anti-cavitation plates

[Rob Ripley]

Do they work on a Seagull? what is a good shape?

[Rex NZ]

Hi Rob

Do they work on a Seagull?

Yes & No

On the seagull, sucking air down from the surface isn't really a problem, so, the anti cav plate doesn't help much here

However, the anti cav plate can be used to benefit as a trim tab to correct a poor hull shape.

Either way I find them kinda ugly, LOL

Rex-NZ

[Buzzook]

Well, if they truly don't work, why do almost all the Kiwi Seagull racers swear by them????

I tried WS motors with and without on the Waikato trip and 'without' one was was noticeably slower and had to have the motor far deeper in the water...

...ergo, they must at least (thru being higher) reduce drag, if not add to thrust as is often claimed...

And an 'authority' in Kiwi-land told me he had experimented with the same motor - different props, different fuel mixes etc etc and the 'anti-ventilation plate' made the most immediate and noticeable difference to boat speed.

But that is, as they say, utterly hearsay.

[Rob Ripley]

Thanks for that Mark, I found another gearbox (that came apart easily), so I plan to graft it onto a 102 power unit, 'cos I like the containment of the water supply pipe.
It has a 2mm thick bakelite gasket that would make the template, so now my question is ... what shape ?

[attachment=0]cavitation plate 003.jpg[/attachment]

[The Tinker]

Yes they work, they stop the exhaust gases been picked up by the prop. the stuff that leaks around the pipe to casting. Also it allows you to run your motor higher and gain a few more revs. Cav plate should be run just below bottom of boat. You can experiment a little by lifting engine higher.

[Buzzook]

Most of the ones I saw in NZ were roughly 'shield shaped' - with a definite point towards the centreline of the keel, peaking just in front of the forrard face of the 'box, while the rear overhang was about 100-200mm, and side overhang was about the same.

So roughly 300-400 long and approx 200-300 wide.

Some of the kiwis, and I note on some of the Bermudan motors as well, turn down the lateral edges of the plate - those parallel with the keel - by about 20-30mm, at anything from 45-90deg angle from the horizontal.

You can one fairly clearly on H-A's motor in the thread on his new dory here viewtopic.php?f=15&t=2138&start=15

The attached from that thread also shows the aprox depth - said by the experts to be about 50mm above the cav plate. except there is not load in the boat....

But if you look at the other pic - showing the motor on Paul Gilchrist's boat racing on the Waitara River in NZ last November - you can just make out that the cav plate is barely below the surface, at speed, whilst laden. And this and Graham Keegan's similarly-equipped and operated Porta-Bote, were two of the quickest boats on the river.

Okay, so they probably have done other things to their motors, as well, but......this is how the 'experts' do it.....

[Collector Inspector]

B