Sailing yacht - what RPM to run engine at?

[ghostrider5408]

Originally Posted by az_r2d1

It won't matter at all. Most engines fail due to other factors like losing cooling, running out of oil, running with coolant in the oil , etc etc.
In the hypothetical case where a marine diesel would have all other issues resolved before they become an issue, it will outrun the life of the boat.

I would worry more about everything around the engine like coolant, exhaust, hoses, clean fuel, etc.

One thing I always keep in mind on turbo engines is to let them idle for a few minutes before shutdown. Keep an eye on that exhaust manifold too.

This is true as a retired professional Caption with an unlimited 500 ton license running larger private yachts and delivery I guess I assumed that a knowledgable boat Captan would always to a recheck before and after starting as well as an hourly engine room check and of course monitoring their systems gauges. Sorry for that.

[a64pilot]

Originally Posted by cyan

Oh, it is indeed alternative physics, as in perpetual motion machine physics. a64pilot, you generously added the text per revolution to the horribly incorrect statement:
"It use the same amount of air, at idle that it does at full rpm."
in a post that started with the announcement:
"First you need to understand the concept of a diesel."

That was just plain friendly of you, but there's some serious funny somewhere in the original.

I understood what he meant, not what he said specifically.
Like Advocating.
I catch most thing this IPad does, some slip by, some doozies too, you have to wonder how the heck does it come up with that.

[a64pilot]

I’m thinking that racing the engine has to do with a water cooled exhaust, however the generators don’t ever race theirs?
I assume there are some Yanmar powered generators?

[Wotname]

I think that comparing a diesel powered generator running 1,500 rpm to diesel powered sailing boat running at 1,500 rpm using the same engine is similar to comparing apples with oranges.

The boat engine will be pretty much unloaded at 1,500 while the generator engine load could be significantly higher, being dependant on the electrical load of the generator.

At least, this is my current understanding
Yes, pun intended...

[a64pilot]

No, if propped correctly it should be well loaded, not over or under loaded.
It’s the reason boats do not need transmissions that shift gear ratios like an automobile.
However a controllable pitch prop or a self adjusting pitch prop is good for when conditions change, like bashing to windward, or motor sailing, but no wind flat water you can select a prop that matches the power output of a normal engine quite nicely, or you would see a lot more controllable pitch props or transmissions with varying ratios.

It’s the generator that will run widely varying loads as it can’t change RPM, average generator is rarely heavily loaded, and often running essentially unloaded

[Wotname]

Originally Posted by a64pilot

No, if propped correctly it should be well loaded, not over or under loaded.
It’s the reason boats do not need transmissions that shift gear ratios like an automobile.
However a controllable pitch prop or a self adjusting pitch prop is good for when conditions change, like bashing to windward, or motor sailing, but no wind flat water you can select a prop that matches the power output of a normal engine quite nicely, or you would see a lot more controllable pitch props or transmissions with varying ratios.

It’s the generator that will run widely varying loads as it can’t change RPM, average generator is rarely heavily loaded, and often running essentially unloaded

Hmm... my understanding that a fixed pitched prop should be selected to be fully loaded at max continuous rpm which in the engines we are considering, will be 3,000 plus rpm. Hull speed in calm waters will be achieved well below max continuous rpm (but is dependant on size of engine fitted). Thus my assertion that at 1,500 rpm, the engine will be lightly loaded, certainly some load but not a lot.

The load on the generator is dependant on electrical load so yes, if the generator is running unloaded, the engine is also unloaded however if the generator is being used at max continuous electrical load, the engine should be delivering it rated 1,500 rpm hp.

It the end, it depends on how the generator is used I guess. I'm not sure why someone would run a generator unloaded but then again, I'm often surprised at what some other folk do!

[a64pilot]

The power the engine will produce pretty closely follows the HP required, not perfectly of course, but closer than you would think.
This is my engine, a 4JHE, but as you see the curves follow each other pretty well.

