Prop RPM question:

[LOW MN]

If the Mercruiser 165 HP is at 4,000 RPM, what would the Prop RPM be?

 

[1980Coronado]

Re: Prop RPM question:

You'll need to find out what the out-drive gear ratio is. Divide the engine rpm by the gear ratio and you'll have the prop rpm.

 

[Texasmark]

Re: Prop RPM question:

Following along with the thread, curious as to why you want to know the prop rpms? Just a question.

Mark

 

[Dhadley]

Re: Prop RPM question:

He's probably wants to calculate slip %.

 

[1980Coronado]

Re: Prop RPM question:

http://www.mercstuff.com/drivegearratio.htm
http://forums.iboats.com/attachment.php?attachmentid=67861&d=1284734471

From these two links it looks like you probably have a 1.65:1 ratio which means at 4000 rpm the prop is spinning at 2,424 rpm

 

[Robbabob]

Re: Prop RPM question:

Knowing the gear ratio, engine operating RPM at WOT, actual speed, boat weight, (and I think engine HP), properly input into the right prop calculator will give you slip %. I haven't heard of a tach to measure actual prop rpm while at speed, so I am curious about the benefit of knowing.

 

[1980Coronado]

Re: Prop RPM question:

Knowing the gear ratio, engine operating RPM at WOT, actual speed, boat weight, (and I think engine HP), properly input into the right prop calculator will give you slip %. I haven't heard of a tach to measure actual prop rpm while at speed, so I am curious about the benefit of knowing.

You need to know the gear ratio to input into the slip calculator. Engine RPM divided by gear ratio x prop pitch (inches) x 60 min/hr divided by 12"/ft divided by 5280 ft/mile will give you theoretical miles per hour. Compare those numbers to the actual measured speed and you can figure slip.