Rudder or motor steering in small cargo boat?

Discussion in 'Boat Design' started by Tijs Eysermans, Mar 19, 2015.

  1. Tijs Eysermans
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    Tijs Eysermans Junior Member

    Hello bloggers, I am a graduate student at KULeuven Belgium.

    For my thesis I am looking at what is best to use when steering a 6meter long cargo boat.
    What I was wondering, do any of you have experience with both systems? (a steerable motor or a rudder steering system).

    If you have, what are the difficulties with one in comparisson to the other? Which is better to use, or is easier to use when going forward or backwards?

    I need this imformation to verify these answers with what I find from simulations. This would help me an awfull lot!

    Thank you in advance!

    Tijs Eysermans

    PS: I also put this post in stability, because I think stability can be an issue with a rudder when going backwards, is this correct?
     
  2. gonzo
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    gonzo Senior Member

    If you have an outboard, then there is no need for a rudder. If the setup is a propeller on a fixed shaft, you need a rudder. I don't see why a rudder would create instability in reverse, can you explain?
    Outboards will steer better, because 100% of the thrust is used. A rudder deflects some of the thrust. In reverse, rudders are usually not effective until a fair speed is attained. Outboards are effective at zero speed because they don't need water flow for steering.
     
  3. Tijs Eysermans
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    Tijs Eysermans Junior Member

    Thank you for your response.

    I was thinking that when you go forward, your rudder balances itself out by the water going allongside it. If you don't have a stable shaft and your rudder would divert a few centimeters it would be pushed in the straight position by the water that pushes against it.

    But the same effect would happen if you go backwards, but this time if you divert from the straight position a little bit, the water coming from behind would push your rudder even more into that wrong direction. Do you see what I mean?

    We have a steerable inboard motor. We could still use a rudder if we wanted and fix this inboard motor. That is why I'm looking at the differences.

    Thank you again for your response
     
  4. TANSL
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    TANSL Senior Member

    Sorry to disagree with what is said in this paragraph. This is my opinion:
    The rudder can not divert any thrust. There's a serious misconception. The propeller thrust is transmitted directly to the hull by the thrust bearing and therefore, it is impossible that this "deviation" occurs. The rudder features a drag but similar to, perhaps less, that exerted by the structure of the tail of the outboard.
    At zero speed rudder or outboard are equally ineffective. Until the outboard not start to move the ship is not able to govern. If there is no water flow around the ship, you can not handle it, except by pulling it with ropes.
     
  5. Tijs Eysermans
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    Tijs Eysermans Junior Member

    Is it then not so that when you turn your rudder you get more drag that with a turneable motor? I mean if you want the same turning angle.
    With a steerable motor you use the thrust created to steer the boat. With a rudder you count on drag effects to steer your boat, or am I getting this wrong?
     
  6. gonzo
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    gonzo Senior Member

    Rudders do not steer by creating drag, that is just part of the inefficiency of the system. They divert the flow of the water, thus changing the direction of the thrust. Outboards don't need forward speed to steer. For example, they can work as a thruster does. When docking, it is common to attach a bow line and then power up the outboard motor to move the stern towards the dock. This maneuver is done without any forward speed. Anyone with experience handling boats has done this many times.
    I understand what you mean about locking the rudder when in reverse. Rudders often have locks or stops to prevent damage. On very high thrust applications, like tugs, a rudder may be damaged in reverse.
     
  7. TANSL
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    TANSL Senior Member

    Both rudder and outboard experience a certain amount of drag, do not know which is the biggest, but they are very different topics. The outboard uses the whole thrust to steer the boat while the rudder is using a force that is proportional to the square of the fluid velocity around its blade but that does not mean that the rudder deflected a portion of the thrust of the propeller. That is wrong. The propeller produces a thrust that has nothing to do with whether or not there is a rudder. Instead, think of a boat being towed by another: the rudder is effective without diverting any thrust. In the case of shaft / propeller the whole thrust, I repeat, the whole thrust is transmitted to the hull through the thrust bearing.
    The outboard seems, then, far more effective to steer the boat but it is also true that produces more lateral sliding of the boat than the rudder. Arguably, this sliding also "diverts" the thrust of the propeller.
    An outboard produces thrust while the boat is stationary, obviously, no one disputes that. A rudder on a stationary vessel produces no drag nor can maneuver the boat. A rudder may be damaged in reverse and an outboard propeller may be damaged in reverse. All that are platitudes.
     
  8. whitepointer23

    whitepointer23 Previous Member

    Rubbish.
     
  9. TANSL
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    TANSL Senior Member

    Well, I'll shut up. Thank you for your expert analysis and opiniĆ³n.
     
  10. whitepointer23

    whitepointer23 Previous Member

    Of course an outboard can steer a boat without moving. And mincing words about thrust is rubbish to. Normal people always talk about thrust being the water pushed by the prop. Which does get deflected by the rudder. A rudder can turn a stationary boat in forward but not reverse. Like when you tie off a bow rope and walk the stern in with thrust and rudder. Tansl and his constant argument with gonzo just confuses the people asking the questions.
     
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  11. fredrosse
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    fredrosse USACE Steam

    Thrust Forces

    There are two thrust forces at play here. For a conventional inboard, the force of the rotating propeller on the propeller shaft, which pushes on the thrust bearing is one of them, and this force does not steer the boat. However the forward running rotating propeller directs a jet of water aft, across the rudder, and if this jet of water is deflected by turning the rudder, then there is another thrust force generated, a thrust force on the rudder, which transfers as a force on the rudder shaft, and a force moving the stern of the boat in one direction or another.

    If the prop is rotating in reverse, then there is very little "jet" of water directed at the rudder by the propeller, so in this case the rudder is much less effective in producing any steering forces, at least while the boat has no reverse velocity. As reverse velocity builds, then as this flow of water, now with velocity relative to the rudder, then the rudder begins to have an effect, again because it deflects a flow of water to the right or left of the stern.

    The thrust force associated with deflected fluid jets: F = Mdot * dV, Force = Mass flowrate being deflected multiplied by the change in Velocity, a vector equation.
     
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