[a64pilot]

I’d tell you most generators are run on power boats and often run pretty much all night unloaded, cause the only load they get is from air conditioning. Average 16 K AC pulls what about 11 or 12 amps?
Average power boat has usually much larger than a 7.5 KW generator which is a 62 amp generator?
A better question I have is what do people use these 12KW generators for? 12KW is 100 amps, what on a boat is going to use 100 amps?

I’d say few generators are loaded very much at all.

[donradcliffe]

I've done about 5,000 miles delivering the Merlin, which currently has a 75 HP 4JH4-TE. In economy mode, I run it at 22-2300 rpm, where it burns 4 l/hr at a boatspeed of 7-7.5 knots. On the trip back from Hawaii, I ran it at that RPM for about 50 hours, but when I closed the coast and wanted to bump it to 8.5 knots at 2800 rpm I found I couldn't get more than 2400 rpm out of it. My first thought was that the turbo had coked up, but I took a look at it and it was clean as a whistle. The second thought was that I had a fuel problem, and replacing the fuel filters solved it.

Since then, I have no qualms about running that boat at 20-2200 rpm for a couple of days, then half an hour at 25-2800+ to clean things out.

I also agree with the comments about letting the engine idle for 5 min to cool the turbo off before shutdown.

[Wotname]

Originally Posted by a64pilot

The power the engine will produce pretty closely follows the HP required, not perfectly of course, but closer than you would think.
This is my engine, a 4JHE, but as you see the curves follow each other pretty well.
Attachment 176052

I'm not sure of your point, I would have thought this was self evident but perhaps I am missing something obvious.

[Wotname]

Originally Posted by a64pilot

I’d tell you most generators are run on power boats and often run pretty much all night unloaded, cause the only load they get is from air conditioning. Average 16 K AC pulls what about 11 or 12 amps?
Average power boat has usually much larger than a 7.5 KW generator which is a 62 amp generator?
A better question I have is what do people use these 12KW generators for? 12KW is 100 amps, what on a boat is going to use 100 amps?

I’d say few generators are loaded very much at all.

I bow to your knowledge of what power boaters do; it does appear the generators in this usage would be lightly loaded.

Maybe they think bigger is better

[Lepke]

Originally Posted by a64pilot

A better question I have is what do people use these 12KW generators for? 12KW is 100 amps, what on a boat is going to use 100 amps?

I’d say few generators are loaded very much at all.

My boat was used for custom trips and came with 2 14kw generators. As it was set up - electric stove, 50 gallon water heater, watermaker, bar reefer, aux ice maker, all lights were 120v, 240v anchor winch, and so on.
And they still managed to glaze the cylinders on both generators.

[a64pilot]

Originally Posted by Wotname

I bow to your knowledge of what power boaters do; it does appear the generators in this usage would be lightly loaded.



Maybe they think bigger is better

Big Lagoons come I believe standard with an 11 KW Onan, and I’d bet there likely is a bigger one optional.
I had to look that up, cause I didn’t know how big they were, but I have been astonished before on how big and heavy the generators are on some Cats

When I started looking at a generator I was almost universally told to get at least a 5 KW.
Well a 5KW is a 50 amp generator, My Boat is a 30 amp Boat, so why would it need a 50 amp generator?
I installed a smaller, lighter and cheaper 3.5 KW generator, which is a 30 amp generator. I still can’t figure out why I would need “at least” a 5 KW generator?

[nknowland]

Yes ... except for fuel efficiency. Big difference between 1800 rpm and 2400 rpm at the gauge.

[Compass790]

Originally Posted by a64pilot

I’d tell you most generators are run on power boats and often run pretty much all night unloaded, cause the only load they get is from air conditioning. Average 16 K AC pulls what about 11 or 12 amps?
Average power boat has usually much larger than a 7.5 KW generator which is a 62 amp generator?
A better question I have is what do people use these 12KW generators for? 12KW is 100 amps, what on a boat is going to use 100 amps?

I’d say few generators are loaded very much at all.

Perhaps the bigger the generator the bigger the annoyance radius you can project at 0400 hrs in a crowded anchorage????

On a more serious note almost all the diesel generators we worked with years ago had stickers on saying something like o not run with no load for long periods of time