High Speed - Low Speed Small Disp Diesels

[tranmkp]

Is there any difference? My 70hp Westerbeks (Mazda) is over 200 lbs more than a similar sized Yammar.

I would suspect the block is thinner on the high speeds? They should then be noiser? Is less rotating mass on a boat diesel a good thing?

[TeddyDiver]

By the way it's originally a Perkins, lisenced to Mazda. And it's a bit "oldfashioned" model compared to Yanmar (which I don't know well), but perfect for displacement boats as it is..
Noises (and vibrations) are not directly compareable to masses involved, it's more an issue of air intakes, exhausts and unbalanced shafts etc

[FAST FRED]

The difference is usually in the rpm required for a power output .

IF you compare the larger engine with the smaller usually the swept volume (cylinder displacement times RPM ) will be about equal for the same HP.

SO A LARGER ENGINE MAY CRUISE AT 1800RPM WHILE THE SMALLER NEEDS 2800 FOR THE SAME POWER OUTPUT.

Both will turn the same prop at the same speed , by selecting a "deeper" reduction gear for the small engine.

Amazingly service life will be similar for displacement cruisers.

The use of a high speed auto conversion as Yannmar does with BMW diesel engines ( 300hp or more), will result in auto style service life , say 2000 hrs to rebuild time, with excellent maint.

FF

[StianM]

Is less rotating mass on a boat diesel a good thing?

A larger rotating mass would require longer time or more power to acelerate/deacelrate.

It's interesting if you are using the engine for running a generator since the class has requirements for maximum rise/droop in voltage and frequency.

The rotating mass should also be taken into account when callcultaing short circut currents.

For pure propulsion I don't think it will make anny difference as long as the engine don't come into overspeed conditions.

[tranmkp]

FF makes and interesting point - if the hi speed is cruising at a higher rpm then it would also have more piston travel. You dont want that do you? Of course if you gear it down, to a lower rpm it would be equal I guess.

another point on th eside is - I have owned to "perkins" motors, this 70 hp and a 30 hp. Both have been butter smooth, esp on the higherpower band. I have owned 3 Yanmars - all newer, some 3 and some 4 cylinder. All have been reliable engines but they just not as smooth. Could be the Yanmars have more stroke and the perks are short stroke engines? Ill need to look at the specs.

[FAST FRED]

Most engines still need to be under 2500fps piston speed at cruise .

At the same swepped volume , you may find the piston speeds near equal.

FF

[apex1]

FF makes and interesting point - if the hi speed is cruising at a higher rpm then it would also have more piston travel. You dont want that do you? Of course if you gear it down, to a lower rpm it would be equal I guess.

another point on th eside is - I have owned to "perkins" motors, this 70 hp and a 30 hp. Both have been butter smooth, esp on the higherpower band. I have owned 3 Yanmars - all newer, some 3 and some 4 cylinder. All have been reliable engines but they just not as smooth. Could be the Yanmars have more stroke and the perks are short stroke engines? Ill need to look at the specs.
Bore and stroke are not directly related to smooth operation. Although long stroke tends to smoother ride.
The butter smooth riding has to do a lot with their old fashioned and beefy design. Although those grandpa´s engines are not the best choice on planing boats, they have their advantage, they last nearly for ever.
One of my boats is propelled by a 5,5 ton 43 ltr. 300 hp @ 300 rpm "Buckau Wolff" raw water cooled engine, that clocks about 140.000 hrs since 1939 which translates to more than 5,5 hrs. every day with absolutely not a single piece (except the so named blower / indicator´s) ever renewed! Not even the (gear driven) pistons on the raw water pump have ever been touched!:?:
So, do not worry if your gear is a bit outdated, as long as it is in good condition, it will serve you well. As long as you have only one life to live, trust in the knowledge of your grandfather! He did the products to stand even your use ´till you may go. Nowadays products are for sure much more clever engineered and have their place in the market, but they are NOT produced to stand a life time. I hear the nay sayers already..........:p
Beat a "Buckau Wolff" U boat engine in terms of TBO or reliability or keep your mouth shut.:o

All the FF statements valid IMHO

[xpol]

hello to everyone!!
It's my first message in this forum. I'm a student in Naval Architecture and marine engineering school of national technical university of athens and I have a project at this time concerning the study and designing of the whole mechanical installation of a tanker. First of all I must find the appropriate diesel engine. The demands are: 8500kW-128rpm and 6800kW-124rpm. The engine must have 20% power margin because of bad sea condition. Could anyone help me find a low-speed diesel engine because all my efforts have been unsuccesful? Thanks a lot!!

[apex1]

hello to everyone!!
It's my first message in this forum. I'm a student in Naval Architecture and marine engineering school of national technical university of athens and I have a project at this time concerning the study and designing of the whole mechanical installation of a tanker. First of all I must find the appropriate diesel engine. The demands are: 8500kW-128rpm and 6800kW-124rpm. The engine must have 20% power margin because of bad sea condition. Could anyone help me find a low-speed diesel engine because all my efforts have been unsuccesful? Thanks a lot!!

hello, and welcome.
have a look here:
http://www.manbw.com/category_000077.html
you may also have a look at the TIER II programme, same page on the left "Marine"
Regards
Richard

[xpol]

I've searched every man, wartsila and caterpillar engine with no result!!
I would appreciate if you could suggest me an other company which produces marine diesel engines.

[JRM]

Xpol

Why is the RPM for your design so fixed ?

