Motorsailing & occasionally revving engine

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srah1953

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There's an article in the August edition of YM on motorsailing. In this the author says that his preferred cruising rpm is 1,600 and that, as recommended by Yanmar, he will periodically slip the engine into neutral and increase to 3,000 revs for a few minutes. Would it not be just as good for the engine and more productive for forward progress to drive in gear at 3,000 rpm?
What am I missing?

Thanks for your thoughts.
 
Agree The purpose of periodic running at higher revs is to put full load on the engine which does not happen in neutral.
 
Its totally pointless old wives tale nonsense.

The engine will be running cleanly and efficiently at cruising rpm so does not need any 'cleaning out'.

Do you, as a motorist occasionally drive at 120mph after many miles at the speed limit to clean your engine out?

I think this o.w.t. must have stemmed from the days of steam when the boilers needed a good roasting to clean out the soot....!
 
ianj99 said:
Its totally pointless old wives tale nonsense.

The engine will be running cleanly and efficiently at cruising rpm so does not need any 'cleaning out'.
Click to expand...

Not true. Diesel engines running at cruising revs are not at ideal stoichimetric conditions. Some of the fuel is not burnt fully and is deposited as carbon. In a car, with variable revs, acceleration, etc., this situation does not occur. In a boat, that may run at the same revs for many hours, it can become a problem. Yanmar recommend running at maximum revs for five minutes every two hours to burn off carbon deposits on exhaust valves and ports.
 
Suggest you look at the propeller power curve for your engine and you will see that you are not fully loading the engine at constant cruising revs so the fuel is not being burned fully. As Vyv says it is a mistake to compare marine use with automotive cycles which is why Yanmar make the recommendation about periodically running the engine at full load.
 
Interesting comments so far!

1. It is good practise to run the engine at high revs from time to time, to clear the insides out. All diesel engines deposit carbon which why at 5 - 8,000 hrs the cylinder head usually comes off as part of the servicing.

2. Yep running at 3,000 rpm is not economical. You get to a point where you reach the hull speed and more revs just = more fuel burnt. Even up to the hull speed the curve of boat speed v fuel flow will flatten as you pass the cruising speed.
 
alahol2 said:
Can you explain that please? Not doubting you, I just don't understand, seems very inefficient.
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The torque curve of most diesel engines peaks at very close to maximum revs. At this point the conversion of fuel into CO2 and water is at its most efficient. At cruising revs, which might be 2/3 of maximum, some of the carbon is not fully oxidised with the result that some CO and some C is produced, the former emerging with the exhaust gas and the latter depositing somewhere in the engine. Hence the Italian Tuneup, running the boat at maximum revs for 15 minutes or so, is a well-known way of improving performance where it has deteriorated due to carbon accumulations. It also works for gasoline powered vehicles that are rarely revved anywhere near their maximum.

In a diesel car the same thing happens as with the boat, but every time you take your foot off the throttle the combustion chamber is flooded with oxygen, burning the carbon away.
 
A tractor is used to launch the local boats, it gets constant repeated short runs and never gets to operating temp. The engineer recommended lending it to a farmer to use for a day ploughing.
 
My Vetus brochure shows nothing of the kind for the majority of its engine range - usually max. torque occurs at about half max. power revs. An exception is the M2.C5 11bhp.model.
 
1600 rpm with some drive also produced by sails (hence probably less load on engine than motoring at 1600 alone) is a very very fuel-efficient way to make progress, but a horrible way to use a diesel. OK if given a full-load or near full load burst at times, but certainly not in neutral.

Have seen two identical Volvo 2000 series diesels, one at about 1000 hours, very hard to start, rattly, lots of blue smoke on startup and a trail of smoke behind when running, on stripping found to have badly glazed bores, big bill for strip and rebuild.

Other at 5000 plus hours, started instantly, ran sweetly with little smoke on startup and none once running. The low-hours one had an owner who ".... never ran it more than half throttle" and idled it to warm it up. He thought he was being kind to the engine, till he got the big bill. The other was an ex-charter boat....

