Optimal cruising RPM

[FPNC]

Originally Posted by captain69

I was running my Volvo D – 20 at around 2200 RPM, give or take 100. Now y’all have me scared and thinking I should run it in 1969. Does anyone know what the manual actually reads? I can’t find that anywhere, this thread is the closest hit I could find.

Peak torque on a D-20 is between 2200 and 2600 RPM. This range is where you are using the engine to it best efficiency. Carry on.

[TeddyDiver]

Originally Posted by FPNC

Peak torque on a D-20 is between 2200 and 2600 RPM. This range is where you are using the engine to it best efficiency. Carry on.

It's not just the right RPM for max torque but also the right loading which is usually somewhere near 70-80% of the max loading to be most fuel efficient.

[NorthCoastJoe]

Most of these engines are based upon industrial engine standards. That means work it, don't spend a lot of time at low loads. Even the old VW diesel rabbit engines worked hard 48 HP would get you to 70 mph max in about 2 minutes of constant petal to the metal driving. They were running WOT all the time on the highway and got 50 mpg.

I run engines and baby them by making sure oil is changed and filters are clean. Nothing worse for an engine that dirt, it's that simple. Old VW air cooled gas engines had no oil filter and had an expected lifespan of 40,000 to 50,000 miles. The main reason for short life dirt would get sucked in through the rear of the crankshaft, they had no seal just an oil slinger. I rebuilt over 200 engines gas and diesel and swear by clean.

When loading a engine you need to keep RPM up to circulate the cooling water faster. Never work an engine hard at low RPM. So more about clean, clean ,clean.

I used a point and shoot IR temperature gauge on my Beta 50 exhaust flange where it leaves the heat exchanger to check loading. With a dirty prop and hull the EGT was almost double for the same RPM as a clean hull and prop.

[FPNC]

Originally Posted by NorthCoastJoe

Most of these engines are based upon industrial engine standards. That means work it, don't spend a lot of time at low loads.

Well designed modern engines are so very different than old design diesels.

Diesel generators run at either 1800 RPM (USA 60hz), or 1500 RPM (EU 50hz), continuously, for the entire life of the engine. This is way below the oft promoted 80% of max RPM and also below the peak torque RPM. I have seen small generators that run over 20,000 hours. The electric ends wear out before the engines wear out. The common denominator is that the engines are run very often for long periods and the oil is kept hot and changed regularly. This is pretty much opposite to how your average sailboat propulsion engine is operated and maintained.

[barnakiel]

There is no benefit in running any engine "faster" - as long as the oil and engine temperatures are good.


Older engines were longer lasting also due to the fact that they were slower running. (* see my note on vibrations)



One other factor not discussed but well worth note is that most engines run smoother at some revs - and this depends both on the engine and on the installation. It is good for the engine and the whole installation to be run at low vibrations - and this you can judge by just looking at the engine and listening to it as you go.


b.

[Bowdrie]

Originally Posted by barnakiel

One other factor not discussed but well worth note is that most engines run smoother at some revs - and this depends both on the engine and on the installation. It is good for the engine and the whole installation to be run at low vibrations - and this you can judge by just looking at the engine and listening to it as you go. b.

Good point, yes, many if not most engines will have an rpm "band" where they "feel good", and also some place where they seem "buzzy".
I guess it has something to do with "nodes".
There's a field of engineering that has a focus on balance/vibration.

[Dockhead]

Originally Posted by GordMay

The 3500 - 4000 RPM is the Full Throttle (WOT) operating RPM.
The RPM/Torque curve indicates highest efficiency between 2000 - 2500 RPM, which is where I’d typically cruise, provided that’s high enough to get the engine up to operating temperature.
➥ http://www.sra-moteur.com/uploads/ca.../md22-md22.pdf

Remember though how "maximum efficiency" is defined here -- not according to liters per mile but according to grams per kW/h.

So that's not the whole picture of efficiency. At that engine speed you'll be getting more power per amount of fuel burned, but you may be needing much more power than at a lower speed. Hull speed is a Speed-Length Ratio of 1.34; anything over 1.0 is uneconomical -- the power vs. speed curve is already rising steeply. Most cargo ships today operate at speeds significant less than 1.0 S/L. And ships, with much finer hulls than our boats, have much better Froude numbers, so the power vs. speed curve goes up more steeply for us.

So long story short, the slower you go the fewer liters per mile you will consume, going down steeply until you get below at least 2/3 of hull speed. And the variation of efficiency here, in these speed ranges, is going to be an order or two of magnitude more than what you can get from operating the engine in a slightly better regime.

So it boils down to a question of how much of a hurry you're in. If I am in no particular hurry I typically operate my boat at around 2/3 of hull speed (or a little more than 6 knots), at about 1800RPM, when motoring in very calm weather. I can get 8.5 knots motoring on a calm day at about 2800RPM (all this assumes a clean bottom), but I will be using at least 3x as much fuel per mile. YMMV.

That economical cruising RPM (1800 out of a 3800 redline, turbo motor) is not harmful to the engine but you do have to blow it out every few hours with a few minutes near redline (as per the Yanmar manual).


Yet another factor in efficiency is the prop. The engine will work better (i.e. more efficiently) if it's loaded up somewhat, and that varies with the prop curve. But even here, the possible gain in efficiency is far less than what you gain in hull efficiency from lower speeds.

[FPNC]

[QUOTE=
So that's not the whole picture of efficiency. [/QUOTE]

This is correct for fuel efficiency. If you truly want excellent fuel efficiency, run at about 4.5 kts. Above that speed and water drag on the hull starts to increase steeply. Over 6 kts and drag increases much more. When you start to near the hull speed, drag and wave resistance get to be a functional limit.

