140 prop

[biggjimm]

'76 mercruiser 140 in a 16 foot Rinker open bow tri-hull. I don't remember what rpm's it runs at right off hand but it has a 23 pitch prop on it. Standard alum. 3blade. Does this sound right for this boat? I've got a couple mercruiser props that are 21 pitch that were off of a older deep V rinker 140 that I used to have & a couple 17's (a regular 3 blade & a cupped 3 blade) from I don't know where. It always seemed to do fine but I didn't know if that 23 was right for it or not. Gonna take her to the lake maybe tomorrow so I'll see how she runs out.

 

[Bondo]

Ayuh,.... Ya need whichever prop that'll allow the motor to pull 4600/ 4800 rpms, at Wot,....

 

[steelespike]

23" seems like a lot of prop but by all means test it. Fairly small boat so a 23 may work.May be sluggish but if it runs within the wot rpm it may produce the most speed.

 

[biggjimm]

Ayuh,.... Ya need whichever prop that'll allow the motor to pull 4600/ 4800 rpms, at Wot,....

Bondo, hopefully we're going to the lake tomorrow to check her out & see where she's running.

Steelspike, that's what I was thinking but you never know. I've never really thought it had a problem as far as the prop is concerned although my old 19' deep V rinker with what was supposed to be a mercruiser 120 would smash this boat out of the hole & by the time he got planed out I'd have him by about 4 or 5 boat lengths but he'd eventually overtake me. I was running a Michigan wheel 21p 3 blade cupped prop that is one of the props I still have. We're gonna try to splash her tomorrow & after I get the carb adjusted I'm gonna do some prop testing & see what we come up with. Kind of excited to get this ol' boat wet again. She's been setting for a couple seasons but I'm getting my "priorities in order" & have her all ready to go to the lake! Thanks guys. Jim.

 

[airshot]

Be sure to watch that tach closely.......if you over rev you may be replacing more than a prop!!

 

[biggjimm]

Well took her to the lake & it ran like poop. Could only get about 3700 rpm's out of her & that was only when I found good water. In the chop it was more like 3300. It just didn't seem to have any power. Then when it got good & warmed up it didn't want to start. So I come back home & check the timing & it was way off. Which didn't make sense cause I put a new distributor in it about 3yrs ago & had set it then. Long story short, my father in law informed me that he & brother in law took it out shortly after I changed distributor & bro in law decided it should have "more power" so he set the timing by ear on the water because when I did it with the timing light I must have done it wrong! Ugghh! If he'd a been here I'd have throat punched him. So now I've got the timing set properly & I'm going back to the lake today.

 

[biggjimm]

Got her on the water again today after setting the timing & I could get right at 3800 rpm's out of her. Took the 23 pitch prop off & tried the 21 & that took her to about 4200. Forgot my GPS but it seemed to lose a little top end speed. I think the other prop is a 17 pitch cupped prop but ran out of time today. I'll slip it on next time out just to see what it does & figure where to go from there. Water sure was slick today. Real nice water to boat on. I'll get her back out in a couple of days & see where were at. Thanks guys. Jim.

 

[fishrdan]

I could get right at 3800 rpm's out of her. Took the 23 pitch prop off & tried the 21 & that took her to about 4200. I think the other prop is a 17 pitch cupped prop but ran out of time today.

Looks like you're getting the text-book 200RPM/inch of pitch change. Along those lines, a 19P prop should bring you to the 4600RPM upper end of the WOT RPM range, while the 17P prop you have will be over-shooting your WOT RPM at 5000+rpm (bad).

If looking for top-end speed try a 19P, if looking for hole-shot (water sports) try a 18P 4-blade prop. I'm running a 18P Alpha4 prop on my 140HP Mercruiser and LOVE IT, running 34-35mph at 4500-4600RPM, awesome hole-shot on my heavy 18' fishing rig.

 

[biggjimm]

Yeah I figured the 17p would run it too high. That being said, I thought I read somewhere that a cupped prop would essentially run the same as the next size larger standard prop? As in my 17 cupped prop would perform like a standard 18p?? Perhaps I've got my information mixed up though?? I'll recheck my sources tomorrow.

 

[steelespike]

Yeah I figured the 17p would run it too high. That being said, I thought I read somewhere that a cupped prop would essentially run the same as the next size larger standard prop? As in my 17 cupped prop would perform like a standard 18p?? Perhaps I've got my information mixed up though?? I'll recheck my sources tomorrow.

It is suggested that a cupped prop will perform like 1" higher pitch

 

[Rick Stephens]

I run a current 17 inch which gives me a leetle over 4700, and a 4 blade 15 inch when pulling skiers. I can over rev the 15 and it doesn't come near the top speed of the 17, which is around 34 mph.

 

[biggjimm]

I don't know how weight figures in to the formula but the previous owner had written in the owners manual that it weighs 1700 lbs. But I've got some other issues to work out now. I got some bad gas & now it's popping thru the intake & exhaust a little so I'm not sure if it's got some carbon & crud built up on the valves or what but as soon as I get that resolved I'll try the 17 & see what it does. Thanks guys.