'Cruising RPM' - What Does this Mean ?

[moggie]

WOW!!
You guys got your numbers right?
My MD2030 (30HP) used 1/3 GPH. @ 2500 RPM
My new D1-30 @ 2200 RPM burns 1/3 GPH.
How are you guys using so much fuel?
Please don't tell me.

[estarzinger]

There is a theory that 'cruising speed' in knots should be roughly = square root of waterline (in ft). The concept is that above this speed a displacement hull mostly starts digging a deeper hole in the water and hull drag increases dramatically above this speed.

On our boats we pick an RPM that is high enough that the engine is running at operating temperature, while low enough to keep noise and vibration to a 'reasonable' level. For our Yanmars this is around 2200-2400 rpm, while also happens to move us at about the knots mentioned above (sqrt of waterline).

If you are looking to absolutely maximize fuel economy (either per mile or per hour) it has been my experience that in flat water dead slow is always the fuel maximizing rpm. Hull drag increases with speed, increasing slower at low speeds, but rapidly with faster speeds.

[Tia Bu]

Here's another way of thinking about it. Attached is the product brochure for my engine, a Volvo MD7A, downloaded from Volvo's website. If you look on the right-hand page, there are three graphs in the data section.

Maximum RPM is 2600. Maximum fuel efficiency (in gallons/hour) is around 2000. Maximum torque is around 2100.

For months, I ran at 2100 sort of going by "ear". As you can see (as I later discovered with these graphs), this gives maximum torque with very little increase in fuel consumption. The boat averages 6 statute miles per hour on the ICW at that speed, burning 1/3 gallon per hour. Offshore, speed depends on the wind, waves, and (often) sails. But the consumption per hour stays the same.

At maximum RPM, the boat runs about at hull speed in calm water. So the prop must be sized about right.

If you're wondering (as everybody here seems to be doing), the performance graphs at least give you something else to wonder about .

[David M]

There is not a quantitative definition. It is where the speed you are getting is not excessive for the amount of fuel you are consuming. This is somewhere in your engines upper RPM range.

Making your cruising speed very low will eventually kill your diesel.

[barnakiel]

Originally Posted by SPARK

It seems to me that the rpm is almost irrelevant to engine loading.
(...)

This would be so if drag / speed were linear.

b.

[Juniper]

Originally Posted by David M

There is not a quantitative definition. It is where the speed you are getting is not excessive for the amount of fuel you are consuming. This is somewhere in your engines upper RPM range.

Making your cruising speed very low will eventually kill your diesel.

Is it RPM or load that is at issue?
I know poeple that run thier diesel at 200 rpm 24-7, but they are fully loaded at that speed, it's also a stationary engine running an alternator, but rated speed is 650rpm, an 6 HP Lister, a different beast,
It looks like the Volvo @ 1800 is adeal if you are using 7KW, now with a prop that is using 9.5 KW at 2600rpm should be using about 4 KW @ 1800 so @ 1800 rpm it's not fully loaded.

[sigmasailor]

Speed through water is a third power curve in relation to HP required or fuel consumption. This formula is valid for boats sailing up to hull speed. We have all experienced that at certain point boat speed does not increase but noise and waves do; that's a sign you are trying the go over hull speed (waste of fuel).
Suppose we have a boat capable of a hull speed of 8 knots; it might burn two gallons of fuel per hour or 8 miles. Reducing speed to 4 knots theoretically reduced fuel consumption 8 fold (0,25 gallon per hour).
Running any engine at 100% of its rated capacity will wear it down more quickly than running at say 80%. At 80% you will still burn a lot of fuel. 4 Knots is probably a bit slow for any prolonged cruising but sailing at 6 knots (when compared to 8 knots) will still save well over 50% (42%) fuel per hour or mile. In my book that's cruising speed; it conserves fuel and is not to hard on the engine. I do not believe glazing can be avoided by running at 80% exclusively. Running at 40% load will not harm an engine; occasional loading at 80% will do it good.

