'93 Bowrider 3.0 Mercruiser underpowered?

[floutier84]

Hi everyone. First post here on the forums.
I recently purchased a '93 Larson Bowrider with a 3.0L I/O Mercruiser. The motor has 460 hours.
I am currently unsure of the prop size since it's out on the lake but when I go WOT, it gets up on the plane in about 2 seconds, however the top speed is only 31mph and the RPMs get to 5100-5200.
A) That doesn't sound like the top speed is where it should be and B) I was tracking the RPMs should only be at about 4800.
Oil pressure it good and the motor runs great. Smooth idle and no sputtering at WOT.
Also, this is with the motor trimmed all the way down (I'm new to boating and assuming that is correct)
Thanks for the help
David

 

[airshot]

What is your definition of under powered? You have a very heavy hull, your rpm should be in the 4500/4600 range and change pitch accordingly to keep it in that range at WOT. The 4 stroke I/O are not designed to run at WOT all day long like 2 stroke outboards are. Running hard like that will drastically shorten engine life.
Most will run an average cruising speed in the 3300/3400 rpm tops for all day running with an ocassional wide open run at WOT for only short periods of time to maximize engine life. You did not say how big your Larson was but they are a heavy hull so low 30's should be about right. I boat on lake erie in Ohio and rarely can we run more than 25mph due to roughness of the water and normally run around 20-22 mph to handle the waves so the 3.0 is fine for my boating conditions.
If you enjoy fast high speed boating then another choice of boat and motor may be in order. Your Larson was not designed for that type of boating. Keep in mind boats built in late 70 thru much of the early 90's were designed to save fuel so smaller more efficient engines were popular for that reason.

 

[floutier84]

I appreciate the answer. It is an 18ft (180 SEI). My concern isn't that it goes fast enough for me since I take it out with my wife and 2 small children. I would typically keep it between 3500-4000 to cruise.
The reason for the questions was more for the concern that there was an issue with the motor or that I was running an incorrect setup.
If that speed and RPM at WOT sounds normal for the weight of the boat, then I have no real concern.
Thanks again!

 

[Fastatv]

I appreciate the answer. It is an 18ft (180 SEI). My concern isn't that it goes fast enough for me since I take it out with my wife and 2 small children. I would typically keep it between 3500-4000 to cruise.
The reason for the questions was more for the concern that there was an issue with the motor or that I was running an incorrect setup.
If that speed and RPM at WOT sounds normal for the weight of the boat, then I have no real concern.
Thanks again!

If your tach is correct...then you have a concern. I believe that particular engine is supposed to max at 4800 RPM. I recommend you verify your tach operation. If your tach is correct, and if your turning those rpms ( 5100-5200 ) , trimmed all the way down, then you will need a prop of a larger pitch.

 

[fishrdan]

Also, this is with the motor trimmed all the way down (I'm new to boating and assuming that is correct

Trim all the way down to take-off, the trim up for speed once on plane.

Sounds like the previous owner put a low pitch prop on the boat for water sports; too much WOT RPM, planes really fast, low WOT speed. What is the prop pitch, or what are the numbers off the prop. (I'm guessing they have a 15P prop on the boat, normal would be 19P to 21P)

 

[midcarolina]

I agree with fisherdan........ sounds like it was repropped for water sports, a heavy hulled 18' jumping on plane in 2 seconds? problably propped way down....... Does it also have a hydro foil fin mounted on the outdrive?

Keep in mind 3500 rpm is 3500 rpm... motor does not know what prop is turning........ If you like the way the boat performs simply back off on throttle so as not to over rev past stated max rpm's.......... If you change the prop to higher pitch hole shot time will increase fairly dramatically and top speed will increase

 

[Fleetwin]

I have a Larson with a 3.0. Yes, they are heavy.

Mine runs WOT right at 4600 with a light load-40 MPH GPS.. I have a 19" prop on a V-P SX drive. I assume the MC would be simliar in performance.

 

[floutier84]

I have a Larson with a 3.0. Yes, they are heavy.

Mine runs WOT right at 4600 with a light load-40 MPH GPS.. I have a 19" prop on a V-P SX drive. I assume the MC would be simliar in performance.

What year is your motor/boat? Also what pitch is your prop?

 

[floutier84]

Got my boat out of the water today. On my prop says Quicksilver 48 16314 A4 17P
It has 3 blades and measures about 15 inches across. I'm assuming that the 17P means the pitch is 17. That would be low and cause my RPMs to be too high/not so quick top speed, correct?

 

[Grub54891]

Got my boat out of the water today. On my prop says Quicksilver 48 16314 A4 17P It has 3 blades and measures about 15 inches across. I'm assuming that the 17P means the pitch is 17. That would be low and cause my RPMs to be too high/not so quick top speed, correct? Yup,see if ya borrow a 19 and test run it. I bet it preforms in the proper rpm range.