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Old 17-02-2008, 09:44   #16
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Elie
I think your automobile analogy is quite accurate and we tend to theorize too much when it comes to things like engine rpm, prop pitch and what type(s) oil to use. As a practical matter, it's hard to argue with your logic. As I said, our Maxprop is somewhat over=pitched for reasons we believe to be sound and with a little knowledge, can operate the engine so as to not incur the wrath of the bearing angels such that if/when we ever need emergency thrust, it's there.
I had a Peugeot which ran perfectly for 162,000 miles before my wife made me get rid of it and with consistent oil and filter changes, never had a problem with it running at a nominal 2000 rpm which was it's sweet spot.
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Old 17-02-2008, 10:19   #17
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Elie,


Thank you very much for your respone. The point you are making had occurred to me too. We have two diesel cars with 2L engines and one of which actually gives a gear recommendation on the dash. This generally suggests a gear change at around 2000rpm. When cruising at around 80mph the engine still only does 2200rpm and consequently these engines would rarely go above 3000 although their max revs are 5000rpm. This is a very puzzling compared to the situation of the boat engines.


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Old 17-02-2008, 10:58   #18
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Thank you, S/V Illusion and FJVDO, for concurring with this running diesel speed. I may suggest that these comments comes from Nigel Calder's writings and a few other relevant points based on much older marine engines. This Perkins Volvo for example is another beast altogether compared to older P407 or P408. Overhead camshaft, sturdy main bearings, high revolution, direct injection, dual stages injectors, aluminum heads, etc. Nothing compared to my previous boat MD17HD, or Perkins, Budda and Sabb seawater cooled monsters of another age, heavy, slow revving, and low tech. These raw water cooled engines needed to be pushed to get rid of low temp carbon deposits, ring sticking etc...? I may very well be ignorant of some reality,and maybe someone will provide solid argument on why modern marine diesel engine has to run fast or die, while the same motors in cars are ok? In the same idea, prop match with the engine announced max speed, is again in my opinion another generally accepted idea that need to be qualified. Unlike cars transmissions where we have 5 or more speeds to modulate power with speed AND load , or boats transmission/props are primitive single speed devices that in fact can operate efficiently at one speed only: the top speed and power of the engine. Speed where in fact we hardly run at all. Why? Serious commercial vessels use variable pitch props to adjust power to speed, as propeller driven planes. Autoprop, and some other prop makers propose some ways to adjust in fact the pitch while running, as a tentative increase a bit the efficiency of this propulsion system. In fact, over pitching a Maxprop is not such a bad idea in most of the time.
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Old 17-02-2008, 11:30   #19
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What many may confuse as a strategy unique to operating a marine diesel from that of an automobile diesel is in the distinction between the two cooling systems. While it may be true that running a marine diesel at close to max RPM may prolong the life of an exhaust elbow, some may extend that argument to the engine itself and therein lies the myth. I am not certain either way other than to look at it from a practical perspective in comparing marine and automobile engine life and draw what seems to me to be the only supportable conclusion.
I know others may disagree but I have yet to see any real evidence to support either argument. My experience with my overpitched prop and engine nominal operating range around 2000 rpm (which is approx. 60% of max rating) is the exhaust elbow, upon periodic inspection, suffers no appreciable build-up after hundreds of operating hours.
Regarding the engine lube oil issue, it is inherent in a diesel's design that it burn some limited amount of oil over time and it seems like speculation what an acceptable rate should be.
I've also read some interesting comments on what brand is best but I must say, having been involved in the petrochemical refining business for decades, it would be virtually impossible to take blind oil samples and attach their brand name based on analytical data. About the best one could hope to accomplish is to identify viscosity.
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Old 17-02-2008, 11:49   #20
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I just can't believe that drivers of these cars has to drive their machines at the speed required to drive hard their diesels on British roads where speed limits exist
No it is a compleatly different thing. Firstly, the power is matched to the gear. That's why you have a gear box, not just one gear to start and go to full speed. Second, the upper RPM is matched to the speed the vehicle is designed to reach. Thirdly, and the realy major big difference is, the engine has a variation of load/RPM. It doesn't sit at a "set" rpm for long hrs at a time. Even say a truck on a long distance across country run has variances in road conditions, wind, rain, slightly up hill, down, gradiants and so on. All changing the way the engine works. In a boat it is very very different. You simply can not compare the two. But most importantly for the boat, it is matching the power availabe to the "traction" in the water. Better power coupled to the water means a better handling boat in tight marinas.
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Old 17-02-2008, 11:57   #21
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Slightly OT

