Engine at full revs?

[Lakesailor]

Occasionally, punching a strong tide I have run it at sustained 3000 revs. when it can achieve close to 8 kts. through smooth water, which seems to be almost above the theoretical hull speed, (On a good broad reach in flat water we can just manage the same) but it is noisy, the steps vibrate and it gets very warm and it then burns 7 litres per. hour.

Secondary harmonics can make engines rough as old boots.
Car manufacturers often fit balancer shafts to try and iron out vibration at certain revs. Different cylinder configurations or crank plane geometry give unavoidable harshness at certain revs.
If your motor vibrates at certain revs, don't use that range.
Running a motor in the range of secondary, out-of-balance forces will knacker it.
I agree that running a motor at lower speeds will lead to a longer life.
Not revving a motor when cold will help. It can take 10 or more minutes to get the oil up to a proper protective temperature.
Anything that disturbs or creates additional wear to the bearings will shorten engine life. Delaying oil changes will adversely affect engines as will running for a few minutes to clear you moorings and then shutting down. Ideally an engine should run up to it's optimum operating temperature, for a while, every time it is run.

Look at motor cars. Would you buy a car with 100K miles on the clock if it was a boy-racer's car? Of course not
Would you buy a granny's shopping car? best not.
Would you buy a motorway cruiser that has sat at 3000rpm (rev limit 6000rpm) for hours and hours, not used it's brakes or steering or clutch and gears? Good bet.

 

[haydude]

You don't say what your engine is, but most modern engines max at 3200-3600, or even higher for some automotive derived engines such as the Perkins/Volvo MD22.

So, if you are reaching max speed at 2000 either your rev counter is way out or you are seriously overpropped (or possibly way overpowered).

If the engine is correctly sized to the boat then the optimum prop will achieve hull speed at close to maximum rated revs. You will then get a comfortable cruise speed at revs that equate to roughly 70% max power.

Volvo Penta D2-55 max cruising revs is 2000 which coincides with max torque. It can get to 3000, but it is not advised due to fuel waste and vibration. Hence every boat with a D2 have the prop sized to provide max speed at 2000 revs.

 

[penfold]

Volvo Penta D2-55 max cruising revs is 2000 which coincides with max torque. It can get to 3000, but it is not advised due to fuel waste and vibration. Hence every boat with a D2 have the prop sized to provide max speed at 2000 revs.

Is the D2 a commonrail injection/ECU controlled engine? They don't really have the same characteristics as engines with mechanically controlled injection systems; you have much greater freedom to adjust fuel quantity/timing throughout the rev range, allowing the manufacturer to achieve a very wide range of power outputs/service speeds from the one set of engine components.

 

[Quandary]

I don't believe that a 28hp engine can possibly use 7 litres an hour.

The running time for the engine, revs. used and journey time is always recorded in the log. The fuel used is also recorded at every fill up. The trip in question was returning single handed under motor only after the last race in West Highland Week in 2008 down the Sound of Mull and the Sound of Luing with a brief stop to drop my crew off at Oban. I was also in a bit of a strop when all three crew hopped off to drive back to the same destination when only two of them had an urgent appointment. I was pushing really hard because I had called Crinan sea lock and they had agreed to wait for me as there was foul weather coming in. It was against a strong tide and for a time I was getting just over 3 knots over the ground when the log was showing 8.1, the throttle lever was right down and I was dodging the rocks to stay close in down the Luing shore and went through the Dorus Mohr within a couple of metres of Craignish point. The tank had been filled at Tobermory and was topped up again next time we left Crinan. The pump at Tobermory has a cut off while the one at Crinan does not so if anything there is a tendency to underfill. I will accept that 7 was a rounding up but the decimal was closer to 7 than 6 so the fuel went somewhere. Next time I am on the boat I will review the calculation but it is basic arithmetic so hard to get wrong.
The VP 2003 in my (heavier) previous boat could use 5 litres/hour at times but I do not recall that it was ever pushed as hard as this.

 

[Tranona]

Volvo Penta D2-55 max cruising revs is 2000 which coincides with max torque. It can get to 3000, but it is not advised due to fuel waste and vibration. Hence every boat with a D2 have the prop sized to provide max speed at 2000 revs.

There is a difference between "maximum cruising revs" (not a very precise term!) and maximum speed. Go back over my earlier explanation. 2000 revs is drawing approx 60%+/- maximum power of the engine which will give you a good cruising speed. However, you should be able to access the other 40% that the engine can produce by fitting a prop that allows the engine to get there and gives you a maximum speed close to the theoretical maximum of the hull. You are right that 2000 is probably the maximum torque, but you will also probably find that the torque curve is very flat between that and 2500, as is the specific fuel consumption curve. Above that specific fuel consumption rises, and the marginal increase in speed relative to power declines.

So, would be normal to choose a cruising revs of between 2000 and 2500. Clearly the lower revs use less fuel per hour, but speed is lower, so fuel/distance varies very little. Many engines have what is often referred to as a "sweet spot" - I ran one boat where it co-incided with the grid on the cooker just stopping vibrating! So most people choose cruising revs where they get the balance of speed comfort and fuel consumption they are looking for. This does not alter the fact that the prop should allow the engine to develop its full power - even though you rarely need it.

