How low will a diesel run (and not be harmed)?

[Oyvind Snibsoer]

I'm considering dumping the old smoking, leaking, raw water cooled Volvo Penta in my (goop) sailboat for something fresh and new. Recently added to my short list is Beta Marine, which is based on a Kubota engine.

My current engine is a 13hp VP MD7A. Now, looking at the specs for different Beta models, I see that the Beta 20hp model, which has a max rated RPM of 3600, will deliver just above 13hp @ 2200 RPM, and will consume a whooping 1 liter/hour at that speed.

However, I've heard that diesels should be run fairly hard. My question is, would running a diesel at a little less than 2/3 of max RPM be harmful for the engine in the long run? Would I be better served with a slightly smaller engine? The next engine down is 16hp, and it consumes ~50% more fuel when delivering 13hp.

You may see the engine specs here: http://www.betamarine.co.uk/Templates/sea_going.html

And, yes, I know I'm supposed to sail in a sailboat. But I also need to get places for the benefit of my not-always-so-patient family and the wind seldom seems to be there when you need it...

[John B]

I wouldn't be too worried provided it wasn't labouring with the current prop. I'd always go the extra hp if it was an option and it fitted.
The worst that would happen is that you'd need to budget another propeller with less pitch into the equation.
2200 to 2600 or so often seems to be a comfortablecruising kind of rpm on a boat.

[Tinman]

Diesel engines are built to last, and last and last and last. As long as they have fuel, oil and a cooling medium, they will run. Sounds to me like the engine you have will work just fine. Just don't drive it like a sports car, let it warm up thoroughly before you put it under load, and you should be good to go. Oh ya, one more point. Don't run it out of fuel. An airlocked fuel system is not a fun thing to deal with, especially at sea.

[Scott Rosen]

Running a diesel low does not harm the engine.

Running it at lower than normal operating TEMPERATURE can cause problems. As I understand the chemistry, a cold-running diesel causes the crankcase oil to become acidic, which can corrode the metal parts.

If you run the diesel cold, you'll need to change oil more frequently.

Running it at half or two-thirds should not be a problem.

It's better to have extra power than to be underpowered.

[Bob Adams]

Good advise all, and I'd add you want to give 'er a good hard run once in a while to clear things out.

[Ron Williamson]

If you check out the HP rating differences between Light duty,Heavy duty and Continuous duty for the same engine,you should notice that it is a function of the RPM.
Like this
http://www.marineenginedigest.com/pr...uggerl1276.htm
So as long as you treat it decently,it should last longer.
R

[Oyvind Snibsoer]

Thanks,
the more I look at the Beta, the more I like it. The company doesn't have a large dealer network, but claims that since the engine is built around a Kubota tractor engine, parts can be had pretty much anywhere in the world. Also, the engine comes with a "self service" warranty, which basically means that you service the engine yourself and that doesn't affect the warranty. In addition it's cheap, and the way the pound is going these days it's only getting cheaper.

[Lew Barrett]

My observation in a few years of running a Diesel has been that indeed, they do enjoy a hard romp once in a while. As usual, Scott has provided a good response, but so has everyone else.

In any case, we seem to have two modes for running. One is the "Lake Union romp" which is below 7 knots, and frequently for very short distances, the other is transiting north, which is by far the more enjoyable.

The biggest problem with low speed running, it seems to me, is that you never really get up to temperature. Oh sure, the cooling water gets there, but the block itself never does. You just don't get enough load on to really get the cleanest possible burn. Now, take her out and run her at 75% and everything smoothes right out, with clean, smoke free exhaust, no visible unburned fuel, and a happy pair of motors. Running her up to say 80% of output once in awhile seems the right thing to do to keep her internals clean and happy. Unlike gas engines, which enjoy being pampered, Diesels seem happiest when running in that slot around 2/3 to 3/4 power.

But as all have said, you simply need to give her the stick once a bit every now and again and all comes right. More power is better within reason, and I promise you, if you have it, you will find times that you are really glad you do.

I do hear the new common rail devices can meter fuel so precisely that low speed running can be highly efficient, but that doesn't stop me from loving my recent but older school Cats.

