Scratch built cat - cavitation problems

[MrEvoMan]

New to boat building, was giving it a shot.

Took a design that is floating around on here for a cat, I think it was Sharky's design or something to that effect.

I am using a single strut in the middle of the tunnel with a rudder combination from the head of the river guy on Ebay.

At first, I thought the strut was too high up and that it wasn't low enough in the water. I relocated it so it's almost 1.5-2" below the bottom level of the boat.

When I apply full throttle right away, I can get it to sound like a blender in the water, but it doesn't go anywhere. If I feather the throttle, I can sometimes get it to take off but any little water distortion causes it to cavitate again and slow down to almost nothing.

I see a lot of these cats on here, with props close to the water line and can't imagine how mine is still cavitating with as low as it is in the water.

I've worked on the balance of the boat as well, trying to get to that 30%, but it really seems that it doesn't seem to affect my problem at all.

Suggestions?

Pictures have been added a couple posts down.

[Jesse J]

need pics of setup, will help with your responses and our evaluation......

[NativePaul]

Move the strut back up so the bottom of the hub is level with the bottom of the hull and flat for a starting point, and try a prop 1 size larger. Evaluate performance and temperature then if neescisary go 1 size larger on the prop again.

[MrEvoMan]

Pictures!

Yes, I know it's not perfect yet like some I see on here, but I'm more trying to get it to work before I make it beautiful.

[MrEvoMan]

And I've tried a number of props... from small 2 bladed ones, to large 2 bladed props almost 2.5" in diameter varying in pitch to the 3 blade that's on there now. No real huge difference is noticed.

And yes, I've tried sharpened and unsharpened.

The boat is approximately 38" long.

[NativePaul]

Wow, strange looking hull, it looks like theere is a piece sticking down into the tunnel as low as the sponsons can you even get the strut up high enough to have the bottom of it level with the bottom of the sponsons?

Is the hardware poking even below that protrusion? If so that will add huge drag.

It looks like you are using a through the hull water pick up, which would still work fine and have less drag if it was flush with the hull bottom, but you also still have the water pickup on the bottom of the strut, which is the worse possible place for it as it distubes the waster flow to the prop, first thing I would do is file that off, then fill any holes that leaves.

It looks like a surface drive prop opperating fully submerged but only just, 2 things here surface drive props and submerged props are different and while both will do the job in the other mode they wont do it well. also a submerged prop that close to the surface will allways suck air down, having a hull above it prevents that for most submerged drives but when behind the boat ie an outboard, you need deeper submersion and a cavitation plate above it to make sucking air down harder.

If it was mine I would cut off that triangular piece at the top of the transom allowing you to move the hardware up the transom off the protrusion that sticks down ito the tunnel, then cut that protrusion off too.

None of the above will work if the power system isnt spinning the prop fast enough or isnt spinning a big enough prop to make the boat plane out properly but it sounds like its only just not planing and a few small and free improvements may push it over the edge so it is just planing.

How heavy is the boat RTR, how big is the motor and what KV is it, how many of what cells are you running it on, what ESC are you using? 2.5" should be plenty of prop to push a 38" cat/trimaran of reasonable weight if its spinning fast enough.

cl # 9 = IS

[MrEvoMan]

Wow, strange looking hull, it looks like theere is a piece sticking down into the tunnel as low as the sponsons can you even get the strut up high enough to have the bottom of it level with the bottom of the sponsons?

The part in the middle is something I recently added due to the attempt to lower the strut significantly.

While your suggestions seem good, I believe you are under-estimating my problem. Its not just a little speed I'm missing, it's almost any speed.

Unless I'm sitting perfectly still and the rear of the boat is sitting in calm water, I can't hardly move. When even given only half throttle, I can visibly see an instant cavitation of the water under the boat. It's almost like the prop setup is soo strong, that it can evacuate all the water from under the boat, before it can move and get into new water.

[Fluid]

The problem is your choice of props. None of the props shown are appropriate for a sport catamaran and all will give cavitation problems in most boats. They may not even be appropriate for the power system you are using.

We cannot give you good suggestions until you answer the questions we have asked. I'll summarize:

What motor, exactly? Brand, model, Kv.
What voltage/cell count?
What ESC are you using? Brand, model.
How much does the cat weigh ready to run?

