Prop for 18' Larson Lazer

[lefird]

I'm looking for a new prop for my 1990 Larson Lazer, 185hp Mercury. The prop that came off was a 14.5" X 17pitch. Seem to work well. My RPM at top speed is 4600 with that prop. Mt range is 4400 to 4800, so I'm right in the middle currently. Have been looking at some 19p props around that same size, some down to 14". What difference would that make?
Any suggestions would be great. Thanks.

 

[steelespike]

Re: Prop for 18' Larson Lazer

I assume when you said Mercury you meant Mercruiser.
We need a gps speed and remind us of your rpm.
From my limited experience I think your motor should be revving more, perhaps over revving with a 17" prop.
The typical 18' 4.3 would produce speeds approaching 50 mph.probably with a 19 or 21" prop.at peak rpm.
I suspect perhaps a motor issue.
It appears presently that a 19" prop would take you below your minimum rpm.It would slow hole shot and not work well with water sports.
Others may be along with more specific info.

 

[lefird]

Re: Prop for 18' Larson Lazer

My current Max RPM WOT is 4600, The manual says it should be between 4400-4800 WOT RPM. Yes I meant Mercruiser. It's actually 175 horse power. My speedo is not working currently, I think I'm maxed around 40mph. With a higher pitch would I get more top end and less hole shot? Is that how that relationship works?

 

[steelespike]

Re: Prop for 18' Larson Lazer

Very basically Assuming the motor can turn the right rpm a higher pitch would increase speed
and likely lose some hole shot. I feel your present rpm is low for a 17" prop. At this point a 19" prop would lose about
300 to 400 rpm putting you too close to your bottom rating.
Is the boat a typical bow rider? "LaZer" sounds like it is intended to be fast.Do you know the weight.
Is it a deep V ,Modified V. shallow V is it intended to go fast?
Unless your boat is a real tub I would expect your motor to be over revving with a 17" prop.
A 17" would normally be used for water sports with a load and caution about over revving with a light load.
Is the motor in excellent tune? good compression?Does the throttle open all the way?
Is the timing set right and advancing all the way?Bottom clean and smooth?
I just checked your dry weight is about 1900 lbs. NADA says 165 hp which makes me wonder if
it is a V6 or straight 6?