What is the deal with running a marine diesel at full throttle? I presume there is no "red line". Is there a downside to running it full out? For long periods? for short bursts? Does revving it all the way up in neutral harm it more than idling in neutral (no load)?
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Old 17-02-2008, 13:03   #22
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Interesting debate and I'm not 100% confident who is correct but just want to note that when you said "Even say a truck on a long distance across country run has variances in road conditions, wind, rain, slightly up hill, down, gradiants and so on" that you may be unfamiliar with Interstate 80 in the U.S. where constant prolonged driving at a constant speed over level terrain with no traffic is the norm and is the route by which most east/west cross-country trucks here traverse. I'm well aware that Kiwis are the most travelled folks in the world so you may already be familiar with I80 here and will hopefully agree.
They also consistenly run their engines at idle all night while sleeping alongside the road although that is slowly changing primarily as a result of more restrictive environmental regulations here

Defjef
Running any engine at redline for a prolonged period is not recommended. The conventional wisdom to which I believe you refer is the belief that one should run a marine diesel at 85% of max rated RPM for a number of reasons not thoroughly proven.
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Old 17-02-2008, 13:35   #23
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Alan
Interesting debate and I'm not 100% confident who is correct but just want to note that when you said "Even say a truck on a long distance across country run has variances in road conditions, wind, rain, slightly up hill, down, gradiants and so on" that you may be unfamiliar with Interstate 80 in the U.S. where constant prolonged driving at a constant speed over level terrain with no traffic is the norm and is the route by which most east/west cross-country trucks here traverse.
I-80 is by no means level. There are hills and even mountains out west. Theres a seldom heard of pass on I-80 called "The Donner Pass"
Even so west bound trucks crossing the plains can face headwinds that will cost them 1-2 mpgs. This is work, just like pulling 40,000 lbs is work. if you were to let off of the accelerator on a big rig at 70 mph. It will slow, the engine needs to work to keep it moving. Traffic can and does change the engine speed regularly. Idle time is another matter.
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Old 17-02-2008, 14:23   #24
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Never intended or implied that I80 is entirely flat for it's entire 2500 mile length and frankly, didn't think anyone would infer otherwise anymore than you would want anyone to infer the "Donner Pass" is typical of it's entire length.
What I said was " is the norm" as it applies to my description as it being generally flat.
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Old 17-02-2008, 14:31   #25
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[quote=Alan Wheeler;135291]No it is a completly different thing. Firstly, the power is matched to the gear. That's why you have a gear box, not just one gear to start and go to full speed. Second, the upper RPM is matched to the speed the vehicle is designed to reach.
Allan, When a car reaches its cruising speed in general it does so on last gear, at rpm that is far from its full power(lets say 2000 rpm at 100k/h). That true even while accelerating to acquire cruising speed. And going at top speed on top gear as you infer is a sure way to end up with a big ticket and worst. In fact the engine is kept in a 'reasonable' rpm range among other things to be fuel efficient, the top rev speed being only used on urgent maneuvers: after all nobody's racing!. Further more, boat are also subject to various waves, winds, so demand on engine varies also. On the other hand I know that big slow cargo ships diesels runs at almost full speed most of the time. But our engines are for most of them car engines with an added different cooling system, in my opinion much more efficient. I thing theses little brutes have no knowledge of where they are used, they should behave like car engines: unless you tell them . Seriously, I just can't find any technical verified arguments to sustain such requirement, other to try to make good use of the fixed pitch propeller that is design to be perfectly matted to engine at only one speed: the max.
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Old 17-02-2008, 14:42   #26
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I'd second the recommendation to see what Volvo themselves--and no one else--states is normal oil consumption for this engine. Someone once complained that their new Mercedes diesel (car) was consiuming a quart every thousand miles and the Mercedes-US reps said that was normal. The car owner said no, that was unheard of in the US for cars. The Mercedes-US reps said ah, but this is not a Detroit gasoline engine, this is a Mercedes diesel, and it is intentionally built with loose rings so that it can suck oil instead of wearing the rings, this way all you need is oil, not new rings, for the first million miles.

