Does running a planing boat at low rpm damage the engines?

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Eren

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Hi,

I have stepped up from an Azimut 47 with 2 x 575 HP Cat C9s to a Sunseeker Manhattan 70 with 2 x 1550 HP MANs. I am naturally quite happy with this upgrade but, I am also worried about the very possible increase at my yearly fuel budget. To keep the fuel bill under control, I decided to cruise at displacement speeds. The rev at idle is 600. I heard that diesel engines must be under load for healthy usage. So I plan to cruise just at 700 rpm, where I burn around 3,5 lt/nm and the engine is a little bit under load. I am aware that, for healthy usage at low rpm, I should first bring up the engine to nominal working temperature, and also I have to speed up now and then to throw away the carbon deposit accumulating at the exhausts. I will soon be covering 750 nm to Montenegro and I plan to cruise at such low rpm.

My questions are:
- Will I be ruining my engines if I cover such long range with such low rpm?
- Are there other people at this forum who has been cruising this way for many years and have seen the effects on the engines?

I will be grateful if you will share your experience about this.

Thanks.
 
This is complex area ,historically yes , -but -these days with electronic this that and t,other ,it all depends if the electo-twackery does not over fuel it at such low rpm -if that happens overfuelling then the excess un burnt fuel washes the lub oil film in the cylinder , which is not good .
Any how assume MAN - modern electro -twackery is your friend .
Next prob is oil pressure in both engine and may be gearbox ?
700 is really low -thought about 8 something like 875 ?
I mean what is max D speed within reason before a big bow wave makes its counter productive ?

I have 1/2 your 24L engines in 14m or thereabouts ( smaller wetted area with 700 ,, 6 cylinder ) modular -so your pistons fit mine etc

Occasionally we do D but 8-900 ,from memory 50-60 % load about 20L /side 'so 40 L / h ish - total 'gives 9.+ something Knots .
Only because we want to arrive not too soon ie want a longer cruise .,not to save fuel

The sweet spot is 1760 -1850 rpm turbos nicely spooled up -all gauges at optimum
At tick over 600 the oil P is 1/2 or 1/3 where it is at cruise "chat "17-18 00 rpm
I think 8-9 00 itmoves up enough
But 700 eek feels low .Since oil is needed to flow round and be cooled /filtered -I may be wrong buts that what would worry me . Oil circulation ! There is not any electro-twackery here -simple mech oil pump relies on RPM
Temps -cruise 86 ,
8-900. Drops to85 --- so I,am not worried by going this low
Tick over 600 in marina mooring after coming back ( hot ) drops to. 82 in a minute . Tells me cooling system is optimised for high speed .
I know 82 to 86 is nowt really -I mention temps cos the MAN book says do not let them idle for for than 5 mins -turn them off -due to over cooling !
Before I get shot down -I did not write the owners manuel !-just quoting it !

I would say 750 Nm @ at 700 rpm ---hmm --- I would not do it - 8-900 rpm -yes - would do as oil P looks right and load @55% seem ok ish .
What's your burn rate @ 8-900 ?

I did Naples to Cannes @ 1750 rpm

Loads a funny thing it's zero at tick over in N - 600 rpm first click in F - suddenly 35 % same rpm 6-7 knots
About 55 % at 875 rpm 9 + knots
About 85 % at 30 knots
Above 36 knots its 95 % ,topping out a 98% -38 knots 2200 rpm
feels like its a curve ,not linear
Those V12 -24 Lumps are going to need deep pockets @ annual service compared to D9 ,s
Fuel bill like for like 3-4 X as much
 
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First, congrats that is a lot of boat and a lot of engine.
The link from Nick H talks about glazing and loading the engine. First, the article says as long as the props are turning then it's under load. But to me the way to calculate a maximum load that can keep the bores from glazing would be to look at the range of engines from the same manufacturer using the same block (or half block). What is the lowest HP output from a non-turbo model (usually from the engine that is rated for continuous duty). And at what revs is it achieved at. That number (actually probably about 80% of that number) is what it needed for the health of the bores for those short burst. And this is done without revving the life out of the turbos and drinking fuel at a rate of several liters per minute.
 
