Outboard mounted Midships..yes /no ?

Discussion in 'Boat Design' started by pistnbroke, Jul 16, 2010.

  1. Submarine Tom

    Submarine Tom Previous Member

    PB,

    Is there a compromise that you could mount it just aft of centre allowing for directable thrust steering and eliminate the need for a rudder?

    -Tom
     
  2. DCockey
    Joined: Oct 2009
    Posts: 5,072
    Likes: 551, Points: 113, Legacy Rep: 1485
    Location: Midcoast Maine

    DCockey Senior Member

    A motor mounted "just aft of centre" will not steer the boat effectively. In order to steer a boat, an outboard needs to be mounted away from the longitudinal center of the boat.

    An outboard doesn't turn a boat by directly exerting a moment on the boat. Rather, when an outboard is turned it pushes sideways against the boat WHERE it is mounted. So if it's at the stern that causes the boat to start to move sideways. The effective center of resistance to the motion of the boat is forward of the stern. This seperation of the thrust vector of the motor from the effective center of resistance results in a moment (couple) which in turn causes the boat to turn.
     
  3. Submarine Tom

    Submarine Tom Previous Member

    DC,

    You're so clever. I did say "compromise", not all the way to the stern and not midship. Get it?

    -Tom
     
  4. DCockey
    Joined: Oct 2009
    Posts: 5,072
    Likes: 551, Points: 113, Legacy Rep: 1485
    Location: Midcoast Maine

    DCockey Senior Member

    Submarine Tom, interesting tone to you reply.

    "A compromise that you could mount it just aft of centre" which provides reasonable steering control without a rudder would need to be close to the stern (or bow) and thus not close to the center.

    If there was a reason steering by a center mounted outboard is needed then one way to improve the steering performance would be to add lateral resistance forward.
     
  5. troy2000
    Joined: Nov 2009
    Posts: 1,743
    Likes: 170, Points: 63, Legacy Rep: 2078
    Location: California

    troy2000 Senior Member

    You also said, "just aft of center."
     
  6. Submarine Tom

    Submarine Tom Previous Member

    I believe you have missed my point.

    I tried.

    -Tom
     
  7. keysdisease
    Joined: Mar 2006
    Posts: 794
    Likes: 43, Points: 28, Legacy Rep: 324
    Location: South Florida USA

    keysdisease Senior Member

    The difference between theory and practice is excersised everyday here in South Florida. The mullet skiffs I reference have their motors mounted anywhere from center under a console to forward (as the boat pictured in my earlier post) and while thay have different steering characteristics than a transom mounted powered boat under certain conditions (docking) their handling can be superior to a transom mounted powered skiff. They also plane very nicely.

    These boats have been in common use as net tenders here in South Florida for 40 years.

    For the intended use with the motor mounted anywhere on centerline with steering fixed straight and an appropriate sized rudder aft the vessel should steer just fine.

    Steve
     
  8. David Cockey
    Joined: Sep 2009
    Posts: 9
    Likes: 1, Points: 0, Legacy Rep: 36
    Location: Michigan

    David Cockey Junior Member

    Do the mullet skiffs have a rudder, or are they only steered by turning the engine?
     
  9. keysdisease
    Joined: Mar 2006
    Posts: 794
    Likes: 43, Points: 28, Legacy Rep: 324
    Location: South Florida USA

    keysdisease Senior Member

    No rudder, steering is conventional push-pull teleflex style steering. These boats typically have 90HP and below.
     
  10. keysdisease
    Joined: Mar 2006
    Posts: 794
    Likes: 43, Points: 28, Legacy Rep: 324
    Location: South Florida USA

    keysdisease Senior Member

    these are also common in gulf States like LA ans AL
     

    Attached Files:

  11. Lake Trolling
    Joined: Jul 2010
    Posts: 3
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Redding, CA

    Lake Trolling New Member

    I have considered a forward mounted outboard in a rectagular well placed approximentally one third back from the bow and steered by a rear mounted rudder. The boat would be built like a shallow vee hull but with a tunnel the width of the well running to the aft through the transom. We are talking a bit less that 20 foot boat. The sides of the well would also feature dagger board that could be lowered to help hold the boad from side drift when fishing near the bank. There are other features but I will post this and wait for the responses.
     
  12. pistnbroke
    Joined: Jan 2009
    Posts: 1,405
    Likes: 34, Points: 48, Legacy Rep: 404
    Location: Noosa.Australia where god kissed the earth.

    pistnbroke I try

    dont see any point in the tunnel unless you are trying to make a tunnel drive or unless the boat is heavy enough to totally fill the tunnel and will only go slow ie not plane ....if the prop is through the bottom and below the hull thats fine but if you fit a tunnel it would need to go forwards and backwards or the water flow would be bad and may draw in air ....
     
  13. Lake Trolling
    Joined: Jul 2010
    Posts: 3
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Redding, CA

    Lake Trolling New Member

    I know I'm not explainning this very well but from the well back there would be a six inch rise in the bottom (not a tunnel as I first indicated, bad choice of word on my part). If you could see my hands helping to discribe what I'm saying it would help. Let's look at the transom and the bottom portion would look like a W or kink of like that with the center of the W six inches tall by the width of the well and squared off on the top. The deck of the boat would be flat across the to of the what I called a tunnel without the bottom section so maybe you would call it a very deep grove. Darn it is hard to draw a mental picture of the bottom of the boat.
     
  14. pistnbroke
    Joined: Jan 2009
    Posts: 1,405
    Likes: 34, Points: 48, Legacy Rep: 404
    Location: Noosa.Australia where god kissed the earth.

    pistnbroke I try

    yes I follow but the flow in front of the prop is as important as the flow behind and if this boat planes it could pull the prop out of the water and the tunnel pull air in at the front ....bad idea not neccessary...just make the centre line of the boat a ride pad about a foot wide and stick the motor through that ....
     

  15. Lake Trolling
    Joined: Jul 2010
    Posts: 3
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Redding, CA

    Lake Trolling New Member

    You are absolutly right and that is why the prop will drop down below the front well transom and the wake will be cleared through the hump of the hull. So when you are trolling at the slow speed you use when trolling you are not seeing much of the wake or prop wash at the stern. And when you have a hook up you are not as concerned about landing the fish and the stern mounted prop. Some of the lakes turn over in my neck of the woods and the unfouling of weeds in the prop is easier when the outboard is brought up in a well, rather than leaning over the aft of the boat and trying to clear the prop. when you are bass fishing and are in the water close to the shore it always seems that the wind is blowing you into the bank, even if it is a gentle breeze. With a dagger board set up at the sides of the well you can put down the dagger boards and help maintain position much like a sea anchor and yes an electric motor is also useful for this. Keep the thoughts coming as I do appreciate them.
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.