Lower prop pitch to get higher RPM?

[rbestsaints]

I have a shop telling me to get a lower prop pitch - currently running a SS 14.25x19 on a 1993 Caravelle 20ft, 4.3L Mercruiser (180hp) and Alpha One drive. My problem is at WOT I'm not hitting 4000rpm. I've had this boat 3 years - and I do general maintenance, but I seem to remember that I used to get over 4000rpm and a speed closer to 38mph. This season I'm maxing at about 32mph trimmed and empty around 3800rpm. Can a boat need a different size prop after some time to get maximum performance?

 

[dingbat]

I’m not an I/O guy, but if memory serves me you should be getting 4700-4800 rpm at WOT.

Engine problems or wet foam are the usual culprits

 

[Scott06]

Here is a link to the stickie on low wot

List of possible causes of low WOT

Here is a list of possible causes of low WOT (Wide Open Throttle) rpm. In no particular order Engine Won't Reach Operating RPM. Check 1. Fuel condition. Type and Octane possibly old fuel 2. Propeller pitch or diameter, damaged blades 3. Restricted fuel pickup tube or anti siphon valve Fuel...

forums.iboats.com

id verify the tach against a shop tach and go through the list before buying a prop. under the line item of ignition components set check the timing and total advance. If the igntion module is not advancing or timing is retarded it wont make full power.

 

[Texasmark]

On the prop the cause...nope. If it were the problem it would be with you when you install it.

 

[zachary958]

Here is a link to the stickie on low wot

List of possible causes of low WOT

Here is a list of possible causes of low WOT (Wide Open Throttle) rpm. In no particular order Engine Won't Reach Operating RPM. Check 1. Fuel condition. Type and Octane possibly old fuel 2. Propeller pitch or diameter, damaged blades 3. Restricted fuel pickup tube or anti siphon valve Fuel...

forums.iboats.com

id verify the tach against a shop tach and go through the list before buying a prop. under the line item of ignition components set check the timing and total advance. If the igntion module is not advancing or timing is retarded it wont make full power.

Thanks for the information, I will try to figure it out for more. Keep sharing such informative post keep suggesting such post.

 

[richter4859]

I have a shop telling me to get a lower prop pitch - currently running a SS 14.25x19 on a 1993 Caravelle 20ft, 4.3L Mercruiser (180hp) and Alpha One drive. My problem is at WOT I'm not hitting 4000rpm. I've had this boat 3 years - and I do general maintenance, but I seem to remember that I used to get over 4000rpm and a speed closer to 38mph. This season I'm maxing at about 32mph trimmed and empty around 3800rpm. Can a boat need a different size prop after some time to get maximum performance?

Hi,
If your WOT RPM is too high or too low, you'll want to try a prop with more or less pitch. Adding propeller pitch will decrease wide-open throttle (WOT) engine speed, while subtracting pitch will increase WOT RPM.

 

[racerone]

Best to find out why you now have the lower RPM.-----Possibly the boat has added weight ( water logged foam ) or the motor has lost POWER.-----Putting on a lower pitch prop will get the RPM back up.----But is not the correct approach.