Why it's so hard to run a diesel properly.

[Roger Long]

Always wonder which is better? I do run the engine up to 2800 for periods of time.

I do the same thing you do and I think you are probably OK. The engine will take a good deal of under 80% running as long as it isn't the majority of the operation and it gets some 80-90% operation frequently. Engines very so this is just a broad brush generalization.

Many people never run their engine over 40-50% power and often don't even let it get up to full operating temperature in the short jog in and out of a marina. They will almost always have problems before their time.

When I've run for a long period at 2100 to conserve fuel, I try to run hard for a couple hours at the first opportunity after refueling.

 

[jibes138]

Not to be picky but the purpose of insulation is to slow down the HEAT TRANSFER RATE also known as Q dot in some textbooks (Q with a dot over it). Energy always flows from high potential (hot) to low potential (cold) so the purpose of the insulation is to keep the heat in the pipe and out of the engine compartment. The side benefit is that when fishing around for the screw you dropped and the engine is still hot because you just got back and your arm accidentally brushes up against a very hot pipe you don't get a burn.
So regardless what a couple old geezers like me and Stu want to argue about, wrap the pipe in exhaust pipe insulation please as a safety precaution, regardless of how the original design was installed.

 

[kloudie1]

In a few cases, insulation is for personnel protection, but most insulation is to keep things hot (or cold) that are designed to be that way.. This street ell is not designed to be insulated, so I wouldn’t put any on.. You are absolutely right, Jibes.. BUT when ya limit Q dot with insulation, the Delta Tee (across the shell wall) goes toward 0.. so the temperature of a plain cast iron part approaches the exhaust temp of the engine.. The mixing elbow runs cool enough to put your hand on (in most places), MUCH cooler than the street ell..stuff downstream of the mixing point and the mixing ell itself is cool enough to not cause a fire hazard .. Ya don’t insulate the cast iron exhaust manifolds in normally aspirated cars/trucks because ya don’t want the material to be too hot..

 

[Roger Long]

The side benefit is that when fishing around for the screw you dropped and the engine is still hot because you just got back and your arm accidentally brushes up against a very hot pipe you don't get a burn.

I reached way in through another opening and touched it right after docking, much sooner than I could have removed everything from the cockpit locker and crawled in there. It didn't even hurt. It cools off fast with all that water cooled mass on both sides.

wrap the pipe in exhaust pipe insulation please

Pretty please?

 

[kloudie1]

KABOOM!! Roger threw in the grenade and stepped back a little to watch.. LOL

 

[Snotter]

Always wonder about these setting. We power sail quite often, the wind is about 3-5 kts from abeam and we have a 6hr run ahead of us. I will run the 3gm30 about 2000rpm and get a good apparent wind speed that will push us along about 5 sometimes 6kts. If I run the rpm upto 2800 which for me is 80% rating the apparent angle goes to far forward and the sails luff. Always wonder which is better? I do run the engine up to 2800 for periods of time.

I think the moral of the story is to occasionaly push your engine hard and don't be afraid or timid about it. Truckers always idle there engines at rest stops; however, they also run them pretty hard.

 

[Maine Sail]

Last season I spent a great deal of time on the phone with Westerbeke engineers, not the distributor but the actual engineers at Westerbeke in Taunton. I had ordered a new prop and it was over propped by about 300 RPM and was over loading the engine. Another problem was that I had some fast "at idle" speeds which is not good for docking or picking up moorings or in and around the anchorage. While waiting for the replacement prop the safest route was to run the engine at exactly 300 RPM less than what I normally would as I was up on hull speed that much sooner. It still smoked more, even when cut back by the 300 RPM I was off, but less than running it at 70-80% max rated RPM.

Long and short of it was that Westerbeke was absolutely adamant that I have my engine within 100 RPM of max rated at the worst, with a clean bottom and prop. They really prefered within 50 RPM. They had NO vested interest in this as my engine had 2900 hours on it at the time. The previous prop was well within 50 RPM of max rated.

Over proping, according to them, can lead to plugging of the exhaust elbow as well as the bad fuel can that was previously mentioned. If the prop is to big it will lug the engine nearly all the way through the RPM range not just at max throttle or cruise, mine did even off just as much as 300 RPM.

My prop is now nearly perfect yet my engine driven refrigeration adds some decent load to the motor. The motor has no visible diesel smoke with the current prop, or transom soot, yet when I flip on the fridge compressor I can literally watch it begin to smoke as the combination of the compressor and perfectly sized prop luggs the engine. It will do this throughout the RPM range excep at very, very low speeds (like 1000-1200 RPM) where there is minimal prop load to hull resistance. Our fridge compressor only needs about 30 minutes a day and once ot shuits off I run her hard to blow out any soot..

