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RPM VS Knots

DIESEL engine, transmission and generator repair and maintenance discussion forum.
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RPM VS Knots

Postby bobfino » October 10th, 2014, 1:49 pm

Hello, I have new to me the 2005 Carver 396MY with the D6 Diesel 370 inboards. Trying to get a feel with other owners on what they are seeing for optimum RPMs and Knots or MPH.
Thanks for the help.

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Re: RPM VS Knots

Postby Lyndon670 » October 10th, 2014, 2:09 pm

With the Volvos, they are designed to run best at 90% of WOT. So if your WOT is 2300rom, dial back 230 RPM and that will be your best cruise speed. As for the knots - there are so many factors that it's hard to calculate. Climate, load, sea conditions etc.

At the end of the day, it's your choice.... Although my 506 will run fast, I enjoy the rip almost as much as the destination.
Last edited by Lyndon670 on October 21st, 2014, 7:49 pm, edited 1 time in total.
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Re: RPM VS Knots

Postby bobfino » October 13th, 2014, 11:13 am

Thank you Lyndon for the information, I appreciate it.
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Re: RPM VS Knots

Postby Hugo » October 21st, 2014, 11:34 am

Lyndon670 wrote:With the Volvos, they are designed to run best at 90% of WOT. So if your WOT is 2300rom, dial back 230 RPM and that will be your cruise speed. As for the knots - there are so many factors that it's hard to calculate. Climate, load, sea conditions etc.

At the end of the day, it's your choice.... Although my 506 will run fast, I enjoy the rip almost as much as the destination.


Where did you hear this?

I don't see why a Volvo diesel engine is different from a CAT or a Cummins or any other diesel engine. In these boats they're all high performance high HP diesel engines. Why would you want to run your engine harder and burn more fuel knowing that the average high HP high performance diesel engine has a life of about 30,000 gallons of fuel burned? The higher the rpm, the higher the boost, the higher the fuel burn. Doesn't make any sense to me unless you're in a hurry and don't care about engine life...

Best for the engines is hull speed but if you're going to run on plane find the lowest rpm at which the boat runs the best and gets the best fuel economy. On a good portion of these carvers I think you'll be at 400rpm off the top at a speed between 16 and 18 knots. If the engine is mechanical with EGT and Boost gauges you can monitor the load on the engines. Some of the electronic ones will show you the load.

More important than anything is make sure you're propped to at least 50rpm over WOT volvo specs checked with a photo tack unless your tacks are AETNA digital.
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Re: RPM VS Knots

Postby Lyndon670 » October 21st, 2014, 7:10 pm

"Where did you hear this"

1) My Volvo 7.4TAMDEDC users manual, page 71.

2) The Eastern US/Canada Volvo master tech who did the engine survey on my engines.

3) The Volvo section on Boatdiesel.com

I could go on, but that would be silly to continue this discussion on where I heard that.

"I don't see why a Volvo Diesel engine is different from a CAT or a Cummins or any other Diesel engine"

First off, what makes it different is cam profile, engine computer programming, fuel delivery system, waste gate sensor/timing, boost sensor setting, cylinder displacement, which all amounts to torque curve. At what RPM the engine is most efficient developing the most torque using the least amount of fuel.

Second off, I won't can't comment and won't comment on a CAT diesel or Cummins diesel because I don't own one, don't have any experience with them. Because I am not informed on them, I am not qualified to offer advice. Being informed on Volvos, I am able to offer the poster the information that I have gained through the aforementioned research.

These engines are designed to run hard and actually need to be run hard to stay clean - which ensures longevity. Can they run at hull speed? Of course they can - but they will load up and need (as per page 73 of the owners manual) to be run up to speed to stay clean.

I'm sorry, I must have missed the model/engine on your signature at the end of your post....
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Re: RPM VS Knots

Postby Hugo » October 21st, 2014, 10:40 pm

Well, I'm sorry to say but your mis-informed. Especially if you're reading on boat diesel. Any of the top guys there will tell you that any diesel engine will burn a certain amount of fuel in it's life at which point the oil burn will become so high it's not usable. Like I said about 30,000 gallons for these sized engines. Knowing that the higher the rpm the higher the fuel burn (common sense) how can you believe engine longevity is in running at 90% of WOT or 200 rpm off the top? You will go through 30,0000 gallons of fuel much quicker at 90% of WOT than you would at lower rpms. My Volvo manual says that the engine can be run at 200 rpm off the top also. Volvo is in the business of selling engines and the techs are in the business of repairing them.

Also consider any failure where you loose cooling(hose blowing off, belt, impeller, etc) at those rpms and you are guaranteed to toast the engine. The more horsepower you ask of the engine the more heat is being created and more chances of risk. Plenty of examples of that on boat diesel.

I'm not saying it's wrong to run it that hard, just don't expect to get 10,000 hours or what most people think they will get out of a diesel engine. The guys getting those hours are running at hull speed or if on plane not asking the engine for every bit of horsepower it can make.

There is a reason a race car engine is rebuilt after every race. It's because it's run hard and any mechanical piece of equipment like an engine run hard will wear sooner. My Volvos are at almost 30 psi of boost running 400rpm of the top, that's race car boost! I've talked to Tony about this and his guess is to expect at best 2,000 hours before an overhaul is needed making that much horsepower. Our Carver boats tend to not have enough engine and that means we are asking the engine for everything it's got to keep the boat on plane at 16-18 knots. Now, 2000 hours for the average boater equates to a lifetime of boating provided you keep the boat that long...

