Motorsailing tactics

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RJJ

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Hi all

When on a long passage and we have, say 6 knots of wind on a beam reach, we would often motorsail. The choice is then: run the engine at low revs (typically 1800 or so), sip fuel and do normal motoring speed...or run the engine at normal revs (2200), enjoy a little more speed but burn diesel at the normal cruising rate.

Clearly if we're in a hurry it's option 2. But if we want to save fuel...option 1. In that event, should I be worried about running at prolonged spells at low revs? Appreciate any thoughts, thanks
 
If you are still getting some drive from the sails, you will not be burning the same amount of fuel with the engine at 2,200 revs as without sails. The engine will only use the amount of fuel that's required to get to whatever revs you set: consumption is (essentially) load dependent, not revs dependent.

If you really want to save maximum fuel the options are go somewhere else, turn off the engine and take whatever time it takes, or stay where you were.

The consensus seems to be that (except with new engine) running the engine at low revs & loads is OK, provided that you vary the revs from time to time on passage, and give it the occasional blast with revs and load. For the expert view see Vyv Cox's site on the topic here, especially the 'Avoiding Problems' section at the end. http://coxeng.co.uk/engine/bore-glazing-and-polishing-in-diesel-engines/
 
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There is little doubt that running a diesel at low load, especially when the low load is at high revs (which it can be when motor sailing) can cause bore glazing and excessive ring wear which in extreme cases can result in compression loss and excessive crankcase pressure blowing oil out of the breather etc. There is no need to get paranoid however. As long as such use is not continuous and you also work the engine hard from time to time most engines will tolerate this use. The trick is to give the engine some real work to do at normal revs and power when possible and the bore glaze is broken and carbon build up avoided.

addendum;
i have just read vyv cox article referenced in the previous reply and commend it as an excellent summary. I have just rebuilt 2 TMD41a volvos that were blowing oil. On strip down the bores were measured and found to be within .0005in of new size but very heavily glazed. The pistons themselves were also within tolerance but the ring gaps were oversized by up to .030ins indicating excessive wear. All bearing shells were almost unworn. The previous owner had used the boat on Dutch canals for most of its life and I suspect was only ever run at light loads from new. Honing the bores to restore an acceptable finish (as in Vyvs photo) and fitting new rings with the correct gap has restored both engines to as new performance. I also agree that the "running in" period is critical. My above comments regarding "Its OK for short periods if you give them welly after" should be read as applying to a properly run in engine.
 
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6kts of wind should be fine but also consider the max angle of heal the engine can work at; 15 degrees max is quite common. Also, the oil alarm my not go off if a damaging oil emulsion is created.
 
dom said:
6kts of wind should be fine but also consider the max angle of heal the engine can work at; 15 degrees max is quite common. Also, the oil alarm my not go off if a damaging oil emulsion is created.
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Thanks Dom and mike, very useful and also for the link to Vyv's advice.

What's the issue with heel angle? Other than water intake (which is near-as-dammit on the centerline). Surely the engine is designed to operate at heel to charge batteries when sailing close-hauled?

That said, if healing more than 15 Deg I probably wouldn't be motorsailing anyway.
 
If you frequently motor sail, it is worth considering fitting a Bruntons Autoprop. As the name suggests, the pitch of the blades is automatically adjusted to suit the conditions. Say that under power 1000rpm gives you 2knots. You are sailing in light conditions, and only doing 2knots. With an Autoprop, if you switch on the engine as well, at 1000 rpm, you will do 4knots, because the propeller has automatically gone to a coarse pitch, thus putting some load on the engine. I have no connection with the company, other than being a satisfied customer.
 
RJJ said:
Thanks Dom and mike, very useful and also for the link to Vyv's advice.

What's the issue with heel angle? Other than water intake (which is near-as-dammit on the centerline). Surely the engine is designed to operate at heel to charge batteries when sailing close-hauled?

That said, if healing more than 15 Deg I probably wouldn't be motorsailing anyway.
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The angle of heal is limited by the angle at which the sump oil pickup pipe starts to suck air. Sometimes one receives a low-pressure alert from the oil pressure alarm but not always. BTW I know you’re talking about lighter winds here, so I’m really only mentioning this as one can easily either wreck an engine, or seriously shorten its life this way.

That said, the max continuous angle is usually specified in the manual although 15 deg is a reasonable rule of thumb. I have however seen max angles of 12 degrees, so best to be sure.
 
RJJ said:
Thanks Dom and mike, very useful and also for the link to Vyv's advice.

What's the issue with heel angle? Other than water intake (which is near-as-dammit on the centerline). Surely the engine is designed to operate at heel to charge batteries when sailing close-hauled?

That said, if healing more than 15 Deg I probably wouldn't be motorsailing anyway.
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Some engines don’t like motor sailing for longer periods especially Volvo MD11 etc as they do run short of accessible sump oil and return of sump oil as I recall
If you add heeling and sea climbing you can end up with excessive angles
Short tacks would help
 
Turns out Volvo penta D2 55hp is spec'd for 35 degrees tilt while running, both sideways and fore-and-aft.
 
