would outboards mounted midships just break tradition, or other risks?

Discussion in 'Boat Design' started by why crazy, Aug 10, 2020.

  1. why crazy
    Joined: Aug 2020
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    why crazy New Member

    Any reason not to mount outboards midships on a sailing multihull? Picture a trimaran with outboards on midships akas.

    Naturally, there may be a really good reason that most boats put drives at the stern. Since sailboats are typically trying to keep weight out of the stern, any reason not to simplify fore-aft weight distribution in this fashion?

    Would a lightweight 40-foot daysailing trimaran have different related considerations than a 40' bluewater trimaran?
     
  2. BlueBell
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    BlueBell Ahhhhh...

    Welcome.
    Not crazy.
    Hard to ventilate a prop amidships, even from extreme pitching.
    A loss of directable thrust, but you've got rudders, right?
    Why engines plural? You only need one.
     
  3. why crazy
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    why crazy New Member

    Thanks for your reflections on the question and for the welcome. This is my first posting, tho I've lurked for a few years.

    Good point about keeping the props below the surface when pitching. And yes: rudder(s) included.

    To reply to your query about number of engines: 2 for directable thrust (e.g. downspeed handling with 1 in forward and 1 in reverse), redundancy, balance either side of the central hull.

    Another choice that I think has reasonable upsides, but about which I feel some uncertainty: the choice of outboards vs the traditional inboard. Hoped for upsides: tilt out of the water for less drag, service more easily, swap more easily if they can't be serviced.

    Aside from cost, what factors do you feel favor going with only one?
     
    Last edited: Aug 10, 2020
  4. A II
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    A II no senior member → youtu.be/oNjQXmoxiQ8 → I wish

    You can have the outboard up to the bow if you want to, no rudder needed for motoring as steering then can be done with the motor, or rudder(s) if you already have them for sailing, I've recently posted the below on the thread A Small Skiff for Coastal Florida in post #24, cited here just for info...

    ‘‘ Mullet Skiffs in eg. the South Florida region used to have the outboard well near the bow for setting nets quickly out of the stern when a school of fish was sighted, and to have as much deck space as possible to store and work the nets behind the motor. Nowadays most regions have banned that kind of shallow water netting. They were primarily used for catching mullets, even though some are now used for catching live bait to sell to sport fishermen.

    [​IMG]

    ‘‘ . . . two Florida Mullet Fisherman coming in with their net on the transom of their Mullet Skiff taken in the 1930s . . . ’’ (Spira)

    [​IMG]


    [​IMG]

     
    Last edited: Aug 12, 2020
  5. Mr Efficiency
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    Mr Efficiency Senior Member

    Naturally, an engine at the bow has steering function, just the reverse of a stern mount. Loss of steering ability is what limits mid mount installations.
     
  6. why crazy
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    why crazy New Member

    Several good points about the lever arm between axis of rotation and propellor. The midships configuration would of course mean no lever arm for a swinging outboard operating at higher speeds. Directed thrust would struggle to affect steering. My sailboater's brain had rudders and independent throttles. Useful to recognize my assumptions. BlueBell said it in the first reply, but it didn't fully register. Thanks for the additional examples!
     
  7. BlueBell
    Joined: May 2017
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    BlueBell Ahhhhh...

    Ya, don't be confusing directable thrust with differential thrust.

    One vs two o/b's aside from cost: weight, unnecessary complexity.
    You don't need to balance it with a second engine.
    There are lots of ballast type things you can place to compensate.
     
  8. brendan gardam
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    brendan gardam Senior Member

    my cat has the outboard 18 inches behind the mast to keep the prop in the water when pitching. pretty standard i think .seawind 24 has a similar setup.
     
  9. tlouth7
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    tlouth7 Senior Member

    You have to cut a hole in your hull which increases drag unless you can make a really neat fairing.

    The hole/well provides a location for flooding to occur. This can be mitigated if the hole is in the cockpit.

    You end up with an engine in your saloon or cockpit (not relevant if in amas). If you want to raise the engine then you need even more space.
     
  10. JSL
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    JSL Senior Member

    If you are operating amid debris you will have a shorter avoidance reaction time
     
  11. Mr Efficiency
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    Mr Efficiency Senior Member

    There is also the matter of fumes to consider, the further away from the passengers, the better.
     
    fallguy likes this.
  12. Rurudyne
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    Rurudyne Senior Member

    From a practical standpoint I'm guessing your bow wave system will tend to have to need to mount the outboard a bit lower, or maybe have an adjustable height mount, to work properly so you don't end up with a accidental surface piercer at times.
     
  13. JamesRiver
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    JamesRiver New Member

    This isn't a crazy idea. That said it is rarely done for a reason. There is a high penalty to pay in terms of hull efficiency, but if you aren't too worried about speed under power it may be a price you are willing to pay. Naval architects use the term hull efficiency to combine two other terms, wake fraction and thrust deduction. Thrust deduction is the idea that when the propeller accelerates that water and this water runs along the hull of the boat faster than it otherwise would, it increases the friction on the hull which increases resistance. Wake fraction has to do with the relationship of speed of the water the prop is in to the free stream speed. A prop works by accelerating water backwards to move the boat forwards. The slower the water going into the prop is moving, the bigger the delta of speed in to speed out the prop will generate and the more efficient it will be. Moving the propeller forward negatively affects both of these concepts. The wake fraction is largest at the back of the vessel (the local water is moving slowest relative to the free stream) and at the extreme bow it is moving at the free stream, it varies along the hull but is generally best at the stern. Similarly, moving the propeller forward moves the propeller race (the water accelerated behind the propeller) forward. This race then impinges on much more of the boat than before, generating a lot more resistance. Thus if you move your engine to the forward half of the boat it will either cost you a good deal of speed or require a bit larger engine. That doesn't mean you shouldn't do it though, that depends on the cost/benefit analysis of your use case.
     

  14. brendan gardam
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    brendan gardam Senior Member

    So tell me at 20knts how much difference is there .
     
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