[Jeroen]

Originally Posted by D33-PAC

One question
How do people survive this business as sailors ?
.

I don’t know how it is in India. But I was just reading an article in a Dutch magazine, titled How much is your Diploma work. It was a comparison of all college and university degrees. Showed which courses get you jobs the easiest and the quickest, which degree pays the most initially and after 10 years.

Merchant officer qualification (which is a Bachelor qualification) scored very high on all aspect. So people graduating have no problem getting a job. Their starting salary is really good and after 10 years they are earning considerable more then most of their peers in other disciplines.

Still, no matter what, you are away from family and home for extended periods of time. Money doesn’t make up for it, but it is nice even so!

Jeroen

[Brumby]

Hello Friends,

As you can see I'm a newbee here and yet to post my introduction, but when it comes to anything marine I just cant stop myself. I know there a a lot of mariners here but had not stumbled upon a topic which discussed marine engines. Its really amzing to see the diversity of the topics discussed here and truely justifies my desperation to be a part of this wonderful forum.

I am working as an engineer in the marine transport industry since 1998 and have been on Capesize bulk carriers (150K-200K deadweight), general cargo, a large container 5000 TEU(twenty equivalent units), product tankers, gas carriers, infact on one of the largest one, a 260K cubic meter LNG carrier, car carriers, forest product carrier etc..

The largest engine I worked on was a 66120 bhp at 92rpm, a 12 cylinder inline 2-stroke engine consuming 180 tonnes of fuel oil everyday. The ship was capable of doing 25 knots and under favourable conditions (wind and curren from aft) did 30 knots as well. There are engines capable of producing 100K bhp and have applications in power generation plants and container ships.

The marine 2-stroke diesels are slow speed engine which though high on power, are actually high on torque. A super long stroke engine has a bore to stroke ratio which is more than 1:4. So these are actually torque giants. This makes us understand why torque is more important while starting from standstill condition or when immediate increase in speed is required.

Presently I am on board trading in Venezeula and looking forward to being back home in a few weeks.

Cheers

[V.Narayan]

Originally Posted by Brumby

Hello Friends,

Its really amzing to see the diversity of the topics discussed here and truely justifies my desperation to be a part of this wonderful forum.

Welcome Sir. From ghosts to pet dogs to diecast models and tax advice you'll find it all. Like the Tata Steel advt of earlier days T-BHP can say '...and we also review cars'
Thank you for this data. It really is quite marvelous - so much bhp at such an incredibly low rpm. Can you help answer my questions in post 290 of this thread. Thanks - Narayan

[Brumby]

Originally Posted by V.Narayan

Questions for the Aquatics from an Avian:


(1) How much underwater is the depth of a typical cargo ship of say 100,000 tonnes dead weight ie how deep underwater is the deepest part of the ship (keel or props I assume)
(2) Some container ships I believe are quite fast like 25 knots cruising speed. If 25 knots is the cruising or sustained speed I assume the top speed must be quite a lot more. What would that be in range of - 28 knots or 30 knots or 33 knots.
(3) Lets say a cargo ship is cruising at 18 knots and then encounters heavy seas then how much more power do you need to put out to keep cruising at 18 knots - 10%, 30% or 100%. I realize the phrase 'heavy seas' is as ambiguous as it gets but just trying to understand the magnitude.
(4) The sheet metal that your hulls are made of - how thick is it -20mm or more like 50mm. May be the thickness varies at different parts of the hull.

Apologies in advance if the questions are poorly framed. I look forward to your answers and learning more of the 'other' fluid.

Hello Sir,
I will try to make the answers as simple as possible.

1)Underwater depth is called draft of the vessel and varies depending on the size and condition of the vessel (laden or ballast). A typical 100000 tonnes deadweight ship (oil tanker) will have a loaded draft of around 18 meters, where as an LNG carrier (Q-max) which carries around 130000 tonnes of liquid methane has a draft of around 14 meters when laden.

2) Basically there are various speeds that the vessels operate on, like chartered speed (as per the lease agreement), MCR (maximum continuous rating), CMCR (contracted MCR). Most of the ships operate at the chartered speed and these are usually close to CMCR. The container trade is the fastest one and hence thay have speeds in the range of 20-30 knots (depending on the size). The difference between the CMCR and MCR is not much due to the slippage factor which increases with speed (a different fluid you see). The propeller of a ship is like a screw moving in a material which yields, so the slip is higher during the starting periods and in the top range.

