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]

Originally Posted by tranmkp

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

Originally Posted by 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.

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..........
Beat a "Buckau Wolff" U boat engine in terms of TBO or reliability or keep your mouth shut.

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!!

Originally Posted by 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!!

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]

Originally Posted by 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...