Outboard motor transom design

Discussion in 'Boat Design' started by mainsail, Apr 2, 2013.

  1. mainsail
    Joined: Apr 2013
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    mainsail New Member

    I am a yacht designer and am currently designing an 8meter motorised catamaran tender. The outboard motor will be centrally mounted at the correct transom height. The motor is 50hp and I envisage cruising at 15 knots with a top speed of around 22 knots.
    My question is do I just leave the outboard motor with no transom or should I create a transom that is finished at the water level similar to most planing power boats. What is most efficient to get the best thrust from the motor?
    Thanks for your help.
     
  2. J Feenstra
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    J Feenstra Junior Member

    Welcome to the forum mainsail,

    I would create a transom for the backwash when in preplanning mode, Why do you suggest those two sollutions? what are the benefits of them? maybe some pictures help to understand the question.
     
  3. Richard Woods
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    Richard Woods Woods Designs

    For sure add a transom!!! otherwise you won't do anything except make spray at speeds over about 8 knots

    My Skoota designs may be similar to what you are doing. With a central 25hp my Skoota 20 does 15 knots, with a central 50 my Skoota 24 does 17 knots. My just launched Skoota 28 with twin 20hp does 16 knots

    You can see videos of these boats here

    http://www.youtube.com/user/WoodsDesigns?feature=mhee

    and more on my website of course

    Hope your project goes well

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     
  4. garydierking
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    garydierking Senior Member

    You definitely need something in front of the outboard leg or you will lose much of the motor's thrust through ventilation.
    This photo shows the fairing I put in front of the outboard on my motorized Tornado. I could not find any data on the best design for this but this one seems to work pretty well.

    [​IMG]
     
  5. Fanie
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    Fanie Fanie

    I'll consider placing the single motor behind the one hull, I doubt it's going to make much difference.
     
  6. Richard Woods
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    Richard Woods Woods Designs

    Gary's setup is very similar to what I made

    I don't think one engine on a transom is a good idea. The boat will tend to motor in circles, so you will always need a slight helm angle. A worse problem is that manouverability will be difficult, especially when turning to port

    Richard Woods
     
  7. mainsail
    Joined: Apr 2013
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    mainsail New Member

    Thanks for the feedback

    Hey thanks guys for the great feedback on my query. The consensus is that I should have a transom in front of the motor. In fact I have designed a central hull for both structural purposes and to help with quietening the waves under the cockpit sole that finishes in a transom for the outboard. I have designed the transom to finish at WL level.
    The next question is how wide should it be to help the water flow over the prop.
    Thanks for the video and photos, it helps a lot to get such good feedback on this.
    The boat will be built using a foam core to keep it as light as possible.
    I have attached a couple of raw renderings.
     

    Attached Files:

    1 person likes this.
  8. CDK
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    CDK retired engineer

    You don't actually need a transom. An upside down pedestal carrying an anti-ventilation plate is all you need to prevent air reaching the low pressure area in front of the prop.
    Even an enlarged plate on the outboard itself or a shroud around the prop would be sufficient, but that may lead to an unacceptably high load in case the trim angle is less than optimal.

    There surely must be data on the internet to determine the size of such a plate for a given prop depth, speed and engine output, but that is not my field of expertise. I do know however that the props of my boat, located at the end of two tunnels, create a vortex with a length of approx. 40 cm sucking air when reversing at any rpm above 800. Especially when turning the bow against the wind with one engine in reverse and the other forward, this severely limits the prop thrust on one side.
     
  9. michael pierzga
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    michael pierzga Senior Member



    The OP should research the design work of Kotaro Horiuchi

    http://[​IMG]




    Also

    http://www.elektrofoil.se/bilder/bilaga2_1.jpg

    http://www.elektrofoil.se/bilder/bilaga2_2.jpg
     
  10. CDK
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    CDK retired engineer

    Very nice Michael!
     
  11. michael pierzga
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    michael pierzga Senior Member

    The system may require trial and error . Obviously a XL shaft OB is needed. as far as I know only Honda makes a 50hp extra long shaft.

    I would think that the experimental fairing and cavitation plate could be a simple plywood glue, sheet metal, rivets .structure.
     
  12. mainsail
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    mainsail New Member

    Ventilation plates

    Michael that is a most interesting boat and it goes to prove that a transom is not necessary.
    As I read from CDK the transom or in fact a plate stops ventilation at the prop. I kinda like the idea of a plate as I can experiment and adjust it as required.
    Any idea of any research on the size and shape of anti-ventilation plates?
     
  13. HJS
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    HJS Member

    This solution I've had in my own boat, and it works in all conditions.
    With the experience I have, I would advise against a long and rounded central hull.
    It should be a wedge with a flat underside slightly above the floating water surface.

    js
     

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  14. Easy Rider
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    Easy Rider Senior Member

    I've had considerable experience powering canoes w OB engines. For years I used the clamp to the gunn'ls mount that positions the powerhead of the OB just to left of the helmsman sitting aft in a tandem canoe. Ballast fwd being a person or water jugs.

    The steering w a 3 hp Johnson (Angle Drive) was only fair ... usable but poor. I bought a 2 hp Suzuki 2 stroke w the typical small/skinny OB leg and course changes over 10 degrees resulted in complete cavitation. Almost totally unusable. Other OBs were a mix of the two examples above.

    I came to the conclusion that if performance well above paddling speed and maneuverability was required a square stern was unavoidable. I purchased a square stern 18' canoe (somewhat bigger than most w a small transom) and all OBs I've used have resulted in flawless performance. I don't recall any of them caveating while turning and I've used 2 to 8 hp.
     

    Attached Files:


  15. garydierking
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    garydierking Senior Member

    Down with ventilation!

    The problem of ventilation when side mounting an outboard is avoidable. I too had poor performance until I added a home made fiberglass foil to the leg of the 2hp motors. It eliminated the problem entirely.
    A building tutorial is here: http://homepages.paradise.net.nz/garyd/outboard.html

    [​IMG]
     
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