Working out optimum engine revs for cruising

Nostrodamus

Nostrodamus

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www.cygnus3.com
Is the an easy way to work out your optimum engine revs when cruising?
How do you know when you have too many or two few revs on when using your engine for several hours.
There are times when I have to motor for a prolonged time and I am not sure if I am using the engine to the best of its capabilities. If I am using too little (which I tend to think) then I may not be getting the best from the engine or pushing it hard enough but how do you know when you are using too many revs and how many is too many?
I have a Volvo MD22 and tend to cruise at about 2200 revs. If anyone has the same engine can you tell me what revs you use.
Finally, if motor sailing the main is obviously helping your speed and taking some of the strain away from the engine. Does this mean you should decrease, increase or leave your revs the same to get the best from the engine as it is not working as hard.
Thank you for any replies.
 
Nostrodamus said:
Is the an easy way to work out your optimum engine revs when cruising?
How do you know when you have too many or two few revs on when using your engine for several hours.
There are times when I have to motor for a prolonged time and I am not sure if I am using the engine to the best of its capabilities. If I am using too little (which I tend to think) then I may not be getting the best from the engine or pushing it hard enough but how do you know when you are using too many revs and how many is too many?
I have a Volvo MD22 and tend to cruise at about 2200 revs. If anyone has the same engine can you tell me what revs you use.
Finally, if motor sailing the main is obviously helping your speed and taking some of the strain away from the engine. Does this mean you should decrease, increase or leave your revs the same to get the best from the engine as it is not working as hard.
Thank you for any replies.
Click to expand...
I regard cruising as 2600-2800 rpm on an engine with a max revs of 3400. This gives me 6-7 knots depending on how clean the prop is.
 
Nostrodamus said:
Is the an easy way to work out your optimum engine revs when cruising?
How do you know when you have too many or two few revs on when using your engine for several hours.
There are times when I have to motor for a prolonged time and I am not sure if I am using the engine to the best of its capabilities. If I am using too little (which I tend to think) then I may not be getting the best from the engine or pushing it hard enough but how do you know when you are using too many revs and how many is too many?
I have a Volvo MD22 and tend to cruise at about 2200 revs. If anyone has the same engine can you tell me what revs you use.
Finally, if motor sailing the main is obviously helping your speed and taking some of the strain away from the engine. Does this mean you should decrease, increase or leave your revs the same to get the best from the engine as it is not working as hard.
Thank you for any replies.
Click to expand...

How long have you owned her & how many miles have you done :o
 
sailorman said:
How long have you owned her & how many miles have you done :o
Click to expand...

Two years but I try and sail when I can...
What I think may be right and what may be best for the engine might be different. I was just looking for informed opinions, especially with someone with the same type of engine.
 
Get the engine performance graphs.

It's all about how much fuel you want to use, but most people seem to cruise at max torque.

As you can see from our graph, we tend to use 2000 rpm

556e0cb4.jpg
 
Nostrodamus said:
Is the an easy way to work out your optimum engine revs when cruising?
How do you know when you have too many or two few revs on when using your engine for several hours.
There are times when I have to motor for a prolonged time and I am not sure if I am using the engine to the best of its capabilities. If I am using too little (which I tend to think) then I may not be getting the best from the engine or pushing it hard enough but how do you know when you are using too many revs and how many is too many?
I have a Volvo MD22 and tend to cruise at about 2200 revs. If anyone has the same engine can you tell me what revs you use.
Finally, if motor sailing the main is obviously helping your speed and taking some of the strain away from the engine. Does this mean you should decrease, increase or leave your revs the same to get the best from the engine as it is not working as hard.
Thank you for any replies.
Click to expand...

Nostrodamus, I am certainly not an expert but based on my understanding I could suggest the following:
- Check the manual and find out what is the max rated rpm of your engine.
- On a calm day and your boat/prop clean and reasonably loaded (well I know you are travelling, you lucky guy, so let's assume your nice Oyster is reasonably loaded) run your engine at full rpm and see if max rpm you can achieve is close to what the manufacturer says. If yes, then you have the correct size prop and everything is fine.
- Assuming everything is fine it's quite safe (for the engine) to run it at cruising rpm 75-80% of maximum.

I will leave it to the more experts now....
 
There is not an "optimum" - the more power you use, the faster you go and the more fuel you use. As already suggested the rule of thumb is around 70-75% max power on the power curve, which is usually the point of maximum torque. If your prop is correct and you can get maximum revs and maximum hull speed then that should give you a comfortable cruise speed. Does not have to be exact as most installations have sweet spots and you should have found that by now - and it may be slightly different from the same engine in another boat. If you want to use less fuel, reduce revs and go slower; if you want to push hard in adverse conditions use more revs and consume more fuel.
 
Looking at ExSB's nice graphs, it supports running relatively slowly particularly in flat water.
Even if fuel cost is not an issue, it's nice not to have to fill up too often.
Personally I have an opinion that always running a the same RPM for a long time can be related to premature bore wear.
Also sometimes I place more value on getting somewhere fairly quickly, other times I am happy to be out there taking my time.
Quite often we motor because we are already behind where we hoped to be, because the wind has died, so we are then in a mood to make progress.
If we set off to motor somewhere, e.g. if we know there is to be no wind, we'll have loads of time and want to have the motor quieter.
 
