Prop for a 2002 Mercury 115 4 stroke

[Cpope3]

Hi
I just purchased a 2002 Sea Fox 18 ft Flats and really like it. But I only get 4200 rpm wot. Need to know what is the stock prop size to see if that is what is on the boat and what prop size I need to get. What else would cause the rpm to not come up to max? Which I assume is around 5600/5900.
Had it back in the shop and they said its the prop but they don't know what the stock prop was and said the motor runs perfect. It does seem to run very good, with only 250 hours on the motor everything seems OK except it will not go over 4200 rpm wot.
Thanks for any help.
Charles
US Coast Guard Retired

 

[steelespike]

In order to tell how your setup is working we need your lightly loaded wot rpm and gps speed.
We need your prop size ,near the base of the blades or under the prop nut.
Your gear ratio is 2.07.
If I had to guess I would estimate your pitch as about 19-21"
What is the speed at 4200?
Guessing a bunch of stuff a 19" prop at 4200 rpm assuming 13% slip speed would be about 31 mph

 

[Cpope3]

Prop size is 13, numbers are RUBEX25 13 and part number is 94411-133-17 4200 rpm speed is around 29/30

 

[steelespike]

A 17" prop would very likely over rev on a 18 ft flats boat.
At the least you should be at 6000 rpm over 40 mph.
Assuming the tach is accurate you would have to drop 9" to get to 6000 from 4200.
I think the tach is accurate calculator says 17" @4200 with 13% slip is 28.9 mph.
I suspect some sort of motor problem.
Could be as simple as a bad plug or a plug wire off.
Maybe the throttle isn't opening all the way.
I think a compression check is the first thing to do.
Note the location and condition of each plug.