Scratch built cat - cavitation problems

[MrEvoMan]

I would use 4s with a 2 blade X445 prop. Your pitch is to much for the hull weight and will cavitate. Mount strut(center of prop) even and parallel with the bottom of the hull. move the weight forward, then back as needed. Should run around 30mph.

By "bottom of the hull", you mean the bottom of the sponsons?

[Tuslow1]

Not that I know anything but it looks like your prop is running way too deep in the water and not only is the prop dragging but everything else is to. I say use a X447 2 blade and raise your gear out of the water and set the prop so that it's even with the rear of the sponsons on a flat surface and that should be a great starting point aswell as playing with your center of gravity (mainly cell placement) I tried to take some pics for you to see but they look like crap, sorry.

[ReddyWatts]

Yes, bottom of the sponsons.

A X445 prop is calculating to use around 80 amps at 1100 watts and might get to hot. With a 442 prop it calcs to use 60 amps and 900 watts.

[Fluid]

I guess I'm not getting it... how does too small of a motor relate to cavitation problems?

No problem, no one is born knowing all about FE boats. If the motor is not powerful enough to drive a large enough prop fast enough, the boat will not accelerate well and the prop will slip - "cavitate" - sucking air and reducing thrust even more. A large enough 'right' prop driven fast enough will accelerate the boat well. Too small of a motor or prop will not, that is what you are seeing.

A 1665 Kv motor on 3S will have a theoretical loaded rpm of only ca. 17,000 rpm - that is pretty slow by FE standards. A 540-sized motor will not produce enough power on 3S to drive a decent-sized prop - 45mm minimum diameter - fast enough to accelerate the boat.

On 4S you are closer but the 540 motor is probably pretty weak. Not all brushless motors are created equal, and some brands are poor regardless of their published specs. A 10 lb 38" cat is pretty big and will run much better with a more powerful (larger) motor on 4S minimum. A Feigao 10XL would be a good choice and would probably work with your ESC. The longer 'LX' motor will have a lot more torque than a generic 540 does, will pull a larger prop, and will have fewer excess heat issues.

I'd start by putting the cat on a flat table and adjust the strut bottom to be flat on the table. You will probably be able to raise it some later, but start here. An "X" series prop is your best bet as they accelerate well on about any boat. You just need a large enough prop for the hull size/weight and enough power to spin it rapidly. Keep us posted and keep asking questions.


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