Cavitating Cat

[r1wheel]

Hi i am just starting into electric boats and am unsure of why my boat is cavitating?

The boat is a 29" cat from ebay, running KB45 11XL (1000kV) on 6S1P(5000mah).
Cat weight is 3.6kg (8lbs) rtr is this too heavy ?
Cg is about 30%

I have adjusted the strut height above and below sponsons didnt really make a difference.

Boat cavitates permenantly on x442 or less.
Boat cavitates for at least 30 yards on x445 and will cavitate very easily if i loose speed.

The boat cavitates a little (5 yards) on X447, however my current draw with this prop is about 130amps = blown speedo. I did get to 40mph before bang

Is such a large prop required to move this hull? I have read many forums with cats of similar size running much smaller props.

[rockwerks]

Got any pics? what is the name of the hull?

Hi i am just starting into electric boats and am unsure of why my boat is cavitating?

The boat is a 29" cat from ebay, running KB45 11XL (1000kV) on 6S1P(5000mah).
Cat weight is 3.6kg (8lbs) rtr is this too heavy ?
Cg is about 30%

I have adjusted the strut height above and below sponsons didnt really make a difference.

Boat cavitates permenantly on x442 or less.
Boat cavitates for at least 30 yards on x445 and will cavitate very easily if i loose speed.

The boat cavitates a little (5 yards) on X447, however my current draw with this prop is about 130amps = blown speedo. I did get to 40mph before bang

Is such a large prop required to move this hull? I have read many forums with cats of similar size running much smaller props.

[r1wheel]

Hi rockwerks,

The cat is actually the boat you were inquiring about (red cat - goblin 29" post).
Have attached some photos. Any ideas?

thanks

[lonewolf]

1st off you need to angle the rudder to 90 degrees of a bit on the trailing side.
That much kick under is causeing the hull to lift in the rear when you turn. 90 degrees will be neutral a good place to start....

[ReddyWatts]

FeCalc says this setup should only use 60 amps and run 40 mph. What is the amp rating for the controller? If it is using 130 amps, check drive line alignment, vibration and binding.

Are you sure it is an 11XL?

[r1wheel]

I think the cat is drawing about 70amp average(5 min runtime), however the wattmeter indicates the peak amps was 132amp. My original esc was 100amp OEMRC, which burnt i think due to the peak amps.

I can run full throttle out of water with no vibration, alignment is fine and shaft is easy to turn by hand(also have thrust bearing on motor).

I am now running the Himodel 200amp speedo with watercooling, so far so good, but i want more speed, so i figure if i can run a smaller prop i can either increase the battery from 6S1P to 8S1P with a HV esc, or change motor from 1000kV to 1400kV.

[Dr. Jet]

1st off you need to angle the rudder to 90 degrees of a bit on the trailing side.
That much kick under is causeing the hull to lift in the rear when you turn. 90 degrees will be neutral a good place to start....

Actually, the pivot appears to be parallel to the transom, so I don't think the apparent visual angle will cause the rear to suck down as it would if the pivot were angled forward. It looks like you have a "kick-up" rudder that is positioned incorrectly. Regardless, it should be correct and the pivot should be perpendicular to the water surface. The photo is not 100% clear.

Imagine the pivot line were angled 45º in the apparent direction in the photo, and you turned the rudder 90º. You would have a scoop that pulled the back of the boat down. Smaller angles in either the pivot or rudder deflection reduces this tendency, but the effect is still there; unless the pivot is perpendicular to the water. The shape of the rudder really has no impact on this phenomenon, just the angle of the pivot.

Regarding the cavitation issue, are your props right out of the bag, or are they sharpened and balanced? This could be the source of the cavitation.

[ReddyWatts]

The 11XL on 8s and a M545 should get it to 50 mph using under 100 amps average, but you will be adding more weight to the hull.

[r1wheel]

Hi,

All my props are sharpened and balanced, reedywatts thanks for your suggestion, i too have experimented with combinations on FEcalc my problem is a 45mm diameter prop takes forever to get on plane and cavitates easily??

Could the overall weight (8lbs) be the cause of the cavitation problem with props smaller than 47mm?

[ReddyWatts]

Yes, weight and design of the small hull.

[Eodman]

Stupid question --- have you checked CG yet ... maybe you can play a little with battery placement!

Start neutral ... get good temp readings consistently then play with the other things!

CG should be 29 to 33 percent of boat from transom!

Being new here last year the best piece of advice given to us was start slow then make it fast! Understand what your boat does oe does not want to do is the first step

[rockwerks]

what about trying a higher revving inrunner, like a 2350KV and smaller prop with more thrust maybe maybe a x642 or X645 and move your weight to maybe 25% CG

[ChaKane]

Have you thought about changing the throttle curve of your ESC to give a move gentle start and acellerate once it's on the plane?

[rockwerks]

this rudder is to the side of the prop shaft, I believe for that configuration the the front edge of the rudder should be inline with the front edge of the prop/drive dog?

or moved inline to the rear of the prop........could the cavitation be caused by the rudder being out of position?

could the prop wash be building up against the side of the rudder?


Thinking out loud here

[r1wheel]

Hi,

appreciate all your advice, i have always had the esc set to soft start to help minimise cavitation.

Rockwerks i bought the rudder and strut combo, i didnt know that there was a distance the rudder had to be relative to the prop? I cannot adjust there distance from the transom.

I have an 1800kv motor but thought i would cavatiate more because it would be turning almost twice the speed, with a smaller prop? I am not sure how thrust works, is (small prop + high rpm) = (large prop + low rpm)?