how to determinemax rpms

C

cpj

Ensign
Joined
Jun 14, 2005
Messages
958
Gentlemen,<br />I have seen quite a few posts on this site about max rpms relating to prop diameter/pitch.Everyone who is wanting to help wants to know what your max rpm is.My question would be, how do you determine what max is.On my boat with the motor trimmed all the way down(or in,if you will) it is about 4800-4900 or so.As I trim it up, I can get it to at least 5800.I say at least because I dont want any extra holes in the side of my motor so I dont trim any farther :D .I trim it till it reaches 5500 and stop because it doesnt go any faster after that and seems to slip.Maybe I have setup issue with the boat,but I was just curious on how max is figured. :confused:
 
180shabah

180shabah

Rear Admiral
Joined
Mar 26, 2005
Messages
4,995
Re: how to determinemax rpms

To find max RPM run boat wide open and trim the drive up without allowing the prop to blow out(run out of the water. A quick look at the tach will show your max RPM. NOTE: this should be done in water that is as calm as possible for an accurate result.<br /><br />This max RPM is a function of several factors(horsepower, weight, Prop pitch, prop diameter, etc.) and must be known before any meaningful suggestions can be made about prop selection.
 
QC

QC

Supreme Mariner
Joined
Mar 22, 2005
Messages
22,783
Re: how to determinemax rpms

Yes, 180shabah is on it and so are you. It appears you have found it with the 5500 RPM spot as that is where trimming up does not give you any more speed AND it starts to "blow out". It is possible with less weight, weight transfer etc. that you may be able to increase the RPM and speed, but it seems like you are pretty close. The next issue is to compare that to the engine manufacturer's recommended wide open throttle (RPM). Then you can determine if your are propped correctly.<br /><br />Take a look at the Engine FAQ section. There is a thread on prop selection and WOT. That may help clarify this for you.
 
C

cpj

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Jun 14, 2005
Messages
958
Re: how to determinemax rpms

Thank you both ! that helps me out
 
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