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Why do some boats lean into turns?

  1. Aug 19, 2012 #1
    Hi, I do quite alot of sailing, and I have recently noticed that when sailing boats make a sharp turn, they heel towards the outside of the turn. However, power boats lean towards the inside of the turn.

    I assume it is something to do with the fact that they have differnt types of hull; sailing boats have displacement hulls, while power boats have planing hulls, and perhaps something to do with the fact that sailing boats have keels, while power boats do not.

    I am only a keen GCSE student so please try and keep answers simple :D
     
  2. jcsd
  3. Aug 19, 2012 #2

    PhysicoRaj

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    The outward lean might be due to the ordinary inertia (centrifugal force), which is indeed increased with the presence of a deep long keel on sailboats. But for the inward lean of power boats, a little thinking must be done. Since powerboats have no keels, they experience less inertia but they do not experience 0 inertia, since it must be compensated for the speed the have!
     
  4. Aug 19, 2012 #3

    rcgldr

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    I thought it was because the thrust and/or rudder is below the center of mass of the boat.
     
  5. Aug 19, 2012 #4
    The keel keeps the sailboat on an even heading and also without a keel any slight side wind would tip the sailboat over.
     
  6. Aug 19, 2012 #5

    K^2

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    Yes, it's just that. Typical power boat will have its prop bellow its center of mass and turn with the rudder. When you make a turn, this turns into a torque turning the boat inward. Simple as that.

    Reason for it is also pretty straight forward. Power boat will ride pretty high at speed, so center of mass is above the surface. If it wasn't for leaning inwards, it would basically tip over due to centrifugal effect.
     
  7. Aug 19, 2012 #6
    ok, thanks that definitly clears it up for me :)
     
  8. Aug 19, 2012 #7

    sophiecentaur

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    I think the two examples in the OP are probably extremes. A sailing dinghy will have its rudder hung on the transom and a large proportion of its area will be above the centre of pressure of the water on the hull. (I think this centre of pressure is at least as important as the CM, already mentioned) Hence there will be a little roll induced and this could well be in a sense to roll 'out of' the turn. I sail a keel boat with a deep rudder which is all below the hull. This could well cause roll in the other (the powerboat) direction but I must say I have never noticed this. I must make a note when I remember and I'm next sailing but generally things are less dramatic on a sailing cruiser and she tends to turn elegantly on the proverbial sixpence rather than executing the flashy curved path with all that wash you get from the fast boys with their hundreds of kW engines.
    A "power boat" will often have an outboard drive (or steerable propellors) which acts well below centre of effort of the hull (when the prop is not actually tipped up to avoid grounding), giving the expected 'roll into' the turn. I wonder whether large motor vessels (such as fast warships) would also lean into turns as their props are not steered and they are not below the hull.
     
  9. Aug 19, 2012 #8

    Bobbywhy

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    There is an extreme example of "high center of mass" and that is the aircraft carrier. When she is in a hard turn she leans outward dramatically...you need to "hold on" during this maneuver!
     
  10. Aug 19, 2012 #9

    sophiecentaur

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    That's the beauty of PF. There is such a wealth of experience here. That's brilliant info. Not just the Isle of Wight Ferry - a frickin Aircraft Carrier!!
     
  11. Aug 21, 2012 #10

    sophiecentaur

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    I have just re-thought this Aircraft Carrier situation. No ship can have a centre of Mass above it's centre of flotation - or it will capsize. Rather like a double decker bus, an Aircraft Carrier appears to have a high CM but it just can't have. I suspect that an aircraft carrier needs to have a deep draught (keel) to reduce its roll when travelling in a straight line (to help keep it level for landing planes on it). Its propellor will not be far below the water line and so the deflected water from the rudder will probably be high up relative to the centre of pressure on the keel - causing it to roll out of the turn - possibly worse than a sailing dinghy.
     
  12. Aug 21, 2012 #11
    A turn is an acceleration, and for that the sum of the forces must add up to a vector in the direction of acceleration. Draw a free body diagram, and you will see that the direction of that resulting force underwater is in the opposite direction for sail and power boats.

    That applies to the typical planning hull power boat. For a displacement hull power boat, the forces will add up in the same way as the sailboat, which is also a displacement hull.
     
