How many RPM's should I be getting?

[bobbymcgee88searay]

I just purchased an 1988 Searay S21. I no nothing about boats. It has a 5.7L Mercruiser inboard/outboard. It will only go about 35 mph. 40 if I push it. How fast should this boat go and at what kind of RPM's? Any help would be appreciated.

 

[95yj]

Re: How many RPM's should I be getting?

Welcome to IBoats!

I would think you should be able to get into the high 40 mph range or maybe even 50 mph with the boat. The motor is probably set up for Wide Open Throttle at around 5000 RPM although it could be rated at 4100 - 4800RPM depending on the ignition and setup. There should be a sticker on the motor that lists the WOT if it's still there. With everything running correctly, the propeller is going to govern your top speed vs. hole shot vs. RPMs. There should be markings on your propeller that state the size and pitch. Do some reading in the propellers forum and you'll understand the concept and be able to tell what you have.

 

[Don S]

Re: How many RPM's should I be getting?

The 85 to 88 engines have a recommended WOT rpm of 4200 to 4600 rpm

 

[thrasher]

Re: How many RPM's should I be getting?

Just in case, WOT stands for Wide Open Throttle, it is the maximum speed the engine can achieve whilst driving the boat with the throttle in its maximum opened position. Your top speed will be very dependant on how well you trim the outdrive, as you say you know nothing about boats, you may find your boat will go faster if you learn to use the trim correctly. Of course you might already be trimming it correctly and then it indicates you have a problem as your boat should go faster than 35mph.

Gary

 

[dgopetactical]

Re: How many RPM's should I be getting?

My buddies got the same boat WOT 4,500 14x19 prop he gets 35-40 on normal choppy water and 45 maybe 50 on calm glass with trim tabs and the whole nine yards. I dont think your too far off. Play with your trim alittle bit you can be amazed how with a little tweek you can jump 10mph.

 

[bobbymcgee88searay]

Re: How many RPM's should I be getting?

Hey, yall have been great. I wasn't sure if the rpm's were to high or to low. From the way it sounds, I'm not to far off. I might play with the pitch of the prop and see what happens then. Thanks alot for yalls help.

 

[95yj]

Re: How many RPM's should I be getting?

Thanks for that clarification Don, I give some generalizations and you know the exact specs for that setup. It's nice to have that many people with specific knowledge in this forum.

It sounds like your boat is real close to what it should be according to everyone here. Possibly it could use a slightly smaller prop than what's on it. Also remember that the stock tach and speedometer in most boats are about as acurate as a two dollar watch. Check your speed with a GPS and check your RPMs with a shop tach and you'll have a truer picture of what you're getting.