Optimal cruising RPM

[Budawang]

Originally Posted by GordMay

The 3500 - 4000 RPM is the Full Throttle (WOT) operating RPM.
The RPM/Torque curve indicates highest efficiency between 2000 - 2500 RPM, which is where I’d typically cruise, provided that’s high enough to get the engine up to operating temperature.
➥ http://www.sra-moteur.com/uploads/ca.../md22-md22.pdf

Good to know. In the absence of knowing my exact exhaust temperature, I might now aim to cruise at the higher end i.e. 2,500 RPM (if range isn't an issue) with the occasional short blast at 3,000 RPM to clear the carbon deposits.

[mvweebles]

During a sea trial in anticipation of purchase, the engine should be run at WOT for at least 5 mins (preferably 10) to demonstrate it won't overheat or self destruct. If properly propped, it should reach about 300-400 lower RPMs than no-load WOT.

Older Diesels were designed with a duty cycle and load range in mind (versus gas engines in cars that run at highly variable loads). If properly mated, it should be run at about 80%-90% of the WOT RPM. If the engine is oversized for the application as many auxiliaries are, it becomes inefficient because there are laws of physics that keep the hull from going any faster. For a monohull with about a 3:1 beam to length ratio, a good cruising speed will be 1.1-1.2 x sqrt WL. If you're engine is only at 40% load, we'll, it's oversized.

Carbon buildup is mostly a wives tale these days, a holdover from the old 2-stroke diesel engines of WW2 era. Yes, long periods of idling at anchor are a problem, but running a 3500 WOT engine at 2200 RPM long term is not an issue.

Peter

[TeddyDiver]

Originally Posted by SV DINGO

Wow, i have a MD22L and i try not to run it any higher than 2000 rpm , most times when motor sailing it runs at around 1500 rpm but is did ask my diesel mechanic about it and he did recommend to run it at a higher rate from time to time to get a higher temp to burn carbon deposits.

If that's the most common use of your engine, and you never actually have the need to run at engine max rpm, then you have a case when over propping is advantageous as you can run the engine at higher loading at lower rpm. But as I said earlier it makes it difficult to determine the new safe max rpm to avoid over loading without knowing the exhaust temperature.

[thinwater]

Originally Posted by Wotname

...Really, there is no point in advancing the throttles once you get to hull speed unless you are "blowing out the carbon" or diagnosing some issue....

But the OP is talking about going 5.5 knots on flat water in a boat with a hull speed of closer to 8.5 knots. We're no were near that point. He is just creeping. He won't even feel the bow wave until nearly 7 knots.

[Wotname]

Originally Posted by thinwater

But the OP is talking about going 5.5 knots on flat water in a boat with a hull speed of closer to 8.5 knots. We're no were near that point. He is just creeping. He won't even feel the bow wave until nearly 7 knots.

Yep, completely agree. At this stage we don't know what rpm is required for his boat to reach hull speed but we can be sure there is 'no point in advancing the throttle once you get to hull speed...' (calm conditions etc).

[Compass790]

Gotta remember the OP's question was best revs for engine longevity, not fuel efficiency or speed.
I'd go with gordmays post #15 but it might be even lower revs for longevity. His key words were as long as it reaches operating temp.
Although any thermostat equipped engine should get to operating temp even at idle in time.
Op can do his full throttle blast in neutral if he wants to do it less loaded.
FWIW i worked for a mechanical engineer who had a naturally aspirated 75hp CAT diesel & he aimed to achieve an EGT of 450oC to get max fuel effficiency. Had a variable pitch prop to adjust to achieve that.



Still think what OP is currently doing is fine but following the discussion with interest.

[mvweebles]

If longevity is the question, chances are that if it is well maintained and serviced semi-regularly, it will go approx 4000-5000 hrs between major overhauls, maybe more, maybe a lot more.

Sailors kill engines by ignoring leaks, and motorsailing with engine on an angle for long periods. Excessive periods of low rpms to charge batteries or similar contribute, but for the most part, it's pretty tough to wear out a decent diesel. You have to kill it. Ignoring any sign of leaks is #1, not low rpm running

Peter

[Budawang]

There's been some great advice on this thread. Do you know what an acceptable rate of heal is for protecting an engine?

Originally Posted by mvweebles

If longevity is the question, chances are that if it is well maintained and serviced semi-regularly, it will go approx 4000-5000 hrs between major overhauls, maybe more, maybe a lot more.

Sailors kill engines by ignoring leaks, and motorsailing with engine on an angle for long periods. Excessive periods of low rpms to charge batteries or similar contribute, but for the most part, it's pretty tough to wear out a decent diesel. You have to kill it. Ignoring any sign of leaks is #1, not low rpm running

Peter

[Compass790]

Originally Posted by Budawang

There's been some great advice on this thread. Do you know what an acceptable rate of heal is for protecting an engine?

It depends on engine design but on our Yanmar its about 25o ( depends on sea state too) as then the oil pump pickup tube can come out. We know if heeled too far as the oil pressure alarm goes off.
If you can get a manual it may tell you in there what maximum angle is for your engine.

[mvweebles]

Originally Posted by Budawang

There's been some great advice on this thread. Do you know what an acceptable rate of heal is for protecting an engine?

Good question and I don't know the answer, but would guess it depends on the engine and if it has a deeper sump pan. Would also depend on where the oil pickup is which may be off-center so more of an issue on one tack than the other. All engine manufacturers give a max install angle, but that's fore/aft take, not angle of heel (generally in the 10-15 degree range, but varies widely).

Im no expert, but the way you describe running your engine - predominantly at around 2200 RPM - sounds about the way I'd run it if it were mine, though at warm-upI'd probably leave it at idle which is likely around 850 rpm instead of advancing to 1500

For the most part, sailors seem to over-think their diesels. Those who run idle/no-load for long periods to charge batteries aren't doing themselves any favors. But the ratio of engines that are killed due to neglect or poor maintenance (impellor and mixing elbow being leading cause of death) versus wearing out an engine is probably 100 to 1.

Use the boat. Enjoy the boat. If you see telltale signs of a leak including white fuzz indicating seawater, address sooner rather than later. Change the exhaust elbow and impellor at recommended intervals. You'll be fine.

Peter

[GordMay]

Originally Posted by mvweebles

Good question and I don't know the answer, but would guess it depends on the engine ...
Peter

See your engine manual.
For instance:
Perkins Installation Manual TPD 1317e
Chapter 1 Page 3
Engine Mounting
1.1 Installation Angles
(a) The engine is intended to be mounted so that the cylinders are vertical, when viewed from ahead or astern (figure 1). The heel angles that are permissible in service are shown in figure 2a for 6 cylinder models, and 2b for 4 cylinder models...
See page 3 ➥ http://s7d2.scene7.com/is/content/Caterpillar/C10351902

[a64pilot]

Wear and engine longevity has been found to very closely follow the amount of fuel used.
For example your engine may last for 10,000 gls of Diesel, whether you burn that amount in 3000 or 6000 hours, many bigger engines now schedule maintenance based on fuel used and not hours or miles. Simply put an engine that’s burning half as much fuel will pretty much last twice as long.

Engine efficiency peak is way over rated, first it’s pretty slight, and secondly its way overwhelmed by other factors like hull drag. Yes it’s technically accurate that your getting slightly more HP per unit of fuel burned at a set RPM, but I guarantee you that your burning more fuel per distance travelled than you will at a lower RPM, try it and see.