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Question regarding cruising rpm's

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Topic author United States of America
denpooch
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Question regarding cruising rpm's

Postby denpooch » June 19th, 2019, 6:02 pm

Hi folks,
I just got back from a trip and ran my 6.0 Crusader's around 3200 rpm to 3400 rpm's. The Mariner weights about 9 tons.
I mentioned this to a friend and he said that I was nuts running over 3k. He said that car engines never run that high and the I will probably shorten the life of the motors.
On my last boat which was a 34 ft express cruiser weighing in at about 13,000 lbs, I never ran the 454's over 3,000 rpm's. I sold her with over 2,000 hours and never had to rebuild the motors.
Am I pushing it at 3200 to 3400? Thanks for your thoughts.


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Re: Question regarding cruising rpm's

Postby Viper » June 19th, 2019, 7:53 pm

You can't compare automotive to marine use to begin with. Even at slow speeds, marine engines work way harder than automotive engines do. I wouldn't worry about it though. What will hurt them more is lack of regular maintenance. Use a good quality oil at recommended intervals and they should be fine. Check your manual for the engines' duty cycle. Typically they're okay to operate at 85% of WOT. I would also add that it's a good idea on long trips to vary your rpm periodically. Find your hull's cruising speed sweet spot as that will be easier on the engines.

How many hours are on them currently?

Topic author United States of America
denpooch
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Re: Question regarding cruising rpm's

Postby denpooch » June 19th, 2019, 8:21 pm

Thanks Viper. I was hoping to hear from you. I am sure many agree that you are a great resource of knowledge on this site.
I purchased in June 2017 with 237 hrs. Now I am at about 560 hrs. I do try to keep on top of maintenance.

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Re: Question regarding cruising rpm's

Postby Viper » June 19th, 2019, 8:44 pm

That's pretty low hours for their age which is usually a red flag but it sounds like they're running strong. Run them, make them work (within reason), that will keep them healthier than just sitting.

BTW, if you're concerned about longevity running them at that rpm all the time, there is no "try" to keep on top of maintenance, it's a must!
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Re: Question regarding cruising rpm's

Postby tomschauer » June 20th, 2019, 12:10 am

My 454's cruise at 3200-3400. Less than 2800 I fall off plane and work them harder than at the higher rpm.Todays car engines do Cruise at lower rpms in the 2nd or 3rd overdrive gear,but in the 70's and 80's before overdrive, they would cruise in the 3000 range.

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Re: Question regarding cruising rpm's

Postby Viper » June 20th, 2019, 6:42 am

Doing 3000 rpm on wheels on a level road is one thing, there's nowhere near the work required compared to going uphill non-stop as soon as you put it in gear, which is the condition a marine engine runs under all the time.
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Re: Question regarding cruising rpm's

Postby bud37 » June 20th, 2019, 9:13 am

denpooch wrote:Source of the post I mentioned this to a friend and he said that I was nuts running over 3k. He said that car engines never run that high and the I will probably shorten the life of the motors.

There is actually some truth to this in your case......certainly not the comparison to the car engine use though as has been pointed out, unless the car is towing a heavy trailer up a long grade for its whole life.

It is more about the small block vs big block comparison you pointed out.......your old boat weighed less and you had big blocks that have considerable more mass and can take more hp with less flexing and loss of useable life.

You now have a much heavier boat with small blocks putting out more hp at WOT than the 454's you had.That says they will not last as long. More hp used equals more fuel.You will within reason use the same hp to move your boat at 20knots no matter what engines you have, the big block will just handle it for a longer time ( hours )in my opinion.

If you want a true measure of how you are using the engines up, get some fuel flow meters ( which may already be there with your captains choice ), that will give you a good gage on how much hp you are using at any given throttle setting.
Two eco spots are below hull speed ( 5knots or so ) and on plane and trimmed ( the sweet spot Viper mentioned )

One thing you do get with these stroked small blocks is great fuel mileage at hull speed, mine is wonderful.
"There is nothing - absolutely nothing - half so much worth doing as simply messing about in boats.”
― Kenneth Grahame, The Wind in the Willows

Safe travels with light winds and calm seas.....

Topic author United States of America
denpooch
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Re: Question regarding cruising rpm's

Postby denpooch » June 20th, 2019, 10:42 pm

Bud I agree the fuel burn rate at hull speed is great. Doesn't burn much more than a trawler. What engines do you have in your Mariner?
When you are cruising on plane where do you run the rpm's?
Thanks.

Topic author United States of America
denpooch
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Re: Question regarding cruising rpm's

Postby denpooch » June 21st, 2019, 10:53 am

Thanks for your feedback. I took a GOOD look at the operating manual for the 2005 Crusader 6.0 engines and they list two important pieces of data:
Preferred WOT RPM is 5000.
Cruising RPM (Max) is 4000.
Seems like the 3400 range is well within safe operating standards for this engine. I do believe that brings me around Vipers sweet spot although I will experiment around a bit.
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Re: Question regarding cruising rpm's

Postby RGrew176 » June 22nd, 2019, 3:53 am

My 81 3007 has twin 270's 5.7's. My max RPM is 4000. I generally like to keep my cruising RPM's no more than 75% of WOT. Of course most of my previous boats were I/O's and inboards are a different animal. For my 3007 I generally cruise in the 3000 to 3200 RPM range.

My cruising speed is 15 to 16 MPH and my WOT speed is 25 MPH.
Rick Grew

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