Low RPM Range with new prop

[juice69]

Threw on a new SS prop, and I'm running right at (or 10-20 rmps below) 4400 which is the low end -as the motor says 4400-4800. At full throttle I'm trimming up (nothing out of the ordinary), and it takes about 10 seconds to get there. I typically run 3/4 throttle (3500-3900) when we go out, and very rarely go full blast. I was qurious what the experts had to say running with a prop that close to the low-end of the rpm range?

 

[steelespike]

Re: Low RPM Range with new prop

It would help if we knew more about the boat and motor.old prop size and material new prop size and model.
Think of it this way.Marine engines work very hard like they are pushing the boat up a never ending hill.
When you prop it with too high a pitch its like making it work in overdrive.This produces poor performance
likely poor gas mileage and could cause premature repair problems.Your not in terrible shape but if you routinely
run with 4 or 5 adults and a bunch of gear you'll be getting even lower in the rpm band.
The motor will perform better and hold up better if allowed to freely rev within its rpm range.

 

[juice69]

Re: Low RPM Range with new prop

Steele - Im running a mercruiser 2004 3.0l on an 18 foot runabout. The old prop was an aluminum 14.5x19, and i was turning about 4900 rpms and could only get about 33-34 mph. I should say i thought the old prop was 14.5 (it said A45 which i thought was 14.5 after further inspection), and i think it was really 14 or 13.5. When i tested the new SS 14.5x21 prop, I got right up on plane and i was running right at 4400 rpms (might of been able to trim slightly more had i kept pushing), and was getting 39.8 mph (which is about 8.5% slippage). I understand adding more weight and losing some rpms, and that was one of my concerns. Typically, it's just me and my wife (and a cooler!). Like I said, we usually travel around 3500 rpms (and this prop at 3500 was at about 35 mph). What do you think, go back to 19 pitch, or keep what I've got?

 

[tpenfield]

Re: Low RPM Range with new prop

So, you went up about 10% in pitch and lost about 10% in RPM . . .

You could go to a 19" pitch SS to get back to 4900 . . . or a 20" pitch SS might put you right in the middle of the WOT RPM range spec.

I would keep what you got, since you are still within 'spec' on the RPM's

 

[steelespike]

Re: Low RPM Range with new prop

If the motor is overpropped its over propped at 3500 or 4400.
I think a 19 stainless will give you really peppy performance should stay on slow plane with less diddling.
May give you back 4800 to 4900 rpm just stick with 4800 except for a little spurt now and then.
Sounds like you just cruise mostly anyway.
Speed at 3500 may be a little less but throttle setting may be less as well.
The increase in rpm should help the slip a little and could result in a touch more speed.

 

[cyclops2]

Re: Low RPM Range with new prop

My boat ?
Have the boat tachometer checked out for accuracy by a good mechanic.
The 2" increase in diameter more than ate up the gains of thinner blades.
You are running MINIMUM load now & if the tacho is right, you are propped to much for any weight increases.
You are wasting fuel. Try to get back to 4800 with YOU ALONE in the boat. Then a few drinks & a boat load of people is OK for the engine.
Check the tachometer FIRST. otherwise you are guessing & hoping for dumb luck. Local prop shop. or Places allow you to test the prop & return it for DIY salesw.

 

[smokeonthewater]

Re: Low RPM Range with new prop

You also have an option of taking the prop to a local shop and having them "depitch" it a little.