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I have read this question and the answers left me with almost the same question about airboats:

Assuming that an airboat CoM is below the center of the horizontal thrust provided by the rudder. Along with some resistance on the hull from the water this should (in my mind) create a torque rolling the top of the boat towards the outside of the turn, however the opposite appears to happen, why?

I can think of a few of reasons why this might be:

  • My assumption is incorrect that the CoM is below the turning force (is that why the drivers are always seated so high)?
  • The driver compensates by using the torque of the motor or by counter-steering.
  • There is a force with a significant upward component acting on the outside-side of the hull. However I find it difficult to get my mind around how this would instigate the roll in the case of a 90$\unicode{xb0}$ chine i.e. a simple rectangular hull cross section. (which many airboats seem to have).
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I think your last guess (about the forces on the hull) might be the reason for what you see.

A massive object wants to go in a straight line unless there is a force acting on it. Let's look at the case of you turning a boat to the right. In this case, the net force acting on the boat must be acting to the right. On an airboat you do this by initially deflecting the airstream to the right. This causes a reaction force on the boat in the left-ward direction. But didn't we just say that in order to turn the boat to the right, the net force on the boat must be to the right, and not the left?

Well, let's look at what happens. The deflected airstream pushes the back of the boat towards the left, which causes the front of the boat to rotate towards the right. Inertia of the boat wants to keep it moving in a straight line, but now you have to think about what the water is doing. When the front of the boat is tilted to the right, then as the boat tries to move in a straight line, it will push a lot of water towards the left. This deflected water exerts a reaction force on the boat pointing to the right. This is the force that causes the boat to move towards the right.

But you are not only deflecting water to the left, but you are also deflecting a lot of water downward, which pushes upward on the outside of the hull and causes the boat to be tilted into the curve.

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  • $\begingroup$ This makes most sense to me so far, while in the turn (assuming the roll is already instigated) the waterline seems to be below the chine of the leading/outside edge and I would expect the upward force to be larger on the inside/trailing edge just because the angle of attack is the same but the wet area is larger (I might have to draw some diagrams later)! $\endgroup$
    – jayjay
    Commented Sep 25, 2017 at 12:49
  • $\begingroup$ Rethinking the question, and watching some more videos of airboats turning, I got to the point AlbertB got, but didn't post, because I don't find the last step convincing at all: yes, the water is pushing the boat to the right, but I don't see it pushing the boat up... $\endgroup$
    – stafusa
    Commented Sep 25, 2017 at 21:28
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It shouldn't be the general solution, but for some of the boats it can be that your assumption about the boat's Center of Mass is not verified.

Many of these boats are known for being top heavy, with not only the seats, but also the engine and propeller often mounted high on the boat.

Besides, the rudder plates are sometimes mounted low behind the propeller, which means the center of horizontal thrust will be rather low.

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