RPM's less than rated, Change Prop?

[crappie14]

Motor is a 1990 88 SPL Johnson
Boat is a 20 Foot Harris Pontoon
Current Prop is a Solas
Stamped info on prop is OD 3X 13.5 X 15 R

Guessing the 3 means 3 blade (as it is)
13.5 is pitch
15 (????? diameter ????)
R (????? right rotation????)

First Question -- Do I have the Diameter and Pitch BACKWARDS????

Manual states this motor on test run was 4500-5500 rpm using OMC 382861 prop. Guessing this means operating range is 4500-5500 rpm.

I recently swapped this motor onto this pontoon, it was previously powered with a 48spl. Prop being used was prop that came with motor which was used on a 24 foot pontoon however I do not know anything about performance on other boat.

On the test run, it was carrying about 750 lbs of added weight, on normal run I expect to double that to around 1500 lbs of people/gear. I have not verified the tach as being accurate and I have not verified that linkage is giving me total full throttle. I think tach is close and I think linkage is also adjusted correctly but this is my first motor swap.

The motor at WOT was running at 4200 RPM, boat was moving at 22.8 mph, boat planed out quick. I am happy with performance.

My question is -- Will running the motor at less than rated RPM damage the motor? The motor didn't sound like it was lugging however I expect the RPM'S to decrease as weight increases. Assuming my tach is accurate, what increase in RPM should I expect to see by dropping to say a 11 pitch prop.

Needless to say running a motor at higher than rated RPM's will damage the motor, I don't see how running it at less than rated RPM will hurt as long as I am not "LUGGING" the motor. I had to run the 48SPL at WOT to even feel like I was moving when loaded, top speed was 10 mph loaded, 17 mph empty. What I hoped to accomplish was to get the boat on plane then back out of the throttle just to the point of keeping the boat on plane. Yes, and to my surprise the pontoon would plane out with the 88SPL quick.

Thanks in advance for your thoughts.

 

[Robbabob]

Re: RPM's less than rated, Change Prop?

13.5 is the diameter and 15 is the pitch. Seems like you should be getting performance out of it with this set up. As you said, there are still things you need to check and verify. Do that and return with confirmation or updated data.

When you add the known additional weight, the rpm will drop further. You want to be within range loaded and empty; saves the life of engine, performance and gas use).

Can't give any info until you get back to us.

Good luck!

 

[Outsider]

Re: RPM's less than rated, Change Prop?

I don't see how running it at less than rated RPM will hurt as long as I am not "LUGGING" the motor.

If your max RPM is below the 'rated' RPM, you're lugging the motor across the operating range.

Q? Is motor height correct? If the 48 was short and the 88 long, you can play with pitch all summer and never get it right. If height is correct, an 11" should (operative word 'should') get you close to 5000.

If it were mine, I'd want it to redline before full throttle when empty, redline at full throttle with a nominal load, and still be well in the op range with max load.

Something to keep in mind, pontoons aren't designed to be speed demons. When powered beyond design limits, they also become accidents waiting to happen ...

 

[sschefer]

Re: RPM's less than rated, Change Prop?

I don't see how running it at less than rated RPM will hurt as long as I am not "LUGGING" the motor.

If your max RPM is below the 'rated' RPM, you're lugging the motor across the operating range.

Q? Is motor height correct? If the 48 was short and the 88 long, you can play with pitch all summer and never get it right. If height is correct, an 11" should (operative word 'should') get you close to 5000.

If it were mine, I'd want it to redline before full throttle when empty, redline at full throttle with a nominal load, and still be well in the op range with max load.

Perfect response.. You get a gold star!