Diesel idling

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Oct 11, 2007
Vessel Name
Circuit Breaker
Vessel Make
2021..22' Duffy Cuddy cabin
For years I have been told, and have read numerous articles on "why you should not let your diesel engine idle too much." I practice this but wonder why truckers don't. When you see an 18 wheeler at a truck stop, he's having lunch while his rig is idling. Doesn't this practice hurt his engine too?*
  1. They don't own the truck?
  2. They don't know any better if they do own it?
I read recently that Wal-Mart was in the process of buying new tractors with small diesel gensets (APU) to replace some of their aging fleet.* They own and operate thousands of trucks and are even investing in hybrid technology.

Maybe this will bring small diesel gensets to market with a better pricing point than we are currently seeing.

The most important thing with a diesel is that it be operating in the proper cylinder head temperature range to ensure complete combustion of the fuel/air mixture. The rpm is not as important as the combustion chamber temperature. An over-the-road truck has a better regulated engine temperature system than most boats. If nothing else, the fabric "shield' over the front of the radiator on many trucks can be zipped closed in very cold weather. Some trucks (and railroad locomotives) also have radiator shutters that can be adjusted manually or automatically to help maintain the proper engine temperature. Also, at least on the over-the-road trucks I had some exposure to in the Yukon and Alaska, the idle setting for keeping them warm is higher than the normal idle setting.

The heat exhangers or keel coolers on a boat are REAL efficient at removing heat, particularly if you boat in colder waters like in the PNW and the NE. Since there's no way to regulate the flow of raw water and coolant though or over them, at low engine speeds the chances are good the engine's going to get cooled more than it needs to be, even with the thermostat regulating the flow of coolant through the engine.

By the way, the big problem for the truckers that hauled oil equipment from Texas to the North Slope in winter was not their engines but their differentials. When they stopped for the night, someone had to come out every couple of hours and run the trucks back and forth in the parking lot for 15 or 20 minutes to keep the lube oil in the differentials limbered up. If they didn't, the trucks weren't going anywhere the next morning until someone lay underneatht them for awhile playing a blowtorch over the differentials. From the time these trucks left the euipment manufuactures' loading docks in Texas until they arrived back in Texas a month or more later, the engines were never shut off (unless they crapped out, which I was told was a very rare occurance).
I think the thing that keeps the 18 wheeler's idling from doing damage is that it's typically preceded and followed by several hours of high output operation. The oil gets hot, the combustion temps get high enough to burn off any varnish that's built up.

The bad things to do on our engines are
- coming down to the slip and idling the engine(s) for a half hour to "warm them up" without taking the boat out at all.
- running for an hour or two while anchored to charge the batteries

In other words - starting, idling, and then shutting down.

Still - there's nothing good to come of a lot of idling.

Fishing for opinions here: how long do folks warm up at the dock before departing? I typically do less than five minutes - basically long enough to check for water in the exhaust, a final visit to the head, pull in the mooring lines, and go. Low power under load - as in the "no wake" zone as you leave the marina - is (IMHO) the best warm up. You may need to stay at lower power once out of the no wake zone to finish getting the temps up - but I've never seen an argument for longer idle warmups being better.
On the advice of the head of the engineering department at Northern Lights/Lugger, who is a good friend who we took with us to California to check out the GB we subsequently bought, a diesel should be warmed up enough to get a decent head temperature before it's asked to do any work.

We needed to learn about diesel operation when we bought the boat, so we used him as our teacher. He knows a lot about FL120s and most other engines in addition to the engines his company makes. His advice to us with the FL120s was to start the engines when we started getting ready to leave. In other words, when we were ready do disconnect the ground power cable, turn on the electronics and set them up, single up the lines, and so on. By the time we were ready to cast off--- which for us is about ten minutes later---- the engines will be ready to work, although he said we wouldn't want to go to a high power loading until the engines were into their proper temperature range. So that is what we have done the last nine years.

While we use our boat year round, there can be periods of a month or more during the winter when the combination of my work schedule and the winds prevent us from going out. Also on the advice of our friend, and of our diesel shop, if we get to four weeks without being able to go out, we'll run the boat in its slip. We run the boat in gear, one prop in forward, one in reverse so there is little strain on the lines, and we run the engines at about 1200 rpm. This gets both engines up into their operating temperature range quite fast and we run them at temp for about 15-20 minutes. We initially did this out of gear and the engines never got up to operating temperature at all. Our diesel shop said do it in gear, and this makes the difference.
I'd start the engines just before you leave the dock. Soon enough to make sure all is ok and late enough so you don't smoke the neighbors out. The engines will warm up as you leave your slip and negotiate the fairway.
Yep, me too. Start the engine, pull in the electrical cord, boarding step, spring lines, breast line, stern line, and off I go. Nothing over 1000 rpm until the temp gets to 140, then up to cruising revs, either 1750 or 1900 depending on the mood. The engine runs real happily at either speed and varies from 2.2 to 2.5 GPH. (per Floscan)

Lehman 120, 5200 hours
I'll let mine idles as I get the boat ready(less than 5 minutes). The toodle out to the bay is another 5 minutes. The boat is nowhere near operating temp. So I will power up to 12-1400rpms for about a minute and then about 1800 and then 2200 and then 2400 and by then the temps are up and set cruise around 2800.....turbo loads up about 2400.
I ate brekfast with a man that owned an over the road trucking company with 17 trucks mostly scattered accross the country at any given time. He came down hard on his drivers not to idle the engines...just several minutes prior to and after being on the road. Many truckers idle all night long while they sleep and most all idle while in a resturant. Have you all noticed how diesel PU owners will let the engine idle for long periods? I think they like the sound of the thing. I think they think it's a manly thing ... grows more hair on their chest. I think it's obnoxious!
It may have been a fairly ligitimate thing at one time. Diesel engines in the 30s, 40s and possibly the 50s probably had starter motors that were probably on the weak side. Going from 6 to 7 - 1 to 14 to 16 to one was a huge increase in cranking resistance. In those days one may not get going again. Idling more than a few minutes today is just plain stupid.

