RPM for SAW

[Gary]

I have a question on prop speed for all out speed. It seems that everyone says to stay in the 30-35,000 rpm range on most SAW hydros and other boats. Any time i put more voltage to a motor it always goes faster.My boat will go 50 mph at 30,400 rpm <calculated> and at 45,000 rpm it will go 60 mph. I know the prop probably is not seeing that true rpm but who knows. Now any time i see a SAW boat run, those motors are screaming and it sounds like the rpm is beyond 40,000 Am i way off base here?

[Fluid]

Most high-powered SAW record holders run between 35,000 and 45,000 rpm on the water. This is the true rpm, not one calculated by multiplying Kv by volts. While earlier records were sometimes set with 45,000+ rpm, there has been a trend lately for running in the lower portion of the above range. A lot depends on the prop used - the more pitch the less rpm is needed for the same speed.

Simply adding more cells can increase speed, but eventually you will be outside the efficiency range of the motor. You will have to use progressively smaller props as voltage increases but you will reach a point when the speeds will no longer increase - but heat will.

A common method to find the right motor/prop combo is to decide what speed you want for the voltage you will run, and what props are available or are suitable to the particular hull. Then choose the motor to run efficiently at the rpm needed to attain the speed target. This is very simply put - it is usually far more difficult to do and takes experience and judgement. But it is fun!



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[Gary]

So would you want more or less pitch to run faster. Ive heard smaller diameter and more pitch, is that corect?

[Ub Hauled]

It's a happy medium between the highest RPM you can achieve with the highest pitch you can throw at the motor.

[Gary]

And this can all be sorted out with the help of a Watts meter?

[Ub Hauled]

How well do you know math?
But usually the simple way is having an
onboard telemetry.