisting a tach than the stern sections. But the CS hull form requires some depth up forward if its going to be slender enough.

    I think you must keep in mind 'compromises' , and remember you are seeking out a motorsailer. Here is one of my solutions:

    http://www.runningtideyachts.com/dynarig/Tennant_Hull_V_ChainDrive.php
     
  14. brian eiland
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    brian eiland Senior Member

    Yes that was a good performer. In fact I was asking Roger about a collaboration with one of his hull forms for a smaller 45-50' gamefishing/sailing cat for a couple of clients in Asia.
     

  15. brian eiland
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    brian eiland Senior Member

    Engine Size and placement

    Keep in mind that Cummins is an in-line 6 that will fit in the narrower portions of the hull. Go bigger HP and you will likely have to move the engine further forward to get space beside it to service it....that space further forward may eat into your living space, paricularly if you are using a conventional shaft arrangement. Unless of course you provide a wider hull which defeats your slenderness ratio, and slows you down.
     
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