Varying the revs

I alter from time to time - maybe every two or three hours. Run it up faster for a few minutes to blow out carbon, then back to somewhere around where it was before (which will not be exactly the same).

The explanation I read somewhere is that running at exactly the same revs for long periods will etch a "ring" into the bore of the cylinder which is harmful in some way. Don't claim to be expert in the details.

Pete
 
Yanmar recommendation is to run at around 70% maximum revs and occasional WOT (full revs). Cruising speed at 2400 or so is comfortable but you might want to vary it a bit to hit the sweet spot. On one boat I sailed on that was just when the cooker grid stopped rattling! But, of course modern saildrives like yours tend to be fairly smooth across the range.
 
I'd always thought somewhere between 2/3 and3/4 revs for most of the time then a blast for 5-10 mins at WOT every few hours of running time but have never been scientific about it.

Also to minimise the time at idle, ours idles only when making sure that water is coming out the exhaust, and on leaving or approaching the berth.
 
jac said:
Also to minimise the time at idle, ours idles only when making sure that water is coming out the exhaust, and on leaving or approaching the berth.
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I have never understood the obsession of some people to "warm up" their engine by leaving it to idle for 10 or 15 minutes. It will warm up far quicker if put under a light load (pootling out of the marina or anchorage) and spend far less time attempting to shove thick, cold oil around its internal parts. Oddly enough, it always seems to be the smokiest, smelliest engines that are left idling. You'd think they would cotton on eventually.
 
Its hard to see how varying revs could adjust anything wear wise? Does the piston travel further at high revs? Do the rings change direction at a higher point?
Some things do change. The fuel pump must pump more fuel, the oil pump likewise. The big end bearings must receive more force, which must wear them slightly more (but I doubt it could be measured).
Many land based diesel motors run at a set rpm for days, weeks even. They seem to last a long time like this, longer (in engine hours) than those used intermittently.
 
trapezeartist said:
I have never understood the obsession of some people to "warm up" their engine by leaving it to idle for 10 or 15 minutes. It will warm up far quicker if put under a light load (pootling out of the marina or anchorage) and spend far less time attempting to shove thick, cold oil around its internal parts. Oddly enough, it always seems to be the smokiest, smelliest engines that are left idling. You'd think they would cotton on eventually.
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You must be referring to the boat at Bradwell Marina that runs fishing trips - warms up between 0730 and 0800 and has the dirtiest and noisiest exhaust that I've ever encountered.
 
MM5AHO said:
Does the piston travel further at high revs?
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Supposedly, yes. At least, according to the motoring correspondent of a national newspaper, writing about the potential problems of a car with one "little old lady" previous owner. He reckoned that at high revs the conrod stretched or compressed microscopically at the end of each stroke, due to the momentum of the piston. If the car was only ever driven slowly, pottering down to the shops, the expansion/compression would be less and so the piston would not quite travel as far on each stroke, and the wear on the cylinder bores would only extend as far as that lesser position, leaving a tiny ridge at each end. When the old lady's car is eventually passed on to her 19 year old grandson (or whoever), the more aggressive driving means that the piston is travelling its full compression/expansion distance on each stroke, and the rings are bumping over the wear ridge at each end, with presumably undesirable results.

Just passing the idea on, I'm not a car mechanic :)

Pete
 
The explanation of the Yanmar recommendation is that combustion is incomplete at all except maximum revs. Small deposits of carbon accumulate on various surfaces such as piston crown, exhaust valve, exhaust ports and, to some extent, manifold when running at cruising revs. Running at maximum revs increases the operating temperature and airflow, helping to oxidise these deposits.
 
An engine is far happier if you set it at its normal continuous rating (NCR) and leave it there.
Diesels are funny old things in that they like to be warm before loaded but using their own combustion to warm them leads to bore glazing and endless issues with rig wear and smoke. the best way to pre warm your engine is to use a space hater in the engine compartment or an auxiliary heater on the fresh water cooling circuit to circulate warm water before starting. However on a yacht size engine your just as well starting and leaving the quayside straight away. The engine will cope better with the cold steel than it will the off load running.
If your fitted with a particulate filter (awful things) Then running at low revs will soot it up and you will need to blow it through at high revs now and then. But he NCR will usually be high enough (75%-90% of maximum continuous rating [MCR]) that this will not be an issue.
 
trapezeartist said:
I have never understood the obsession of some people to "warm up" their engine by leaving it to idle for 10 or 15 minutes. It will warm up far quicker if put under a light load (pootling out of the marina or anchorage) and spend far less time attempting to shove thick, cold oil around its internal parts. Oddly enough, it always seems to be the smokiest, smelliest engines that are left idling. You'd think they would cotton on eventually.
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Agree; Idling an engine especially when cold creates oil starvation problems and therefore excessive internal wear
 
During a longish spell under engine one or other of us inevitably catches a foot, trouser leg or whatever on the throttle lever so speed never remains constant for hours at a time. :)

With regard to idling/warming up, we get the boat ready in all respects for departure and then start the engine. Once sure we have good water circulation, 30 secs?, we let go and depart. On arrival we try to stop the motor as soon as we have the boat secured. I cannot understand owners who start their engine and then put the kettle on, wander around slowly getting ready to leave, have a coffee and then finally set off. :mad: Some slight exaggeration there but you get the picture.
 
Leighb said:
During a longish spell under engine one or other of us inevitably catches a foot, trouser leg or whatever on the throttle lever so speed never remains constant for hours at a time. :)
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Yes, especially when you travel with kids!!
Sometimes I wish I had a box installed around the throttle lever....
 
AngusMcDoon said:
Very different engine technology and economics as well. Fuel is a major cost, unlike for sail boat engines, so optimising for fuel consumption is important.
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Large industrial gas turbines, again totally different from diesel engines but closely related to aircraft ones, run for remarkably long periods at constant revs. In LNG compressor drives I know of some that do 18 months between shutdowns.
 
I agree with the suggestion that you shouldn't run an engine at idle for long.

On the other hand I learnt the hard way that it's a mistake to cast off immediately as a cold engine (Volvo single, in that case) can stall if you put it into gear immediately (or the few seconds time it takes to drop the mooring) after starting it from cold! I now always give a boat motor a minute or two to warm first. (Particularly if, as then, I have a rocky shore a few yards away!)
 
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