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

[piratos]

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 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?

Many things have been said, and I just wanted to add that you can ofcourse do slow speed passages, but the max displacement speed of this boat is around the 1000 RPM, and you can not compare with any LRC built for these speeds and they usually do long passages with speeds of less than max displacementspeeds. They have a proper rudder the size of half a door, whereas you have two tiny spades and no keel. With anything else than a flat sea you will need pressure on the rudders to run a straight line. You have to accept the 1000 Euros extra fuel for this delivery trip, and enjoy the boat

 

[Hardmy]

I have 2xCAT3208TA's (so no electronics here) and do occasionally do a 120NM+ trip in one step. When seas are very calm, I don't mind to drive slowly (1300 RPM / 7.5 kts) through the night and others sleep. But then the engines are clearly underloaded, so I give a blast of 20 minutes at 2200 RPM every 2 hours. In deed, quite some soot is pushed out the first 1-2 minutes. My mechanic told me, that the engines are only well loaded (and turbo pressure OK) from 1600 RPM. And it should be the revs I should aim for.

Good Idea Kashurst -will get him out pf jail
If its a twin disc -likely ? - go to Page 29 "windmilling "
http://www.marind.ca/medias/pdf/produits_marins/twindisc/td_operators_manual.pdf

My engines are coupled to Twin Disc gearboxes. I have been told to never allow freewheeling to happen. Now, the manual you posted is less categoric. I will discuss it with my mechanic, because I am really considering running from time to time on only one engine. In our cruising ground (the Ionian) there are days where the sea is flat like a pond. Then we don't mind to proceed with only 7 kts.

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

Some seals, like the PSS's installed in my boat have a cross over pipe. So the cooling of the running engine lubricates also the freewheling shaft. And, if I remember well at low speeds like 6-8 kts, the lubrication comes also from the back pressure of the seawater creeping up the shaft. So you might not need a crossover pipe at all.

Like Hurricane, slow cruises are always finished off with 2200 RPM for 20 minutes. Every second day, I also run the engines at WOT for 5' and check if I get my max RPMs and whether all pressures and so on are OK.

 

[Eren]

...You have to accept the 1000 Euros extra fuel for this delivery trip, and enjoy the boat

Hi Piratos,

Unfortunately there is a difference of EUR 4.000 between cruising with 10 knots and 26 knots for this passage of 750 nm

Your point is fully correct about technical setup of LRCs. However about keeping the line, planing boats are not that bad as well. My previous boats (Azi47 and Princess 64) were all able to keep the line at D speeds at seas of 3-4 Beaufourt. Above that it is better to power up and keep hold of the control against waves.

 

[Hurricane]

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).

Not sure how you would do that with electronically engaging gearboxes.
Not even sure it is possible.

 

[MapisM]

Not sure how you would do that with electronically engaging gearboxes.
Not even sure it is possible.

Good point. Shutting down the engine while at idle but with gear engaged, maybe?
Just a thought - I never actually tried that, but I can't think of any logical reason why it shouldn't work.
In fact, coming to think of it, is there such thing as a REALLY electronically engaging g/box?
I mean, I've only seen electronically controlled g/box leverages, which are a different kettle of fish altogether.
More akin to mechanical governor engines driven by electronic throttles via actuators, if you see what I mean.
But maybe there are also 100% electronic g/boxes nowadays, 'dunno....

 

[piratos]

Hi Piratos,

Unfortunately there is a difference of EUR 4.000 between cruising with 10 knots and 26 knots for this passage of 750 nm

Your point is fully correct about technical setup of LRCs. However about keeping the line, planing boats are not that bad as well. My previous boats (Azi47 and Princess 64) were all able to keep the line at D speeds at seas of 3-4 Beaufourt. Above that it is better to power up and keep hold of the control against waves.

I absolutely agree on all above points, however his intention was to do the 750 nm passage at speeds of less than Max Displacement speeds - and the difference between 800 and 1000 RPM gives the aprox difference of EUR1000 (at Croatian prices ) When you are talking D-speeds of your P64 I take it for granted you mean around 10 knots ant not 7 knots.

A Full displacement boat of 70 can also manage the 10 knots, but at speeds like the 7 or even less the the planning hull is clearly not the best option.

 

[Hurricane]

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).

Well, for reasons that I won't go into right now, this summer we did shut down an engine whilst it was in gear!
I didn't have time to check but I think the gearbox slipped back into neutral.