Cruising speed on diesel engines

[MapisM]

Any experience/suggestion on the MINIMUM cruising RPM with diesel engines?
More specifically, my boat is well overpowered with two Caterpillar 3116, rated 350 hp at 2800 RPM.
In fact, being a displacement hull, it cruises at 9-10 kts in the 1800-2000 RPM range.
The rest of the range (2000-2800) is in practice never used.
Besides, after some trials, I discovered that 8-8,5 knots gives the best fuel economy, with the engines around 1600 RPM.
Now, I heard that this could be too low for those engines, which could suffer more than at higher RPM.
That's the reason for my question in the first line...

 

[LeytonC]

I dont have any exp. of cummins but the general rules I belive about diesel engines are (IMHO)

1) any revs as long as you have load on them, that is the important thing.
If there is no load they tend to polish the bores and then smoke as they burn
a drop of oil.

2) Rule 1 always applies unless the engine is turbo diesel, then you need to run
the engine at high enough revs to get the turbo 'working' ie. have the engine turing
fast enough for the engine to completely burn the fuel so it doesnt produce to much soot. If a turbo diesel runs at low revs the soot produced (becuase TDs have lower compression ratio) gets on the blades and causes balance problems which leads to bearing failure. (and expense)

3) If you always run at low revs and never run at high revs, when you ask the engine to rev high (I see this on the severn with big boats that always run on tick over then go to sea) the engines cannot cope and tend to have things break cos they are not used to the work.

But if your engines are old and have been running fine then i would not worry
about it, or do what i do, which is to take the boat for a realy good run every now and again to make sure she can cope with high demand without overheating etc.

Sorry I cannot be more specific about this, but i hope my answer is food for
thought.

(Please correct as nessesary)




Thanks

Leyton
(River Severn / Bristol Channel)

 

[kimhollamby]

Load is not purely relative to rpm...especially on a displacement hull. If your boat is pure displacement it is likely to be over-engined with those motors. What is the boat and what is its length on the waterline? The latter will give you an indication of the point at which the load is coming on the engines.



kim_hollamby@ipcmedia.com

 

[NorthernWave]

Fancy swapping for two BMW D6.
How do you find the Cummins generally?

Chris

Relaxation starts at the pontoon gate

 

[adarcy]

Re: Which owner which engine ?

Err

in the original thread, they were Cats
then LeytonC doesn't know about Cummins
and then you asked him about Cummins !

Is it me who's lost the thread?

 

[adarcy]

LeytonC is absolutely right.

If they are old/well run in engines don't worry about bore glaze, jsut give them a blast now and again to clear the rubbish out. If they are new, they must have some load much more frquently during the running in phase ( ? 100 hrs).

Engine mfrs specs show that big turbo diesels are getting a good turbo boost as low as 1100 revs so, as lomg as tis is under load not in neutral, that will suffice. In fact, all of the "serious" engines I have seen have a significantly lower rev and so power limit when they are rated for continuous duty rather than leisure rating. As long as you are above 1100-1200 in gear when run in - don't worry, just a blast every now and again.

Anthony

 

[Guest]

Cummins

I know this thread is not about Cummins but I must add (fingers crossed) that my couple of turbo Cummins have never given me any problems, they are economical, start instantly and don't smoke. They have a max rev limit of 3,000 and I cruise them at about 2,200 on longer distance runs.

Bernie

 

[MapisM]

Yes, it is pure displacement, wooden planked.
The displacement is in the region of 80.000 lbs, LOA 53', LWL 50'.
Approx.700 hours on the engines, no probs so far, fingers crossed.
Judging purely by feeling, when cruising at 8,5 kts/1600 RPM the boat actually gives the impression that the engines are almost not working. They're so quiet that from the flybridge, one have to look at the instruments to be sure they are both running...
A consumption of about 30 l./hour (not joking!) confirms that they're not stressed at all.
Over-engined, surely it is. The builder himself (who makes also fishing boats with similar hulls) admits it.
But I'm happy to live with that - as long as a blast now and then, as suggested by LeytonC and Adarcy, is sufficient to keep them in good order.
Would you confirm that, considering also the above figures?

 

[LeytonC]

Re: Which owner which engine ?

Sorry, it my fault, I ment Caterpillars but i was at work!!! and i had my mind in
neutral instead of forward. So I have the hat of shame.

Again I am sorry.



Thanks

Leyton
(River Severn / Bristol Channel)

 

[LeytonC]

Thanks for the confirmation anthony!,

(Please See my other reply thou )

I have nothing against Turbos at all, but the only problem
i see is on the river you cannot get them working hard enough
on correct props (most of the large stuff on my moorings are turbo
and smoke like crazy cos they are never really used hard)
thus they can cause problems when you
use them for sea work.




Thanks

Leyton
(River Severn / Bristol Channel)

 

[kimhollamby]

Your engines will not be coming under real load at much under 9.5 knots or thereabouts on your boat. The 30lph figure also suggests that (when the Cats are developing max power they would burn 140lph, which gives you some idea).

A quick blast will help but you can expect to get problems from bore glazing and sooting turbos if you run the engines at speeds which are most sensible to your boat and wallet (in terms of fuel consumption).

Can you run on one engine?

kim_hollamby@ipcmedia.com

 

[MapisM]

Hmmm... running on one engine at a time?
I never considered that - and fortunately never had to.
Well, the autopilot calculates automatically whether and how much the rudder has to stay port or stb, so I guess that after a while the boat should stay on its course (though I guess that it wouldn't work so well in rough sea).
But apart from that, isn't there any other potential drawback?

 

[kimhollamby]

Yes, you need to check that your gearboxes are okay to freewheel; a lot of hydraulic boxes are but some are not. Look at the manual, it should say.

And you need to check that the shaftbearings will be okay. If they are water lubricated via feeds from the engines you might have a bit of an issue with overheating on the shut down side at sea speeds (slow river speeds not normally so bad as prop doesn't windmill so fast). Answer is to put cross feeds on, but you'll need one-way valves too so that the working engine cannot back-fill the exhaust (and possibly at least one cylinder) of the dead engine.

Another answer is to lock the dead engine's shaft but that's quite a fiddle to organise.

Finally you'll find that your alternators probably do different jobs on each engine and you might have a calorifier plumbed off one. You can reorganise systems on those too (twin coil calorifiers, different electric circuitry etc) if desired, or just run a regime where you understand how long you need to run on each engine to keep all the various requirements up to speed.

Seemingly lots of hassle, but a good way of getting the load up on the engine whilst keeping the hours down (useful if, like lots of displacement boats, you travel distances and log significant hours). Generally quieter too.

Posting this was some degree of experience, having run my twin engine displacement boat for many miles on one engine and also run the MBM Sealine F36 and F37s on rivers on one.

kim_hollamby@ipcmedia.com

 

[MapisM]

Many thanks Kim for the overview!
More than enough to keep me busy in wintertime, checking/preparing all the equipment.
It's a real pity that I'll not be able to make sea trials before march/april next year.
I won't forget to post my findings, anyhow!
Thanks again, Mario.