Working out optimum engine revs for cruising

[micha571]

Interesting post, I have been wondering about cruising speed, too.

I have a Bukh DV 10 LSME (saildrive) with a small two-bladed folding prop.
When cruising, I usually go to something betwenn 2.400 and 2.700 rpm. At 2.400 I'll reach around 5 kn in flat water/no headwind, at 2.700 she does around 5.5 knots. At full throttle she goes 6.8 knots, hull speed is 7 knots. Bukh manual says 2.700 is "75% of maximum output", so I think this should be ok.

Anyone else with a BukH DV10, what are your experiences?

Michael

 

[nigelmercier]

Loads of cars have an MPG readout as part of their computer functions.

I did consider such a device as a project. The first problem is measuring the fuel volume that is being burned, you need two sensors. A way around this is a mass air flow sensor, which measures the air intake. I bought one to experiment with, but in the end decided that a table of RPM against l/hr would suffice for the moment, and now my digital fuel gauge project does this conversion.

 

[Dockhead]

You'll have to experiment.

My Yanmar 4JH3HTE has redline at 3900, but is really happy only at 2200 - 2300, contrary to all the rules about cruising speed being x% of full revs. A Brunton Autoprop will make the best of relaxed low revs, as others have said.

 

[Roberto]

the "optimum rpm" may be very different depending on which variables you want to maximize

as an example, if you need the "highest miles per fuel litre", then almost invaribaly this leads to run the engine at its lowest possible rpm

hull resistance is basically speed to the n'th power, no decrease in specific power consumption can match that (in the engines one usually finds in boats), so miles per liter the slower you go the better it is

is that good for maximizing engine life ? I suppose (and believe) not, but then if you are crossing a calm stretch of ocean, it is up to you to decide where to put the limit between how long you want to wait and swim around the boat, and the number of days you would like your crossing to last

 

[Jamesuk]

70-80% of your engines throttle to avoid damage to the engine. I have recently been told that motor sailing for long periods is like engine suicide and that goes the same for using gori props. Methods to recover is high bursts at near max throttle or adding friction i guess like an additional alternator.

 

[tudorsailor]

I motor my Yanmar turbo with revs about 75% of max. However I have been told that it does a diesel engine good to be run at max revs every so often. Not sure how often that is and how long for. Also by doing this is apparently gives one piece of mind that there is still power in reserve

Any thoughts

Tudorsailor

 

[Jamesuk]

I motor my Yanmar turbo with revs about 75% of max. However I have been told that it does a diesel engine good to be run at max revs every so often. Not sure how often that is and how long for. Also by doing this is apparently gives one piece of mind that there is still power in reserve

Any thoughts

Tudorsailor

Jason down at Sea power (Volvo) said max for 30 minutes should be enough.

 

[westhinder]

Gori

70-80% of your engines throttle to avoid damage to the engine. I have recently been told that motor sailing for long periods is like engine suicide and that goes the same for using gori props.

Could you please explain what is wrong with the Gori props?

 

[Jamesuk]

Could you please explain what is wrong with the Gori props?

I would love to but im not either of the two professional engneers who told me. One of them went further than the other to say crossing the Atlantic under engine using a gori prop/motor sailing in overdrive (low RPM) is like giving the engine an incentive "to commit engine suicide'. A way out is to give it a 30 minute blast each day out of overdrive.

Just google it this s a start. We hover between 1600rpm to achieve 8.5 knots out of overdrive we do 7.5 at 1600 so to take the Perky up to 2300 was advised for 30 mins if we wanted to keep Perky happy the fact he's just started leaking coolant is not encouraging but thats another thread.

http://www.sailingmagazine.net/how-to/technique/1150-diesel-defense

 

[EdWingfield]

I set the throttle to achieve 4.2kn boat speed (10m beneteau)

The wind will eventually return. If it doesn't I'll have done 100M.

I got this from a Nic 32 owner of close to 30 years ownership.

 

[Malabar]

As other have advised, diesel engines like to be running at around 75% of full throttle. As my Yanmar handbook points out, do not operate the engine at low idle or at low speed and light load for more than 30 minutes at a time. Since unburned fuel and engine oil will adhere to the piston rings when operating at low speeds for long periods, this will interfere with proper movement of the rings and the lube oil consumption may increase

 

[Nostrodamus]

I suppose that most boat diesel engines are derived from car engines. When using them in cars and going through the gears they are taken up to near maximum revs before changing gear. In boats this does not happen much. We tend to set the throttle and leave it in one place so the engine is not very often used at high revs. I have heard of the "Italian tune up" where giving it a good blast for a while often does it some good.
I know boats are different to cars but is there a reason they don't have gears. maybe one for manoeuvring in marinas and one for cruising.
Going back to my original post... should you change your engine revs when motor sailing as the engine is often not under so much strain because of the assistance that is given by the sail? All that power to speed/ fuel consumption then goes out the window.

 

[reeac]

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!

Sorry, I misunderstood the statement - I'm well aware of fuel computers in cars having first had one back in 1992.

 

[AntarcticPilot]

I suppose that most boat diesel engines are derived from car engines.

No, they're mostly derived from light industrial engines; a few are purpose built (e.g. Volvo 200x series). Very few (in sailing yacht sizes) are derived from automotive base units. After all, we're mostly looking at BHP rating of around 30; when did you last see a car engine rated that low? This may not be true for larger engines, though.

What that means is that they are designed for running for long periods at constant revs, pretty close to their maximum power-output.

I'm afraid I don't do all the science; I simply run the revs up to the point (about 2,200 - 2,500) where I know that increasing it won't increase my speed through the water very much - it levels off between 5.75 and 6 knots through the water. Every once in a while - perhaps once in a week of sailing - I'll run it up to maximum revs for a short while to clear off carbon and so on. Doing this I get consumption figures that tally pretty closely with what I reckoned from the engine data, and I'm running at or close to hull-speed. However, the Volvo 2003 is notoriously rather more powerful than the Moody 31 needs; the story goes that Moody had bought a job-lot of Volvo 2003ss and it was cheaper to over-engine some of their range than it would have been to buy smaller engines.

I do recall one of my dad's boats (A Trotter made by Grimsby Marine Plastics) having a Briggs and Stratton engine that (with hindsight) was a lawn-mower engine; it was certainly very similar to the engines on two lawn-mowers I possessed. It did have one advantage; it was air-cooled, so no worries about cooling systems, water pumps, through-hulls, engine anodes and so on!

 

[lw395]

I would love to but im not either of the two professional engneers who told me. One of them went further than the other to say crossing the Atlantic under engine using a gori prop/motor sailing in overdrive (low RPM) is like giving the engine an incentive "to commit engine suicide'. A way out is to give it a 30 minute blast each day out of overdrive.

Just google it this s a start. We hover between 1600rpm to achieve 8.5 knots out of overdrive we do 7.5 at 1600 so to take the Perky up to 2300 was advised for 30 mins if we wanted to keep Perky happy the fact he's just started leaking coolant is not encouraging but thats another thread.

http://www.sailingmagazine.net/how-to/technique/1150-diesel-defense

That article has to win the prize for the least basic understanding of a diesel engine :
'Fuel and air are mixed in precise ratios'