  13. Aug 21, 2012 #12
    A more interesting contemplation is why an airboat also leans toward the inside of the turn.
     
  14. Aug 21, 2012 #13
    Indeed not! I recently was on a cruise ship, and I noticed that when it made a sharp turn, it leaned outward (very noticeable on the top deck!).
     
  15. Aug 21, 2012 #14
    Yes. That ship is a displacement vessel.
     
  16. Aug 21, 2012 #15

    Dotini

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    When I tilt my kayak to the left, it turns right. It is a matter of hull design detail.
    Here is the explanation, with diagram:

    If the paddler leans the boat towards the left, the left edge of the kayak digs into the water and causes the kayak to travel along the curved path that is actually the left side of the kayak. The kayak starts to turn to the right.
    http://www.outer-island.com/Articles.html

    Respectfully submitted,
    Steve
     
    Last edited: Aug 21, 2012
  17. Aug 21, 2012 #16

    russ_watters

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    Thats not true for either vehicle: leaning away from the turn is what cells you the com is above the support. Why don't they tip over?

    If a bus leans in one direction, the outboard wheels apply a larger force and inboard ones a smaller force and the resulting torque keeps it from flipping over.

    For a flat-bottom ship, the outboard side of the hull goes deeper under the water in a turn, which increases the buoyant force on that side (and vice vera on the inboard side). This would not work on a boat with a round bottom.
     
  18. Aug 21, 2012 #17

    russ_watters

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  19. Aug 21, 2012 #18
    You can begin turning on a jet ski from a dead stop because the thrust is vectored. If you do this, you begin to lean into the turn almost immediately due to the trust from the jet.
     
  20. Aug 21, 2012 #19

    sophiecentaur

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    My point in comparing a bus with an aircraft carrier was that the outward appearance tells you little about the position of the CM and the real stability of the thing.

    If you take a flat bottomed power boat - like a dory or RIB, for instance, and drive it into a turn, it will always tend to roll into the turn. This must be because of the action of the steering mechanism (the propeller pushing water directly in the direction of the centre of the curve to point the hull where you want it to go) and the prop is low down in the water (beneath the hull by a significant amount) - tipping the boat inwards. This effect happens even with the boat stationary - so there is no centripetal / centrifugal effect at all, at that stage. There is no rudder at all is such boats. However stable the hull is, it will still tend to roll out of the turn when not under power.
    Boats with a rudder and fixed propellers all, either from the simple rudder's motion through the water (sailing boat) or with the propelled water deflected by the rudder at the same level as the hull - will have water deflected definitely not below the level of the hull. So there will be little or no compensating couple to roll the hull into the turn. The amount of roll will, of course, depend upon the hull shape but I can't see how it can be ever 'into' the turn because the speed is low, the radius is large and CM will not be far above the centre of bouyancy. A rounded hull, lightly loaded will be worse than a flat hull with a full load when turning. (That is why they ballast ships with no cargo).
    Plenty of boats have a 'rounded hull' but the stability depends upon the relative position of the CM and the centre of bouyancy (which shifts as the boat heels). Even a barrel can be stable as long as it has sufficient fixed ballast at a low enough point (beneath the CB). I was looking at the Cruiser HMS Belfast today (on the Thames near London Bridge) and she draws 20ft, which compares with a similar amount of freeboard (largely hollow for crew quarters). There is obviously a huge proportion of her mass well below the surface - engines,boilers, fuel and magazine - although the appearance is that most of her is above the water. Warships are not pleasant to be in in bad weather, I am told.

    When a boat is under sail, the effect of the wind on the sail will dominate the roll, whatever point of sailing, and I don't think that anything much can be said about the likely direction of roll. When running (down-wind), a keel boat will roll badly (effect of the wind) but a dinghy will behave much better if you raise the centre board because you are much less likely to 'screw' round into the wind and the boat just 'skids' along and can be controlled much easier with the rudder (castor effect).
     
  21. Aug 21, 2012 #20

    sophiecentaur

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    That is interesting. I guess the jet nozzle is near the bottom of the hull. Does is point up or down? They have a very high power/weight ratio and the bows have quite a noticeable 'scallop', too, which would make the hull ride up on the outside of the curve as soon as it is moving at all. We don't get many of those craft where I sail but I must keep my eyes peeled to see one in action.
     
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