 

[pvb]

This does not alter the fact that the prop should allow the engine to develop its full power - even though you rarely need it.

Somehow, I don't think your message is getting through!

 

[Tranona]

Somehow, I don't think your message is getting through!

Yes, not sure why. Not as if I am David Cameron trying to explain the "Big Society".

 

[Ruffles]

Get the engine up to temperature and then have a high rev (not maximum) run, under load, for 5 or 10 minutes. Like Tranona says, 75% revs should be your target (2400rpm). Half an hour at maximum revs with no load is not right.

I was doing this at the weekend with the boat while tied up. Just serviced the engine. I was moving the entire pontoon against the tide! Only way of making sure I haven't screwed up the fuel filters.

 

[GrahamM376]

not convinced with the theory of knackering an engine by not running it hard

Mine is 39 years old and is still fine

It is a Volvo MD21A and over sized - 50HP@3000 rpm for

It does 6 knots at circa 1800 RPM

It is sweet - It will die of rust and failure of the ancillaries long before it wears out. I do give it a short blast occasionally to blacken my transom but otherwise don't believe in the long term damage to your engine theory.

Have anyone actually worn out the rotating parts on their engine without doing long term cruising?

Not long after buying the present boat, we did Conwy to Northern Portugal and back for our summer cruise. On arrival just off Portugal, the engine blew all the oil out with crankcase compression (Mitsubishi K4D). When I stripped it, found the rings had stuck in No2 cylinder and the exhaust elbow was also partly blocked with carbon.

Phoned Diamond Diesels (UK MItsubishi plant engine importers) for new rings and gaskets. Their immediate comment was "someone's been motor sailing at low revs".... "Always give the engine a blast at full revs every now and then or, the same will happen again"

 

[Interlude]

The Yanmar (YM20) manual also says to let the engine idle for 5 mins to cool down at the end of a run, then rev to max briefly four times to clear carbon from the exhaust ports etc. I am the only person I know who does this, but after the aggro of diagnosing and then replcing the coked up elbow and head on previous boat (1GM10), I do it religiously now.

 

[Latestarter1]

There is a difference between "maximum cruising revs" (not a very precise term!) and maximum speed. Go back over my earlier explanation. 2000 revs is drawing approx 60%+/- maximum power of the engine which will give you a good cruising speed. However, you should be able to access the other 40% that the engine can produce by fitting a prop that allows the engine to get there and gives you a maximum speed close to the theoretical maximum of the hull. You are right that 2000 is probably the maximum torque, but you will also probably find that the torque curve is very flat between that and 2500, as is the specific fuel consumption curve. Above that specific fuel consumption rises, and the marginal increase in speed relative to power declines.

So, would be normal to choose a cruising revs of between 2000 and 2500. Clearly the lower revs use less fuel per hour, but speed is lower, so fuel/distance varies very little. Many engines have what is often referred to as a "sweet spot" - I ran one boat where it co-incided with the grid on the cooker just stopping vibrating! So most people choose cruising revs where they get the balance of speed comfort and fuel consumption they are looking for. This does not alter the fact that the prop should allow the engine to develop its full power - even though you rarely need it.

Tranona,

Seems like you are motoring against the tide so to speak!

Just take comfort in knowing that you are far better informed than your detractors.

Some of the more ill informed or perhaps simply plain nuts comments are worrying.

#1 All mechanical diesel engines can run at no load (high idle) when warmed through for several minutes with zero issues.

#2 If VP dealers are re-powering and propping motors to peak torque instead of rated speed, owner has NO warranty, this is made totally clear in engine installation manual.

#3 All engines have torsional activity, designer just makes sure that destructive torsionals are well outside operating envelope. The harmonic crank damper looks after torsionals. Balancer (Lanchester) shafts are something entirely different. Four pot motors have inhierent imbalance, they tend to want to move vertically, addition of a Lanchester shaft running at half engine speed removes large amount of this imbalance, however these balancer shafts do have drawbacks as they add weight, cost and impose rev limitations. Over rev a balancer shaft motor and there is a point at which the balance weights want to exit to atmosphere!
When Ricardo designed the Ford Duratorqe diesel motor the first thing that Uncle Ford did was take the Lanchester shaft out. If you look inside a broken Duratorque you can see the balancer gallery is still there as the shaft was eliminated too close to production to redesign the block.

#4 Provided you are correctly propped you can operate these little Jap puddle jumper motors, Kubota, Mitsubishi, Perkins Shibura, Yanmar ect, etc at WOT for extended periods SHOULD you want to, or anywhere in between with zero ill effects. Consider this.........JCB mini digger, largest application of little Perkins Shibura, operator comes on site at 8 am, fires motor up and gives it the beans, once throttle is set operator just dives the stick, tracks and slew, engine is at WOT all the time. Operator may shut engine down a few of times during the the day, however sometimes they have their break and just leave motor idling. At the end of the day motor is shut straight down. These little motors will stick this kind of treatment and come back for more.
Over prop marine version of the same little Perkins and you will kill it stone dead in no time. People complaining about sooty transoms when running at part load, ASSUME that they are propped correctly, perhaps they need to check with by hand held tacho and confirm that they are actually getting to rated speed, Yanmar tachos always under read by 100 or so rpm, VP instrument panel, who knows, however if dealer is incapable of propping engine correctly can you rely on tacho being calibrated.

