Zapcat prop

[rik_elliott]

Hello,
Ive been lurking and trying to learn from the various threads, particularly the mini cat prop thread.

i have a zapcat and a 40hp yamaha 2 stroke short shaft (1980s) - I recently replaced the prop as the old one was badly chipped and the hub rubbers pretty rotten - expecting this to stop the problems with cavitation I was having.

I got a standard 11x15 g series prop on the cheap brand new - so bunged that one (direct replacement of the existing prop). Its not really made a great deal of difference - im guessing its ventilating really badly. The probs come on at hole shot - i get gallons of slip and really struggle to get it on the plane at all, just wheelies and crazy over revving without any speed. Ive tried fiddling with the trim too - no better. The nose comes up, speed picks up a little, then on comes the noise and bubbles - no speed increase.

The centre of the prop is almost perfectly level with the centre of the highjackers. I assume the shaft length is spot on.

Im guessing the prop doesnt have enough cup - what sort of size / pitch semi cleaver would i need? or am i missing out something else fundamental. Id obviously like to avoid spending shed loads on yet another prop - but i cant really see anything easy ive missed.

Any suggestions re prop dimensions / suppliers in the uk? Or any other things i can try to actually get some performance out of this beast? the other thing ive considered is getting the old prop fixed up and maybe cupped by steel developments?

Thanks very much

[rik_elliott]

Through my investigations I found a company called propeller solutions. I have contacted them and here is the reply I got - I thought I'd post it here for other peoples reference as I have seen a few threads about this sort of thing dotted about and I hope this helps others.

"Hi Dr Elliott,

This is one of the most asked questions we get re Zapcat and the other inflatable cats as the alloy propellers never work.

For all over performance out of the box the Powertech semi cleaver is the best and with a 40HP I would go to a 14 pitch. This will be good for blasting around in the surf as well as calm days.
They cost £230 + VAT and I would have to get a 14 in which takes around 5 working days.

Best Regards,
Thomas
Propeller Solutions"

[Bus_Boy]

For a novice user, i would reccomend that the prop sit's lower than the hi-jackers themselves. If it is level with them, then once the boat hit's a planing angle, then won't be enough prop in the water, and you will experience slip issues. This is where a SS prop, even a cleaver style come into play. An SS prop will flex less than an alloy prop, meaning you can run less of the prop in the water.

With regards to your saleman's reply, thundercats work with alloy prop's if not intended for racing use. I know this from experience with both mini and full size cats.

I'd argue if a salesman is reccomending a cleaver for non racing, general use. Ring another store, and ask them for their opinion on a default 50 HP prop modified for a more aggresive cup.

[rik_elliott]

I did wonder about sitting my normal prop lower to reduce ventilation as it does seem to be on hitting the plane angle it all goes pear shape - but I have a short shaft motor sat as low on the transom as it can go, and its only level with the hijackers. Im in a position where its either new prop or new engine, or chop part of the transom away to drop the engine (scary!).

Ill ask about having the standard prop cupped though - wasnt sure if that was a workable idea or not so thanks for the input.

Thanks for your help - its a steep learning curve this cat hulled boat business - Ive only played with normal sibs before (and commercial hovercrafts but thats another story!) and they seemed pretty idiot proof by comparison.

[Commando C2]

hi
i have the same setup, but a mariner badged 40hp 1985 2 stroke twin.
I have similar problems but can get mine to plane by feeding the power in gently.
are you running the boat with 2 people in and where is the petrol tank.
I put my tank in the front and get my passenger up the front for take off.

i PM'd you also Rik by the way,
dont forget changing the prop could allow you to over rev your engine, unless you have a rev counter in it!!
cheers

[ashbypower]

As this thread is a year old I would guess the OP has it sorted by now, or given up !!!

[Commando C2]

oh yes i just noticed it was 2010, i saw the 7th september and thought it recent!

[jcjcat]

your engine is set too high, the shaft should stay 1 inch above the highjackers lower level

[rik_elliott]

Thanks guys but I sorted the problem with a semi cleaver prop and sold the boat ages ago!