bow steer in fast powerboats

Discussion in 'Powerboats' started by gtflash, Oct 13, 2010.

  1. gtflash
    Joined: Jun 2006
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    gtflash Senior Member

    I am struggling to understand the science behind why a planning hull powerboat bow steers more in a following sea than a head sea... Consider a boat with a deep v aft, that increases gradually forward. I call it warped. I cannot see why the boat would steer more by the bow if it is still outrunning the waves coming from behind. It cant be the shock of the nose stopping as it would then occur in a head sea more.

    Help im confusing myself
     
  2. daiquiri
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    daiquiri Engineering and Design

    It has very much to do with the dynamics of waves, as I have once said in this post:
    http://www.boatdesign.net/forums/po...roaching-following-seas-31548.html#post345871

    To summarize it here, a following wave will:
    - push your stern forward (trying to change your course) if you are running down it's frontal slope;
    - pull your bow forward (helping you to mantain your course) if you are running up it's rear slope;
    - do both actions if you're running in the wave trough.
    It is inherent to the nature of waves and you can do nothing about it.

    Planing hulls (and other hull types with wide, flat sterns) have a further disadvantage of the longitudinally asymmetric underwater volume distribution (and the relative LCB), which is shifted abaft. In other words, the buoyancy (and the reserve buoyancy) of the forward half of the hull is much smaller then the buoyancy of the rear half.

    When such boat pitches down into the water because of the wave swelling from astern, or when it pitches up because of the wave swelling from ahead, it's CLR will shift forward. That's because forward hull sections have a lower reserve buoyancy and a much more pronounced V than rear sections, so they (fwd sections) will sink more in response to inertial forces and by doing so will offer much more lateral resistance. At the end of the dive phase you have a yaw-rotation pivot moved further ahead from CG, and this increased lever require more counteraction by the rudder. If the rudder is unable to develop a sufficient force to overcome this increased yawing moment (becuse it is too small, is moving too slowly, is ventilating or whatever), the boat will broach (rotate to a position perpendicular to waves).

    What you can do is to:
    - avoid running with following waves (of course), particularily in a boat with a wide transom and a deep forefoot and low reserve buoyancy forward (Wally Power alikes comes to my mind, or some axebow concepts I've seen around);
    - apply the throttle in order to increase the waterflow around the rudder, and so increase it's authority.

    Hope it was clear enough... Cheers!
     
  3. Ike
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    Ike Senior Member

    It was clear enough. It was right on the money.

    But to put it in simpler terms, the bow is deeper in the water than the stern, so the boat tends to bow steer.
     
  4. gtflash
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    gtflash Senior Member

    is the bow not deeper in the water than the stern in a heavy head sea?? particular at the point of bow impact with an oncoming wave? Maybe I should have asked why does a boat not bow steer in a head sea instead :D

    Thanks for feedback
     
  5. Ike
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    Ike Senior Member

    It depends on how buoyant the bow is, which determines how quickly it lifts (or rises) to the oncoming wave. A sharp deep narrow bow rises slower than a broad bow with lots of flare.
     
  6. daiquiri
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    daiquiri Engineering and Design

    This is a physical fact:
    Regardless of wether it's head or following seas, the equilibrium position for a free-to-rotate boat in a wave system is in the trough, perpendicular to waves.
    That's how it will tend to position itself, if you take your hands off the rudder and/or throttle control.

    And the following is my personal reasoning (so take it with a grain, or a handful, of salt):
    The way I see it, the difference in the effort required to keep it on track in case of following vs. head seas comes from:
    1) The longitudinal assymmetry (wide transom vs. water-cutting bow) which makes it easier to push the transom sideways, than the bow.
    2) The point of application of the destabilizing hydrodynamic force, relative to the CLR. The CLR is shifted forward in both cases, following or head seas. But in the case of following seas the destablizing force is applied at the rear half of the hull, whilst in case of head seas the hydrodynamic force is applied more forward. So the lever arm is higher in the first case, giving a higher yawing moment which has to be equilibrated by the rudder.
    3) the speed direction and magnitude relative to the wavetrain, which makes for a different encounter frequency. In a following sea the wave acts on the boat for much longer time than in a head sea (if the boat is moving under its own power or under sails), ultimately giving a bigger cumulative course-deviating action.

    Cheers!
     
  7. Fanie
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    Fanie Fanie

    Easy to figger out. Take a flat sheet of something.

    Upright in the water push it sideways. Lots of resistance. Foreward it goes easy. So if this is the keel it's going to steer.

    Horizontal it now and it slides left or right easily, and this is the stern.

    In the case of a deep V (which btw usually has a lot of roll) the bow will steer and will not move sideways. The stern can thus easy slide sideways to overtake the bow. The effect is steering like a forklift.
     
  8. tunnels

    tunnels Previous Member

    A contributiong factor is to do with the balance of the boat as well .
    If it cant lift its bow your in trouble !!
    In a following sea its scary to be in a boat and have it broach !! You really dont want to be there ,to the point where at a little speed can simply make you go swimming like it of not :D.
     
  9. Fanie
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    Fanie Fanie

    Correct. In a following sea you want the bows trimmed up to reduce bow steering and heading into it you trim down.

    Alternative is to get a boat as wide as it it long. Maybe that's why the aliens use saucers. Any direction is ahead :D
     
  10. mark775

    mark775 Guest

    Don't lose count of what Daiquiri said about the nature of waves. This is the gist. You have very little water flowing over your rudder because of the nature of waves. Bow up, down, deep, shallow, whatever...are minor players. A planing hull is pointy up front and fat behind and there is no steerage at or about wave speed when you are on the face (remember that the water molecules are actually going forward past your rudder at this point - You have NO steerage by the rudder when you chop the throttle in fear of broach). Speeding up can work, to a degree, but there is a time where you want to steer a different direction or slow way down to let the wave pass quickly before it gets a chance to kick you around. Plus, fallable as we humans are, if you are going slower, your reaction time is not as critical. Try increasing the mass and depth of the stern to make things happen a little slower - Kind of what the other posters said.
     
  11. gtflash
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    gtflash Senior Member

    Thankyou...
     

  12. Mr Efficiency
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    Mr Efficiency Senior Member

    A boat in this situation, heading in to a wave, is on an upslope, gravity is not helping to alter the course of the vessel as it is in a broach, when the COG is on a downslope. And when it is on a downslope, having passed an oncoming wave, the slope on wave backs is less, and pass quickly under the boat as relative speed of boat to wave train is much greater running into the seas. Little opportunity for yawing forces to grow in to a broach.
     
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