I have a boat that uses a propeler above the water. The propeler blows the wind to the back of the boat and it makes the boat go straight.

There is one problem with this approach: I am using a big propeller and I need to move the motor high so the propeller does not touch the water. As high as I move the motor there is an undesirable increase in the torque generated by the motor related to it's base. I mean, if I move the motor higher, it makes the boat sink the front part of it, if I move the motor down it eases the sink of the front of the boat.

I said it all to ask this question: can you see the red lines and the right angle (in red) at my image? If I make that angle (instead of 90) to become 130 will it reduce the force that makes the front of the boat sink? Or making the angle 40, will ease this force?

You need to remember one thing: when we increase/decrease the angle (from 90) the height of the force becomes lower and lower, so it should reduce the torque but it also creates another force (vertical component) that can make the bottom of the boat "fly" or sink.

I thought a lot about this and I have no idea if I should increase or decrease the angle (red) in order to prevent the front of my boat to sink cause when I increase a lot the power of my motor my boat gets very fast but the front of it eventually gets inside the water and the bloat flips, which is terrible.


I made some experiments today and I tried an angle of 70 degree, in that case, when speed is low the back of the boat sinks but when I go fast the tip (front) sinks. When I tried and angle of about 110 degree and the speed is low the tip (front) of the boat almost sinks with low speed but for some reason when it goes fast the front does not sink much more. What's going on?

enter image description here

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    $\begingroup$ Are you trying to create an airboat? $\endgroup$ – Wasabi Apr 17 '16 at 15:01
  • $\begingroup$ Yeah, I already created it. It works fine, really good. But I want to optimize it cause sometimes the tip (front) of the boat sinks when I go very high and it's not related to waves or wind. It's related purelly with the angle of the motor. I know that as low as I place the motor better and the reason is simple: it reduces the torque that makes the propeller push the boat to be more in the center of mass (vertically). As far (high) I move the motor it creates a big torque that makes the tip of the motor sink. $\endgroup$ – Samul Apr 18 '16 at 13:43
  • $\begingroup$ When you say "go very high", do you mean fast? $\endgroup$ – Wasabi Apr 18 '16 at 13:46
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    $\begingroup$ @wasabi yes, sorry my bad english. When I say "very high", I mean "very fast". $\endgroup$ – Samul Apr 19 '16 at 14:58

In order to decrease the moment that makes your boat front sink, you can decrease the arm of the propeller force (M = F x b).

Compute the torque applied by the propeller around the CG and check if it is a good idea. A brief sketch suggests that if you increase the angle you increase the torque (same force!) and you decrease the contact force with the water and if you decrease the angle you decrease the torque but you increase the contact force. So also the contact point becomes important.

One more thing I can propose is to change the shape of the boat keel in order to make it glide at high velocity (consider the boat as an airfoil and you get the point!) keeping the same configuration for the propeller.


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