It will be loaded at 700 rpm , with F engaged - you can see the % on your instruments .
The v ,s shut down a bank at idle which is 600 , so when does the other bank cut in .? 601 rpm or 750 rpm ? . In running all day on one bank of V12 at low rpm means the redundant bank will cool down ,
But the real issue with sustained low rpm is EGT .
Too low Exhaust Gas Temp and the fuel does not burn property ,leading to fouling of the exhaust valve ,turbo ( which is idle ) -that's what an "Italian tune up " does cleans it all out .
Diesels need a hot EGT to burn properly 500-600 degree not all day @ 300 degree .

Look @ your load and EGT at 700 rpm That's more important l--never mind the L/h
That's what MAN mean by not letting it idle for more than 5 mins --it's the drop in EGT = poor combustion - not water temp

On a long passage like 750 mile I would cruise @1650-1750 rpm .
If you ever do a slow D cruise then give it an"Italian tune up " for the last 10-15 mins
 
you could drive on one engine only for a couple of hours then start the other one and swap over etc. Just check your gearbox spec to check they are OK for a few hours trailing along (they usually are fine). This way you are keeping the fuel burn down and properly loading the engine(s).Going 750m you may find the weather will force you to drive the boat differently as well to keep rolling down. Good luck.
 
We had a customer who bought a Sealine withg two D 4's and after sevral months he came in complaining of high oil consumption on his new engines, we checked the Levels and told him not to refill for two weeks and come back so we could verify the consumption, they were using oil. Then I asked how he used the boat, he was a pensioner and just went out most days with friends and cruised the fjords at idle speed and virtually never went on the plane. He had glazed the cylinders as the rings need the high gas pressure from running under load to make an effective seal and for the scraper rings to do their job.
And on the Princess 95 we operated we ran at about 9-10 kts for a max of 4 hrs them opened her up to get all 3 turbos on each engine working and to blow the soot out. The fuel consumption as max speed of 24 kts was 1000 ltp as opposed to 100 lph at 10 kts, so we used the lower speed when positioning the boat, but would run her harder if the customer was paying for the fuel. The point is we opened her up frequently to avoid problems later.
 
Deleted User made a very valid point a while ago, that any saving in fuel cost has to be set off against higher depreciation related to higher engine hours. If the boat is fairly new and high value, then it may well be cheaper overall to run at planing speeds.
 
Eren said:
I have stepped up from an Azimut 47 with 2 x 575 HP Cat C9s to a Sunseeker Manhattan 70 with 2 x 1550 HP MANs.
Click to expand...

The short answer is that if you can afford to buy a 70 footer with bloody great big V12 diesel engines in it you can afford to run it at planing speed;). But having said that I have the 1200hp version of those same engines in my boat and it is quite alarming to see the fuel burn displayed on the instruments at planing speed! Around 250lph at 22kts and 460lph flat out at 34kts. So I run my boat at 900-950rpm for long periods when I'm cruising which gives about 9.5-10kts which for me is a reasonable compromise between saving fuel, loading the engine and making progress. At that speed the engines are consuming around 45-50lph. However what I always do also is run the engines at planing speed for on average 20mins out of every hour to put the engines under proper load and make some proper progress too because I get bored at 9.5kts. I dont know whether your M70 has stabilisers but what you will also find is that at displacement speeds in beam and quartering seas from astern, the boat is more comfortable at planing speed anyway so there will be times that you will probably want to speed up for your own comfort
 
Hi There,

Welcome to the forum, and congrats with the boat !