[Sailor]

The reason you want to run them hard from time to time is that they coke up. Basically a buildup in the system of undesireable material like soot etc. if it's not working at load, it's not burning effieicently. This leaves unburned or dirty exhaust. By running it under load from time to time it burns out all this dirt. You'll notice a loss in power when it's time to run them hard. VW recommends you bring your car in to be cleaned out when this happens. I know a VW owner who brings it in every 80 000 km or so. She drives in the city mostly and therefore it's not under load. I have put about 148 000 km on mine since I got it and have yet to have any coking problems with it. I do about 90 of my driving on the highway and the engine is under load the whole time. That little bit of city driving I do is cleaned out on the highway and never builds up.
Yeah yeah, that's a car not a boat. Ships often have coking problems. If you have a diesel electric plant or us DGs for ship's service electricity, and the dg is not on high load or is turning a generator that is not properly matched to the engine, you'll have the same coking effects. The solution we use is to "decoke". We shut down all unecessary electrical loads and put all the load on one DG. This runs it at high load while the others are shut down. Once this DG is cleaned out, we move to the next.......
Voila, clean engines.
YUP do a regular "carbon run" and keep those little workers happy.
Daniel

[Bob Adams]

Nice response Lew...I do the "Enterprise" romp, when the old Detroit powered gal shows her transom to the tupperware....and they can't catch up

[Gary E]

Nice response Lew...I do the "Enterprise" romp, when the old Detroit powered gal shows her transom to the tupperware....and they can't catch up

Bob,
Does the Blower on the Detroit help keep it cleaner than a 4 cycle engine?

[Bob Adams]

Bob,
Does the Blower on the Detroit help keep it cleaner than a 4 cycle engine?

Not really, it just allows the 2 stroke cycle to work, it provides the scavaging air.

[emichaels]

Oyvind I have a new Beta 14 (13.5 hp) this is the same engine as the lower RPM model which is rated at 10 hp. Same block and all except the 10 is governed at 3000 rpm 10hp and mine is governed at 3600 for the 13.5 hp. It is a really nice smooth running diesel. I rarely run it above 2200. Usually right there at 2000 is all I need to push my boat.

I have a temp gauge on mine and the engine still gets to the correct operating temperature even if I run it at 1500. It just takes a little longer. The thermostat is what controls this, not the RPM you run the unit at. The RPM just determines how quickly you get there, temperature speaking and speed wise obviously. The only problem with running a diesel with little load, and short time intervals is that it doesn't have time to get to operating temp and the condensation in the cylinders doesn't have time to vaporize and leave the oil enviroment. Simple to solve just warm her up a little more if you are going to short run.

Diesels are built to be lower reving than an equivalent hp gas engine, they are intended to generate the torque and hp for the application at hand at lower rpms. As far as fuel economy goes my Beta used 3 gals of fuel for the entire season last year. We usually sail on/ off our mooring but we always start the engine and warm it up just to be sure she'll go if we need it.

Every diesel, as I am sure you have already noticed, has a sweet spot where she'll run the best. My happens to be right at about 2450 when warmed up. I just go at 2000-2200 because our boat is a canoe stern and much more hp and the stern wants to start to bury.

Eric

[Captain Blight]

One thing I'd be cautious about is the oil; by which I mean that if the oil flow is determined by engine speed, you may not be pumping enough oil at low revs to keep the bearings lubricated, and may wear them prematurely. Mind you, I also like the idea of a little more horsepower available for when you really need it instead of howling along at max power all time at mere cruise speeds. Big engine working easily instead of a little one max'ed out.

[Peter Plones]

Oyvind, I've just installed a new Beta marine engine (60 Hp) in my Lemsteraak ( a traditional Dutch design) It runs very smoothly, around 1800 rpm only 3 liters/hour, according to the Dutch importer of the Beta marine engines, running them at a lower than max rpm isn't a problem at all. Just let the engine run every now and then at it's max rpm.

[ChaseKenyon]

Nice response Lew...I do the "Enterprise" romp, when the old Detroit powered gal shows her transom to the tupperware....and they can't catch up

Yeah and nothin' "nothin'" sounds as good as a two stroker Detroit wound out under proper load at the torque peak.

You should hear my 8V71 in the Prevost 40ft conversion "camper" (Thats what is on the registration guys) with it's close ratio 5 speed Allison winding out through the gears climbing from here at 920 ft above sea level to the next town at Lake Sunape at 1200 something above sea level.

Nice four mile run with 65% of the altitude in the last mile.