Eric

[chris07732]

From Eric post #52 above "I do not believe glazing can be avoided by running at 80% exclusively"

I assume that's a typo, Eric?? Did you mean glazing can be avoided by running at 80%

[Captain Bill]

Reading a few of the responses It seems that some people seem to be interpreting 80% RPM as 80% power output. With diesels this is not the case. 80% RPM is likely less than half of the maximum power output and fuel consumption is substantially lower than 80% power output. The power output curve for my engine running at the recommended 80% rpm is only about 16HP versus 33HP at maximum continious RPM or 39HP at maximum RPM. Running at 80% rpm is in fact running at only about 40% of maximum power output.

[sigmasailor]

Originally Posted by chris07732

From Eric post #52 above "I do not believe glazing can be avoided by running at 80% exclusively"

I assume that's a typo, Eric?? Did you mean glazing can be avoided by running at 80%

What I mean to say is that running an engine at 40 or 50% (after it was properly broken in) will not harm it. Some posters seem to believe that the only way to avoid glazing is running engines at 80% capacity or more; I disagree with that.
On the other hand I do think that just charging batteries (elevated RPM but hardly any load) with propulsion engines is bad practice; engines will foul up internally (and elbow) and may even develop glazing when the engine isn't put to real work regularly.

Occasional hard running an engine will do it good though; carbon deposits will burn and the internals of diesel engines stays clean.

I do not consider using 80% of the rated capacity (note: not RPM) 'cruising speed'; too much fuel is wasted and the speed 'advantage' is relatively small.

Eric

[GordMay]

Originally Posted by barnakiel

Is glazing related to "too low" rpm or is it related to loading (or not) the engine? ...

Originally Posted by Juniper

Is it RPM or load that is at issue? ...

Originally Posted by estarzinger

... On our boats we pick an RPM that is high enough that the engine is running at operating temperature, while low enough to keep noise and vibration to a 'reasonable' level ...

I believe that the root cause of cylinder glazing is low operating temperature, which can be caused by low operating speeds/loads.
Accordingly, I endorse Evans' policy.

[sailorboy1]

Originally Posted by Don Lucas

So what would be considered the "cruising" rpm? And if this 80% thing is so much better why don't we refer to that rpm as the "cruising" rpm?

So what I've learned here from my orginal question is that even though the terms get thrown around alot; there really isn't any real agreement as to what these terms mean.

On my last boat I liked running the engine at 2000 rpm if motoring in a general way in no huge rush. For the boat this was about 5 knots and the fuel and noise were acceptable. The engine noticably purred along at this rpm as opposed to 5.5 knot at 2400 rpm. So to me on that boat 2000 rpm was the cruising rpm.

I guess the only time it would make sense to me to run at less, say the 80%, would be if motor sailing.

[S/V Alchemy]

Originally Posted by estarzinger

There is a theory that 'cruising speed' in knots should be roughly = square root of waterline (in ft). The concept is that above this speed a displacement hull mostly starts digging a deeper hole in the water and hull drag increases dramatically above this speed.

Of course, I think the flat-water rule of thumb would be 5.2 kn for a 35 footer, 5.4 for a 40 footer and 5.6 for a 45 footer. I would not be at all surprised to learn that boats in a similar displacement class would be, at those SOGs, at or near the "sweet spot" of fuel economy at speed and around 2,200 RPM.

So everyone's free to do the math, but most cruisers will hover around an answer of "just over five knots".

[Captain Bill]

Originally Posted by sigmasailor

Occasional hard running an engine will do it good though; carbon deposits will burn and the internals of diesel engines stays clean.

My operating manual recommends cruising at 80% of maximum continious RPM. It does not state that you can not cruise at lower RPMs, but it does state that when operating at lower RPMs for extended periods that one should operate the engine at the recommended RPM for 5 minutes every hour to prevent carbon build up and cylinder glazing.

[Chipg]

Caterpillar says for the larger 3208 you can run it for 5 hours at low rpm as long as you run it at cruise speed and operating temp for 30 min before shutdown.