It really all comes down to duty cycle. Read this article, it explains it well.

http://boatdiesel.com/Articles/index.cf ... s&Search=1

Now if you still don't buy what I'm saying post on Boat diesel and ask Tony yourself, the answer will amaze you :)
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Re: RPM VS Knots

Postby Lyndon670 » October 21st, 2014, 11:09 pm

Thanks for clearing that up. I have just drafted an email to Volvo Canada stating that they need to correct the owners manual, as per Hugo. Also, I will have the Eastern U.S. and Canada Volvo tech reach out to Hugo for his technical advice.

Sadly that also means that all of the owners on the Silverton owners site that I am still a member on are also under the incorrect information regarding their Volvo engines.

Come to think of it, all of the yachts in my Marina are also probably acting on the wrong information from their owners manuals too - this could be an epidemic!
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Re: RPM VS Knots

Postby Hugo » October 21st, 2014, 11:44 pm

Funny...

Did you read the article?

You don't need to update Volvo with any information, you just need to go read your owners manual again. I'm not sure where you got that you should run at 90% of WOT. See attached snippet of the Volvo Owners manual for your engine - pg 25. It clearly says to cruise at a speed of at least 200 rpm below WOT. That's not saying to go cruise at 200 rpm of WOT but recommending that you don't cruise higher than that. I would interpret that to mean if you want your engine to last cruise at lower rpms...

If you take the blinders off and go read what I wrote maybe it'll sink in, or like I said go ask the experts on boat diesel your self but please don't spread bad advice to others if you don't have your facts right.
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Re: RPM VS Knots

Postby Lyndon670 » October 22nd, 2014, 7:56 am

Clearly, you need to take your "blinders" off....

The original posted asked the following

"Trying to get a feel with other owners on what they are seeing for optimum RPMs and Knots or MPH."

As per me, my Certified Volvo Tech, my original Volvo manual....the answer to his question regading the optimum crusing speed is WOT - 10%.

His question was not - "what speed should I run my engines at so they never ever wear out". If that was his question, my response would have been "idle, or shut them off and don't leave the dock".

But it wasn't.

The Best Optimum Speed is that point in the power curve where the torque curve, hp curve and fuel economy cross.

As per the Websters Dictionary, the word "optimum" is defined as the following:

"The point at which the condition, degree, or amount of something is the most favorable"

And that, Hugo, in a Volvo high performance diesel engine is at 10% of WOT - which is direct from the printed Volvo literature.

And as I read the image you posted, it backs up the information that I have infront of me. The 10% of WOT is virtually bang on information you have provided - and as I originally quoted - "2300 rpm, dial back 230 rpm".

Your information states "We recommend a cruising speed of at least 200rpm less than wide open throttle". I believe that 10% of WOT will always give you exactly that.

So, I am not altogether understanding your argument. Now you have not only provided me the information that backs my information up - but close your point suggesting I re-read it?

Ok, I re-read the information that you provided. Volvo suggests a cruising speed of at least 10% of WOT, or at least 200rpms below WOT.

ie - 2300rpms x 10% = 230rpms.

230rpms IS AT LEAST 200RPMS

Have you ever heard of the word "semantics"?

It is defined as "the language used (as in advertising or political propaganda) to achieve a desired effect on an audience especially through the use of words with novel or dual meanings".

As it applies to this post - you have taken it upon yourself to take the information that I have posted and changed the verbage it to say the same thing. And then made an arguing point of it.
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Re: RPM VS Knots

Postby Hugo » October 22nd, 2014, 9:17 am

This is the last post, I'm not here to argue with anyone over something that is common sense and every one of these forums has an "expert" like yourself that probably has never even seen the inside of a diesel engine. You don't back up any of your facts, these are just things you have heard from your also mis-informed boating buddies and like I said you are very mis-informed.

1 - Where is the 90% WOT recommendation from Volvo literature?
2 - Where did you read on boat diesel that you should run at 90% WOT?
3 - Please show me a power curve for any diesel engine where the engine makes optimum torque and HP at 90% of WOT? The torque curve is on it's way down by then, you are way past the torque curve and simply burning a bunch of fuel to fill your need for speed.
4 - Show me a fuel burn for any diesel engine where at 200rpm of the top your at "optimum" fuel burn?

You obviously have no mechanical understanding of the internals of a engine and must just be writing checks to the "Volvo master techs".

"At least 200 rpm lower than the maximum rpm at full speed" does not mean run it there, they're telling you if you run it over that don't expect much life out of the engine.

Anyone on a boating site asking for "optimum cruise" is interested in fuel burn, longevity, etc...

This is very simple to prove, measure your fuel burn and you will see that the closer you get to WOT the higher your fuel burn increases.

To wrap this up once again showing your lack of understanding is a link to fuel burn for your 480HP Volvos. You will see this works for any diesel engine out there. Look what happens to MPG, range and fuel burn as you go past 400rpm off the TOP or 2250 rpm on the graph. At 200rpm off the top you're range and fuel burn are not what I would call "Optimum" per your definition. If you had or knew how to read a boost gauge or EGT you would see that your exhaust temperature is going up, boost is going up along with fuel burn which are all indications of not being at an "optimum" speed.

http://www.boattest.com/Partners/Partne ... s%20yachts

Educate yourself properly and backup your facts before you continue to spread bad information to others.

How many hours do you think you will get out of your Volvos running at 200 rpm off the top? If you think many thousands you will be in for a big surprise... You might not make it to 1000 if you have a simple failure under those loads. Lots of Carvers and Silvertons out there with engines replaced at around 500 hours, owners all as educated as yourself. Combine your 200 rpm with the fact that your boat is probably also over propped and you'll be lucky to see 5 or 6 hundred hours...
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