You should look at the power curve of your engine and use the revs near the maximum torque which is where the engine will provide the best ratio between fuel consumption and power and also will run smoothly.

The engine manual should provide this type of advice. For example a Volvo Penta D2-55 revs up to 3000rpm, but the best cruising rev, as recommended by the manufacturer is 2000rpm.

However often you have to find a compromise by using revs that cause the least vibration. A fouled prop will be a cause of vibrations which will be less at some revs.

In any case my advice during a long cruise is to vary the engine revs at least every 30' a change for a tad more or less will preserve the engine and avoid glazing the bores.
 
Does the manufacturer recommend changing the revs every 30 mins? For running in a new engine maybe, but in service??? Seems a strange requirement as diesels are usually most happy running at constant revs for long periods, indeed thats what a marine diesel is designed to do. If they don't recommend it its almost certainly of no advantage whatsoever.
As for glazing the bore I wasn't aware this was ever a problem at cruising revs - it is (afaik) only associated with prolonged idling (ie when charging batteries). Anyway I can't see how changing the revs would affect glazing even if it did occur at cruise power.

If in doubt, read the manual...do what Volvo says, they are the experts.


edit

As ever a two minute google search reveals that bore glazing is ONLY associated with prolonged running at low revs and/or low power settings - diesels like to be worked hard. It seems to be associated with the cool running conditions experienced with those regimes. A hot diesel is a happy one.
Equally, a search for continuous revs or varying revs comes up with precisely - nada.
 
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I run my MD22 at about 1800 or 1900rpm in smooth water. That gives me about 6.5 knots with a 29ft LWL. I will give it more revs, if required, e.g. to get through a short patch of adverse tide, or strong headwind.

If you use much higher revs, the bow comes up, and the stern goes down (noticeably), you will make a lot more wash, and waste fuel. You will go a little faster

I have a Bruntons Autoprop, so when motor-sailing, I don't have to rev nearly as much as I would have to with a fixed prop.:)

Does that help at all?
 
NormanS said:
I run my MD22 at about 1800 or 1900rpm in smooth water. I will give it more revs, if required, e.g. to get through a short patch of adverse tide, or strong headwind.
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Fuel costs being a major factor in my reasoning, I try to run my MD22 about 1800 and not above 2000 revs until similar situations as quoted by NormanS.
 
The Volvo Penta plate on my boat, positioned next to the helm clearly states:

"Recommended crusing speed 200 rpm below maximum obtained speed"
 
The theoretical solution is to draw a tangent to the engine speed/fuel consumption curve. The tangent should go from the origin to touch the curve and the speed at which the tangent touches the curve is the most economic. In practise this is usually between 70-80% of max engine revs. I had a 25ft mobo with the MD 22 and cruised at 2250 revs which happened to give just under under max hull speed. Fuel consumption was 0.7 gal/hr.
 
Trundlebug said:
The Volvo Penta plate on my boat, positioned next to the helm clearly states:

"Recommended crusing speed 200 rpm below maximum obtained speed"
Click to expand...

The instructions with mine give 4-500 rpm below max, which in my case is 2900-3000, which is higher than I would normally use. I have done 2700 for 24 hours once, when there was a swell against me but 2500 or so is my usual, on a 2030.

The problem gets more complicated when there is a contrary wind or current. It is then worth using much higher revs since a slower speed may actually end up with a higher fuel consumption (ultimately infinite when motoring at the speed of the current). Until someone produces a fuel-consumption meter like some cars, linked to the GPS, I will have to go on doing it by guesswork and feel.
 
haydude said:
You should look at the power curve of your engine and use the revs near the maximum torque which is where the engine will provide the best ratio between fuel consumption and power and also will run smoothly.
Click to expand...

That's superficially true, assuming the prop is well-matched to the engine, and clean, and hull is clean, too, but many other variable come into play. Peak torque revs is always where an engine (diesel or not) is most fuel efficient, but that's not the same as saying that it will be necessarily the best revs for miles per litre. If fuel consumption is an important part of the OP's thinking, he'd be best advised to make careful calculations as to his boat's performance at varying revs.
 
reeac said:
Non-boaty, but haven't heard of this. Which cars have this feature?
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Well my car can tell me what my overall fuel consumption is, or it can tell me what the instantaneous rate is. (Toyota Avensis). that is related to milage from the mileometer.
I think that what johnalison means is a similar device on a boat, related to GPS instead of the mileometer. There are rather more variables with a boat!
 
johnalison said:
The instructions with mine give 4-500 rpm below max, which in my case is 2900-3000, which is higher than I would normally use. I have done 2700 for 24 hours once, when there was a swell against me but 2500 or so is my usual, on a 2030.

The problem gets more complicated when there is a contrary wind or current. It is then worth using much higher revs since a slower speed may actually end up with a higher fuel consumption (ultimately infinite when motoring at the speed of the current). Until someone produces a fuel-consumption meter like some cars, linked to the GPS, I will have to go on doing it by guesswork and feel.
Click to expand...

Very similar to 2030, 2400 which gives 5.4 knots in flat water using a 2 blade folding prop. Fuel consumption in terms of distance traveled is of little use because consumption is a function of power used - and distance traveled is affected by many external variables (wind, tide, sea state) and some internal (cleanliness of bottom, weight, windage). So, in your example running at 2700 would consume more fuel for the distance over the ground covered than in flat water.
 
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