On the flatter part of the run she gets up to the 55 speed limit easily and then as it steepens goes down to fourth and I kick her down to third and as she crest the hill and then manual shift into second to slow down to 30 for the town center speed limit.

[Dale Genther]

Just for comparison purposes. We have a Yanmar 3GM30F, rated at 27 hp at a maximum RPM of 3600 and 24 hp at a max. continuous RPM of 3400. Yanmar reccommends that this engine be run at 85 - 90% of max. continuous RPM, which is 2890 to 3060 RPM. We run ours at 2900. At this RPM we are almost at hull speed (about 6.5 knots). If I open the throttle wide open in forward,in the slip the engine will do 3400 RPM, if I open it all the way open out on the water it'll do 3500 RPM. According to the "experts" I've talked to this is pretty much the ideal conditions as far is prop. diameter and pitch are concerned. It took three tries messing with the pitch to acheive this.

BTW our boat, Radiance, is 37 ft LOA, 25 ft. waterline, 10 ft. beam and about 15,000 lb. displacement.

[Canoeyawl]

All Diesel engines run “on the governor”. The operator can only set the governor.
If the engine is running at say ½ the rated governed rpm it will still deliver fuel for the full-governed rpm when the load demands it. In other words it will self-destruct. This is the real problem with running at less than rated rpm.

If the engine can reach the full load rated rpm (slightly less than “high idle”) with the propeller as installed there will be no problem running at half speed, except you are only going at half-speed. The issue of excess carbon is directly related to more fuel than the engine can use at a given rpm and is an indicator that the engine is hurting. The problem arises when the propeller is matched to the desired operating rpm and not the full rated rpm. This is an overload situation.

Some real problems I have encountered over the years from this are swelled pistons, pulled liners, bent connecting rods, damaged bearings, and the as mentioned excess carbon leading to a long list of problems causing premature failure. Running the engine “hard” to deal with the carbon is a band-aid masking the real problem. Sort of like going out and running a marathon to make up for eating only cheeseburgers for the last six months…
Engine temperature is thermostat controlled. Actual combustion temperature is a little different. This is based on many variables like timing, fuel quality, air temperature and of course load.
All this is further complicated if the engine is turbo-charged.

Sailboat auxiliary engines are rarely driven at full speed and consequently suffer a shortened life span.

[Oyvind Snibsoer]

I'm not sure I really understood that governing thing...
If I repower with a 20hp, assuming I match the engine with a proper propeller that will allow the engine to reach its full operating RPM, but usually run it at speeds well below that - albeit at max torque - is that a bad thing? Should I stick with a smaller engine that will usually run close to its max RPM?

OR, are you saying that sustained running with an oversize propeller, which will never let the engine reach full operating RPM, is a receipe for self destruction?

[Dale Genther]

Sustained running with a propeller so oversized that will not allow the engine to reach full RPM is not good for the engine. Idealy the engine should reach its max. rated RPM and no more at full throttle in gear . If this RPM also coincides with the max. boat speed you ever want to attain, so much the better.

[Canoeyawl]

If I repower with a 20hp, assuming I match the engine with a proper propeller that will allow the engine to reach its full operating RPM, but usually run it at speeds well below that - albeit at max torque - is that a bad thing?

Oyvind, This is fine...

OR, are you saying that sustained running with an oversize propeller, which will never let the engine reach full operating RPM, is a receipe for self destruction?

You are correct, This is no good

Note All Diesel engines are governed, they will not exceed a preset rpm. You cannot overspeed them (unless you are going down a mountain in a truck...)
They will not idle below a certain rpm. They will deliver enough fuel to develop full rated horsepower at an idle if enough load is placed on the engine. Bad - Very bad.
These two settings are refered to as "High Idle" and "Low Idle"
*note that high idle means full throttle, no load.
The high speed setting under load will be something slightly less than high idle but it should be close, usually 150-200 rpms lower dependiong on the type of system. This is easy to verify.

[Lew Barrett]

Jake what accounts for some smoke at idle in an otherwise clean running motor that pulls maximum rpm at full throttle and runs smoothly at all speeds? This motor burns no measurable oil between changes. Nature of the beast or can this be tuned out?

[The Bigfella]

Depends on the colour of the smoke.

Black, blue or white?

