How is it that motorboats lean into a tight turn

Helm turns prop (outboard) or rudder (inboard) outwards causing imbalance of water/hull forces thus rotating the hull methinks
 
ha ! you haven't been on one of Her Maj's best doing an emergency turn :)


Lookit this D skidding round and leaning out. That's what I call a mobo.
 
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I was on a dive boat course at Millport when they introduced us to emergency stops (essentially one of these tight turns but they cut the throttle, so 30kts to 0 in a couple of seconds). I wasn't holding on at the time and apparently did a complete backward somersault to (almost) everyone's amusement.
 
Slowtack said:
Helm turns prop (outboard) or rudder (inboard) outwards causing imbalance of water/hull forces thus rotating the hull methinks
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Not sure about this as I would have thought that the force acting on the hull would create drag or friction on the wetted area causing the boat to lean outwards during the turn and not inwards.

I don't know the answer but would be interested to find out.
 
agurney said:
The same way that motorbikes do, oodles of grip from the propulsion system.
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As I said before I don't know the answer but would be interested to find out.

However I can't believe it is the same as a motorbike. Bikes have to be leant over by the rider to counteract the centrifugal force and to improve the turn angle. In fact the lean is partially induced by countersteering (see http://en.wikipedia.org/wiki/Countersteering ) so the lean is not caused by the propulsion system but rather by the front wheel and the rider.
 
Im no fizist but

In a displacement boat turn the power of the engine is typically insufficient to counteract the centrifugal force that causes the boat to heel outwards in a tight high speed turn (the water on the outward side acting as a pivot ) Her Majesties finest have a lot of weight up top to exacerbate this.

In a planing boat there is little or no resistance from the water when turning, so the engine does overcome the centrifugal force and so pushes the stern up to lean the boat into the turn.

Im not a mobo owner but I suspect that if you pulled the kill cord during a hard turn in a planing boat, it would drop of the plane, bite the water and she could barrel roll outwards because of the above
 
affinite said:
Im not a mobo owner but I suspect that if you pulled the kill cord during a hard turn in a planing boat, it would drop of the plane, bite the water and she could barrel roll outwards because of the above
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It's the occupants that barrel roll when you do that, see my comment above.
In a tight turn the chines are biting the water, so it's not really on the plane to the same extent, especially in a RIB.
 
My experience of playing about either tidal or lake is whilst you are planing the boat does and will lean into the curve whilst it is "skimming" across the surface. I have always put this down to the propulsion is below the centre of gravity of the boat thus acting like a lever pushing the underside of the boat away from the centre line (wherever that maybe)
However, if the boats keel/hull bites the water it can be a very different story, especially if there is a bit of a swell and you are acting a prat!
What always puts a smile on my face is starting from still and turn the helm hard to one side then give it full throttle, the inside back of the boat will drop about 18-24 inches under the water, combined with the grunt of the engine and thrust does give an exhilarating experience, especially if you are sat in the back and all you can see is water above your head height!! :D:D:D
 
I'll have a go off the top of my head and I am sure I will be corrected if wrong. They all try and lean outwards. The displacement hull has an area of low pressure build on the hull at the outside of the turn and a high pressure on the hull inside of turn as the water has to go faster round the outside due to the radius i.e. same principle as a sail and air flow.

The part of the hull in the water at the inside is pushed away from the high pressure to the outside of the turn at low pressure.

This may be fatal in light displacement boats with very powerful engines and planing hulls when a quick turn is made as the boat may high side.

High siding is best observed in moto-cross. As the riders hit the corner they push their bikes right down using body weight and perhaps a groove in the mud to maximise the grip. If they get it wrong the bike flips up and over the high side throwing the rider a la catapult style.
 
agurney said:
The same way that motorbikes do, oodles of grip from the propulsion system.
Click to expand...

Motorbikes lean in to a turn, because the rider leans that way to keep the COG directly over the wheels.

I's perfectly possible to turn a motorbike without leaning it, but it would fall over, so you don't do that do you?

Same as riding a pushbike, you keep the ballance. Something you have to learn, which is why kids fall off their bikes until they have mastered the art of keeping the ballance. Something that once learned, you don't forget.

So the question with a planing mobo is what causes them to tilt?
 
In order to turn the hull must develop a sideways force. It does this by putting itself at an angle to the local water flow under the hull. Planing boats have vee hulls. The sideways skidding causes the side of the boat on the outside of the turn to develop more 'lift' than that on the inside of the turn, so the boat 'banks' inwards. It's exactly the same effect as that given by dihedral on an aircraft wing.
 
peterb said:
In order to turn the hull must develop a sideways force. It does this by putting itself at an angle to the local water flow under the hull. Planing boats have vee hulls. The sideways skidding causes the side of the boat on the outside of the turn to develop more 'lift' than that on the inside of the turn, so the boat 'banks' inwards. It's exactly the same effect as that given by dihedral on an aircraft wing.
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Yes , I think I can live with that, thank you.
 
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