Comparing direct cooled 1GM with modern motors is pointless, Always ran cool and coked up as a result never exactly a brilliant motor, however sold real well because it was nice cheap little throw away engine when much of the competition was utter junk.

 

[Tranona]

Thanks for that. Interesting about the 1GM. I ran one for nearly 20 years and cruised at 3000rpm because my heavy old boat needed that power to get it moving. Had the exhaust pipe off once in that time - clean as a whistle. checked it over when engine was transferred to another boat and still clean. Not so sure the new owner will get the same life as he is a keen racer and only uses it to get to the start line.

Great little engine apart from the well known niggly bits that could be fixed easily if they put their mind to it - like rubbish water pump, steel external oil pipes, poorly made fuel filter housing - but at least all easy to get at and fix. Hope the replacement Nanni lasts as well.

 

[cmedsailor]

Question to Tranona

Tranona, we have the same engine (MD2030) only difference is that yours is with a sail drive and mine with shaft. Just a small question and there's no need for a techical answer as I will not be able to understand much! I run my engine at 2400-2500 because it feels nice and comfortable. 2400 is also reccomended by you. If necessary I can run it upto 3400rpm with the speed keep increasing and no any unusual smoke, meaning the 16X11 2-blade propeller is fine. Would you advice me to run it at higher (than 2500) rpm every now or then? If yes, how often, for how long and at what rpm?
Thanks

PS: The manual, as I am sure you have read, simply mentions that cruising is 300-500rpm below maximum (that is too high for me). Nothing for runing at full rpm for cooling down etc, etc.

 

[haydude]

There is a difference between "maximum cruising revs" (not a very precise term!) and maximum speed. Go back over my earlier explanation. 2000 revs is drawing approx 60%+/- maximum power of the engine which will give you a good cruising speed. However, you should be able to access the other 40% that the engine can produce by fitting a prop that allows the engine to get there and gives you a maximum speed close to the theoretical maximum of the hull. You are right that 2000 is probably the maximum torque, but you will also probably find that the torque curve is very flat between that and 2500, as is the specific fuel consumption curve. Above that specific fuel consumption rises, and the marginal increase in speed relative to power declines.

You are correct. In fact in my case, although I get best fuel economy up to 2000 revs, the max speed is in fact above at around 2700 revs and I can push the revs to 3000 without appreciable increase in speed (which means I reach hull speed at around 2700 revs).

 

[Tranona]

Tranona, we have the same engine (MD2030) only difference is that yours is with a sail drive and mine with shaft. Just a small question and there's no need for a techical answer as I will not be able to understand much! I run my engine at 2400-2500 because it feels nice and comfortable. 2400 is also reccomended by you. If necessary I can run it upto 3400rpm with the speed keep increasing and no any unusual smoke, meaning the 16X11 2-blade propeller is fine. Would you advice me to run it at higher (than 2500) rpm every now or then? If yes, how often, for how long and at what rpm?
Thanks

PS: The manual, as I am sure you have read, simply mentions that cruising is 300-500rpm below maximum (that is too high for me). Nothing for runing at full rpm for cooling down etc, etc.

Your prop is the same size as mine and you achieve the same revs. Don't think there is any absolute about how often or for how long you should run it hard. I think the Yanmar recommendation is 20 minutes every 24 hours - but that all sounds a bit vague to me as 24 hours continuous is very different from 24 hours at normal yacht usage.

However, I think there is a big difference in the pattern of use in the Med where the lack of wind means more motoring (although some might argue that the tides and adverse weather in northern waters might mean the opposite!). I rarely take mine over 3000 except to check that it can still do it - and to check it is not smoking, which is an indication of the exhaust coking up - not a good one, but heavy smoke at higher revs would be a worry.

You have probably realised that the killers for engines are too little running, running for long periods at low revs and light loads and overpropping (plus of course neglect and poor installation). As Latestarter pointed out, these engines are designed to run hard in things like diggers, so they get a good life in a yacht if properly installed and used sensibly.

 

[pmagowan]

I have a beta marine 20HP which works really well on my 28" wooden honeybee now that i have given it a good going over, replaced all the fuel lines and tank, filters etc etc. I dont have a rev counter so use my ear. Most times I'm out I run at 2/3 full power but when I am wanting to get somewhere I often push it up to full for long periods at about 7knots through the water. When I get near destination I throttle down and when I arrive I idle for a few minutes to cool off.
I was under the impression the engines like being run hard. It certainly seems to do OK and no longer smokes, starts easy and has been 100% reliable since the refit.

 

[snowleopard]

Yanmar advise that their engines should be run up to max revs in neutral 5 times before stopping and every 2 hours during slow running. They say it prevents carbon build-up on the injectors.