We have done quiet some distance with our boat at D-speed, but mostly 1100Rpm – 10kn,
Actually this is just slightly above real D-speed. (70ft)
I am in bad position to say that this doesn’t affect the engines, as we had the two engines completely rebuild, ;-)
But imo that has nothing todo with bore glazing, other issues ..;etc
Here is the thread about these rebuilds
http://www.ybw.com/forums/showthrea...st-day-end-last-trip-of-our-summer-holliday’s

In their previous life (20y)
and the first 2 years of our ownership they have been running 99% at 2000RPM -20kn
the inside liner surface looked very well polished, as a mirror !
the last 4 seasons we ran around 50/50% D-speed – P speed.

I’m not sure what you mean by: “first bring up the engine to nominal working temperature”
I assume you won’t do that at big load / planing speed ?


As yours is also a 70ft, I would run her at around 10kn, I guess this will be around 1000rpm.(?)
this has several advantages:
-consumption is less than 50% than @ planning speeds
- you are still faster than most of other D-speed vessels / sailing boats, etc..,
- boat behaves very nice at that speed, at almost any sea state (we have stabilizers)
- your engines are not falling asleep, they have to do some effort,
I have no clue if I have a risc of bore glazing, I still run at planning speed every now and then.
Several people involved with MAN service told me that running at 1000rpm is perfectly OK for these engines.
I have the very old mechanical 12cyl, 24l MAN’s

FYI
we covered quite some distance and hours with her;
when we bought her in 2011 (2300hr on counter) we sailed her from Rome to France,
The next winter again from France to Rome and back,
3 years ago we sailed her from France to Montenegro,
And this last season we went up to Venice and back to Montenegro again.
Now she has 3500hrs on counter, engines were rebuild at 3000 (SB) and 3200 hrs (Port)
I Don’t remember total nm.


Are you coming to Porto Montenegro ?
When?
I’m on berth D23
 
Dear All,

Thanks a lot for your replies. I am really amazed with the wealth of information at your replies!

Portofino: Oil circulation is a good point. I have had a very good surveyor to do the survey before I bought the boat. With his report, he provided a very valuable table which shows engine characteristics from 600 to 2300 rpm with 200 rpm increments. A short summary is below:

RPM - Oil pressure - Load - Consumption - Speed
600 rpm - 1.68 bar - 0% - 2,6 lt/mile - 7,2 kt
800 rpm - 2.28 bar - 14% - 4,3 lt/mile - 9,2 kt
1000 rpm - 3.08 bar - 25% - 7,3 lt/mile - 10,8 kt
1800 rpm - 4.60 bar - 70% - 12,9 lt/mile - 26,0 kt

Gearbox pressure seems very flat around 19-20 bars, throughout 600 to 2300 rpm. So I assume that gearbox is lubed enough at all speeds.

With this, it looks like somewhere between 800 and 1000 rpm looks good.

About Exhaust Gas Temperature, I have to check it during the cruise. That is also a very good point.

Kashurst: I am a bit afraid of cruising with single engine. I want to save on fuel, but don't want to risk things too much.

Deleted User: Yes, I afford to buy the boat :cool: but I also want to make the most out of my fuel budget. Your schedule around 900 rpm looks good.

BartW: My Azimut 47 is still at Porto Montenegro, at C31 (M/Y ELA). I will be there occasionally during winter and will send you a message to see if we can come by. I enjoyed the tax free fuel last summer at Montenegro, EUR 46 cents per liter (yes, fourtysix cents !) and I will take the new boat to there as well. Not to mention the cruising pleasure at Montenegro and Croatian islands!

May be a stupid question but, don't they use the same set of engines at the slow cruising trawlers? I am sure they play with the gearbox ratios but I assume that these boats cruise at very low rpms as well. Any ideas?
 
Eren said:
My Azimut 47 is still at Porto Montenegro, at C31 (M/Y ELA). I will be there occasionally during winter and will send you a message to see if we can come by. I enjoyed the tax free fuel last summer at Montenegro, EUR 46 cents per liter (yes, fourtysix cents !) and I will take the new boat to there as well. Not to mention the cruising pleasure at Montenegro and Croatian islands!
Click to expand...