I tend to disagree, based on my rather indepth conversation with Westerbeke, that overpitcing to achieve 80% load is okay at a lower RPM. All you are doing is loading to 80% at a lower HP output on the curve yet applying the pitch/dia ideally intended for higher point on the HP curve. Pherhaps I misread though..

My 2900 hour exhaust elbow was off this spring and went right back on after cleaning & painting. It had zero build up in it but the original prop was perfectly sized as is the current one. We almost always run at 70-80% max rated and never shut down before the engine has fully warmed up.

According to the engineer I spoke with this was his take: "Over proping an engine in a displacement hull is perhaps the worst thing you can do for it over the long haul. You're actually better to slighlty under propping if you really must choose one or the other."


Over all it cost me a few hundred bucks to re-pitch my prop, all of it the short haul, but one expense I would do again in a heart beat.

 

[Roger Long]

Pherhaps I misread though..

You did. We are in 100% agreement.

Correcting under pitching to avoid consistent running under 80% load is good. Over pitching is indeed one of the most damaging things you can do to an engine.

If I were to buy a prop to match at the 2600 max continuous of my engine, I would try to get the governor adjusted to limit it to that RPM as well.

Another reason why so many sailboats are under propped is probably that getting a close match as the Westerbeke engineers were advocating becomes very critical as you near the match point. The downsides of being under propped will take much longer to show up and the owner may never know that it has anything to do with the engine prop relationship. The boat builder will probably never hear about it. More powerful engines sell boats at shows, larger props don't. There are few downsides for the boat builder in selling a boat with too small a prop and, if you are skimping on engineering, better to be too small than too large.

 

[Ross]

I am bewildered! Most of the engines used in marine service are adapted from engines used in terrestial service. Yanmar builds tractors for agricultural service. Kabota builds industrial equipment for all phases of industry. None of these uses involve operation at 80 percent loading for the major part of the running time. However the engines don't suffer from light duty service. Why is marine service different?

 

[Rich Stidger]

For what it's worth, the East coast Volvo Tech rep told me I could run my MD22L-B (50hp, 4cyl) at WOT-400rpm all the time. WOT is 3000 rpm on this engine, but I usually run at 2400-2500rpm unless I am in a restricted speed area. He also told me to run at WOT for 10 minutes every time I go out. The reason for the 10 minute burn was to clean out the engine and to assure that the system was capable of maximum output. It provides an indication of engine health and prop fouling.

 

[Roger Long]

Why is marine service different?

Load in those land based services is constantly varying. A truck goes down a hill and then up a hill. A marine engine is like a truck going up an endless hill. There is never any downhill or coasting.

The periods of light loading in road service cause similar problems but they are briefer and almost always followed by heavier loading. There are also different expectations about engine time between overhauls.

A reason why it is good to run you diesel close to full power whenever possible if other factors (comfort, fuel cost, vibration, noise, etc.) dictate generally operating at lower power levels is to get a more terrestrial like service profile.

 

[Roger Long]

The reason for the 10 minute burn was to clean out the engine and to assure that the system was capable of maximum output.

The first sign you will see of fuel filters loading up is a drop in maximum RPM. Spotting it by a drop from say 3000 to 2900 will leave you a lot more time to deal with it than discovering the engine won't go over 2100 and is losing power fast.

Another good practice is to run at WOT for a few minutes before getting into a dicey situation like a narrow channel with fast current or a bar entrance.

 

[Stu Jackson]

Phil, I would think that, if anything, the Kubota engines actually probably ARE made for 80% since they (M25 series) were made for tractors. I recall reading here on this board that Yanmars were originally made for large industrial pumping operations. While tractors may not run "fast" all the time (I could never let my father-in-law let me run up and down the meadow at high speed in HIS tractor!) Maine Sail noted an important item: the "auxiliaries" that tractors had to run, like plows and such, although maybe only to raise up and down at idle running speed). Shows ya what kinda farmer I am! And manufacturers do not like to spend money on more "metal" to do the job than they have to, so it would seem they directed the engineers to design and have built engines to do the job at the least cost in the long run.

Others have noted here on this topic that boat engines have gotten larger on the same hull over the years. For instance, my M25 in our 1986 boat is 21 hp. The last C34 built last year had a 35 hp engine. Same bottom. A bit more torque. Pretty much same hull speed, although the Mark IIs can outpace me if they run WOT by about a 1/4 to a 1/2 knot on flat water (rivers). We've never raced under motor! So, it would seem that there may "be a case" that some more modern boats (Generation Mark II for Catalinas perhaps as an example) have engines "too big" for their size. Same props, too.

