Outboard motor on sail boat?

Discussion in 'Multihulls' started by foxfish, Feb 19, 2011.

  1. foxfish
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    foxfish Junior Member

    Hi guys, I am sure this has been discussed before but I could not find the info required via the search button.
    Can anyone tell me the practabillty of fitting twin outboard motors, say 10hp each on a small cat like a Hobie 18'.
    I would like to convert such a craft into a fishing platform to use inshore.
    Thank you....
     
  2. gonzo
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    gonzo Senior Member

    It would work fine. However, if you want a pair, two 3.5 HP or so would be more than enough.
     
  3. foxfish
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    foxfish Junior Member

    Thanks but I would like to go as fast as is safe, are you saying 7 hp would be the safe limit?
    I assume it is about weight on the stern, I am prepared to modify or build a transom if necessary.
     
  4. gonzo
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    gonzo Senior Member

    It's not about safety. 7HP will get you going pretty fast. For example, I had a 25' monohull weighing about 4500lbs that did 4.5 mph with a 3.5. Your cat has a small fraction of the resistance. Narrow hulls can be pushed pretty fast. However, the faster you go, the more they sink in the water. Hobbies don't have planing hulls. There is a point when you become a submarine :)
     
  5. foxfish
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    foxfish Junior Member

    I guess I am just thinking allowed!
    Where I live in the UK there is only one displacement hull design power cat available in the 6 mt class.
    This boat can be powered by twin 15 - 40hp motors & gives about 28mph even on a single 40hp.
    I have examined one on its mooring, the design makes great sense but the high tunnels makes the boat look a bit ugly!
    I am just considering the benefits of a displacement cat, the sea keeping quality & low hp required to get 20 knots.
    I noticed however, that the boat in question, was quite easy to push down with just hand presser while standing on the mooring pontoon!
    In comparison a planning cat is very stable platform at rest.

    In conclusion I was wondering about building a displacement power cat with a steeped out hull just above the waterline?
    On that basis I thought I might try to modify a Hobie as the first step?
     
  6. gonzo
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    gonzo Senior Member

    Do you have a limited budget? It is cheaper to build a new one, that is why I'm asking.
     
  7. foxfish
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    foxfish Junior Member

    Hi gonzo, thanks for your interest.
    I am a semi retired boatbuilder, old school perhaps & traditionally trained but, it just strikes me there is a gap in the market for a twin engine displacement cat, something that is seaworthy & smooth ridding in a chop, but, only requires low hp to get paces quickly for fishing.
    Planing cats already do this but require more hp.
    I don't have a budget, I just want a nice 18-20' displacement boat, I would most like to build something myself or modify an existing boat.
    This is the only one I know of
    http://www.boatsandoutboards.co.uk/fishing-boats/avocet-595-catamaran-JRS007
     
  8. gonzo
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    gonzo Senior Member

    There are hundreds of production power cats. If a gap in the market existed it would quickly be filled. I don't see too many buyers for a narrow catamaran that is wet and very low carrying capacity. Some rowing clubs use old Hobie Cats with an outboard because the make little wake. I think a Vee bottom ply hull catamaran is the most economic to build and will perform well.
     
  9. bad dog
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    bad dog bad dog

    Foxfish, I had a Stingray modified to be daysailer (hard deck, cuddy cabin etc), powered mostly by sail of course, but it had a 4hp outboard - which put it along at max hull speed. Any more would have been wasted.

    I happen to agree about the gap in the market though - for light easily driven boats with a big stable platform to do lots of different things on. See my other thread on converting an A-cat to electric power. It will be powered by either a 1kW or 2 kW outboard. That's either 1.3 or 2.6 HP. Remember that claimed HP is not actual HP, so the 2.6kW option will be the same as 4HP.

    There is another thread runnibg at present re hard decks for small cats.

    Good luck!
     
  10. waltm
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    waltm Junior Member

  11. dstgean
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    dstgean Senior Member

  12. keysdisease
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    keysdisease Senior Member

    Single o/b

    Think about a single motor hung from the aft crossbeam. There used to be a guy that made o/b brackets for small catamarams like the Hobie and thats where they were attached. The mainsheet attached there so there should be plenty of strength for a small o/b and then you don't have to worry about interefering w the rudders or remotes for the 2 motors, + less expense.


     
  13. HydroNick
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    HydroNick Nick S

    Outboard on a multihull

    And to add some useless history. My Dad, back in about '63 sailed Shearwater catamarans across the English Channel, plus racing them. I know that they converted one of the Shearwaters and put an outboard on it to use as a race boat. No idea what hp it was, but my recollection is that it looked pretty big; so, bigger than 7.5 hp.
     
  14. waltm
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    waltm Junior Member


    http://www.cheatabrackets.com/

    Looks like they are for small outboards.
     

  15. keysdisease
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    keysdisease Senior Member

    Thats the critter, and if not similar enough. This mounting lets you not have to mess with the rudders, not have to worry about the weight of 2 motors or the X 2 expense. The weight is further forward and the crossbeams of beach cats can typically handle the thrust of a pretty good motor. If not a little bracing might be in order.

    By contrast I had a MacGregor 36 catamaran with twin 9.9 hi thrust yamaha's on her and in calm water with a tail wind I could hit 12 knts.

    Steve
     
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