My 2030 is only rated up to 15 degrees, which can actually feel quite well heeled. It is rare to reach this figure except when close-hauled, but in these conditions I would normally reef one more than I would when sailing, and probably set the autopilot to steer slightly pinched. My cruising revs are usually about 2500 and I would not normally motor-sail at less than about 2200. I have the impression that my 2-blade folding prop is more efficient at higher speeds, when economy may be scarcely worse.
 
NormanS said:
If you frequently motor sail, it is worth considering fitting a Bruntons Autoprop. As the name suggests, the pitch of the blades is automatically adjusted to suit the conditions. Say that under power 1000rpm gives you 2knots. You are sailing in light conditions, and only doing 2knots. With an Autoprop, if you switch on the engine as well, at 1000 rpm, you will do 4knots, because the propeller has automatically gone to a coarse pitch, thus putting some load on the engine. I have no connection with the company, other than being a satisfied customer.
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Second that. Having had one they do exactly that
 
When off the west coast of Portugal, all the oil blew out of the engine due to stuck rings in no.2 cylinder. Bores were highly glazed and when I phoned Diamond Diesels for new rings, gaskets etc., the first question was "been motor sailing a lot at low revs?" No, we hadn't but the boat was fairly new to us so history not known. Advice from them (they're the UK Mitsubishi engine importers) was that diesel engines need to be run under load and should be given bursts of full throttle for a few minutes periodically or, the engine will be damaged if running at constant low revs/load such as motor sailing or charging batteries.
 
When I saw the title of the thread I was hoping to glean a different result rather than the implications to the engine. Other than running the engine while taking down the sails, I don't believe I have ever motorsailed for any time but have been told I may with my new boat.
My new boat is an Oceanvolt equipped J/88 with the limited range that battery power will give me. Talking to the Oceanvolt people they said I would quite likely find myself motorsailing to extend the range and that using the motor to create apparent wind would be like magic and I would greatly extend my range. Does this make sense? I can see that adding some thrust from the prop will move the apparent wind forward meaning I would have to bear off which goes against the thought on a VMG basis.
Have I go this wrong?
Dan
 
So most offshore racers, beyond 24 hours, get into significant running of the engine without load...is that just tough on their engine? Or do they just cheat...
 
RJJ said:
So most offshore racers, beyond 24 hours, get into significant running of the engine without load...is that just tough on their engine? Or do they just cheat...
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Over long periods, yes it's tough on the engine. Many small boat diesels are marinised generator and plant engines, designed to run alternators or hydraulic pumps at constant high revs. They're inefficient at low revs and unburned fuel gums up the rings and polished bores. Most of us motor sail at times but vary the revs and every now and then bang it up to full throttle to clear it out.
 
I think the main thing is to start by using significant power for the first ten minutes, to get the engine up to temperature.
And turn the engine off whenever it's not doing any real work.
But most people use motor sailing as a chance to get the batteries charged, so want to run the motor for some hours.

End of the day, if your engine is only using a small amount of diesel per hour, it's likely not to be getting properly hot so bore glazing is a risk.
How long does a fast idling dieel take to drop its cyclinder wall temperature? 10 minutes? 30? I guess 'it varies'?

I don't like having the engine, on so when we do, I prefer to get fair benefit from it and keep the speed up. If the wind seems to have come up, turn the motor off and see if we can sail OK.
 
Running the engine at 1800 rpm it's not "low revs". The main thing is to remove accumulated carbon from the cylinders, to prevent carbon ignition and carbon deposits, this is why it is advisable to increase the rpm occasionally to burn out and to remove the carbon (in the case of cars when revving the engine to clear it out, the black smoke is primarily accumulated soot from the exhaust not so much from the engine). Mineral oils will burn and will deposit carbon in the cylinder; synthetic oils will not. I suggest that running the engine at the rpm that develops its peak torque is a good practice (and maintaining engine operating temperature); peak torque on small diesel engines is normally developed at below 2000rpm (most likely at about 1500 to 1900rpm). Of course, if we want to beat against tide and wind we need the engine to develop the max power which is likely to be above 2500 rpm. The main problem with small marine engines is when the engine is idling at say 700rpm for hours. By the way, I motor sail quite often but never push the engine unless there is a reason.
 
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cpedw said:
How often and by how much do you vary the revs? How long is the interval between "every now and then"?

Derek
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It's not possible to state how many revs, how often and for how long. My use isn't typical as we don't day sail and the entrance to the Ria on engine is about an hour so it's always run up to temperature for a while. Thornycroft specify m 1500 rpm minimum constant revs for my particular engine so except when berthing we rarely go below that. Most engines have a sweet spot where vibration is at a minimum (all down to harmonics) and owners usually know at what revs theirs "feels" best and can also tell whether it's under load or not.

If you're adding engine power to sailing speed to just add a knot or so, then the engine most likely isn't running under much load which is bad for it over repeated extended periods. A plant engine may be running at constant revs but the load varies as it works whereas the load on a prop in smooth seas doesn't to the same extent so, vary the revs every now and then and give it a blast against the stop every few days of use.

(When I was in the motor trade, we used to get steriotypical old ladies shopping cars in, "it's just not running very well". A good blast though the gears at high revs a few times usually had it running much better)
 
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