How is it that motorboats lean into a tight turn

[colhel]

Well the only time that I flew a plane I was told to move the joystick sideways to move the ailerons in opposite directions [one up, the other down] in order to effect the banking necessary to make a turn so as far as I know it's the pilot who initiates the banking but maybe the dihedral angle helps.

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If the plane has dihedral the plane will bank on rudder alone. If it hasn't you have to use rudder and aileron to get a nice turn. In fact there is a tell tale on a gliders cockpit to get the "mix" right. Perhaps working on the same principal as sail tell tales?

 

[agurney]

I've seen inflatables [not RIBs] planing and they often have flat bottoms
... do they bank on turns? Anyone know?

My wee inflatable with a 2HP outboard banks on a turn. Give it a bit of welly, push the tiller hard to to starboard and it does its damnedest to push the starboard quarter up in the air while pivoting on the port bow.

 

[Lakesailor]

Work it out from these pics I took

 

[Seajet]

An aircraft has to be "banked" by the pilot in order to induce a turn. The rudder is used to provide secondary corrections only. If you tried to turn an aircraft using rudder alone it would simply yaw. In a way it's like a bike where the rider creates the banking. In the case of a boat though the banking follows automatically from rudder movement. Personally I think that the rudder movement induces yaw which scrapes up a wall of water along the outer side of the boat which leaves a lower water level along the inner side. The boat then adapts an attitude based on this local sloping water surface. This wall will always tend to level itself out and in the case of displacent boats they aren't fast enough to create enough of a wall to matter and centrifugal effects dominate.

Agreed re the effect of centrifugal force.

However aircraft bank into turns as the outside wing - in a turn, even if provoked by rudder instigated yaw - produces more lift, though as Lakesailor points out this can depend on wing dihedral.

Mobo's bank because of the underwater thrust vectoring of the prop' & rudder, but as the pic's show the centrifugal mass of a nuclear aircraft carrier with much of her weight above the waterline can overcome this...

 

[Deleted member 36384]

.... Mobo's bank because of the underwater thrust vectoring of the prop' & rudder....

This can not be true. On a level boat with a rudder at right angles to a hull the thrust should be at right angles to the hull. The result must be a sideways but upright stern slip to the outside of the turn. Inertia will then cause the boat to lean outwards due to the force caused by its mass times acceleration in the new direction.

So where does the force come from that causes the boat to lean inwards against its inertial force that is acting outwards?

Perhaps I am missing something.

 

[Seajet]

This can not be true. On a level boat with a rudder at right angles to a hull the thrust should be at right angles to the hull. The result must be a sideways but upright stern slip to the outside of the turn. Inertia will then cause the boat to lean outwards due to the force caused by its mass times acceleration in the new direction.

So where does the force come from that causes the boat to lean inwards against its inertial force that is acting outwards?

Perhaps I am missing something.

Try thinking about the prop's underwater leverage; in the words of Monty Python, well that's my theory, wot I am sticking to !

Also rudders at 'right angles' cease to be much use vectoring the propwash, becoming brakes with nasty loads, as a last help just assisting the helmsman with a marginal assymetric effect, though prop' torque will make one direction favourable.

 

[Deleted member 36384]

Try thinking about the prop's underwater leverage;

Props are balanced. Any offset thrust direction on the top is countered by offset thrust direction on the bottom in the opposite direction.

All single rudders in the centre line are at right angles to the hull. Twin rudders that may be angled away from the hull, due to the hull shape are also balanced.

I doubt it has anything to do with rudders or propellors. Lakesailors pictures show that there is more water flying away on the outside of the hull. Why? Where does the energy come from to shift more water to one side than the other?