Cruisers Forum

Join CruisersForum Today

Thread Tools Rate Thread Display Modes
Old 30-08-2012, 15:14   #1
Registered User

Join Date: Aug 2012
Posts: 4

On a trawler cruising speed, 80 hp 4 cylinder engine, how many rpm's should it be turning? Anyone? Thanks

boatlover is offline   Reply With Quote
Old 30-08-2012, 15:26   #2
Senior Cruiser
S/V Antares's Avatar

Cruisers Forum Supporter

Join Date: Sep 2007
Location: Annapolis, Bahamas
Boat: 1983 Gulfstar 36
Posts: 1,240
Images: 1
Re: RPM's

What engine?

Will & Muffin
Lucy the dog

"Yes, well.. perhaps some more wine" (Julia Child)
S/V Antares is offline   Reply With Quote
Old 30-08-2012, 15:33   #3
Moderator Emeritus
David M's Avatar

Cruisers Forum Supporter

Join Date: Sep 2007
Location: San Francisco Bay
Boat: research vessel
Posts: 9,844
Re: RPM's

There no such thing as a definitive cruising speed. Cruising speed is where you feel comfortable with the speed you are getting for the fuel consumption. It is somewhere between dead slow ahead and full ahead.

What is your engines maximum governed speed? Cruising speed is less than that. If you want numbers then perhaps 70% to 85% of max RPM.

Life begins where land ends.
David M is offline   Reply With Quote
Old 30-08-2012, 15:36   #4
Registered User

Join Date: Jan 2009
Location: Juneau, Alaska
Boat: Prout Snowgoose 37
Posts: 58
Re: RPM's

The answer is it depends on the engine and the reduction gear ratio. Each engine is different, from old large tugboat engines that redline at 300rpm, and fast gas engines that are meant to run up to 6500 rpm.

Diesel engines in boats are installed, and then tested, to see if they over-rev or under-rev the engine with the hull and propeller. It is important to the longevity of the engine to have it max out at the proper rpm's.

The other factor is how much fuel do you want to burn. While the engine may be capable of turning over at 3500 rpm maximum, with the boat at 9 knots; quite often running 2700 rpm will push the boat at 8 knots using half the fuel. The key is to experiment. Usually around 70-75% power is the sweet spot, but it depends on the boat.

If you give us the engine make/model and the reduction gear ratio, we should be able to give you some idea.
Excalibur5 is offline   Reply With Quote
Old 30-08-2012, 16:41   #5
Senior Cruiser

Cruisers Forum Supporter

Join Date: Aug 2009
Location: between the devil and the deep blue sea
Boat: a sailing boat
Posts: 15,105
Re: RPM's

It should be turning at rpm close to the ones required to deliver the power required to sail at the selected cruise speed. This power can be interpolated from Skene's.

It is fine when you have an engine that delivers this at .5 to .75 of max revs.

barnakiel is online now   Reply With Quote

Thread Tools
Display Modes Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off

Our Communities

Our communities encompass many different hobbies and interests, but each one is built on friendly, intelligent membership.

» More about our Communities

Automotive Communities

Our Automotive communities encompass many different makes and models. From U.S. domestics to European Saloons.

» More about our Automotive Communities

Marine Communities

Our Marine websites focus on Cruising and Sailing Vessels, including forums and the largest cruising Wiki project on the web today.

» More about our Marine Communities

Copyright 2002-2015 Social Knowledge, LLC All Rights Reserved.

All times are GMT -7. The time now is 06:41.

Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2016, vBulletin Solutions, Inc.
Social Knowledge Networks
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2016, vBulletin Solutions, Inc.

ShowCase vBulletin Plugins by Drive Thru Online, Inc.