transom to prop distance (cats)

[ozzie-crawl]

okay i know as a general rule of thumb is 10% of hull lenght but is that just for monos what about cats ?
i know the further out the less chance of porpoising but then more chance of chine walk.
i have seen a lot of diffrent cat setups with the prop being very close to the transom and also some with the prop set rite back.
some gas boaters set the strut in the tunnell so the prop is level with the transom they say it helps stop blow overs, i thought the further back the less chance of blow over
any coments,suggestions welcome
scott

[NativePaul]

10% is good for cats too, but a little bit further back doesn't hurt as much as it does for monos as cats are much less susceptible to chine rolling and you can compensate for prop walk by offsetting the drive shaft. From my experience a longer strut gives more stability helping control the ride angle better but hindering the turns.

[ozzie-crawl]

thanks paul. i wonder why a lot of gas boats run short strut brackets and some even have the strut up in the tunnel
most pictures off struts on electrics that i have found are similar to this and it doesnt look any were near 10%
can anyone tell me how far the prop is from the transom with a speedmaster 21
thanks scott

[NativePaul]

I can think of 3 reasons, (1) their hull has so much downforce they aren't worried about angle of attack knowing that the boat will run flat whatever so they are maximising their turning ability, (2) they have very long hulls and commonly available commercial struts/brackets happen to put the prop in a less than ideal position, (3) they are a bit silly and don't know better.


I actually took the idea of the 45degree strut from a gas boater.

[Fluid]

I can think of 3 reasons, (1) their hull has so much downforce they...(2) they have very long hulls...(3) they are a bit silly and don't know better.

It is none of the above. A recent popular gas hull out of Canada runs very light, is of the standard length:width ratio and some of the top gas racers in the world have mounted the strut inside the tunnel. It works, so who cares about theory?


IME there is no hard-and-fast rule about prop position in cats, just as there is none for monos or hydros. I have run struts on my record-setting cats from under 8% to over 20% with great success. A lot depends on what you want to do with the boat, the type of course and the water conditions you run on, the running caracteristics of the hull, etc. There are sinmply too many variables to ignore.



.

[NativePaul]

What is the Canada hull? Is it standard oval length to width or offshore? Do you have a picture of the bottom, I'm interested in seeing how they maintain a stable angle of attack without the prop out the back.

[Fluid]

It's design is very close to the standard SprintCat/Stryker gas cats, just minor changes to the tunnel floor entry and steps. Nothing you can see from photos, although the deck is different. It's the AC Boats R-42, winning a lot of gas races this season.

[ozzie-crawl]

thanks for your answers guys.