RPM question

[Bayliner-bill]

I'm trying to pick a new prop for my boat and I want to upgrade to get a bit more top end speed out of it. I have been asked time and again what RPM I am getting at full throttle and I haven't a clue. My boat's in storage and in the spring I replaced my tach. I recall the RPMs not being anywhere near what it was with the old tach even though I set it per the instructions when installing it. If I am remembering correctly at full throttle I am about 1200 RPM but it could be lower. I'm trying to find out what this boat engine should be at when at full throttle. Any help would be great!
Thanks,
Bill

 

[Don S]

Re: RPM question

We are going to need to know what engine you have to answer that. Year, make, drive, etc.

1200 rpm is basically a high idle rpm, not max WOT rpm. Max WOT rpm can run from 4000 to 5200 rpm depending on the engine.

 

[Bayliner-bill]

Re: RPM question

We are going to need to know what engine you have to answer that. Year, make, drive, etc.

1200 rpm is basically a high idle rpm, not max WOT rpm. Max WOT rpm can run from 4000 to 5200 rpm depending on the engine.

It's the inline 4 cylinder. 1995 Bayliner Capri 1950, 3.0 Liter Mercruiser, Alpha one

 

[Don S]

Re: RPM question

4200 - 4600 for the 3.0L
4400 - 4800 for the 3.0LX

 

[Bayliner-bill]

Re: RPM question

4200 - 4600 for the 3.0L
4400 - 4800 for the 3.0LX

It sounds like I will be messing with my tech this spring!!
Thank you Don!

 

[Don S]

Re: RPM question

Have a look at the back of your tach, there should be a dial for the number of cylinders you have. Sometimes turning them back and forth a couple times will get the tach back to working. Make sure it's on the right number of cylinders also.

 

[Silvertip]

Re: RPM question

The reason the WOT RPM with what you consider an average load is important is because it is that number when compared with the engine/drive manufacturers recommendation that determines whether or not you have the right prop. You may already have the right one. If the recommended RPM is 4800 and your tach shows 4800 (give or take a bit) you have the correct prop. Changing it to a higher pitch to achieve more speed may actually slow you down because the engine cannot spin that much pitch. You may then think why don't I drop down in pitch so the engine speeds up. Well -- that will happen but the engine speeds up because the prop is taking less bite on the water so the boat moves forward less with each revolution and eureka -- you slow down. Not only that you overrev the engine. So to do this properly and avoid spending money needlessly, a good tune up nd getting rid of excess weight in the boat will do more for performance than a prop change in an attempt to squeeze a couple more MPH out of the boat. With that done, THEN your WOT tach reading with an average load can be used to see where you are at.