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Old 05-12-2020, 15:22   #1
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Optimal cruising RPM

I have a Volvo MD22P naturally aspirated diesel on my boat and the manual says the maximum RPM is 3,500 to 4,200 which seems like a lot. I never push the engine anywhere near that and typically cruise at 2,300 RPM which pushes my 16 ton 43.5 foot yacht along at about 5.5 knots in flat and calm conditions with my 19 inch prop. I sometimes do short blasts at 2,700 RPM or so to avoid carbon build up. Fuel consumption averages approximately 3 litres per hour.

Does anyone know if my cruising RPM is optimal for obtaining maximum longevity of my engine? Are there any other factors I should be considering? When I start the engine I usually keep it idling at about 1,500 RPM for a few minutes before weighing anchor or casting off.
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Old 05-12-2020, 16:09   #2
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Re: Optimal cruising RPM

My guess is what you are doing is fine.
There will be all sorts of arguments here but most say warm up gently & then let it cool down by idling for a few minutes before stopping the engine.
In general more power & higher revs mean more wear. If you have identical engines except for add ons such as turbos or whatever to increase power output the lowest power engine will last the most hours before needing rebuild.
Doing the servicing is vital. Clean ( filtered) fuel, air & oil are all important
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Old 05-12-2020, 16:46   #3
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Re: Optimal cruising RPM

Although I have never had a Volvo MD22P, I would do what you are currently doing. I would probably take it higher than 2700 for short blasts.

I would also take it to WOT for say 30 mins especially just after the having the bottom and prop cleaned. Log the rpm at WOT along with oil pressure and coolant temperature. Note the oil pressure when returning to idle. This will give you a baseline for diagnosis of future problems.

E.G. if your cooling system can't cope for 30 mins of WOT, then something is failing with the cooling circuit. If you can't reach the same RPM at WOT, then your prop or bottom is getting dirty. If you can't reach the same rpm with a clean prop and bottom, you may have developing injector or fuel issues etc
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Old 05-12-2020, 18:28   #4
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Re: Optimal cruising RPM

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Originally Posted by Wotname View Post
Although I have never had a Volvo MD22P, I would do what you are currently doing. I would probably take it higher than 2700 for short blasts.

I would also take it to WOT for say 30 mins especially just after the having the bottom and prop cleaned. Log the rpm at WOT along with oil pressure and coolant temperature. Note the oil pressure when returning to idle. This will give you a baseline for diagnosis of future problems.

E.G. if your cooling system can't cope for 30 mins of WOT, then something is failing with the cooling circuit. If you can't reach the same RPM at WOT, then your prop or bottom is getting dirty. If you can't reach the same rpm with a clean prop and bottom, you may have developing injector or fuel issues etc
That's an interesting suggestion and makes sense - thanks. In my case the max RPM of 3,500 to 4,200 RPM just seems so high that I'd be worried it might damage the engine somehow. This is probably an irrational concern as clearly the engine has been designed to run at these speeds, but the noise and vibrations would be disconcerting.
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Old 05-12-2020, 18:45   #5
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Re: Optimal cruising RPM

Here is a short extract from the Volvo manual.

Operating the engine at wide open throttle (WOT) should be avoided since it is both uneconomical and uncomfortable. Volvo Penta recommends a cruising speed in the range 300 - 500 rpm lower than maximum rpm at WOT. Depending on hull type, choice of propeller, load and conditions etc. the maximum engine speed at top speed can vary, but it should be within the WOT range.
Wide Open Throttle (WOT) range:
MD22L ............ 2,700-3,000 rpm
TMD22 ............ 3,500-4,500 rpm
MD22P ...........3,500-4,000 rpm
TAMD22P .......3,500-4,500 rpm
If the engine does reach the wide open throttle range it may be due to one of the reasons covered in the chapter: Fault-tracing. If the engine speed exceeds wide open throttle range a propeller with a higher pitch should be selected. Ask your Volvo Penta service dealer for advice.



From http://www.catamaransite.com/files/V...ction_Book.pdf
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Old 05-12-2020, 19:12   #6
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Re: Optimal cruising RPM

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Originally Posted by Wotname View Post
Here is a short extract from the Volvo manual.

Operating the engine at wide open throttle (WOT) should be avoided since it is both uneconomical and uncomfortable. Volvo Penta recommends a cruising speed in the range 300 - 500 rpm lower than maximum rpm at WOT. Depending on hull type, choice of propeller, load and conditions etc. the maximum engine speed at top speed can vary, but it should be within the WOT range.
Wide Open Throttle (WOT) range:
MD22L ............ 2,700-3,000 rpm
TMD22 ............ 3,500-4,500 rpm
MD22P ...........3,500-4,000 rpm
TAMD22P .......3,500-4,500 rpm
If the engine does reach the wide open throttle range it may be due to one of the reasons covered in the chapter: Fault-tracing. If the engine speed exceeds wide open throttle range a propeller with a higher pitch should be selected. Ask your Volvo Penta service dealer for advice.



