Props and Rpm

[Supra1]

thank you will print the manual when I get to work hehehehe

[FoiltechLaunch21V]

See if this helps it's from Indmar.

http://www.indmar.com/About/ContactIndmar/faq.html

Propping is not an exact science and may require some trial and error. The maximum RPM your engine can achieve is dictated by propeller pitch. Generally speaking, one inch of propeller pitch equals 150-200RPM. To increase your engine RPM, you should decrease propeller pitch and vice versa. Your local authorized dealer can assist you in choosing the best propeller.
RPM Ranges for current production engines follow:

All carbureted (GM based) engines - 4400-4800 RPM
All Throttle Body EFI - 4400-4800 RPM
4.6 Litre Northstar Engine 5500-6000 RPM
5.7 Litre Cast Iron Multi-Port Fuel Injected Engines 4600-5000 RPM
5.7 Litre with ETX or ETX-CAT Exhaust Manifolds 4800-5200
5.7 Litre (LS1) and 6.0L (LQ9 and LY6) Engines 5200-5600 RPM
8.1 Litre Multi Port Fuel Injected Engine 5000-5400 RPM

[Supra1]

So if I'm hittin the rev limiter with a 14 x 18 I could try 14 x 21 for more speed less rpm correct?

[87SunSportMikeyD]

Correct but call ACME or Nettles to get an accurate pitch.

[Supra1]

will do...... Thanks

[tjonas1]

Supra1, what kind of transmission does your boat have? Alot of the newer boat are running the 5:1 trans, Thats the reason your boat is running such a high pitch prop. With allot of the older boats that have the 1:1 trans we allmost have to run a really low pitch prop to make up for the weight that we are running for a larger wake. A prop size is really easy to understand. The pitch is relayed to the forward movement. For example, 21 pitch prop will move the boat 21" per one full rotation. and a 13 pitch will move 13" per one full rotation and so on.

The best propeller size for your boat and engine combination is based on the recommended operating range at wide open throttle (w.o.t.) for your engine, which you will find in your operator's manual. This will be expressed in terms of a certain horsepower at a certain RPM (revolutions per minute).

The goal in prop selection is to determine what propeller style and size will maximize performance for your boat, while allowing your engine to operate in the recommended RPM range. The correct propeller will prevent the engine from over-revving, yet allow it to reach the minimum RPM where maximum horsepower is produced.

Run the boat/motor at w.o.t. under normal operating load to determine the maximum RPM you are able to obtain. A tachometer is necessary for this test. Adjust the motor trim angle for the optimum performance. If during this test, you begin to exceed the maximum rated RPM of the engine, reduce throttle setting to a position where maximum RPM is not exceeded.

If your test results in your being able to over-rev the engine, you need to increase the pitch of the propeller. Increasing the pitch increment by 1" will result in approximately 200 RPM drop. If your testing shows, however, that you are only able to obtain a RPM somewhat lower than the maximum rating given by your engine manufacturer, you would need to decrease pitch. Decreasing pitch would increase your RPM.

Example:

Operating Range =
5000-5600 RPM

Top End of Operating Range =
5600 RPM

Tachometer Reading =
4800 RPM

Difference =
800 RPM


For every 1" of pitch size, the effect will be approximately 200 RPM. Knowing this, take the difference in the above example at 800 and divide it by 200. The result is 4. The prop to use will be 4" in pitch less than the prop that was used.


Switching from an uncupped to a cupped propeller will also reduce your RPM. The cupped propeller of the same pitch and diameter will typically reduce your RPM by approximately 200.

Once your wide open throttle RPM falls within the recommended range of the engine manufacturer, you have a propeller that is suited correctly for your boat with respect to RPM. If you use your boat for fishing, cruising and skiing, one prop probably won't do all three things equally well. It is best in circumstances like this to have two propellers. One to accommodate one set of circumstances and the other to perform best under the different load. It is imperative that the wide open throttle RPM fall within the range specified by your engine manufacturer.

[Supra1]

Okay .. I follow the logic .... however the issue i have now is the prop that I have 14 x 18 4 blade does not limit my rpm the rev limiter does.I tried the three blade 14 x18 same rpm maybe a little faster mph. So I have a third prop 14 x 18 4 blade that is nicked and out of ballance and I'm thinking of having the prop shop put more pitch/cup in it so that the prop will dictate the max rpm.

anyone try this ? how much pitch or cup did you settle on?

[tjonas1]

Most shop would be able to put around 2 extra inchs of pitch into that prop. That would drop your RPM's by 400 which should put you in a safe zone.

[tjonas1]

Increase pitch, reduce rpm', make boat go faster.. You will pick-up 400 more rpms more before you hit rev limt.

[87SunSportMikeyD]

Ski-it-again.com has lots of used props. If you are looking for a speed prop, over-reving the rpms shouldn't be as big an issue. It is more important if you want power and are raising your rpms. Awesome post TJ!