Commercial shipping and reverse

doris

doris

Well-Known Member
Joined
19 Jun 2001
Messages
2,307
Location
London
Visit site
Sailing with SWIMBO w got into a discussion about how big commercial ships put their engines astern. I seem to remeber that they in efect stop the engine then restart the other way. Do they have gear boxes, as us yachties might know them, or do they have something I know nothing about?
Watching tankers come up the Solent with a couple of tugs attached, backwards to their sterns started all this.
Looking forward to some the more erudite chaps on the forum.
 
On the trawlers I worked on we stopped the engine and restarted,had to makesure there was plenty of air in the bottles and have the compressor running.As one time chief engineer it was touch and go after too many manouvers in sucesseion.On small coasters it was just a big leaver in on the bridge....
 
doris said:
Sailing with SWIMBO w got into a discussion about how big commercial ships put their engines astern. I seem to remeber that they in efect stop the engine then restart the other way. Do they have gear boxes, as us yachties might know them, or do they have something I know nothing about?
Watching tankers come up the Solent with a couple of tugs attached, backwards to their sterns started all this.
Looking forward to some the more erudite chaps on the forum.
Click to expand...

You can also change propeller pitch.

The tugs are usually for steering purposes, not necessarily acting as brakes.
 
It depends on the type of engine in the ship and to the size of the ship as to how they can go astern. On larger ships with slow speed engines the prop shaft is connected directly to the engine no gearing at all so the engine speed is the same as the prop speed. To go astern the engine is literally stopped and started again backwards, under normal circumstances this is done electronicaly using the telegraph lever on the bridge although the engine can be controlled from the engine room.

Smaller ships which have medium or high speed engines are connected to the shaft through a gearbox which is largely similar to what you are used to on a yacht just perhaps a lot bigger.

Some small to medium ships may be equipped with controllable pitch propellers which allow the pitch of the blades to be changed. This means that the prop is spinning in the same direction at a constant speed but the amount of thrust and the direction is changed by the pitch of the blade. This interestingly has an affect of the direction of transverse thrust when going astern as the prop does not actually change direction.

Another method of propulsion which is increasingly popular on passenger and offshore ships is diesel electric where large generators run electric motors which can either be connected to a traditional props or to azipods. Which are rotating pods on the bottom of the ship which contain the electric motor and steer the ship by rotating. these are put into reverse by turning the pod round basicely the same as the small out boards where you turn the engine round to reverse.

Hope this helps but the answer to your question is basicly it depends on the ship.
 
Was lucky to be invited onto bridge of the large ferry from Holyhead to Dun Laoghaire and talking with the captain about the capability of that sort of ship to stop quickly (as expected quite a distance) but what did interest me was his comment that ships generally could do "emergency stops" and stop in far less than the normal stopping distances BUT that there was a certainty of major structural damage in this event.
 
A certain RNLI Severn class coxswain takes/took great delight in demonstrating crash stops to visitors and those out for a 'jolly' on the boat.

The whole boat felt like it was going to shake itself to bits. The look of pain on the mechanics' faces said it all...

PW
 
PilotWolf said:
A certain RNLI Severn class coxswain takes/took great delight in demonstrating crash stops to visitors and those out for a 'jolly' on the boat.

The whole boat felt like it was going to shake itself to bits. The look of pain on the mechanics' faces said it all...

PW
Click to expand...

When the Isles of Scilly boatmen first got a couple of the new fast semi-ribs (or whatever you want to call them), with water-jet propulsion, some of them used to take great delight in showing their boat's stopping capabilities - especially when it was quiet! We travelled on the Wizard, when they'd only had it a short time - we got chatting to "captain" about his new toy - we were his only passengers. He said "would you like to see how quickly she stops?" - from full speed to full reverse, she buried the bow on the sea and stopped instantly! Very impressive, and good job we were holding on tight!
 
An engineer told me, before he died, oh, no sorry, .....When the VLCC he was on dropped a valve or somesuch, with no gearbox they shut the fuel off and then had to wait til the ship stopped, as long as it was moving the engine was rotating, a long time and lots of crashing and banging.
 
seaspeed

One problem with big ships is their ability to engage sea-speed when clear of land. I am not one of the black gang, but I understand the effect is a bit like linking out on a steam recip. It saves a lot of fuel on a long run.
The problem is that the ship cannot easily manoeuvre in this condiditon. It takes about 15 - 20 mins to ease out of sea-speed.
Now, the problem really arises because some ships bound for Rotterdam are known to stay in sea-speed while in the Straits of Dover. Which may be why nobody has ever seen a merchant ship slow down.
In danger with no room to turn, Ease he, stop her, go astern.
My eye.
What they really mean is
If two lights you see in front, Full ahead and sink the dear little fellow.
 
Another couple of quirks that applies when they are at sea speed.
Vessels running on heavy bunker fuel need to change to light fuel AND wait for this to reach the injectors before they can do anything. Otherwise the heavy (hot) fuel will set solid in the fuel lines.
Vessels running constant speed engines with controllable pitch props normally run their alternators of the main engines as well. This makes the main engine revs critical so they generally need to start an axillary generator before they can make any significant speed alterations.
 
Oldhairy said:
Another couple of quirks that applies when they are at sea speed.
Vessels running on heavy bunker fuel need to change to light fuel AND wait for this to reach the injectors before they can do anything. Otherwise the heavy (hot) fuel will set solid in the fuel lines.
Click to expand...

There is no seaspeed as such; there will be a range of engine speed which must not be dwelt in as it corresponds with the resonant frequency of the crankshaft and other large bits of engine, staying in this range risks expensive damage. In order to pass through the 'at-risk' speeds quickest, the engine is throttled up to just below the lower critical speed and then is a pause while the ship accelerates to a steady speed, then the telegraph is advanced to Full Ahead and you accelerate through the critical speed range and on to FAOP; Full Away On Passage.

Most ships built in the last decade or so are capable of burning HFO all the time(fuel circulation and a lot of usually leaky steam tracing), so no need to change to diesel unless the ship is entering a special area where there are emissions controls(like California) or being prepared for cold lay-up.
 
Yes and in California, the Baltic, etc. we burn low sulphur heavy, not diesel or MGO.

Rule of thumb for yachtspeople:

Ferries, big tugs and offshore support vessels when not towing or running anchors can usually back up quite fast.

Everything else takes rather a long time.

(steam turbine vessels are now almost extinct apart from older LNG carriers and nucelar submarines but they need(ed) to engage the astern turbine which usually had 1/3 of the power of the ahead turbine...)
 
You must log in or register to reply here.

Share this page

Top