outboards in nacelles

Discussion in 'Multihulls' started by fiunery, Aug 13, 2014.

  1. fiunery
    Joined: Apr 2014
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    Location: Nelson, NZ

    fiunery Junior Member

    Luff Tension, That looks like a great setup and thanks for offering to talk it over with me. I think that I will end up going with two outboards in the cockpit lockers. The Banshee only has 1.4m height in the cabin and a nacelle in the cockpit would be quite claustrophobic. I have been trying to work out the appropriate height at which to mount the motor.

    Assuming that the centre of rotation of the boat is 3m forward of the engine and that the maximum pitch of the boat is 10% (?) then that is about 50cm of movement vertically (?) up and down from the horizontal. I know that's far more that occurs in reality (because the water surface moves too) and far more than an outboard can manage. What kind of vertical movement should outboards in a cat be capable of accommodating?


  2. Cat2Fold
    Joined: May 2012
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    Location: Felt, Idaho

    Cat2Fold Junior Member

    I have two Yamaha 9.9's, long shaft, high thrust on the transoms of my cat and they have to sometimes deal with traveling 2m+ vertically.
    That's more or less from fully submerged to several feet of air under the prop. :rolleyes:
    Having the maneuverability of two motors spaced far apart is priceless!!!
    I wish I could EASILY move my motors forward...:mad:
  3. fiunery
    Joined: Apr 2014
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    Location: Nelson, NZ

    fiunery Junior Member

    That's a fine looking boat Cat2Fold. I am still trying to work out some kind of guiding principle regrading distance from pivot point in the boat and height at which outboard should be mounted. There must be some basic rules for this?
  4. saltifinch
    Joined: Mar 2016
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    Location: British Columbia

    saltifinch Junior Member

    Feels a bit weird reviving an old post, but here goes.

    I'm looking at building a 12m cat, and am definitely sold on the idea of twin outboards. I am a huge fan of what appears to be called the " Bob Oram mount", cut into the sheer panel. Moving the mounts forward of the steps increases safety for passengers, and reduces cavitation for sure.

    Except one big problem. The outboard's cooling system usually shoots out the starboard side. Which means the starboard motor is going to be running steaming hot water right onto the hull.

    Any way around this? Are these motors configurable?
  5. Mr Efficiency
    Joined: Oct 2010
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    Location: Australia

    Mr Efficiency Senior Member

    "steaming hot" is not correct, imo. Not hot enough to be a problem, imo. If you are still worried about it, a simple deflector of some kind should fix it. The tell tale can be re-directed without too much effort.
  6. Russell Brown
    Joined: Jul 2012
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    Location: washington state

    Russell Brown Senior Member

    Would like to see an image of the "Bob Oram mount". I like his designs.
  7. Corley
    Joined: Oct 2009
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    Location: Melbourne, Australia

    Corley epoxy coated

  8. rogerf
    Joined: Apr 2012
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    Location: port stephens

    rogerf Junior Member

    Tim Clissold has an innovative way of mounting;


  9. DennisRB
    Joined: Sep 2004
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    Location: Brisbane

    DennisRB Senior Member

    That Clisold way looks fancy. I like it, but I also like a bunch of other stuff I would not want to pay for, build or maintain.

    Groper posted good pics on the first page of this thread. http://www.boatdesign.net/forums/multihulls/outboards-nacelles-51157.html#post700423 It seems to be the best way as it gets the engines forward, low and presents almost no drag to the sea, and it keeps them pretty dry. When I went over to visit Alan and Brian to check out the rebuild on "Schools out" I asked how many hours it took to retrofit that setup (previously one central engine) and the number of hours was not trivial. A lot more than converting fixed rudders to kick up AFAIC when I asked about both.

    Allan's 44c was built with this OB setup from the start. I believe it was actually Allan's design (or at least design optimization) and Bob then used it on subsequent designs.
  10. garydierking
    Joined: Sep 2004
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    Location: New Zealand

    garydierking Senior Member

    I wish people would stop using the term "cavitation" The problem is ventilation. Cavitation is a very different thing and causes pitting in metal propellers.
  11. brian eiland
    Joined: Jun 2002
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    Location: St Augustine Fl, Thailand

    brian eiland Senior Member

    Interesting installation, but a question?.
    When I look at the 3rd photo of the underside fairing I see a pretty flat spot that is presented to the water at a very low height,...is that correct? It doesn't present itself that way in the first photo of those 6?

    Attached Files:

  12. luff tension
    Joined: Aug 2010
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    Location: auckland

    luff tension Junior Member

    There is a flat flat spot on the base but is more of a fairing around the engine clamp bracket more than anything else. The structural base of the pod in inside that bit, and the gap is filled with foam. The only time waves hit it is when motoring directly into a slop, as soon as we are sailing it is living well clear of the waves and in the 3 years its been there hasn't posed a problem. The boat's out of the water currently so I can get some more pics if you like.

  13. brian eiland
    Joined: Jun 2002
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    Location: St Augustine Fl, Thailand

    brian eiland Senior Member

    Thanks, more photos would be nice. Just trying to keep a little file on various installations.
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