Maybe you need to alter your propeller design slightly to come up with more suitable RPM.

If you have some more freedom to choose the RPM, you will certainly find a diesel to suit you ? Designers of real ships manage to do this with the engine makers available to you.

If you have no freedom to alter the RPM, then maybe you need a gearbox.

Finally, to repeat some of the advice you have been give above, the following engine makers do what you need;

Slow speed c. 100rpm
- MAN/B&W
- Wartsila-Sulzer

A Sulzer RTA48T 6 cylinder engine has output of 8730kW at 127 RPM

A 5 cylinder MAN 60ME-C has 8900kW at 123 rpm

medium speed - up to 1000 RPM (so you need a gearbox)
- MAN/B&W
- Wartsila
- MAK (Caterpillar)
- Bergen

If you cannot find a solution from these, then I am afraid that there is something wrong with your project - or the way the question has been asked

[xpol]

I spoke with the professor who teaches the lesson and told me that I should reduce the rpm down to 94-110 if I can't find a suitable engine, so JRM you were absolutely right!! Thank you for the assistance!

[tranmkp]

Im still trying to grasp whats going on -

for example a
Perkins M92B - 4 cyl - 82hp - 2800 rpm - weight= 933 lbs
Yanmar 6LPA - 6cyl turbo.intercooled = 315 hp 3800 rpm - weight = 890!


what is going on here????

[FAST FRED]

what is going on here????
Reply With Quote

LONGEVITY

[tranmkp]

Perkins M92B - 4 cyl - 82hp - 2800 rpm - weight = 933 lbs
Yanmar 6LPA - 6cyl turbo.intercooled = 315 hp 3800 rpm - weight = 890 lbs!


look at the difference in weight between the two engines - why is the little one so heavy? or the bigger one so light?

Thats what I would like to know...

[SeaJay]

For a good education on the subject, check out David Pascoe's site:

http://www.yachtsurvey.com/GasDiesel.htm

Here are a few of his thoughts -

"The Problem With Light Weight Engines There is a direct relationship between service life and the weight of engine blocks and cylinder heads. The heavier, or thicker the castings, the longer they will last. That's one of the reasons why older engines just seem to go and go, while we often refer to the engines of recent years as "throwaways."

The problem with light castings is a problem of both strength and heat distribution. Diesel engines, with their 350-550 lb. internal cylinder compression, develop tremendous heat within the cylinders and heads. When castings heat up they expand, and when casting thickness are unequal, this can lead to cracking. It follows then, that the thinner the casting, the weaker it is, and therefore more prone to heat distortion and cracking.

This has been one of the major problems of trying to adapt light weight automotive engines to marine use. Because the loads are much greater, more heat is generated, and therefore more distortion of the castings occur. And when distortion occurs, the close tolerances of the moving internal parts such as crankshaft, bearings and journals, rods, pistons and cylinder walls goes out of whack. The end result is an early demise of the engine. Therefore the move to adapt high speed, light weight small truck engines to marine use results in an engine with a decidedly shorter service life. One of the most common problems that we see with light engines is the frequent cracking of cylinder heads, which is the first place that designers seek to reduce weight."

[daiquiri]

look at the difference in weight between the two engines - why is the little one so heavy? or the bigger one so light?
Thats what I would like to know...

Tran, the difference is essentialy written in the reciprocating engine power equation. It essentially states that the power output of an engine is given by:
Power = K x Pm x D x RPM
where:
- K is a constant depending on engine geometry (comprising efficiencies, multipliers, conversion factors and other things you don't need to consider for this analysis)
- Pm is a mean effective pressure acting on pistons
- D is the displacement of the engine
- RPM is rpm.

Now, the Yanmar in your example has a displacement of 4.2 litres, Perkins has 4.4 litres. That's pretty similar, so you can reduce the equation above and say that, in order to compare those two engines, the power output is proportional to the product of essentialy two numbers, Pm and RPM:
Power = K * Pm * RPM

Since the RPM of the Perkins is just 74% of Yanmar's RPM (2800/3800=0.74), if Pm was equal between two engines you would expect Perkins's power to be 74% of Yanmar's which means 233 HP. Since it is not the case, it means that Perkins' design has a much lower mean effective pressure.

How did Yanmar obtain so much higher mean effective pressure? By doing two things:
1) shorter piston stroke (Yanmar: 100 mm, Perkins: 127 mm)
2) use of turbocharger.

So what is the mean effective pressure? It is a fictitious constant pressure, produced by fuel combustion, which pushes at the piston during the stroke phase of the engine cycle. The actual pressure acting on the piston is highly variable during the cycle, so we adopt this mean pressure to simplify the math.
The point is this:
- a higher Pm means more mechanical stress on the engine structure (that's intuitive, think of thin plastic bottles for soda water vs. thick, heavy equipement needed for scuba diving)
- a higher RPM means more mechanical stress on the engine structure (because of inertial forces).

Since modern fast diesels adopt both design features, they are much more mechanically stressed (or loaded) than the traditional, slow revving and naturaly aspirated engines.
And that's essentialy the physical reason why that old Buckau Wolff engine (post #7 from Apex1) has had (and probably will yet have) a long and happy life, while a modern small fast diesel's lifetime is comparatively short.

At the end, it really all reduces to Fast Fred's concise remark - the difference lies in longevity.

[tranmkp]

thank you very much - I even understood it.

[FAST FRED]

"thank you very much - I even understood it."

Aint this board group GREAT!

FF