Prop advice

[Nitro2014]

I currently have a Rebel SS 3 blade prop. I have a 1989 150 hp evinrude outboard. The max RPMS I can reach is only 4700 rpm @ WOT. My max speed is only 58 mph. The boat is a 1992 charger 180 TF. 18 feet long. 3 batteries. 20 gallons of fuel. 500lbs persons and gear onboard. The motor runs fine. Hole shot is ok, but not great. I am assuming this is a prop issue as I would think the rpms should be higher. Any suggestions are appreciated!

 

[steelespike]

You did a great job getting us the info but we need the prop size. Usually under the prop nut or near the base of the blades.
Is the speed by gps?

 

[Nitro2014]

Sorry about leaving some information out. Yes, the speedo checked out via gps. The prop size is a 15d x21p.

 

[SkiDad]

if you are really hitting 58 your tach is off.... if the tach is correct you are probably running more like 45 mph.

 

[steelespike]

You can't go 58 at 4700 with a 21" prop and 1.86 gear ratio. Guessing 12% slip I get about 6100 rpm.
(more to come)
If we assume the prop is cupped and calculate like a 22" pitch we still get 5884 rpm at 58.
The motor is rated at 4250-5250m rpm.6100 very bad.
You may be able to borrow a shop tach to compare rpm.
Its not unusual for a tach to be off.
Usually there is a selector switch on the back. If you note the position and operate it a few times to clean the contacts it may help.Assuming my figures are right and speed is accurate. you need a higher pitch prop.

 

[Nitro2014]

I really appreciate the information here! I think I am due to start looking at repairing or getting a new tachometer so I don't end up running the rpms too high.
Thanks a lot SkiDad and steelespike!

 

[steelespike]

Good morning and welcome. You need to start a new topic using the "New Topic" tab at the top of the prop forum.
You should get more help than in a topic not addressed to your problem.
The Volvos are pretty much a mystery to most of us I can tell you it's a small motor 2.1 Displacement
The 17" prop is probably typical depending on what the Sea Sprite is.
You should read the stickys at the top of the forum try to get us as much info as possible.
You need a working tach and a gps speed.
We need your lightly loaded wide open throttle rpm and gps speed.Also look on the outdrive for the gear ratio
and on the motor for the rpm rating I'm guessing 5000 on that little motor.
Being 35 years old the motor may be tired and the hull may have problems.