prop pitch vs rpm

[sjenkins]

i have a 22 ft CC Tahoe with a 200 Merc Optimax. It came with a 21 pitch 3 blade prop. WOT rating is 5750 rpm. 52 mph at 5800 rpm with WAY more throttle left. Got a 23 to bring down RPM, but maintain speed, if not increase it. A slight decrease in RPM, but still alot of throttle left (can still push it to 6200). Not much more speed though. Will a 25 do any better, as far as not over revving, but getting max speed? i want to travel, but not blow it up. Thanks in advance for your help

 

[PROPer]

Re: prop pitch vs rpm

Welcome!
I am a bit surprised that you are getting that many RPM out of your set-up, but it is not a terriable problem to have. The 25 pitch should bring the RPM down, but I would be a little concerned that your hole-shot will be really poor. How is the hole-shot now? As far as your top end speed, I would have to imagine you would get a little more but you must be pretty close to the max hull speed. Do you know of anyone getting more speed than you with that hull?

 

[QC]

Re: prop pitch vs rpm

Actually I think there is something wrong with the numbers . . . tach and/or speed. I am getting 21% slip at 5800 / 52 MPH with the 21, and the 23 would be worse. How are you getting your speed? GPS? The Tahoe website shows a 215 CC and with a 200 Opti @ 52 - 55 MPH. BoatTest.com has this test on their deckboat with your power setup: http://www.boattest.com/boats/boat_video.aspx?ID=818#Test-Result

This tells me that a 19 with your setup got it to 49 MPH, so 52 with lots left doesn't make sense to me. All numbers must be dead on to work with properly. Also, I have a hard time believing that a 23 would give you acceptable hole shot if all else is close to the boat tested. Maybe the CC is lighter, but I would not guess by much.

Oh, and the recommended WOT range is actually 5000 - 5750 so that 5750 is the upper limit. Not necessarily the target RPM. This is another reason I don't believe the numbers are right. I don't see how you could be merrily passing 5750 with a 23, when a test setup was maxed at 5500 and 49 MPH with a 19. Yes, I am picking something at random as close as I can find to your setup, and it may not be close, so help us with that.

you must be pretty close to the max hull speed

I haven't really heard of "hull speed" when referring to planing hulls. I understand there are some walls, but elaborate a little on this for us. I would think most V-hulls in this category could easily hit 60+ with mega power. I am NOT suggesting the OP's setup could get there, but I would think the hull could.

 

[sjenkins]

Re: prop pitch vs rpm

I also looked at the data on my setup before buying the boat. I was VERY surprised at the actual differences. I have an acceptable hole shot with the 23. I measured mph with a gps, but i haven't verified the tach. I don't have any real precise data, sorry. Just don't want to waste money on a third prop if it won't help. How do i check accuracy of the tach?

 

[QC]

Re: prop pitch vs rpm

It may be right as is, but you have to use a handheld tach of some sort. Two cycles mess me up in this regard, so best to ask that in the OB section vvvvvvv

What precise model is your boat? Year? Still hard for me to believe it will pull the 23 that well. There is a bass boat with a 24 on the Mercury site, but it runs 67 MPH at 5700 RPM. Same gear ratio, same motor, that's a long way from 52 with just one inch. Maybe this prop is ventilating? Triming in woudl make it faster then, but then hull balance woudl be the next thing to work on i.e. more weight asft so you could get the bow up without blowing out the prop . . .

 

[sjenkins]

Re: prop pitch vs rpm

i have a 2008 tahoe 215 CC. I'm starting to think that the heavy hull may have reached its top speed no matter what prop i use. not really that much of an issue. I will have my tach checked though, just to ease my mind. Just dont want to blow it up.