Eric Henning
Thorne Bay AK
Not being a M>M guy we have to idle for a couple of min to generate the Hyd pressure to get up the ground gear, after that its 800RPM (by Da DD Book) till 130, then a slow increase in throttle till the water has got to 180F.

From a dock , its start , depart , and 800rpm for the first few min .

For my Cummins 6BT5.9: Start engine, then 5-7 minutes until underway, another 3 minutes outside the breakwater, then bring up to cruise rpm's based on water temp. 120F = 1200rpm, 140F=1400rpm, etc. On average, it takes about 25 minutes to get to 1800rpm cruise with a cold engine. 10 minute slow down prior to entering the marina results in a good cool down.

Minimal idling, slow build-up, adequate cool down and occasional hard run.

My engine loves me.
Jay: It seems we all are doing about the same thing but you are the only one that has a formula! That is a great system, especially for the "Newbies."
The video that came with my Cummins 6BT5.9M instructed about 5 minutes of idle time after a cold start, and then keeping engine speed below 1400 RPM until the coolant temp is greater than 140F.* Five minutes is about what it takes to check the water flow, get up to the bridge, look for other traffic, wait if necessary, cast off, motor out of the marina and clear the entrance channel.* I have to be a little more disciplined if I'm just dropping a mooring pennant.
I think starting the engines and leaving the dock is asking for problems as most signs of problems/trouble will show in the first*5 to 15*minutes of running.* I hope that you at least check things out while under way?*I feel, look and listen for any tell tale signs. I start the engines first and let them fast idle while tuning on and checking other things.* Then before leaving the dock personally check the engines, engine room and bilge.**5 to 15*minutes of idle is not going to hurt a engine vs.the problems it will avoid.* **

Given the choice of let them sit or start them up, I start them up.* I think colds starts are a bunch of bunk, as ALL starts are a cold start to SOME degree.* I start the engines, 671 DD 2 stroke, and Perkins 2 stroke, ever month and run at a fast idle until oil pressure is up, about 5 minutes, and then shut down.* The main reason is to move/work the cylinders/impellers and pumps.* On a two stroke, 1 or 2 cylinders are open to the damp air through the exhaust and intake, so to prevent pitting/rust it is recommended to at least bump the engines to move the pistons, impellers/pumps and the fluids. Some old salts will bump/start the engine every week or two on the older two strokes in the PNW.


Just make sure you run the engines hard/hot when you do take them out.* Shoot even my mother knew she had to take her big 350 HP olds out on the highway ever once in a while and blow it out after putzing around town at 25 to 35 MPH.* I swear she was the little old lady of LongviewWashington.* My 30 years old 2 strokes do not set off the smoke detectors in the engine room, start at the turn of the key, and do not smoke.* I dont get why people think that a big block engine is going to fall apart because its in a boat, but will ruin the engine in their 30 to 50 grand cars and think nothing off it.* Dah!* *

Phil Fill

Do you really have a two stroke Perkins ? Never heard of that.
Is it like a Detroit w roots blower and four valves per cyl ?

Eric Henning
30 Willard
Thorne Bay AK
* glad some one is checking.*
It's a four stroke but I still start*it for other reasons, plus it drive the hydraulic pump for the bowthruster and*I like to make sure it's producing AC power.* Usually during the winter the power goes out and we have to use the gen set.* I maintain the Eagle so it can leave the dock at anytime, and with out starting every once in a while I just can not stand not knowing.* I will say nope I am not doing to start things up, but until I do it will drive me crazy.*
So for peace of mind I end up starting them.**
* If they did not start or there was a concern/problem they would be looked at*ASAP.* **

Hye, these funny faces a pretty cool!

Yea, we could tell you liked those!
You guys really rev up your engines! We start up our old girl, idle her at 150 - 200 rpm, let go the lines and chug out of the marina at 400 rpm, when we put the hammer down at 700 rpm, sometimes we red line her and push her all the way to 800rpm!!!
You got to love those old english Gardners, an old 5L3, 15 litres and 3.5 tons of cast iron. At least she only burns 6 litres per hour at 6 knots, then again we can only pull 7 knots flat out.
We make sure we run her at least every month or so, by run her against the mooring lines until she is up to temp (60 degree C), the shut down. She takes abot 6 hours then to return to room temp. lol.
Is it like a Detroit w roots blower and four valves per cyl ?

ONLY the newer higher power DD have 4 valves per cylinder , the lower power rated older engines only have 2 valves per cylinder , and are a bit more efficient at low speeds.

If you cruise at 1200 rather than 1800 the 2 valve is better.

For 2100 max power use the 4 valve makes more power , with big injectors.
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