Ship driving


#1

I hardly ever go on the bridge but I recall leaving Valdez in some particularly heavy seas and was told “they” didn’t want to take it on the stbd. quarter as it could bend the ship in half… well, yea, you could see her bending and i’ve seen it from below but i saw a sputnik news article saying some large bulk carrier that went down should be taking them on the stbd quarter? and some other ‘stuff’ about the article prompted me to write the author and suggest he spend some time on a mariners website! We must be a ‘silent majority’ of some kind?


#2

I’ve never been aboard a ship where the actual stress could be measured in real time. I have only computed stress on the stability program. We do find (and repair) stress cracks in the same areas (midship) that the program shows the highest stress. But we can’t associate specific cracking to a single weather event.

AFAIK the stress calculations are based on simple hogging and sagging. Max hog/sag would be in the case the ship ends were supported or the middle was supported by a swell. In that case if those situation were to be avoided the tactics chosen would depend on the relationship between ship length and wave length.

In other words pick a course where having the ends supported is avoided.

I’ve always been more concerned avoiding high acceleration, how the ship was riding rather than hog/sag stress. I do try to avoid pounding.


#3

The Keystone Canyon and I believe, the Atigun Pass had strain gauges fitted to the hull structure to measure strain, and pressure transducers on the bulb to detect slamming in an effort to reduce or eliminate the problems with high strength steel construction.

The TAPS run from Valdez in the winter played hell with those ships and cracking was an almost certainty during the kind of weather typical of the Gulf in winter. When weather was good enough to go on deck you could find the cracks by the line of IG bubbles when the deck was wet.

There was a radar like display on the bridge with a polar display that showed the current level of strain and the limits. Of course the mates used the limits as a goal rather than keeping a good margin.


#4

Without stress gauges I suppose the flexing of the hull could be watched and with some experience the crew could develop the ability to be able to relate the movement of the hull with the cracking damage. It’d be a little more subtle then having gauges but it seems likely a tanker crew could develop such skills.


#5

Experience showed quite the opposite.


#6

On tankers the flexing of the hull was visible by the movement of the catwalk in the midship where it ended. The catwalk was not hard connected to the midships as it would have been damaged by the flexing but had a sliding connection.

The catwalk shown is of the Shell tanker Ondina on which I sailed.

The Ondina is a famous name here as the predecessor by the same name sank a Japanese cruiser during WWII with shots fired from the single gun on the poop deck.

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A photo of the gunnery crew which was taken after the incident in 1942. The text on the banner speaks for it self!


#7

Also a special case of ship driving was shown by the Dutch tanker Mijdrecht during WWII, I sailed on the new Mijdrecht which was my last ship before moving to a shore position.

A hole large enough to make a turn with a horse and carriage. Despite this the ship made it safely back to Glasgow for repairs.

The ship was torpedoed in the cofferdam just in front of the engine room by the U-70. At the time of impact the captain was on deck busy with the survivors of another torpedoed ship which they had just picked up. After the hit the Chief Officer, who was on the bridge saw signs of the sub surfacing and without a moments hesitation yelled at the Radio Officer who was behind the steering wheel: “Hard to starboard. We are going to ram that bastard”. The sub never expected this manoeuvre, certainly not by a victim, and could not dive in time to get away and was hit bull’s eye at the command tower and periscope position. The U-70 rolled over and disappeared under the Mijdrecht’s keel and sank. The moral of the story is never mess with the Dutch…

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This painting of the moment of impact was hanging in the Officer’s Mess of the new Mijdrecht.


#8

Sailing from US West Coast Ports ( Portland) towards the Unimak Pass in the winters, the sea and the swell could be anywhere from the port bow-beam. There are always at least two storms in the vicinity. These have made it across the Pacific, so they are well developed and energetic.

Firstly, agree that pounding must be avoided at all costs by reduction of speed if necessary. That generates a large amount of email traffic with the charterers and owners who want to know ( from the plush offices in downtown somewhere) WTF you are not making charterparty speed of 14 knots?

There is little anything else one can do, except adjustment of course to find the heading that the ship rides most comfortably at. With 8m swells, there is nothing one can do about stresses; the ship has to take the punishment and survive.

Also there is not much choice sometimes regarding alteration of courses, especially when closing the Aleutian islands.


#9

On the San Clemente tankers you could easily see the flexing of the ship from forward to aft by observing the hand rails in heavy weather.


#10

The chain hand rails also showed the change in hog/sag. When the ship was light, the midship rails were tight, when loaded, there was some slack in them.


#11

Here is a video showing how a Container ship flexes in heavy weather. View along the internal walkway:

You may switch to the external view when half way into the video.

Another video with similar internal and external views:


#12

Flexing was very easy to see from the wheelhouse on tankers but a history of very expensive repairs and the threat of structural failure due to the high strength steel (along with the sunset of single hull tankers) that finally ended the life of the ship could not have been prevented by a mate estimating strain by watching the rail or a kingpost wriggling around.

The best view I ever found was in the tunnels of the APL C-8s. From the watertight door at either end the opposite door would move in and out of view as the ship twisted and bent.

Ah just saw the Bug posted a video …


#13

Another vid, very well made.


#14

great post Dutchie on the Mijdrecht, many stories exist like that that should never ‘tire’ of being told.
I guess now it’s called “instant Karma” or something! Subject matter could fill a category!


#15

Yeah, that’s my question. Can/does the crew observe the flexing of the ship and then make the correct course/speed adjustment? I’m thinking unless observations are made in conjunction with reading of some stress gauges then working by seat of the pants methods is going to be a lot of hit or miss.

One issue is that presumably these cracking problems are going to be by number of cycles, how can the mate or captain at sea estimate that?


#16

One advantage of fwd houses is that it help deter the propensity of some to drive the ships too hard. Nothing like being being up close and personal to the action to change ones perspective.


#17

They can’t. That is the job of a strain monitoring system that measures and records real-time conditions.

No, that would be kind of like determining boiler pressure by the funny creaking noises it makes when the pressure is too high.

The strain monitoring system they put on the TAPS tankers displayed the actual strain relative to maximum allowed and historical number of cycles. The materials engineers came up with projections for strain vs cyclic loading and failure margins.


#18

The focus in this topic has been on bending forces and moments of ship hulls but there are also other forces that play at the same time an important role. A ship operating in real seas is subjected to shear forces and bending moments due to wave load along it length in both the vertical and transversal planes. Apart from vertical bending there is also horizontal bending and torsion or twisting taking place which are part of the hydroelasticity properties of each ship. Google on ‘hydroelasticity of ships’ for more information.

Ships with large deck openings, for instance container ships and bulk carriers, are especially sensitive to twisting in waves due to considerably reduced torsional stiffness. This problem has been analysed by many authors.

Numerical simulation tools have been developed to account for heave, pitch, roll, vertical and horizontal bending and torsion, also by classification societies like Lloyds and DNV.

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One measure which has been adopted for large and slender container ships is the 'torsion box’. The principle is comparable with the curled rim to strengthen a plastic or steel bucket which we use for house-hold purposes. This curl is similar to the torsion box used in ships.


#19

Ship driving is one thing but bicycle driving is something that requires special navigational skills in today’s traffic. And safety helmets are for pussies…

Also off topic: Start of the winter, first snow of the season.


#20

In Amsterdam most appears to follow rules. Not so in Saigon:

PS> Only one rule apply for pedestrians crossing the streets; walk at a steady pace, don’t stop and don’t look around.