Reed Jensen

It is common knowledge in the airplane world that if you rotate a prop faster than supersonic, (at the tip where the speed is the fastest, not at the root, or equal to the airplane's speed) it will vibrate it's self to pieces. Hence, shorter prop blades on faster planes. But what is the maximum speed at the surface on props for boats? Is it better to run a more shallow pitch at a higher rpm? Or is it better to run a steeper pitch at a lower rpm? Or is it all dependant upon diameter of the prop? I'm familiar with the difference between surfaces piercing props and submerged props, and how they work. I know that a higher pitch prop like a 33" is harder to get on plane than say a 23" prop. But what would happen if you just spun the 23" prop faster at the top end? Would it cavitate horribly? Is there a formula for prop speed like there is a formula for prop speed on airplanes? I've seen lots of discussions on optimum props for boats at different conditions. I was just wondering if there was a 'static formula' for figuring props. It seems that most of the information I see about props and their selection is mostly anecdotal and based upon personal experience. Thanks for any information.

HighPriority

Get in touch with Matt at Throttle-Up, he will be the best man to answer your questions.

throttleup

Great question Steve,

There is no clear cut answer. It comes down to testing, testing and more testing.

There is an ideal blade loading, too much or too little blade loading and the prop loses efficiency.

Sometimes it's better to turn a lower pitch prop faster than a higher pitch prop slower and other times the opposite is true. There are advantages with either and it comes down to the boat/motor set-up. If a boat requires less stern lift it may advantageous to spin a lower pitch prop faster to minimize the stern lift.

Other considerations are the motor. Some motors generate max power at high rpm's, thus a lower pitch prop than another engine that develops the same horsepower but at lower rpm's.

Turbine powered boats spin their props at a very high rpm while turning a relatively low pitch prop. It all comes down to the setup.

We don't have to worry about the blade tips going supersonic due to the small diameter of a boat prop compared to that of an airplane. A 15" prop turning at 3500rpm only has a blade tip speed of 163 mph. It would take a lot to get a boat prop supersonic.

If you have any more questions please don't hesitate to call. It's difficult to get into all the details on this forum.

You said it best;
Matt
877 842 6855

Reed Jensen

Thanks for your imput Throttleup. Yeah,,,,, I know it can be complex, and one set up for one boat won't work for another.Also when it comes to stern lift, I'm assuming prop rotation on twins comes into effect too. Such as whether props turn in or turn out. I'm assuming that if they turn it the stern will rise more. Any more imput? Anyone? I'd like to hear as many opinions as possible. Thanks guys!

throttleup

As far as stern lift, when the props turn in that's true for V bottom boats and most cats. With cats it depends on the location of the prop in relation to the tunnel.

You are correct with the theory regarding the smaller diameter prop. As a simple formula the fastest prop is the one with the smallest diameter, highest pitch and least amount of cup. Sounds simple.... but the hard part is finding what the boat runs best with. So when fine tuning and trying to get that extra 1/10th mph out of a prop for a kilo run it takes a lot of trial and error.

In reality to get the best prop you have to go too far and then start with a new prop and go back to where you were before you went too far. Since it's not practical to add diameter to a lab prop if you go too small then that prop is no good for that application anymore.

The variables are truly endless and it comes down to time and money. You could spend months on a single boat trying different set-ups and props to get the best set-up.

Matt