Why high speed diesels instead of medium speed?

The thread regarding the crankcase explosion/bearing failure made me wonder about something. Why do boat/ship makers opt for high speed diesels instead of medium speed diesels when the required power is enough to give you medium speed options? For instance, it was posted that the engine that exploded was ~2000kw. Why install a 16 cylinder, high speed, V-type engine (a 3516 Caterpillar) when a 6 cylinder, medium speed, inline engine could give you the needed output? For instance, a MAN 6L27/38:

l27-38-propulsion-eng.pdf (man-es.com)

could deliver 2040 kW @800 rpm and 186 g/kW-h fuel consumption.

I didn’t find any info on the 3516B but for the 3516C (page 28 of the attached file):

LEDM3457-27.pdf (pon-cat.com)

your ~2000kw comes at 1600 or 1800 rpm and 200-208 g/kW-h.

16 units vs. 6 units; total overhaul cost?
hours between overhauls for the two engines?
Can you purify the LO of the HSD like you can the medium speed? LO consumption/disposal costs?
Reduction gear cost comparison for engines in those speed ranges?
Any other considerations?

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medium speed hd always struggled to pass epa imo regs etc.
They got a pass a few years ago as manufacturers all said maybe they can never pass but a few have now.
Plus running them variable speed to save fuel emission and wear, med speed loses out.
Also becomes a subject if DC rather then fixed speed AC

You can pull them out of a ship and drop in another. Down time versus lost day rate.
The subject has been talked about lots lately when offshore stuff started going DE so boat has many engines instead of 2 large

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Because there already are a bazillion CAT 35xx engines in this world. You’ll pay gold prices, but you can get your CAT parts and fixed anywhere on this earth. That’s why.


Ah so similar to the US build requirement… Not driven by economics but by other factors (in this case convenience).

Pay a premium for the dealership to maintain inventory rather than maintaining it onboard.

Pay a premium for dealer service rather than training your crew.

Accept a higher SFOC and higher LO replacement/ disposal costs as I’m assuming the customer typically pays for fuel and lubes on top of the dayrate.

All in the name of convenience… Interesting but at the same time somewhat disappointing.

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Interesting question. According to this article High-speed engines gain ground on medium-speed products tugs engines spend a lot of time at low loads and high-speed diesels can respond quicker to increased loads.

harbour and terminal tugboats, up to 80 per cent of this running time is likely to be at engine load factors of 20 per cent or less. Taking this into account, the improved fuel economy of high-speed engines has become more attractive to operators. Although medium-speed engines actually return better relative economy at load factors of around 60-70 per cent, the amount of time most tugs are operating at these higher powers are limited. At lower loads, where more operating time is spent, high-speed engines show significantly reduced fuel consumption figures.

High-speed engines operate in a wider speed range and acceleration of the engine to full speed and load is faster. Caterpillar quotes one comparison where a high-speed engine can accelerate from 380 rpm to 1,800 rpm – equivalent to 100 per cent thrust – in 15 seconds compared with a lower-speed engine taking approximately three times longer to achieve the same.


From the Ocean Gladiator incident report, page 10:

“Caterpillar original equipment manufacturer replacement bearings were not available for 85 days, so aftermarket bearings were purchased, delivered 2 weeks later, and installed.”



I worked on a research vessel/tug once that had a Gardner Diesel. The thing was huge for the power it produced at quite low RPM. At first I thought it was an odd antique from the the 1890s or something, but I grew to really like it. It had access ports so you could fix just about anything but a broken crankshaft in place and the thump-thump sound beat a whiny Detroit Diesel all to hell.
It probably would make a modern emissions control engineer just die from fright :roll_eyes:

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Do you have a link to that report?

Ocean Guardian bearing failure thread.

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Deep sea long runs are different than harbor work but here is a example

Company I worked for had two identical mid size RoRo’s on time charter with Sulzer low,speed,diesels. Chartered 10 years then bought the vessels & ran then 15 more. High standard construction they served well. When they were under construction yard built two similar ships same configuration lower standard cargo gear and Pielstick high speed diesels, less expensive. They operated under many charterers until we bought them at about 15 years old.

Engine problems the Sulzers did not have, service speed was lower and the ships were slightly sharper built to compensate for the Pielsticks lower power.

Result was higher operating cost fuel, repairs etc and less cargo capacity due sharper lines. Less expensive when built would have cost more long run except we bought them cheap.


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and the emissions footprint?

some of the cats give me the heeby jeebies when I have to be along side them for long!
that’s a lot of iron swinging fast and the vibrations are sharp and actually violent, I am amazed they last as long as they do.


CAT used to charge a premium compared to other high speed engines because of the parts availability and the service network. They still have the service network but parts availability is another question, more on par with the issues all engine manufacturers are seeing… average engine load on a harbor tug in the US is something like 8-12%…Many harbor tug operators see value to the response time of a high speed engine vs medium speed… globally you see some large anchor handling tugs with medium speed engines but they typically opt for CPP Z-drives which helps medium speed engines avoid the lug scenario in the middle of the engine/prop curve, CPP Z drives have never really caught on in the States for harbor tug applications…Then there is the emissions part of the question. High Speed engines are mostly all category 1 engines… Much of the medium speed engines are category 2 where the market segment in the states is small and does not justify the cost of taking each engine through EPA emissions approval (C280 and GE are about it)… Above 15/L cylinder gets you into Category 3 where you do not need to comply with EPA but rather IMO but those engines are much larger than what you see in most tugs… There are alot of considerations. Many places did not care so much about the fuel savings because the chartering company paid for fuel… they cared much more about up time and the parts and service network was more valuable then the fuel savings…


This is something I was actually curious about… Are purely domestic/inland vessels (River pushboats, harbor tugs, etc) subject to IMO? I was under the impression IMO requirements only applied to internationally trading vessels… Is that not the case?

There’s a couple things there. Domestic vessels need to meet EPA. If vessel ever ties up internationally whether for Drydock, Panama Canal, or operating between countries it should meet IMO.
My comment was more in regards to category 3 engines which are greater than 15 Liters per cylinder. EPA simply does not have regulations for engines of this size so the defer to current IMO regulations


EPA Category 3 engines are at or above 30 liters per cylinder. Category 2 is between 5 and 29.99 liters for Tier 1 and 2 and between 7 and 29.99 liters for Tier 3 and 4.

Don’t confuse Category and Tier levels or EPA and IMO equivalencies. In the real world, because the hardware to meet Cat 4 is rarely if ever available or can physically fit in existing machinery spaces, exemptions are the rule rather than the exception.


Exemptions are the rule on repowers*. I would fully agree that it is regularly argued for repowers but not so often on new construction. There were tons of vessels that purchased engines and laid keels but only recently put into service so it may look like exemptions but not.

I know the difference between the date of keel boats and exempted newbuilds … I am in the marine diesel emissions business.

In the real world, because the hardware to meet EPA Tier 4 (or IMO 3) is rarely if ever available or can physically fit in existing machinery spaces, exemptions are the rule rather than the exception.

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I get that people use exemptions all the time on repowers and that it adds a ton of burden on the owners to install the SCR systems and the additional complexity of T4 engines vs T2/T3 . im my experience it is complete BS to say 1) its not available and 2) doesn’t fit… I’ve helped end users make the argument but in reality it usually has more to do with the fact that they do not want the SCR system, to deal with urea, the additional costs and the additional complexity of a T4 system vs T2/T3… just my experience and I know you obviously have yours.

Show us a list of SCR systems that available for marine generators between 99kW and 300kW, and propulsion engines between 500kW and 5000kW that fit existing vessels between 50 and 60m.