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[Hotglove]

I have tried a few props on my Dumas Chris Craft 1930 Runabout. Eventually I settled for a 38mm CNC alloy design which gives superb performance, quick, quiet and cool running (motor is a Turnigy 3648 1450 brushless outrunner running on a 3s Lipo)
Everything is great except that I get cavitation when turning hard in either direction, you can see a trail like a torpedo as soon as you apply a firm turn.
The rudder is very close to the prop (just clears the nylon nut that secures the prop.
One thought that occurred was to create a rudder assembly that has a cranked rod so that the rudder is moved back by, say, 1/2 to 3/4 inch.
This would have the effect of causing the rudder to swing across the propwash rather than pivot within it.
Has anyone ever seen this configuration used? If so, I would expect that perhaps a more robust rudder servo might be required.
All input welcome, thanks

[Umi_Ryuzuki]

You may be getting aeration, rather than cavitation.
As the rudder swings to either side, the thrust vector will tip the boat raising one side and
possibly catching air under the boat at the outside of the turn, and forcing it into the prop.

[Hotglove]

Thanks for the quick reply, I am watching as carefully as possible to try and analyse exactly what is happening, there is also a rattling noise, at first I thought I had a mechanical problem, but the sound is definitely caused by water.
Other than that the boats performance is very pleasing.
It is probably becoming more noticeable because I am a novice and am just beginning to relax a bit and try tighter turns.

[chas]

You could try putting a little ballast at the stern. I would split the weight into 2 and putt it along the chines by the transom instead of along the keel.
Chas

[Hotglove]

Hi Chas, thanks for the suggestion, will certainly give that a try next time out, it would be great to find such a simple, no-cost solution.
All further input most welcome.

[red181]

I had a similar problem it was the profile of the rudder, it was acting like a brake. If you can narrow the blade progressively, a bit at a time, maybe you can buy a spare one, and work with that.
I also use dual rates on the transmitter when turning at speed, meaning less servo travel. If turning slow, then I turn the rates off to get max servo movement

[Akira]

I have to agree that your issue sounds much more like ventilation than cavitation. Even on full scale boats the same effect can be replicated. To prevent it try reducing speed a little before the turn and/or reduce the the rudder throw command. I think both have been mentioned before, but try them before modifying your build. If all else fails, take a look at the different rudder designs. Yours probably has the old(1950's) tear drop blade design. Today's "fast" hulls use an unbalanced and small swept design especially for higher speeds. The draw back is the high speed rudders give lousy handling at low ( idle) speed, while the old spade rudder draws air in from the chine and ventilates the propeller.
As an aside, you are probably running your hull a lot of scale speed and that cause the effect to happen. For jiggles try running at a scale speed and see if the effect is replicated.

[Hotglove]

I tried various ideas put forward by the kind people on this site, but the problem was definitely cavitation, and only when turning right, even a small deflection of the rudder was enough to start it.
So I made a crude prototype rudder, as per photo.(I did not want to drill the hull and fit two new rudder tubes)
This has cured the problem and I plan to refine the idea over the winter. 0ne idea is that the flat part of the assembly, at the top, that bridges the two blades could be extended and angled to form a trim tab (without drilling the transom) I also plan to move the blades closer together.
On the lake today she seemed more stable and her wake was smoother, altogether an improvement in handling, also steered better in reverse.
Thanks again for previous input, all comments welcome

[chas]

I think that's a brilliant solution, one that hadn't crossed my mind. I'm looking forward to the results of your experiments.
Chas