[Lew Barrett]

It's a fuel burn issue, Mr. Big. I haven't been able to tune it out. In the small quantities that are present, it varies between gray and blue-black (I realize this isn't helpful) depending on if it's backlit, and from which angle it is viewed. It seems to have gotten a bit better as hours have been built on the motor, but has not gone 100% away. Motors have a bit over 500 hours.

It disappears during medium and high speed running and when off idle, only to return shortly thereafter at idle. Canoeyawl suggests to me in private that if the quantity isn't large (it isn't) I might be best off ignoring it if I feel the engines run well otherwise, which they do. Engines burn no oil whatsoever between changes and motor spins to the governor with perfect response, and by the book fuel economy. I was hoping for a 100% clean burn at all speeds; perhaps unrealistic for this mechanically metered and governed design. Cat techs came out and said "runs good....leave us alone." Maybe I set my expectations too high. She does not make clouds at idle; you can just see it against the backdrop of the water at idle. I also get a bit of unburned fuel on the water after start up. None of this is in the spirit of a fresh re-power in my book.

[TimH]

How much more (on average) could one expect from a
Westerbeke 21, 3 cylinder diesel with 1700 hrs before a rebuild would be needed?

[Dale Genther]

TimH - It is tough to say as it is more a matter of proper maintenance than hours. But I'd say about 5000 hours. Remember more small sailboat type diesels die from abuse than die from use.

[ssor]

Farm tractors probably get the toughest treatment. They often run for hours with very light loads and sometimes pull full rated HP for days at a time when draging a plow. I know of several that have more than 20 years and 5000 hours showing on their clocks.
Just one thing to clear up- The governor will call for fuel to maintain rpm not to maintian hp.

[Canoeyawl]

Just one thing to clear up- The governor will call for fuel to maintain rpm not to maintian hp.

Sorry if I was not clear, this why low speed operation is dangerous. The governor can deliver enough fuel [quantity] required to make the full rated horsepower even at idle.
(I am referencing a stationary engine type governor (tractor or marine), not an automotive type. They are different in that the governor only functions at idle rpm and max rpm. With an automotive type governor everything between these settings is directly controlled by the operator).

[ssor]

Sorry if I was not clear, this why low speed operation is dangerous. The governor can deliver enough fuel [quantity] required to make the full rated horsepower even at idle.
(I am referencing a stationary engine type governor (tractor or marine), not an automotive type. They are different in that the governor only functions at idle rpm and max rpm. With an automotive type governor everything between these settings is directly controlled by the operator).

Quite true. It wouldn't take long to distroy a modern high speed diesel if you applied a load equal to its rated hp while at an idle. If the propellor is properly sized for the boat and the engine you can expect very long life from the modern diesel.

[Canoeyawl]

I have a couple here that (incredibly) are pushing 10,000 hours...
Still working every day

[John B]

So you're going for the bigger motor then, Oyvind.

We have a 80 horse turbo in Riada and I'm damned if I can make the thing use more than 2.5 litres per hour on average. We typically run at 6.5 to 7 knots and mostly around the 2600 to 2800 rev range.
I've measured the fuel into the tanks by dipping every 10 litres on fill and run a tell stick record plus log ,so I have a good handle on whats in them. I've measured the fuel useage systematically over 2 seasons and I've taken the fuel as total used in both tanks owing to idiosyncracy of the fuel return feed , and I still get figures in the 2.2 to 2.5 L range. Pretty light really.

[Oyvind Snibsoer]

I suppose I will. The price differential between the 16 and 20hp is quite negligible, the engine is only slightly larger and it weighs in at about 1/2 the weight of the old Volvo, so there should be no issues there.

On the downside, I was hoping to keep my VP S110 saildrive, which would fit to the Beta through an adapter. However, the S110 has a gearing ratio of 1.66/1, which was well suited to the max 2600 RPM of the MD7A. Fitted to the Beta's 3600 RPM, however, the propeller will spin way too fast. I spoke with a Kiwiprop representative yesterday, and he confirmed my fears that this was not a viable solution.

So that basically means that a new saildrive must also be added to the equation, which increases the cost by some 50%. Add a new propeller to that, and I'm looking at a total cost that is slightly more than 1/2 of what I paid for the boat a few years back... And that does not include a new propeller plus the various bits and pieces - wiring, hoses, fittings etc. - that will necessarily follow...

Dunno, may have to go back to the thinking box. OR sit back and watch the pound fall some more...