You do however get engines that seem to survive regular low-rev low-load use, not sure why, but more that suffer from it.
 
jwilson said:
1600 rpm with some drive also produced by sails (hence probably less load on engine than motoring at 1600 alone) is a very very fuel-efficient way to make progress, but a horrible way to use a diesel. OK if given a full-load or near full load burst at times, but certainly not in neutral.

Have seen two identical Volvo 2000 series diesels, one at about 1000 hours, very hard to start, rattly, lots of blue smoke on startup and a trail of smoke behind when running, on stripping found to have badly glazed bores, big bill for strip and rebuild.

Other at 5000 plus hours, started instantly, ran sweetly with little smoke on startup and none once running. The low-hours one had an owner who ".... never ran it more than half throttle" and idled it to warm it up. He thought he was being kind to the engine, till he got the big bill. The other was an ex-charter boat....

You do however get engines that seem to survive regular low-rev low-load use, not sure why, but more that suffer from it.
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Worth looking at Vyv's web page about bore glazing and bore polishing (different phenomena), which underlines your anecdote. A quote from there: "I believe the fundamental cause of glazing to be the condensation of combustion products onto cold bores when the engine is first started". He also suggests that over-gentle running-in is a major contributory factor.
http://coxengineering.sharepoint.com/Pages/Boreglazing.aspx
 
alahol2 said:
Can you explain that please? Not doubting you, I just don't understand, seems very inefficient.
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Yes, it is inefficient. It is the propeller that moves the boat and absorbs the power that is produced by the engine. However, vast majority of props are fixed pitch so can only use the power at one speed. At all other speeds it cannot use the power produced at full load for the revs. So the propeller power curve is much lower. On my Volvo 2030 at 2400 it is using around 12hp compared with 20 or so that it could produce if loaded fully. The governor reduces the amount of fuel to produce that power. The two power curves cross close to maximum revs. It will however, use maximum power (and therefore maximum amount of fuel) at full revs (3400-3600). That explains also why fuel consumption increases rapidly once you get above about 70% max revs - but of course speed does not increase at the same rate!

One answer is a variable pitch propeller which can be found on some commercial boats and have been fitted in yachts in the past, particularly Scandinavian, but they are complex and expensive. The nearest practical alternative is the self pitching Bruntons Autoprop which varies pitch according to load and speed. Popular for motorsailing because it can change the pitch and put more load on the engine at lower revs. Some other feathering props such as Gori and Max Prop also have an "overdrive" which changes the pitch for cruising speed at lower revs.
 
vyv_cox said:
The torque curve of most diesel engines peaks at very close to maximum revs. At this point the conversion of fuel into CO2 and water is at its most efficient. At cruising revs, which might be 2/3 of maximum, some of the carbon is not fully oxidised with the result that some CO and some C is produced, the former emerging with the exhaust gas and the latter depositing somewhere in the engine. Hence the Italian Tuneup, running the boat at maximum revs for 15 minutes or so, is a well-known way of improving performance where it has deteriorated due to carbon accumulations. It also works for gasoline powered vehicles that are rarely revved anywhere near their maximum.

In a diesel car the same thing happens as with the boat, but every time you take your foot off the throttle the combustion chamber is flooded with oxygen, burning the carbon away.
Click to expand...

I suggest Vyv you look at a few torque/power curves and revise that comment - the 3YM Yanmar produces maximum torque @ 2400 rpm and maximum power @ 3600.
You'll find most modern marine diesels have a similar distribution.
I'd agree that, unlike old-fashioned long-stroke diesels, the torque and power curves are closer together with more modern diesels.

In fact it's a matter of opinion as to whether one should run modern diesels flat out on a regular basis - if you talk to the Yanmar designers you'll find that they suggest running the engine @ as close to maximum torque is the optimum - with an CI engine more welly = more fuel = more carbon.

The stricture about opening up occasionally from low revs, has nothing to do with carbon deposits but everything to do with oil distribution and bore wear.

I'm surprised to see you promulgating this urban myth about carbon deposits!!!
 
vyv_cox said:
In a diesel car the same thing happens as with the boat, but every time you take your foot off the throttle the combustion chamber is flooded with oxygen, burning the carbon away.
Click to expand...
Can you explain this, please. I thought boat and car diesels were basically the same, except for necessary marinisation.
Thanks
 
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