Well...that's one design philosophy. And one reason why diesels in general haven't been able to get EPA-certified for passenger cars in the US for many years now. I'd hate to guess what Volvo and Perkins think is "normal". I would use "volvo" and "normal" in the same sentence too often, or quickly.<G>
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Old 17-02-2008, 15:57   #27
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I am not sure who is arguing what here but there are differences in boats and cars that are obvious. What might be getting buried is the speed matching piece.

A car or truck can obviously change gears to match the power range (torque) available to the speed of the car. Your car runs in a range from probably 2000-2600 rpm as you go through the gears. The reason for that is the power band is broad between those two rpm. We all know what happens when you accidentally select 3rd gear to start - the engine stalls or lugs because you are not in the power band of the motor. However as you accelerate eventually you settle in on a Cruise of 2000-2200 at 100kmh or something. This is matched to the wind and rolling resistance of the car and at that speed there is a required horsepower that is being extracted - all figured out by the engineers. As you go up a slope or even in some cars hit a headwind, you may need to downshift to again match the power band.

In the boat you generally get one pitch and you would like to optimize it for the maximum horsepower extraction from the engine. In other words based on the hull and the hp/torque/rpm curve you want all that 90hp available and pushing the boat long at hull speed. This gives you the "maximum" horsepower for say - clawing away from the lee shore.

The 80% rule is rule of thumb and says that you don't want to be running around at redline all the time because you don't need to.

But once again the idea is that the the prop is matched to the engine and the hull so that when hull speed is reached you are close to rpm redline.

I will admit that I am overpitched slightly. It takes a couple of hundred rpm off and I am reaching hull speed about 200 rpm before redline. This is for noise. The trade off is that when the hull gets dirty and the boat drag goes up, I blow a little black smoke and lose a further 100-150 rpm. This for me is a great indicator that the hull needs to be cleaned...

The other difference with the car - especially one that does 100s of 1,000s of kilometers is that they are given a chance to thermally stabilize and we have all heard that highway miles are easier. Same for a boat engine. Long stretches of power are better than starts and stops.

I dont think any of this is inconsistent with anything Wheels, Never Monday or any of the engine experts are saying.
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Old 17-02-2008, 16:10   #28
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HelloSailor - your comments on intentional oil consumption is something that has been suggested to me before regarding small marine diesels. That some are designed to lubricate higher when run at low speeds or under load. If any truth in that I don't know but fits the pattern of our own engine.

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Originally Posted by defjef View Post
What is the deal with running a marine diesel at full throttle? I presume there is no "red line". Is there a downside to running it full out? For long periods? for short bursts? Does revving it all the way up in neutral harm it more than idling in neutral (no load)?
Building on S/v Illusion's comments most modern small marine diesels are high rated by increasing fuelling and are only meant for non continuous duty. Typically they may be rated for a maximum of , say, 2 hours running at full throttle in 24 hours. That will vary as the engine though (for example our own engine is lowly rated in that it's max rpm is 2700-3000 whereas other engines the exact same basic engine are fuelled for higher output at much higher revs - one assumes that the lower rated engine can be driven on the red line longer).

The only continuously rated engines are found in the likes of heavy work boats such as tugs or fishing vessels that tow (eg trawlers). Engine manufacturers then have higher rated engines for various services to which they give various names - these are for vessels such as fast ferries, patrol boats, etc where the engine is not expected to operate at high output for long periods - a typical such rating is called "Maximum Continuous Rating" ("MCR" - MTU and Cat use this term and I will here too). The higher ratings attained by increased fuelling, increased governed max revs and turboing, often being from the same basic engine as a more lowly rated one (ie the same basic engine may be available in both high and low rating).

But these engines are not rated for running at full throttle continuously either - for vessels such as patrol boats a maximum run time per 24 hours at full throttle will be given and for things such as fast ferries often the stop/start nature of their service provides the same effect. Large yachts (power and sail) say 80 foot and above class will usually use engines of this rating also, but they are not meant to be run at full power for continuous periods. Fleet owners with fast ferries, etc will often operate the vessels at 85% MCR ie 85% of max revs, for normal operation to extend time between maintenance or to conserve fuel (but many modern designed fast vessels use no more fuel per mile run when flat out as compared to cruise so maintenance is the main concern for running below MCR).

Pleasure boat engines are more highly rated again and for most you will kill them if run for extended periods at high outputs. Personally I am of the view that the less time run at high output the better, so preferably not at all, but views on that differ. I base this on the experience with less highly rated engines where constraining revs (eg to 85% maximum) definitely extends life between maintenance.