From http://www.catamaransite.com/files/V...ction_Book.pdf
Thanks, so that means I should be cruising at somewhere between 3,000 and 3,700 RPM depending in my propeller and hull type. Even the lower end seems very high and I'd be using a lot more fuel and it would be noisy. I wonder why Volvo recommend such a high cruising speed?
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Old 05-12-2020, 21:31   #7
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Re: Optimal cruising RPM

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Originally Posted by Budawang View Post
Thanks, so that means I should be cruising at somewhere between 3,000 and 3,700 RPM depending in my propeller and hull type. Even the lower end seems very high and I'd be using a lot more fuel and it would be noisy. I wonder why Volvo recommend such a high cruising speed?
I suspect it is because some boats need the hp developed at theses revs in order to go fast enough for their gin fizz owners.

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.

And likewise, it is OK to travel below hull speed for the sake of sanity.

My last boat had a Yanmar 2GM20, max continuous 3,400 and max short term 3,600. Most of the time I used 1800 to 2200 depending on circumstance and about 2800 if in a hurry. Only ever went above 3000 in adverse conditions. However I occasionally would run at WOT (3600) just to know that everything was still OK (cooling, engine mounts etc).
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Old 05-12-2020, 21:42   #8
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Re: Optimal cruising RPM

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Originally Posted by Wotname View Post
I suspect it is because some boats need the hp developed at theses revs in order to go fast enough for their gin fizz owners.

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.

And likewise, it is OK to travel below hull speed for the sake of sanity.

My last boat had a Yanmar 2GM20, max continuous 3,400 and max short term 3,600. Most of the time I used 1800 to 2200 depending on circumstance and about 2800 if in a hurry. Only ever went above 3000 in adverse conditions. However I occasionally would run at WOT (3600) just to know that everything was still OK (cooling, engine mounts etc).
I guess the only other argument for running at higher continuous revs is that higher engine temperatures are better for the engine?
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Old 05-12-2020, 21:49   #9
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Re: Optimal cruising RPM

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I guess the only other argument for running at higher continuous revs is that higher engine temperatures are better for the engine?
Well, the coolant temperature will remain the same as it is controlled by the thermostat however cylinder chamber pressures will be higher at higher rpm.

I will leave to the diesel experts to argue one way or the other about the benefits or otherwise of that.
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Old 05-12-2020, 22:46   #10
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Re: Optimal cruising RPM

Certain your engine will have a longer life running at 2300rpm cruise than 3000-3700 for the simple reason more power more wear plus the fact the the crankshaft does more rotations & the piston goes up & down more often in an hour.
As Wotname says the coolant temp should be set by the thermostat but your oil temp, which is a vital part of the cooling process will be a lot higher if you run at 3700rpm than 2300.
Of course your journey time will be shorter but go back to Wotnames thought re hull speed.
Pretty sure the journey time at 3700 rpm wont be half that of at 1850rpm.

The higher engine temps are sometimes aimed for economy reasons rather than longevity. You can get more efficient fuel burn & less oil drag ( the viscosity drops ) as temps rise.
I dont know if its true but I was told F1 car engines use SAE 10 oil for reduced drag but the engines dont last long.

But this is just from internet research. I'm not a mechanical engineer.
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Old 05-12-2020, 23:28   #11
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Re: Optimal cruising RPM

No need to go WOT but high enough to be on the right running temperature. But you have no way of knowing what that is without a pyrometer in the exhaust manifold. I don't remember what it should excactly be but around 500C. To be sure would check that..
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Old 06-12-2020, 00:07   #12
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Re: Optimal cruising RPM

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No need to go WOT but high enough to be on the right running temperature. But you have no way of knowing what that is without a pyrometer in the exhaust manifold. I don't remember what it should excactly be but around 500C. To be sure would check that..
Thanks for the tip. That sounds HOT! It would be useful to install a temperature gauge in any case as advanced warning of any overheating issues.
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Old 06-12-2020, 00:14   #13
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Re: Optimal cruising RPM

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Originally Posted by Budawang View Post
Thanks for the tip. That sounds HOT! It would be useful to install a temperature gauge in any case as advanced warning of any overheating issues.
That's also the solution for "over propping" to know for sure you don't overload the engine. Over propping is a way to get more efficiency running at lower rpm's..
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Old 06-12-2020, 02:32   #14
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Re: Optimal cruising RPM

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.
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Old 06-12-2020, 02:45   #15
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Re: Optimal cruising RPM

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 Iíd typically cruise, provided thatís high enough to get the engine up to operating temperature.
http://www.sra-moteur.com/uploads/ca.../md22-md22.pdf
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