Marine Diesels, longevity

[SEEKER]

Ran across an interesting article on marine diesels and recreational users. I would love to hear some comments from some long time diesel owners and get their perspective. I'm fairly new to the "high speed" diesel performance since I came from a sailing world where anything more than 5-6 knots was something you stayed awake for. If this article is right than it makes me concerned about running my ranger any faster than 6-7 knots.

http://www.sbmar.com/articles/continuou ... rspective/

 

[walldog]

Ran across an interesting article on marine diesels and recreational users. I would love to hear some comments from some long time diesel owners and get their perspective. I'm fairly new to the "high speed" diesel performance since I came from a sailing world where anything more than 5-6 knots was something you stayed awake for. If this article is right than it makes me concerned about running my ranger any faster than 6-7 knots.

http://www.sbmar.com/articles/continuou ... rspective/

Pretty conservative views. He does say near the end of the article that our common rail engines put less stress on engine components at the higher speeds. Even though they are converted automotive engines. BMW engines were in motorcycles before they were in aircraft before they were in autos. Volvos has similar reputations. Now if it were an Oldsmobile with a converted gasoline to diesel engine (which I owned), I would then be worried, it is not.

We run our boat at 6 to 7 knots most of a days cruising, unless we have to meet a schedule, darkness or weather. Then we put it on plane and cruise between 14 to 16 knots. We always bump it up to WOT for at least 5 minutes each hour or two during a cruise. Then we make sure our temperature has dropped to idle temperature before shutting down.

An aside on working your engine hard. Keep your bottom clean and I mean squeaky clean. It does not take much growth below to show up in your top end and therefore the rpms necessary to achieve plane. Weight carried around also makes a difference. Sometimes it is unavoidable, but do your best to be as light as possible. You will be happier with the performance and your engine will not work as hard. This summer, we plan use the truck more as a convenient storage unit for items not needed for "local" cruising. Loading up only for long itineraries which can be taken at a more leisurely pace.

Enjoy your boat at hull speed. If necessary do not hesitate to bring it up on plane, trim it out for optimal fuel mileage, and enjoy the ride.

Interesting article, thanks for sharing. I will go back and read it again.

 

[knotflying]

An interesting article and I guess time will tell. I usually cruise at around 2,500PRM and that gives me between 7 to 9 kts depending on wind and current. Having owned a plane, which the rule of thumb is rebuild or new at 2000 hours, getting more than that is gravy train. I think with good preventive maintenance our engines should service us well and as I said earlier, time will tell.

 

[Levitation]

Tony seems a bit hysterical on this - publish or perish syndrome, I suspect.
I have respect for Tony's knowledge as a writer, but he is pushing the edge of the power band on this one.

First the engine manufacturers are right up front about this topic. Go to their web sites and the first selection you have to make (talking diesels) is whether you need a commercial engine (high duty cycle / continuous use) or an auxiliary engine (low duty cycle).
For example: https://marine.cummins.com/marine/publi ... age.action

Second, if you need 130 continuous horsepower to push the boat to a given cruise speed, you don't buy a 130 hp engine and no boat builder (with a brain) would market a boat with an an engine that has to run 100% continuous. The boat will be equipped with a 150, or better yet a 180.
If that is a commercial fishing/dredging/etc. boat that will have the engine running for months continuously at 130 hp it will have a 300 hp engine.

For instance my airplane engines (gas) are rated to run at 100% for 5 minutes at a time - yup, that's it, 300 seconds - and then has to be throttled back and cooled down.
But reduce throttle to 80% of maximum HP and the engine is rated to run for 2000 hours - and if used often enough to avoid dry starts and rust, it will go well beyond that. BTW, the difference between 80% and 100% is roughly 7 mph.

Now, a diesel making 30 or 50 or 80 HP per liter that can go 5000 hours BTO can make 160 hp or 320 hp or more per liter- just not for long.
A Top Fuel dragster engine will make 6000 HP - for 4 seconds - then they need an overhaul 😱
Limit that engine to 500 hp output and it will likely run for thousands of hours.

My QSD2.0 150hp rated, is a high speed engine. And I know that it will never go 5000 hours BTO (or at least is unlikely) especially if run at high throttle settings all the time. But I have to idle out of the marina and idle for miles down the river and in rough water I have come back to just barely above idle. In fact is is probably less than 20% of the time on the boat that either the regulations, or the sea condition, or just plain comfort level, will allow me to run full throttle. For me to wear this engine out will take far longer than I will own it. (assuming I don't let it sit so long between uses that it rusts and has dry starts)

For commercial longevity you need a naturally aspirated, low RPM , direct drive engine that weighs a zillion pounds.
For fun you need a light weight engine with a turbo, or two, and a gear box - wheeeee :mrgreen:
For us cruisers, somewhere inbetween is the sweet spot.

 

[NewMoon]

Our engine is a Volvo KAD44P diesel, relatively light weight and high RPM, 260 hp from only 3.6 liters (about 72 hp/liter), WOT 3800-3900 RPM. It's 15 years old, with 5100 hours so far.

