prop length behind sponson

[steveo]

trying to finish up a wood cat i bought awhile ago, it came with an octura strut and the prop sits just behind the sponson, is there a percentage of where the prop should behind the sponsons, the trend seems to be a mile away from recent pics i"ve seen on some hulls
also i will use an offset rudder how deep should it be

[Fluid]

It all depends on the design of the cat. Putting the prop far off the transom makes the cat slightly less prone to blow over and allows the hull to run at a higher angle of attack. But Ed Hughey ran his prop close to the transom and he's long had some of the fastest, best handling cats around.

The rudder length again depends on the hull size and design. For the average 27"-29" cat the rudder should be at least 1.75" below the bottom of the sponsons. Shorter rudders are often used, but they usually result in a cat that hooks easily. The speed penalty for an extra 1/2" of rudder is virtually unmeasureable.



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[steveo]

well i guess i will try what i have for now since i am going to use a spec power plant, thanks

[Simon.O.]

I'll jump in with what I have tried and now use.
I was given a guide of 10-15% LOA aft of the rear as a place for the prop. Further reading and measuring by photos gave me the same info.

I now run my cats within the 10-15% parameter and have a lot of fun.
I can also change them to run the prop closer or further away as I see fit.

My bigger cat runs right out on the 15% and will soon go out more to, as Jay said, reduce the incidence of blowover.

My small cat is a straight out prop rider and my bigger cat is going to be set up as a more sedate fun boat. Still looking for 45+mph from it.

There are many parameters that will make a cat perform and I am no expert in any of those. I have read a LOT and know that there is a lot to learn and know about making a Cat go fast..............reliably and with stability.