3.0 mercruiser prop

[picker523]

I've got a 17.5 foot glastron with the 140 hp mercruiser and I'm able to top out at about 38mph. I could go faster but I'm worried about rpms.
The boat is a narrow 87 model and I wonder if I should get more speed out of it. Does anyone know what this engine tachs out at? Whats the best pitch prop to retain a respectable amount of power and speed?

 

[tommays]

Re: 3.0 mercruiser prop

The motor should run in the 4400 - 4800 RPM range WOT (i really dont like to run it at that RPM for very long

What WOT RPM do you have ?

With the 3.0 or older 140 and its lack of HP your limited as the best prop for top speed will result in pretty poor holeshot peformance


I have a used a 1995 3.0 in a 19 ft boat for 12 years and water sports need a 17 " prop which limits your speed to around 32 MPH

A 19" prop works well for all around use with a OK holeshot and a top speed of about 41 MPH BUT not the best midrange fuel use

If i am going on a long day trip 90 + miles i use a 21" prop which gets going ok with 2 people on the boat and gives a much better MPG at my normal long trip speed


Your boat is lighter BUT this should give you more info to ask more questions


Tommays

 

[picker523]

Re: 3.0 mercruiser prop

At the risk of sounding dumb...what does WOT mean?(wide open throttle). It's a 87 3.0 . Will too high a pitch cause it to over- rev or put too much strain on the gears? Im not sure wot rpms are. I will probably have an average of 4 people in the boat when pulling a skier. Would a upgrade to an electronic ignition help performance?

 

[MJ76]

Re: 3.0 mercruiser prop

The most important consideration when choosing a prop is to make sure that you don't lug the motor. You don't want to over-rev, either, but I have seen many, many more motors damaged by over-propping.

A lower-pitched prop will allow the motor to wind up more freely and will give you better holeshot performance. A higher pitch will give you more top end, but only up to a point. It's a balance that depends on a number of factors that include how much weight you put in the boat and what it's primary use is. For example, if you do a lot of skiing, it makes sense to use a lower pitch to get that power at the bottom end.

W.O.T. means wide open throttle rpm, and that's what you need to know. Take the boat out with the prop you have, with a normal load, and run it wide-open, even if you are not in the habit of doing so. The '87 140 has a w.o.t. range of 4200-4600. You need to be in that range. A little bit over is better than under. From there you can determine what you need, based on the prop you have.

As a rule of thumb, figure that 1 inch of pitch equals 250 rpm, give or take. For example, if you are running 3900 with a 19 pitch, a 17 will bring you up around 4400. If you ski with it, aim for the high end of the range - 4800 won't hurt it. You may end up using a different prop depending on how you use the boat.

 

[picker523]

Re: 3.0 mercruiser prop

so am I to assume I can run at 4800 rpms for extended periods without hurting the engine? If it will reach rpms of 4800 or 5000 does this mean the prop is not pitched too high for the boat?

 

[MJ76]

Re: 3.0 mercruiser prop

If you can get to 5000, I would go up in pitch. The reason you want to aim for the high end of the range is that's where you're going to have the best power for skiing, if that's what you primarily use the boat for.

You might find that a higher pitch that puts you closer to the lower end of the range gives you more speed on the top end, and if you can still satisfactorily pull up a skiier, then great.

The great thing is that the props that fit your boat are the most common props out there. Borrow a buddy's or buy a used one. Experiment to see what works best for you. The important thing is to watch your w.o.t. rpms, and don't use a prop that loads the motor way down.

I probably wouldn't run it at 4800 all day long, unless I just wanted to eat up fuel. As I said, a higher pitch would probably be better for cruising. Do you know what you were running, rpm-wise, with the prop you have?

 

[tommays]

Re: 3.0 mercruiser prop

so am I to assume I can run at 4800 rpms for extended periods without hurting the engine?


NO NO

It is a simple motor built for entry level boats even the best HP Race motor has a life in hours at its WOT


The motor will last allmost forever below 4000 RPM

Running at 4800 RPM steady will use it up in a few years at best




Tommays

 

[MJ76]

Re: 3.0 mercruiser prop

True, but what I'm saying is don't prop it to keep the rpms below 4000, just pull it back a little. Running wide open at 3500 is way worse for the motor than wide open at 4500, guaranteed.

 

[jedimaster]

Re: 3.0 mercruiser prop

I have an 88 X-17 glastron with a 3.0 mercuirser 140hp, and mine will do around 45 with a 17" prop on a calm day lightly loaded at WOT.
I am going to try out a 19 and 21, Likley the 19 will be best.

What model of boat do you have? Might be the same as mine. What prop do you have on yours right now?