I'll be on Blue Angel (D23) the week after next,
pls send a message when you're around,

in may 2017 my 3 year booking will end, but
we love the region and the marina so much that we are considering to buy / lease the berth.

the best price for fuel I had at the beginning of last summer was 43ct ;-)

when will you sail the new boat to PM ?
 
Eren said:
May be a stupid question but, don't they use the same set of engines at the slow cruising trawlers? I am sure they play with the gearbox ratios but I assume that these boats cruise at very low rpms as well.
Click to expand...
Yes and no. I mean, it ain't just a matter of different g/box ratios.
Let me make an example I remember by heart, with Cat C12 (sorry, coming to think of it I never came across any trawler with either MAN or Cat C9 engines).
The pleasure boat version of the Cat C12 is rated for intermittent usage, and is capable of 715hp @ 2300rpm.
Its heavy duty brother has the very same block, but is rated for continuous usage, and can only produce 340hp (i.e. less than half!) @ 1800rpm.
Actually, there's much more than this behind engines ratings, but just to get the gist of it... :)
 
BartW said:
when will you sail the new boat to PM ?
Click to expand...

I am occupied with minor works of the M70 (120 lines of minor works :) ), so it is difficult for me to get there soon. I have one guy there who does the guardiennage and sends me photos every week. That is the best I can do now. But I envy you. I am wondering how beautiful it can be to cruise at the Kotorski Zaliv at this time of the year!

I am planning to cruise the M70 by May.

Have fun in MNE !
 
I think you are right MapisM. I have heard the same thing for Cat C9s. The generator version of it works at 1500 rpm forever. They stop the engines once for oil change every 6 months and then start it and another 6 months goes by.
 
CONCLUSION :

With the valuable inputs that I received here, with my talks with marine engine specialists and with my own previous experience, I decided to do this:

- Going out of the marina, I will warm up the engine by cruising at 1200-1300 rpms. If this is not done the engine stays cold at around 70 degrees (optimal temps are around 90-100 C) and this accumulates soot at the exhaust.
- Once the engine is at optimal temps, I will slow down to 900 rpms for an economical cruise.
- Every 2 hours, I will gradually increase the rpm up to almost full throttle for around 10 mins, so that the accumulated soot is thrown out.
- I have engine oil analysis records of my boat for the last 2 years. I will again take oil samples, will make the analysis and will share the results with you.

Frankly speaking, I had used my Azimut at similar style for the last 3 years. No engine issues, no turbo issues, no muddy oil. But when it comes to servicing, I pamper the engines like babies. I have the feeling that the marine engines are very tolerant, cannot be broken that easily.

As I said, I will share the results with you.

Thanks a lot once again for your valuable inputs !
 
kashurst said:
you could drive on one engine only for a couple of hours then start the other one and swap over etc. Just check your gearbox spec to check they are OK for a few hours trailing along (they usually are fine). This way you are keeping the fuel burn down and properly loading the engine(s).Going 750m you may find the weather will force you to drive the boat differently as well to keep rolling down. Good luck.
Click to expand...

IMO, this is a no no.
Most modern installations have water cooled glands.
Just running one engine may damage the other engine's gland.

Generally speaking, we cruise at two speeds - either 10 knots or 25 knots.
Never just at "tick over"
We always finish a slow (10 knot) cruise with about 30mins at 25 knots and more often, we alternate between slow and fast cruising.
 
Hurricane said:
IMO, this is a no no.
Most modern installations have water cooled glands.
Just running one engine may damage the other engine's gland.
Click to expand...
M, isn't it sufficient to leave the non-running engine in gear, to avoid that problem?
Afaik, the drag of a locked prop isn't much higher than the drag of a self-spinning prop, anyway (counterintuitive as it might seem).
 
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