I have a Chrysler Marine 225 in my 19' boat and am trying to find the RPM limit so as to get more power from a new propeller.
Does anyone know the answer or can direct me to a site that would have this information?
Thanks in advance,
Aaron

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What type of carburator is on your engine currently?
Is it the Carter BBD 2bbl?
DD

Wow! what kind of boat?

In a marine application, do you try to run at peak HP or peak TQ?

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Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

depends on the propellor you are running. the diamater of the prop and its 'pitch' are the factors

http://www.propline.com/Propeller-Gener ... nology.htm

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.

In short, emsvitil, Yes.


A boat equipped with a prop that has lots of pitch would perform better with an engine built to produce torc, while the same boat with a short-pitched propeller would do better with a HP built engine...

It's a bit of a balance that we (boaters) have to dance around out over time to get right...

Thanks for all of the responses but I still am looking for the max RPM for the engine.
I understand the prop pitches and I understand the difference between cupped/non cupped, variable etc.
The carb on it is a 2bbl Autotune.
This engine is completely rebuilt to stock specs but I am running at about 4400 max RPM and I believe that there is some left in it.

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Was the engine bored oversize during the rebuild? Do you know it's new displacement?
Do you know the cfm rating of the Autotune carb?
Do you have any info on the camshaft used in the rebuild?

The engine is an 'air pump', knowing the engine displacement and carb size (cfm) we can get a good estimate of RPMs needed to move that amount of air.
DD

No I don't but I'm pretty sure I can find out.
I bought it used with 2.5 hrs on it.
Thanks for your help.
I will get that info and repost.

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