 

[Ross]

Sometimes a farm tractor will be pulling a plow or harrow all day at a full load often lifting the front wheels off the ground for very brief periods. Other days they will be mowing with a sickle bar and loading perhaps 20 percent. pulling a rake is the same way but bailing is a heavy load. Pulling the hay wagon is light work.

 

[jibes138]

Hey Ross,
How much of a load on the engine when they manure the fields? I drove by a field in Farmville today that was freshly manured apparently, took about ten miles to get the stink out of the truck.
Cast iron temperature of an exhaust pipe effected by insulation? Cast iron melts at well over 2000 degrees F so an exhaust temperature after passing through the exhaust manifold with coolant around 180 degs. F is probably around 300 degrees (just a guess) that is further cooled by the mixing elbow. I really doubt the heat will effect the cast iron structural integrity. I think the very short run of Rogers pipe is probably why his elbow stays relatively cool, 300 degrees though will cause a considerable third degree burn instantly on unprotected skin. I am not looking for a war, just want to make sure everyone stays safe, I've seen too many accidents in my career not to speak up when I think a risky situation is evident. In the end you have to make a personal choice to be safe or not.

 

[Ross]

The exhaust manifold in most cars and trucks is cast iron and gets very hot. We had a Cadillac that had porcelain enamel on the exhaust manifold. My Volvo engine is set up quite differently from the Yanmar, there are no hot spots on the exhaust.

 

[Snotter]

I am bewildered! Most of the engines used in marine service are adapted from engines used in terrestial service. Yanmar builds tractors for agricultural service. Kabota builds industrial equipment for all phases of industry. None of these uses involve operation at 80 percent loading for the major part of the running time. However the engines don't suffer from light duty service. Why is marine service different?

Marine engines can do light duty no problem--just load them up on a regular basis to blow the stink out of them.

 

[Ross]

Marine engines can do light duty no problem--just load them up on a regular basis to blow the stink out of them.

That is true for all engines. The local tugs on the gravel barges are pushing empty barges half of the time. My car is redlined at 6000 rpm but runs about 2300 rpm at 65 mph.

 

[druid]

Roger: Good Stuff! The prop curve is usually not well understood, and you explained it well. But in my experience, most sailboats are OVER-propped, because the salesman likes to say "See how fast we're going at only 1500rpm!". And pushing an overpropped engine hard is bad for it.

Dean: sounds like you're overpropped. The Universal M25 should get to 2800 easily (governed to 3000 iirc), so if it's overheating at 2200 you have a problem: either it's overpropped and lugging or there's a cooling problem. You should be CRUISING at 2200, going up to 2400 or so for "fast cruise" and 2700-3000 WOT. If you can't get over 2500, you should reduce pitch. If you can, check the cooling system (and exhaust). Also if 2200 "feels" bad, that's another sign it's overpropped and lugging.

And while I agree with Roger that diesels (especially Yanmars, for some reason) "like" being run hard, I don't think it's a "requirement" - Auto and truck diesels rarely run 80% power for any lengh of time and they seem to last. But certainly, pushing it hard for at least 15 mins or so every outing is A Good Thing.

druid

 

[Roger Long]

But in my experience, most sailboats are OVER-propped, because the salesman likes to say "See how fast we're going at only 1500rpm!"

I don't think that's true. You are confusing hull /engine match with engine / prop match. You can have too big an engine for the hull and a perfect prop for the engine and reach hull speed at low RPM without taxing the engine. Having sized a lot of props, I see most sailboats as not having sufficient clearance to swing props that will be too big for the engine without pitch / diameter rations that are out of the norm. There are many exceptions but they are just that.

being run hard, I don't think it's a "requirement" - Auto and truck diesels rarely run 80% power for any length of time and they seem to last.

"seem" is the operative word. Engine life is primarily a factor of the total amount of fuel runs through the engine. Since the auto and truck diesels spend so much time coasting and at low power, that number is rather low relative to total running hours compared to boats that never go down hills. Those autos and trucks also come up frequently to 80% power or more going up hills and accelerating.

Running your boat diesel at 40-60% power is not per sec bad for it. However, always running in that power band and never getting it up above 80% is bad for it. Many people never go above because of habit and mistaken belief that they are being kind to the engine. Many sailboats can't get above 80% even if the operator wanted to.

The more time your engine spends around 80% power, the longer it is likely to last and the less maintenance it is apt to require. The individual owner, without any good means of comparison, may not realize that they are doing major maintenance a year or too early as a result of always keeping their engine at low output.