About 7 years ago, at about 2200 hours, we started traveling most of the time at 6-6.5 knots, 1300-1400 RPM, rather than on plane at 3300-3400 RPM. With our heavy 26-footer, 6-6.5 knots is enough to make the engine work a bit, and keep it running reasonably warm (175-180 degrees vs 190-195 at 18 knots). For this particular engine at least, this takes better care of the engine than running a lot at idle or very lower power levels, at which it would run too cool. We also cruise at planing speeds occasionally, and after a lot of slow speed running we take her up on plane for a while to get her good and hot and blow her out thoroughly.

It seems to be working - we get 3X the mpg at the slower speed, and the engine still runs like a top. No reason at this point to think it won't go a few thousand more hours.

 

[baz]

Look after your engine and follow all the tips given by the manufacturer and given here on this forum. A new engine for R-25 and R-27s plus installation will likely run at least $30K today.

 

[SEEKER]

So it sounds like the consensus is that most of whom have posted are following the recommendations of the author which is to slow it down, which makes sense however when I had my orientation with Ranger there was discussion on how the engine runs better under load and ideal is running at or around 85-90% load-that seems high to me according to what I'm reading and hearing. Obviously it goes without saying that this is a less economical means of running your boat at such high load and subsequent speed but my concern is of running my engine outside of it's design parameters? Temperature wise I'm running in pretty cold water and I've never seen my temp below 190° even at 5-7 knots, it usually stays pretty steady at 192-193°once we've been running for 10 minutes or so.

 

[knotflying]

I'm not 100% sure, but isn't there a thermostat in the engine that would control temperature, similar to an automotive engine. If so, then wouldn't temperatures be somewhat consistent regardless of seawater temperature? My engine seems to operate at a constant range of 194 to 197 degrees no matter what the RPM.

 

[Levitation]

Mike makes the point that the engine's coolant temperature is controlled (partially) by the thermostat - lets say by a nominal 190 F thermostat. (different engines have different heat range thermostats selected by the design engineers - I'm picking 190 just for discussion)
At idle or low throttle settings you may never reach 190 because the engines internal heat production is not high enough to make up for the rate of heat loss through the engine block surfaces, runs of piping, radiation loss through the cabin heater under the helm, and from the bypass flow through the thermostat. (the thermostat does not totally shut down coolant circulation even when it is closed )
At high throttle settings, hot day, warm water, etc. you may exceed 190 even with the thermostat wide open. A minor increase over our discussion thermostat of 190 - say 10-12 degrees - is normal for high throttle/load settings.
Too much a temperature rise and you will get an overheat alarm - and be privileged to join the rest of us in the search for the cause - bad pump / clogged exchanger / faulty gauge / etc.

The point the factory makes about the diesel engine being most thermodynamically efficient at a throttle/load setting 80%-90% of maximum HP is valid - though it has little bearing for our boats. No wake zones, passenger comfort, and wave conditions do more to limit power levels than anything else.
My diesel tractors run heavily loaded much of the time when pulling a plow or a disc. They tolerate being run nearly at full throttle with a load that makes them grunt. One has 6000 hours on the engine and the other is unknown but has to be near 10,000 hours based on average use per year multiplied by it's age. Both engines run without burning excess oil or showing signs of needing an overhaul.

Cummins did a big study and determined that diesel engine wear limits are based almost solely on the amount of fuel burned through the engine. The number they publish for time between overhauls is not hours, but gallons - they say 40,000 gallons of fuel and the engine will need overhauling.
How this number translates to our high rpm auxiliary engines is unknown to me. But my gut feeling is that we recreational users will never wear out the engine.
My best advice is run the engine at the power level appropriate to your needs and then run the throttle up for a few minutes on the way back to the marina to blow the crud out of the exhaust system and combustion chamber, and don't worry about it.

 

[Irish Mist]

From what I have heard the turbo needs to get used so it doesn't seize up. I think the full throttle every now and then to heat things up makes a lot of sense... I do believe the older non turbo engines will last a lot longer then the newer ones trying to squeeze more ponies out of a smaller lighter engines . Hopefully we get 5K hours out of them I think we would all be happy at that 🙂

 

[Todd]

Running at full or almost full throtle might not change enging temp much but the exhast temp goes way up and cleans stuff out.

 

[commander bill]

I have an exhaust thermocouple on my Ford truck (7.3 diesel). This provides an excellent indication of engine load. Wish I had one on my boat. Truch exhaust temps run below 275 degrees at idle and will never heat above that no longer how long you idle it. Modern diesel engines burn so little fuel without load they will not heat up enough to make the heater function on cold days without putting load on engine. However, under heavy load (pulling my tug up hill) temp can momentarily climb quickly up to 1100 degrees (most engineers consider 1200 degrees as the upper limit/engine failure). I have only seen that temp for short periods and downshifting will lower the temp to about 800. The reason I mentioned this is that no matter what the exhaust thermo reads the engine coolant temp never varies from about 190 while seeing from 300 to 1100 on the exhaust temp gauge. They are intended to read differnt things.