Reasonably seaworthy cruising powercat conversion

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

  1. svmegatron
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    svmegatron Junior Member

    I was wondering if any of the bright, experienced minds on this board might be able to give me some advice on possibly converting an older sailing catamaran (something like a McGregor 36 or the like) for use as an outboard powered cruising cat.

    My idea is to remove the rig, put a small cabin on the bridgedeck, and mount twin 30-50hp outboards. My hope is to be able to cruise at 15 knots, topping out at 20 knots, and be able to cruise areas like Maine, the Chesapeake, Long Island Sound, and the Bahamas.

    Is this a pointless idea? I understand enough about hull shapes to know that I can't necessarily just slap a couple of outboards on any pair of hulls and expect to wind up with something that meets my seaworthiness, speed, and efficiency goals. Are there any sailing cat designs that would be well suited to this kind of project? Or am I better off saving my money until I can afford a boat that was designed as a powercat in the first place?

    Any pointers would be greatly appreciated!
     
  2. masalai
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    masalai masalai

    Hi svmegatron,
    That would almost qualify as pushing "the-proverbial" up hill with a pointy stick, unless you had the power source of 'megatron' itself... However, cruising is seldom comfortable above 12 to 15 knots and most sail boats are happiest plugging along at 6 to 10 knots, and very very fuel efficiently at the lower end (twin 20 to 30 hp saildrives with fixed blade propellers)

    The reasons being - - to achieve 20 knots will be a very expensive exercise as
    - 1) - The hulls and general structure is probably not designed or engineered for that...
    - 2) - The horsepower would be HUGE as the hulls are not designed to operate efficiently at that speed...
    - 3) - Where would you store the required fuel and additional weight?
    - 4) - Would it be wise to cruise those regions with something that would be lucky to push 12 knots in perfect conditions, and in "slightly-rough-seas", have the propellers half the time pushing nothing but air and over-revving? - and - being unable to do 4 knots....

    Something like 36 ft LOA is best pushed at "hull" speeds and by a propeller that is well located under the water to push that weight... If you want, - - "A 200hp outboard on a bathtub", is similarly preposterous as it does not fall within DESIGN PARAMETERS...

    To discard the mast and be content with 6knot cruise to possibly 10knot max is appealing and achievable... - just retain the existing diesel engines...

    AN AFTERTHOUGHT, as I found an image.... - - - If the attached image is a McGregor 36 http://www.australiaboats.com.au/listings/listing.php?id=15561 - it is a little narrow at 18 ft but with a tent could cruise quite economically as is... Add a bridgedeck cabin is to destroy any potential as it would become quite heavy (read low bridgedeck clearance), - - If those do not come with diesel inboard/saildrives then small outboards around 10 hp each, could possibly work, but need sails as well...
     

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  3. Mr Efficiency
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    Mr Efficiency Senior Member

    Where are the outboards going to be installed ? You would need to be getting a cheap deal to go to the trouble of converting it, even if it proved satisfactory, and 20 knots seems quite optimistic with those motors. There are very real problems mounting those engines on the back of a sailing cat that has the bottom rising to the waterline.
     
  4. brian eiland
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    brian eiland Senior Member

    Go For It

    Not pointless at all, you just need to do a little research. Have a look around the boating forms and goggle for the number of Hobie cats and other brands that have been converted into yacht club tenders, race tenders, and other utility uses. Scale that up a little.

    McGregor 36 hulls might be a good souce, particularly as some can be found quite cheap I believe. have to get creative in the motor mounting, but you don't intend to go out and challenge big seas.

    Or if you really want to go the extra effort, why not build something like this 'picnic/weekender' as I called it out of some Polycore hexagonal polypropylene core sheets I've found very interesting.

    http://www.boatdesign.net/forums/powerboats/weekender-picnic-powercat-33751.html
     
  5. Mr Efficiency
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    Mr Efficiency Senior Member

    My concern would be the likelihood of excessive hobby horsing going into the seas, which of course is a new experience for the boat, and they don't learn by experience !
     
  6. philSweet
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    philSweet Senior Member

    If you are eyeing a McGregor, you need to keep it very light and must not load down the aft end. a poptop pod for camping and a single 20 on a sled mounded well forward of the transom might do, but would not be more than a camp cruiser for very good weather. There are better hulls to start with if you want to do more than camp, like an Iroquois 30 or 33. Send a message to Keysdisease here. I'm pretty sure he had a M36 in Key Largo. It was modified for power, but maybe by a subsequent owner. I only knew the boat after Keys had sold it.
     
  7. svmegatron
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    svmegatron Junior Member

    Thanks for the replies, everybody!

    I'm definitely not wedded to the M36, I just picked it because I remember it having a reputation for speed. I want to mount an outboard on each transom, so it sounds like I would need something less weight sensitive than an M36 anyway. The Iroquois sounds like a good idea, although I am hoping for more headroom than the Iroquois is built with. I suppose I could try raising the ceiling of the Iroquois cabin, maybe.

    Is it true that, in general, I should look for designs with little rocker and fuller sections aft?
     
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    I'd have thought you would be better to avoid the banana profile underwater, as this would likely hobby-horse madly, especially if the ends were fine. But, maybe I'm obsessing a little about that aspect, though it would be very annoying for long periods. I have seen power cats that had a straight run aft, but with rounded hulls that porpoised annoyingly, and if it looks bad looking from the outside in, it is surely worse looking from the inside out ! In that case the lack of a chine was the main problem, but cats are, because of the slimness of the hulls, not that stiff in pitch. In a lighter hull where dynamic lift comes more into play, a boat that doesn't present a greatly different profile to the water with changing angles of incidence, will hobby-horse, like a planing monohull with too much rocker.
     
  9. Doug Lord
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    Doug Lord Flight Ready

  10. brian eiland
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    brian eiland Senior Member

  11. Richard Woods
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    Richard Woods Woods Designs

    In general sailing hulls do not make good powerboat hulls. The latter tend to have more buoyancy aft, partly to increase the prismatic, partly to help offset the weight of engines and fuel tanks.

    Years ago I acquired a set of lines for the Iroquois. It is not a suitable hull for an efficient powerboat.

    A typical cheap Macgregor 36 will cost around USD13000. Then you'll add maybe USD5000 adding a cabin. Plus engines. Say USD20,000, so total near USD40000 assuming you do the work

    And you'll have a bodged together boat that won't be efficient motorboat, nor ideally laid out.

    So why not build from scratch? My Skoota 28 powercat would do the job you want. And I'm sure there are other similar designs out there

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     
  12. brian eiland
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    brian eiland Senior Member

    This looks like a MacGregor conversion of some sort
     

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  13. brian eiland
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    brian eiland Senior Member

    Found this somewhere...I think in goggle images
     

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  14. svmegatron
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    svmegatron Junior Member

    Brian, thanks for the links! I will check those out. The first one in particular looks really good.

    Richard, your Skoota 28 is at the absolute top of my list right now, and probably the direction I will go if I can afford to. I'm just feeling a little gun shy about building a boat and want to explore the other options. Building a cabin onto an existing boat is somehow less daunting than building the entire thing.
     

  15. svmegatron
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    svmegatron Junior Member

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