Mericruiser 3.0 prop question

[jmarines]

Sorry if this is redundant but I'm trying to get a little more top end speed from my 95 searay with a 3.0lx motor. I currently have a 19* prop that was on the boat when I purchased it last year. I currently run it at about 4500 rpms at around 35 mph give or take a few with a full load of gear and a few passagers. From my understanding I need to get a 14.25x21 prop for more top end speed .. is this correct? Just making sure about my decision before I drop $$$ for a new prop. Thanks in advance

 

[alldodge]

Current WOT rpm is at 4500. Going from a 19 pitch to a 21 will lower your WOT rpm to about 4100 rpm, so your going in the wrong direction. Start by doing an compression and leak down test to check health of the motor. You might be working against your self

 

[jmarines]

The motor is good.. compression test came back great. All maintenance is done. My wot is more than 4500 rpms due to prop size but I never take it above 4500. I thought going to a larger pitch prop would gain top speed but loose rpms and going to a smaller pitch prop wold loose top end but gain rpms.. maybe I'm misunderstanding this.

 

[alldodge]

Your WOT should be between 4400 to 4800 rpm, so if you can go over 4400 but not exceed 4800 your good. Increasing pitch just lowers your rpm but not speed. Even if on a rare occasion you were able to increase speed at lower rpm, this just puts more load on the motor. More load, less life

 

[jmarines]

So what would be a good prop to go if I wanted to increase top speed?

 

[Scott Danforth]

The motor is good.. compression test came back great. All maintenance is done. My wot is more than 4500 rpms due to prop size but I never take it above 4500. I thought going to a larger pitch prop would gain top speed but loose rpms and going to a smaller pitch prop wold loose top end but gain rpms.. maybe I'm misunderstanding this.

So what would be a good prop to go if I wanted to increase top speed?

first and foremost, what is the top speed of your boat at WOT with the handle smashed against the stops. need to know RPM and speed verified by GPS.

your boat can not go faster if you do not take it faster. for all you know by your own admission your boat motor may stop reving at 4501RPM or 1000000000 RPM since you dont know since you dont take it above 4500

so without this key piece of information, how can we help you?

take the boat out, go for a long run, take the average in two directions at WOT (which stands for Wide Open Throttle - as in the throttle is against the stops) documenting the motor RPM and the GPS speed. then come back and ask your question on the prop.

 

[jmarines]

Wot is 4900 rpms at 38mph (rpms may be off a bit due to the original gauges age) .. that's gps verified I never take it that high in rpms because I don't want anything to happen motor wise. This was verified last boating season . All maintenance has been done but I haven't been able to take it out on the lake this season due to flooding in our area. Most local lakes are closed to boaters that are motor operated. We are fixing to head up to lake Greer in Arkansas at the end of the month for vacation and I wanted to see about purchasing a new prop that would give me more top speed. I currently have a 19* 3 blade for everyday use... 4 blade prop for skiing. . Now I'm wanting to add a prop for top speed.

 

[Maclin]

Well, my PerfectWorld (tm) prop calc-er shows that if your engine can spin to 4800 with a 21p and keep same 13% slip then the speed would be 41 mph.

 

[Scott Danforth]

Well, my PerfectWorld (tm) prop calc-er shows that if your engine can spin to 4800 with a 21p and keep same 13% slip then the speed would be 41 mph.

and in my Perfect World, I eat candy and poop rainbows..... jumping 2" in pitch will drop the motor RPM down 400 RPM (close to the bottom).

I would look at a more agressive 19P, most likely a stainless prop. 4900 is right on the cusp of 4800 that adding a cooler of beer and a buddy will lower the RPM

 

[Maclin]

Yes, exactly. My observation for jmarines is definitely heavy on the if here... "...if your engine can spin to 4800 with a 21p..."

 

[jmarines]

What I'm gathering is I shouldn't even attempt a 21* pitch prop because I won't be able to turn it very high in rpms...

 

[Scott Danforth]

What I'm gathering is I shouldn't even attempt a 21* pitch prop because I won't be able to turn it very high in rpms...

Bingo. Just going from aluminum to stainless will drop your RPM and gain you some speed.

I ran a 19P stainless on my 3.0 and did 41mph at 4800 RPM. 21p brought the little motor to its knees. However I did have an OMC .

My suggestion is to test some props. Most good marinas have test props