[Audeamus]

Some real world figures to regarding efficiency supporting the last two posts.

Yanmar 4LH 170hp in 54ft 27ton displacement monohull (rather overpowered but what to do)

After dialing in the prop we make hull speed at 2900-3000rpm - close as to WOT.

At 1800rpm about 6L/hr with hull speed 6knots.

At 2200rpm 11L/hr with hull speed 7.5knots.

I don't want to think how much it would drink at 2800rpm - for the proponents of "work it hard"

The performance curve for the engine indicates the engine is at its most efficient from 2200rpm to 2800rpm if recall correctly - so you would think 2200 would be the best as the efficiency really improves there - yes most efficient but not most economic.

(For interest MPG at 1400-1500 is about same as 1800)

It's a dilemma. I think in the day Yanmar void warranty if you don't operate in the higher rpm range. If you think about it, we set the prop pitch to WOT or thereabouts. This means by default we are potentially over propped at all rpm below this

Something to be said for proper variable pitch props and exhaust temp gauges but those toys reserved for the big boys to play with.

[TeddyDiver]

Originally Posted by Audeamus

Some real world figures to regarding efficiency supporting the last two posts.

Yanmar 4LH 170hp in 54ft 27ton displacement monohull (rather overpowered but what to do)

After dialing in the prop we make hull speed at 2900-3000rpm - close as to WOT.

At 1800rpm about 6L/hr with hull speed 6knots.

At 2200rpm 11L/hr with hull speed 7.5knots.

I don't want to think how much it would drink at 2800rpm - for the proponents of "work it hard"

The performance curve for the engine indicates the engine is at its most efficient from 2200rpm to 2800rpm if recall correctly - so you would think 2200 would be the best as the efficiency really improves there - yes most efficient but not most economic.

(For interest MPG at 1400-1500 is about same as 1800)

It's a dilemma. I think in the day Yanmar void warranty if you don't operate in the higher rpm range. If you think about it, we set the prop pitch to WOT or thereabouts. This means by default we are potentially over propped at all rpm below this

Something to be said for proper variable pitch props and exhaust temp gauges but those toys reserved for the big boys to play with.

You are "under propped" to be efficient on lower rpm's, but over propping you can't push the lever further anymore and CPP's are $$$. There are some available for boats your size and even smaller from Norway, maybe from Denmark too.
ps. also Autoprop

[barnakiel]

Originally Posted by FPNC

This is correct for fuel efficiency. If you truly want excellent fuel efficiency, run at about 4.5 kts. Above that speed and water drag on the hull starts to increase steeply. Over 6 kts and drag increases much more. When you start to near the hull speed, drag and wave resistance get to be a functional limit.

on fuel efficiency


and going even slower, you get even higher miles per gallon (all other things same)


just that most people are in hurry and they want to go as fast as they can


which is reverse of fuel efficiency


have cake and eat cake, one cannot


b.

[Audeamus]

I do not think we are under propped - cannot quite make 3200rpm WOT and just seeing black smoke about 3000+. Its near enough for me. Engine has 2500hrs and new Turbo recently. We started (when we got the boat) way over propped - black smoke at 2400rpm (I wasn't prepared to push harder on the throttle to see how high it would go)- 3.5 knots at idle LOL and slowly worked back from there. Previous owner did 1400hrs tootling around at 1500rpm. Happy to discuss offline - always learning.

[Dockhead]

Originally Posted by TeddyDiver

You are "under propped" to be efficient on lower rpm's, but over propping you can't push the lever further anymore and CPP's are $$$. There are some available for boats your size and even smaller from Norway, maybe from Denmark too.
ps. also Autoprop

Indeed. A fixed pitch prop, adjusted to allow redline to be reached, will be under, not overpropped at lower RPMs.

Variable pitch props improve efficiency and engine health, but I wouldn't do it for just for the fuel savings. The really big advantage of variable pitch props is for pleasant and quiet motor-sailing, where you can pull the revs right down if you are getting some power from the sails.

I have an Autoprop, which is really good for this. But Autoprop has other disadvantages, for example, very expensive, fairly involved maintenance requirements.

But the king of variable pitch props is the Hundested, which has manual rather than auto pitch control. Minimum size of that one is for 150hp, so Audeamus could have a Hundested. Prepare a lot of $$$, however. I would say worth it if you've got it.


At the other end is the Gori two-speed prop, which we have on another boat. Only two pitch settings, rather than a wide continuous range, but already a great leap forward compared to a fixed pitch prop. Simple, inexpensive, easy maintenance. Also worth a look.

[Dockhead]

Originally Posted by Audeamus

. . . Previous owner did 1400hrs tootling around at 1500rpm. Happy to discuss offline - always learning.

Those engines are fine at 1500RPM so long as you blow them out every few hours with a few minutes at high RPM. It's in the Yanmar manual.


Does sound like you want a variable pitch prop, though. If you have the money (you have to buy a special gearbox for it, too -- $$$), you can get a Hundested to fit your boat.

[Dockhead]

Originally Posted by barnakiel

on fuel efficiency

and going even slower, you get even higher miles per gallon (all other things same)

just that most people are in hurry and they want to go as fast as they can

which is reverse of fuel efficiency

have cake and eat cake, one cannot

b.

There is a sweet spot at about 0.7 speed/length, which is where ships mostly operate when at economical slow cruise. Below that there is not all that much savings; above that speed the efficiency falls off rapidly. That's the point where the drag curve turns up steeply. I think it's about 2/3 of hull speed.