Off Topic First run in boat think needs prop

[skinny]

Well got boat out for first trip yesterday
Most seemed good with it but I did find it seemed to rev quite a bit
The boat is 3.0 4 cyl and omc electric shift outdrive 18’ aluminum

It came p on plane well but seemed to rev high or let me rev it high while under way
It was very easy to reach 5000 rpm but it didn’t feel like under load or pushing

Who is a prop guy on here. I’m sure finding omc props going to be bit of fun
Pics attached of boat and prop

 

[Outdorman]

you need to know what your motor's suggested top RPM range is, what your existing top RPM is and keeping track of your top speed is a good starting point. On my outboard the suggested WOT RPM is between 5500 and 6000. When I experimented with a 4 blade SS prop (that was too big a diameter) the top RPM dropped to 4800 and the speed dropped 5-6 MPH. Sorry, I don't know any specs for inboards. It Isn't to too expensive to re-cup a prop smaller for better performance but real tough to make them bigger. I tuned a 4 blade on my old 50hp 16 footer and got the boat to plain at 12-13MPH and come out of the hole like a bear. A good prop shop can tune your prop if it's too big down to a couple of hundred RPM of what you need but you have to give them existing RPM and speed info.

 

[Old Cut LongPointer]

How old is the prop ? They do wear down after a time and that will effect performance.

 

[Honkey Donkey]

Hi Skinny
I have a prop for sale Check it out, posted in Classified.

That appears to be an older 80's era Merc Sterndrive prop:

https://www.crowleymarine.com/parts/228627.cfm

https://www.mercruiserparts.com/79574a4-prope

 

[Twister]

Some of those old props had a rubber center that can slip. I think you'd want to make sure the old prop and the trim is functioning properly before you spend on a different prop. Not a prop guy but you should find one before you buy the wrong prop a couple times. WWW.go2marine has a prop size calculator that might help.

 

[Walter Fishonn]

Had somewhat of an issue like that once. Turned out one of my plugs was done and was only running on 3 of 4 cylinders.

 

[Derkd9]

Also looks like a lot of grass wedged in there
Take it apart and take photos for us to see

 

[Bent Pole]

I could be wrong, but the pitch of the prop changes RPM. Maybe you have the wrong pitch of prop.

 

[Schmick]

I think if you have a good hole shot and are revving out to max rpm without hitting a decent ‘top speed’, you could likely pitch up the prop a little. There are some online calculators to assist. Read until your eyes bleed to avoid unnecessarily purchasing a dozen screws though! Good luck.

 

[Outdorman]

Bent Pole is right that the pitch changes RPM, but don't forget that prop size will also change the top RPM. It's a balance. In my specific case the prop was changed from a 13" to a 12" and re-cupped it also. The results were outstanding but it took lots of stats on performance and RPM provided to the prop shop that gave them the info that they needed.