Once you answer the questions we can be of more help. Right now all I can say is to use an appropriate prop design - an Octura "X" series with at least work with any boat. Size depends on the answers you give.


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[MrEvoMan]

The problem is your choice of props. None of the props shown are appropriate for a sport catamaran and all will give cavitation problems in most boats. They may not even be appropriate for the power system you are using.

We cannot give you good suggestions until you answer the questions we have asked. I'll summarize:

What motor, exactly? Brand, model, Kv.
What voltage/cell count?
What ESC are you using? Brand, model.
How much does the cat weigh ready to run?

Once you answer the questions we can be of more help. Right now all I can say is to use an appropriate prop design - an Octura "X" series with at least work with any boat. Size depends on the answers you give.


.

I'm using roughly a 2000kv motor (I think it's around that) with a couple different batteries, either a 3s or a 4s pack.

I believe the weight is a little high. It is a heavy boat, so estimate on the heavy end of things.

Props I've tried:

x645
v947
2250
x452

[Fluid]

You are making this too difficult, we are not mind-readers and I don't really like guessing games. We need to know what the motor is, Kv alone is virtually meaningless. I'll try again, for the final time:

What is the brand of the motor?
Is the motor an in-runner or an out-runner?
What is the diameter of the motor ?
What is the length of the motor body (excluding the shaft)?
What ESC are you using?
Does the boat weigh 5 lbs? 10 lbs? 15 lbs? "Heavy end of things" is meaningless, we need measurements to help you.




.

[MrEvoMan]

You are making this too difficult, we are not mind-readers and I don't really like guessing games. We need to know what the motor is, Kv alone is virtually meaningless. I'll try again, for the final time:

What is the brand of the motor?
Is the motor an in-runner or an out-runner?
What is the diameter of the motor ?
What is the length of the motor body (excluding the shaft)?
What ESC are you using?
Does the boat weigh 5 lbs? 10 lbs? 15 lbs? "Heavy end of things" is meaningless, we need measurements to help you.
.

The motor is an ebay Xpower brushless 1660kv 66a 540 size motor, inrunner, running on a Seaking 90a ESC.

The shaft is a 5mm diameter and the motor is approximately 50mm long.

The boat fully loaded with a battery weighs about 10lbs, +/- a pound or two.

I guess I didn't look up the exact details, because I feel I'm missing something obvious and was hoping someone just had a "smoking gun" kind of answer from what information I already posted. I'm generally pretty technical and fairly good with engineering (I build my own race cars - real race cars), but for some reason this first attempt at a cat boat is troubling me.

[domwilson]

If the hull is 38" long, then that motor is too small. Pushing that size hull with a 90 amp ESC is eventually going to give you grief. You may want to go with a larger motor. In addition, the strut, looks a little too deep. You want to start with the strut being parallel with the sponsons including a neutral angle. Minimize the curve in your stuffing tube. It looks like you may have too much of a bend. Those are the first things to start with.

[MrEvoMan]

If the hull is 38" long, then that motor is too small. Pushing that size hull with a 90 amp ESC is eventually going to give you grief. You may want to go with a larger motor. In addition, the strut, looks a little too deep. You want to start with the strut being parallel with the sponsons including a neutral angle. Minimize the curve in your stuffing tube. It looks like you may have too much of a bend. Those are the first things to start with.

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

If I somehow get some clean enough water and start with as straight as possible and slowest acceleration, the boat gets up to plane... but otherwise I just get a lot of noise and a lot of cavitation and no movement.

Please, I'm not looking for a 60mph boat here, even 35mph, just something consistent.

The strut WAS mounted flush with the bottom of the transom previously, the only reason it's lower now was an attempt at getting it deeper to solve the cavitation problems.

[NativePaul]

That KV is more apropriate to 6s but the motor and ESC are probably to small to run enough prop on 6s to get you moving.

A surface drive prop will allways cavitate off the mark, and the higher the pitch the longer it will do so, with zero forward speed any prop speed is 100% slip angle, the prop isnt screwing itself through the water but throwing water backwards for thrust untlle the slip angle lowers, and for a 10lb boat you have to throw a lot of water out the back which requires more power than it does to keep it on the plane.

[ReddyWatts]

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.