another question about RPMs

Cool It

Petty Officer 1st Class
Joined
Feb 13, 2004
Messages
284
Hello <br /><br />I was out today and at 3200 rpm my speed was about 21 mph -- to get to about 25-27 I need to push the rpms to 5000-5500 is it ok to ride at that high rpms----and shouldnt I have better speed at the higher Rpm--1988 5.0 mercruiser<br /><br />Thank you<br />Cool IT
 

achris

More fish than mountain goat
Joined
May 19, 2004
Messages
27,468
Re: another question about RPMs

Were to revs this high when you first got the boat? What size prop are you running? If you let me have that in the next few hours I'd do the calculations and let you know what I come up with. (I'm off to China for a few weeks, so won't be able to answer anything after Monday). The max revs on these motors is 4200-4600. <br /><br />Chris.........
 

cobra 3.0

Lieutenant Commander
Joined
Jul 31, 2003
Messages
1,797
Re: another question about RPMs

What kind of boat and what's your boat length? I too think its reving too high and the pitch needs to be higher.
 

Cool It

Petty Officer 1st Class
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Feb 13, 2004
Messages
284
Re: another question about RPMs

Thanks Guys<br /><br />The boat is a 22 foot W/A The problem is I dont Know what pitch I have now-- also can I change prop in water by tiliting up because I do not have a trailer -- or am I going to have to have it hauled out??
 

Spidybot

Lieutenant Commander
Joined
Apr 4, 2002
Messages
1,734
Re: another question about RPMs

The current prop specs can be derived from the part # stamped into the prop.<br /><br />It is possible to remove/refit a prop while on the water. Take the boat to a nice calm beach and stand behind the thing. Take care not to drop nut or washer.<br /><br />Rule of thumb says one inch in pitch changes rpm by 200. Higher pitch = lower rpm and vice versa.<br /><br />Sounds like a 4" increase would be right, provided all else is OK. A good prop shop will allow you to test a couple of props so you'll have to buy only one to get the right thing.
 

Cool It

Petty Officer 1st Class
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Feb 13, 2004
Messages
284
Re: another question about RPMs

I was out today and I relize that even when I tilt out drive all the way up my prop still does not come completly out of water-- am I still ok pulling it off like that - and will I still be able to lube the propeller shaft Before I put it back on, being that it will be in the water ????<br /><br />Also one other question --the book tells me that I need to tighten the nujt a minimum of 55 lb torgue --well I am no mechanic do I need a torque wrench or can I just snug it nice and tight with a cresent or socket???<br />Thanks<br />Cool It!!!!
 

achris

More fish than mountain goat
Joined
May 19, 2004
Messages
27,468
Re: another question about RPMs

Relubing the shaft in the water? yes, smear the grease inside the prop, then slide it back on.<br /><br />
or can I just snug it nice and tight with a cresent or socket???
I'd prefer the socket. ;) <br /><br />Chris...........
 

Cool It

Petty Officer 1st Class
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Feb 13, 2004
Messages
284
Re: another question about RPMs

Any other help before I try to replace the prop in the water???<br /><br />Cool It
 

Dunaruna

Admiral
Joined
May 2, 2003
Messages
6,027
Re: another question about RPMs

Coolit, there is no special way to change a prop in the water. You and your tools are going to get wet - period.<br /><br />As uu said, shallow water so you don't lose stuff, and greasing is o/k underwater - remember, that is where the prop lives.<br /><br />Aldo
 

Cool It

Petty Officer 1st Class
Joined
Feb 13, 2004
Messages
284
Re: another question about RPMs

OK - hopefully this will be my last question- But I have never done this before and I am not to handy -but I would like to try to do some things on my own rather than calling a mechanic every time. The question now is about the hub-- is the hub inside the prop when I take it off or is it on the shaft and do I need to replace with new one for the new prop or will the same hub fit new prop. hope this does not sound like I am an Idiot!!! and if there is anything else I should know about this whole procedure I would apreciate the info.<br /><br />Thanks Very much<br />Your non mechanic<br />Cool It!!
 

Doug Durako

Chief Petty Officer
Joined
Nov 17, 2002
Messages
519
Re: another question about RPMs

Cool---do you have a basic Merc manual? It will show you a detail of all the prop parts and how they go on the shaft. This is not hard to do, but first time changing in the water might not be a great idea. You need the correct washer in front of the prop on the shaft to keep your prop hub from eating into the shaft or welding itself on there.<br /><br />Do you have a friend who trailers their boat and has the same drive leg you do? Try it on dry land first to get the hang of it.
 

Cool It

Petty Officer 1st Class
Joined
Feb 13, 2004
Messages
284
Re: another question about RPMs

Thanks to all <br /><br />I did change the prop in the water and it was actually pretty easy- especially after all the question. I went up from a 17 pitch to a 19 pitch It help somewhat but I think I still need to go up - as my RPMs are still to high. Although the prop dealer said that 21 pitch is to high for a 22 foot W/A Cuddy ??? <br /><br />Any opinions on that???
 

Spidybot

Lieutenant Commander
Joined
Apr 4, 2002
Messages
1,734
Re: another question about RPMs

Prop dealers advice should be considered. Your own experience from testing beats it, however.<br /><br />If he says "no 22" and your tests proves otherwise, the only truth is that he was wrong.<br /><br />If you like the hole shot you have, do not go higher. Keep within wot range by keeping an eye on the tach and reduce throttle accordingly.
 
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