Low RPM or am I just too new to boating

[Twinz]

I have a 2003 Bayliner 185 with a 3.0L Mercruiser with about 135 hours on it. This past weekend I got a chance to open her up. With just me and my wife in the boat she topped out a 37 MPH with he tach reading 4300-4400 depending on how I trimmed the outdrive. I have no idea what pitch my 3 blade prop is but do those RPM's seem right? I thought these little 4 cylinders would rev to at least 5300-5500?

Maybe I am just too new to boating.............

 

[JustJason]

Re: Low RPM or am I just too new to boating

Your rpms are spot on.

 

[Philster]

Re: Low RPM or am I just too new to boating

You could squeak another 100-300 RPM out of her. That's it. You'd need a perfectly tuned prop, the perfect light load and have 'er trimmed way up/out. Hull has to be nice and clean and she'd need to be in great tune. After all this, you could start tinkering and figuring out all she has to offer. You might even wind up swapping props, changing manufacturers, waiting for the right weather, running her WOT under different conditions, with various props, etc.

::shrug:: That is a lot of dough, effort, testing and retesting for 1.5 - 2 MPH.

 

[jerryjerry05]

Re: Low RPM or am I just too new to boating

That motor was designed to run at 4400rpm for best performance and economy.You could re-prop and bring up the rpm but your motor won't last.
It sounds like your doing good with what you have.J

 

[dirtyoldman]

Re: Low RPM or am I just too new to boating

You could squeak another 100-300 RPM out of her. That's it. You'd need a perfectly tuned prop, the perfect light load and have 'er trimmed way up/out. Hull has to be nice and clean and she'd need to be in great tune. After all this, you could start tinkering and figuring out all she has to offer. You might even wind up swapping props, changing manufacturers, waiting for the right weather, running her WOT under different conditions, with various props, etc.

::shrug:: That is a lot of dough, effort, testing and retesting for 1.5 - 2 MPH.

If it was just the two of you and you plan on taking more people later in the summer, I think it might be worth going down one inch worth of pitch. You are a little on the low side of RPM anyway, and that will only get worse with a loaded boat.

 

[91whitestang]

Re: Low RPM or am I just too new to boating

I have a 2003 Bayliner 185 with a 3.0L Mercruiser with about 135 hours on it. This past weekend I got a chance to open her up. With just me and my wife in the boat she topped out a 37 MPH with he tach reading 4300-4400 depending on how I trimmed the outdrive. I have no idea what pitch my 3 blade prop is but do those RPM's seem right? I thought these little 4 cylinders would rev to at least 5300-5500?

Maybe I am just too new to boating.............

Those RPM's are about where you want to be. You could probably get it to about 4600 but much more than that, your motor won't last. I have a 3.0L Mercruiser as well, maxum 17ft. and with 6 people on it yesterday with a 21 pitch prop, it was doing about 35mph at 4500 rpms.

 

[Philster]

Re: Low RPM or am I just too new to boating

Note on car engines vs boat engines:

Most car engines will absolutely NOT 'red line' when approaching top speed in top gear. You can hit red line in lower gears, but it is unlikely for this to happen when in top gear.

A boat is in top gear the whole time. Maybe a street version of a given 4 cyl marine engine will spin out to 5300 rpms in 1, 2, 3 and 4th gear, but max out at 125 MPH at 4400 RPMs when trying to reach top speed in 5th.

The top gear has to provide cruising performance and blend in a decent top speed. On a boat, it is a very tricky balance. Power drops off at the peak RPM anyway, so it's common to hit a wall short of the proverbial red line.

As noted, you are in the sweet spot for max operating RPM range.

 

[Twinz]

Re: Low RPM or am I just too new to boating

Thanks for the advice. I guess I am just too much of a newbie

 

[jkust]

Re: Low RPM or am I just too new to boating

Now grab the gps and get the suprise most of us get with pitot speedos. They read generally higher than actual at higher speeds. Your 37 could actually be 34.