3) Cargo ships are not designed to maintain speeds, they are designed to be operated a continuous rpm for a time until a maintenance schedule is not due on the main propulsion engine. So during the heavy seas we just cruise at an rpm which does not overload the engine (thermally or mechanically), speed becomes irrelevent and safety of the ship becomes important. The mother nature is unforgiving, heavy pitching and rolling takes its toll on the srtucture and may lead to failure. The only advise to seafarers during heavy weather is to avoid it, take de-tour, maintain a course where in you can avoid pitching and extreme rolling or in extreme case if possible take a shelter. So a weather working against us results in drop in speed.

4) The sheet metal has a variable thickness all through the structure of the hull, it progressively decreases from the underwater part of the hull to the part that remains out of water above the load line. Another factor that decides the thickness is the size and trade that the ship is designed for. Strengthening of the structure is equally important and the hull is a heavily framed structure with extra thickness for the steel plating and extra strengthening in key areas. The ships are like long beams and they are allowed to sag and hog within limits.

And for the last part, its the answer which is right or wrong, good or bad, because the question is always right

Hope I have not confused you much and the answers were simple enough to understand.

[Sawyer]

The low rpm is an effect of engines as big as houses. High RPM, and they will fall apart before getting there. And considering the surface area of the propeller to be made to start moving against water resistance and the mass/weight of the ship to start to move, BHP is nothing here without high torque as well.

Question: with these 80-100 rpm speeds, is the main shaft to the propeller a direct drive? Or is there a reduction gear box in between? If so, does it have just one ratio or many as in a car? And presumably, reverse must mean a gearbox and not the engine turning in the other direction as shown in Titanic with its steam powered engines?

[vibbs]

Originally Posted by Sawyer

T

Question: with these 80-100 rpm speeds, is the main shaft to the propeller a direct drive? Or is there a reduction gear box in between? If so, does it have just one ratio or many as in a car? And presumably, reverse must mean a gearbox and not the engine turning in the other direction as shown in Titanic with its steam powered engines?

Yes, on ocean going merchant ships most certainly always the propeller is directly coupled to the engine. Reversing is done by engine actually turning in the other direction!

[Sawyer]

How can that be on a two stroke diesel? It has to have some limit for rpm above which the IC cycle can be sustained? And it can't reverse speed surely - must have a gearbox that allows the shaft to do so? Or no?

Whereas as seen in Titanic, one can see the con rods come to a halt and then start moving in the other direction. Now that I think of it, how is that achieved though?!

[Sawyer]

Originally Posted by Brumby

So during the heavy seas we just cruise at an rpm which does not overload the engine (thermally or mechanically), speed becomes irrelevent and safety of the ship becomes important.

Relevant to also point out here that if caught in heavy seas, the engines become critical for survival. Loss of motive power can be fatal at these times, just as it is most of the times in an aircraft. Unlike in a car, where one can pull over into a safe place and park with the engine off. In a ship, regardless of what the wind and the waves are trying to do to you, you do not want the waves crossing you left to right or vice versa if the ship is to not rollover. Usually you want to be running in the same direction as the waves and only sustained engine power availability can let you do that successfully over the period of the storm, which can be hours or even days at times.

[V.Narayan]

Originally Posted by Brumby

Hello Sir,
I will try to make the answers as simple as possible.

2) Basically there are various speeds that the vessels operate on, like chartered speed (as per the lease agreement), MCR (maximum continuous rating), CMCR (contracted MCR). Most of the ships operate at the chartered speed and these are usually close to CMCR. The container trade is the fastest one and hence thay have speeds in the range of 20-30 knots (depending on the size). The difference between the CMCR and MCR is not much due to the slippage factor which increases with speed (a different fluid you see). The propeller of a ship is like a screw moving in a material which yields, so the slip is higher during the starting periods and in the top range.

So theoretically if a screw turned through a solid there wont be slippage. Right?