I agree a lot with Alan's comment that one should be careful to match the prop to the engine so that the engine is properly loaded for the vessel's typical duty (for pleasure sail boats normally taken to being able to pull full revs at full throttle without too much smoke ). An easy way to see how duty affects prop selection is to is to apply full power while tied to the dock so turning oneself into a tug boat - it will likely (actually should)put out black smoke plus, if a wet exhaust, carbon in the cooling water whereas the same revs on flat sea will not (assuming a sail boat propped to pull max revs without overloading).

There is a dilemma here though for some vessel services a clear example of which are small fishing vessels that tow (eg trawlers) and which often do not have variable pitch props. This becasue the duty changes during the course of a voyage. The vessel leaves port with fish holds empty (or maybe iced, but the ice is carried home too) and full fuel and water cruising to the fishing grounds at displacement speed. It gets to the fishing grounds and then starts towing the trawl net at slow speeds so a completely different duty more like a tug. Then it returns with fish holds full (hopefully) but low fuel and water again at displacement speed. So, it is not always possible to do a good match between engine and prop.

To a lesser extent the same dilemma can apply to sailboats in that for many their main running is at low speed in and out of the marina so maybe such boats would be best over propped? I don't know. And other sailboats, especially of the cruising kind, are expected to be able to power into heavy seas and winds giving a different duty again.

So rounding all that twaddle up for pleasure vessels it is not wise to run them at high output for anything other than very short periods. In my view best to not run them at full revs at all. In our own case we normally cruise at 2100 revs with engine max revs being around 2800 or so and that keeps us at displacement speed. We have a high aspect ratio fixed 3 bladed prop optimised for sail boats and that suffers badly if fouled (as it usually is, like now, when the boat is due for its 2 yearly lift) and that knocks a knot off our cruise at 2100 rpm. We suffer the loss of the speed rather than push the engine harder for the reasons above.

Phew, time for a beer .

John
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Old 17-02-2008, 17:36   #29
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Thanks, Have a cool one on me.

Another question for marine diesel geniuses.

My engine is a MD17D with thousands of hours and a large perhaps 20" fixed blade prop. Don't know the pitch and don't know how to measure it when I haul out. But she move well under power forward and reverse.

IIRC the manual says that the max rev is something like 3400 or so. When in reasonable flat seas and a clean bottom I can do hull speed at 2000 rpms. So when I motor I usually stay at 2000-2100. When I go higher the stern drops down and maybe We can pick up a bit of speed.

The thing is I don't think I can get to anything like 3400 under any conditions, even neutral. I think I might have done a tad over 2800-3000 once or twice to see what happens. Since there is hardly a need to go over 2100 I just never do.

Can anyone explain why this is so based on the info above?
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Old 17-02-2008, 21:55   #30
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This is very complex. Many points and questions have been brought up in the above posts, I will try and broadly cover most.
The relationship between a Marine Diesel and a road Diesel is actually quite different. At least if the Marine diesel has been designed correctly from the begining. Yes some engines come from the Road transport industry, but if the Mariniser has done his job properly, then the engine will have several mods. The first major modification is in the Fuel injector pump in regards to the Governor. Various engines have various mods. The old Ford D series had cast piston liners for Marine and chrome /steel liners for road for instance.
The next area is where power and torque is developed. Road diesels have the torque and power at different cross points in the Rev range than Marine diesels. Please realise that with a boat, the load seen on the engine by the propellor is purely RPM related, not speed through the water related. A Road vehicle has the load increase as the speed of the vehicle increases. A propellor increase in resistance to the engine as the RPM increases. The Road vehicle usually develops it's peak in the torque curve at a lower RPM than marine Diesel. The latest generation Marine high pressure fuel rail computer controlled wizz bang Diesels have a Torque curve that is very broad and quite flat over a wide rev range.
Diesels have usually two rev ranges. Continuous and intermitent. Intermitent is specified for a time. This can be a few minutes to 1hr. You need to read the specs to see what the manufacturer has used. But the intermitent short term RPM is a lot higher and well above the normal max continuos RPM. It is quite normal for the fuel pump to be locked off at the Max continous RPM point. This is to safe guard the engine from over reving. A Marine Diesel's RPM is governed in such a way that it tries to maintain it's set RPM no matter what load is being applied to it.
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