3) Cargo ships are not designed to maintain speeds, they are designed to be operated a continuous rpm for a time until a maintenance schedule is not due on the main propulsion engine. So during the heavy seas we just cruise at an rpm which does not overload the engine (thermally or mechanically), speed becomes irrelevent and safety of the ship becomes important.

Interesting. So the Master doesn't say 'Chief Engineer give me 18 knots' instead he says 'Chief Engineer maintain 120 rpm'. Did I get that.

And for the last part, its the answer which is right or wrong, good or bad, because the question is always right. Hope I have not confused you much and the answers were simple enough to understand.

You have been such a good teacher that I will now plague you with more questions. Why do warships (those which are diesel powered) have medium speed diesels that run (I think) at say 1000 to 1200 rpm. What factors/needs lead to a medium speed diesel being a more appropriate choice for a naval frigate. Thanks in advance for your effort. - Regards, Narayan

[vibbs]

Originally Posted by Sawyer

How can that be on a two stroke diesel? It has to have some limit for rpm above which the IC cycle can be sustained? And it can't reverse speed surely - must have a gearbox that allows the shaft to do so? Or no?

Not sure if this is that you are looking for.

http://www.marinediesels.info/2_stro...ost_motion.htm

But engines are actually reversed. As in when the ship sails astern, the engines rotate in the opposite direction. Large merchant going ships almost never have a gear for reversing.

[Sawyer]

Thank you, exactly what I needed. So essentially the same thing happens in effect as seen in the Titanic movie. The question as to how the engine starts from standstill when connected to a load - the propeller in water - is still open. Any idea?

[Jeroen]

Originally Posted by Sawyer

Thank you, exactly what I needed. So essentially the same thing happens in effect as seen in the Titanic movie. The question as to how the engine starts from standstill when connected to a load - the propeller in water - is still open. Any idea?

Ships engines are started by using compressed air, forward and reverse. As these large two stroke engines are directly connected to the propellor the compressed air will turn the engine, the propellor and the propellor shaft.

Depends a bit on the type, but these engines can run at very low RPMs, so they start at those very low RPM as well.

Still you need a large volume of air at a good pressure. Ships will have huge air tanks and multiple compressors.

The air is introduced into the cilinders by means of special starting valves. They are operated by the cam shaft for correct sequencing or these days electronically controlled.

Jeroen

[Sawyer]

Thank you; so the compressed air takes up the load until the engine reaches a self sustaining rpm, however low that may be.

[sparky@home]

Originally Posted by itwasntme

Great post Sawyer. Das Boot and Perfect Storm have always been favourites and I will definitely look for the 200 minute edition. Couple more books/movies in the same genre that come to mind:

Submarines:
U-571 (overall amazing)
Hunt for Red October (Sean Connery, book is better though)
Crimson Tide (amazing stand-off between Denzel W. & Gene H. in a nuclear sub)

General military:
A Few Good Men (Jack Nicholson, Tom Cruise & Demi Moore all excelled)
The Scarlet and the Black (WW2, under-rated but a good watch)
Saving Private Ryan (sometimes melodramatic but eminently watchable)
HMS Ulysess (novel, haunting last page description of her final charge)

Have you read The Caine Mutiny, the book is a 1951 Pulitzer Prize–winning novel by Herman Wouk, and the movie, starring Humphrey Bogart, is also very interesting. If not do try, it will keep you engrossed.

[Thad E Ginathom]

Originally Posted by Sawyer

Thank you; so the compressed air takes up the load until the engine reaches a self sustaining rpm, however low that may be.

Bear in mind the basic principle of a diesel engine. It is compressing the fuel that causes it to increase in temperature and explode.

Our car engines have become complex with their fuel injection systems. A basic diesel does not even need electricity.

I have seen a single-cylinder small-boat engine of the simple diesel kind. To start it, you open a valve so that there is no compression. You turn it by hand until the flywheel has enough momentum to turn against the compression and close the valve. Boom, it fires, and, hopefully, goes on firing.

(I have also completely failed to hand start a 4-cylinder small-boat marine diesel designed for battery starting. It took skin off the hands of four of us without a positive murmur. We managed to get help of the jump-start kind.)

So, marine engineers, your huge diesels, in principle, would be more like that single-cylinder diesel that I used to know, and really not much like my Polo TDI, I think?