Distorted ship's hull steel panels

I always assumed that when you saw a Naval ship with panels on the hull side dented in, it was the arduous sea journeys that had indented the steel plates between their bulkheads and ribs.

But today I saw a film of the launching of HMS Sheffield (later lost in the Falklands) and it had the same indented hull side panels, perhaps 10 feet square, all down the length of the ship.

This got me thinking. If they were welded at the joins would not the cooling cause the panels to be pulled tight? Why are they all indented at the centres by an amount that is very visible in reflected light?

A similar effect can be seen on locomotive tenders that have been welded. Perhaps an effect that should be replicated on scale models?

Any experienced large scale welders able to comment?

Norm.

Why would the external panels only be welded to each other, and not the internal structure that you can't see?

I wonder how small the variation actually is. Light can be a very sensitive test and show the slightest of variations in surface flatness.

regards Martin

Low res picture, but like this?

I can only find reference to welding distortion, seems strange that they accept it / haven’t got a solution to it.

Edited By Jim Young 2 on 10/04/2021 22:46:58

Edited By Jim Young 2 on 10/04/2021 22:48:31

I can only talk about ships,

the royal navy ones would be as light as possible compared to merchant ships so this may account for some of the panel distortion

They are basically big speedboats with guns

Edited By Ady1 on 10/04/2021 22:58:28

It doesn’t answer your question, Norm ... but there’s a decent photo here: **LINK**

https://www.seaforces.org/marint/Royal-Navy/Destroyer/Sheffield-Type-42-class.htm

... and some useful comment here: **LINK**

https://military.wikia.org/wiki/Type_42_destroyer

MichaelG.

.

[quote] When the Type 82 air-defence destroyers were cancelled along with the proposed CVA-01 carrier by the Labour Government of 1966, the Type 42 was proposed as a lighter and cheaper design with similar capabilities to the Type 82. [/quote]

Its caused by the welding on the 'inside" of the shell. There are frames and/or bulkheads at approximately 1 metre intervals along the length of the hull, and longitudinals at approximately every 600mm going from the sharp end to the blunt end. These cause "panels" all over the external surface of the ship. When the welding happens the "panels"are pulled in by the action of the welding, causing the distortion mentioned. The distortion varies but can be up to about 10mm. The welding of the frames and longitudinals to the ships shell (the outer plating visble from the outside of the ship, is continuous. Shell plating thickness varies considerably from a maximum at the deck edge and the "turn of the bilge" amidships to a minimum towards the bow and stern. I was told by someone who would know that about seven tonnes of epoxy filler is used to smooth out the hull on your average frigate, this improves performance and reduces the hull generated noise.

Ady1 is right about the weight, considerable care is taken to ensure that weight is minimised. Not to sure about the "...big speedboats with guns", though!!

Hope this helps

cheers

Bill

Simple question, complex answer. On most steel and aluminium hulls it is possible "to see her ribs". How apparent that is depends on many factors such as shell plate thickness, frame spacing, longitudinal spacing, whether a frame is a bulkhead with intercostal longitudinalls or if the bulkhead is "cut" round continuous longitudinals. Generally rivetted hulls are less affected than welded.

Structure of ships is governed by Class Rules, linked is an example from DNV for special service craft

http://rules.dnvgl.com/docs/pdf/gl/maritimerules/gl_i-6-1_e.pdf

Other flavours and types available. Worthy of mention that Naval vessels are not bound to be built to Class and afforded other exemptions from SOLAS and collision regulations. However the basic principles of Class standards for construction are usually considered!

Deformation of the shell plate is monitored at routine surveys through the vessel life and again there are limits on allowable deformation before replacement. Deformation of frames and stringers has less tolerance and a badly "tripped" frame or stringer usually has to be replaced immediately it is notified.

Bill is correct that mid ships L/4 is the critical area for longitudinal strength and frame and stringer spacing may be closer in this area.

Lightweight has not always been the order of the day in Naval ships, HMS Blake commissioned in 1961 sent for scrap in the early 80's had 10" armour plate around critical areas of the hull!

Paul.

Pauls addition is interesting and provides an alternative perspective.

One of the reasons that weight is important is the increase in electronics and cabling in modern ships. An attempt to reduce "top weight" by the use of light alloy superstructure proved to be a dead end. The T21 class frigates had light alloy everything above the weatherdeck, and it was realised fairly early on when the first of class (Amazon) had a fire when fitting out, the al. alloy structure just burn't away. Sadly this came home to roost in the Falklands. The superstructure of later ships, is of fairly light gauge steel. Interestingly (well I thought so!!) the paint scheme is very important to prevent rust. It's not just a case of slapping on another coat of battleship grey!

I was only involved with two classes of ship, T21 and T22. They both used constant frame and longitudinal spacing (36" x 24" for T21 and 600mm x 1000mm for T22). The variations required to comply with longitudinal strength were achieved by varying the shell thickness as required.

cheers

Bill

I never knew about this but the moment I read it it made perfect sense

There are also coatings on the upper decks which reduce radar reflectivity/signature

edit

The usas sooper destroyer had such a small radar signature it was a navigation hazard.

At night other vessels thought it was a small fishing boat, not a full sized ship

Edited By Ady1 on 11/04/2021 07:39:14

I noticed the visible ribs effect a long time ago, and wondered what a strong enemy frogman, with a good knife, might be able to do if he could approach undetected...

There would be some noticable marking on a new ship, but the indentations do get more pronounced as the ship gets older. Destroyers and similar do have thin hull plating, and the indentations are normally much more pronounced near the bows rather than towards the stern.

Modern destroyers should not be confused with destroyers of a bye gone age. For instance a modern T45 destroyer displaces over 7,000 tonnes and are about 500' stem to stern. An early "V" class destroyer laid down in 1916 was about 1190 tons and 300 feet long. To provide an effective escort for the modern fast battleships in WW2 the "L" and "M" class were built. They displaced around 2,000 tons and were 360 feet stem to stern.

Even HMS Queen Elizabeth had visible between frame and longitudinal panel warping at the time of her launch

cheers

Bill

I know its a lot flimsier but I remember the feeling of horror when I first saw a Seaking taking off from up close. As the tail rotor takes up the torque the tail boom flexes slightly, and all the rectangular rivet arrangements on the tail boom deform enough to make it look like a diagonal line has been scribed from one corner of the sheet metal to the other in each "box".... hard to describe but you would know it if you saw it. It makes sense of course, but still unnerving!

Thank you all for joining the chat. But I still cannot see why, when they build a big Naval ship, the plates (panels?) all appear indented at launch.

Bill kindly explains the welding might pull in the centres, but that seems odd - couldn't they design the ribs to match the planned curvature in all places?

A big ship like a liner with heavy plates has them all rolled to match the curvature; I don't think the same plate distortion is seen. So, are we thinking that the Naval ships are made with much thinner plates, to make a lighter and faster ship, and try as they might they cannot make them neatly curved all over, and have to resort to seven tonnes of epoxy filler?

So why can't thin steel panels be rolled to a correct curve, and welded to the curved bulkheads and ribs, so that it all looks neat? There is something we are not understanding, and that was the whole point of my first question.

My guess is that localised heating and cooling of the skin from the sun leads to expansion that either has to pop in or out, and standard practice says to make them all pop in. I wonder if this is why the same panel distortion is seen in welded locomotive tenders?

But, if the panels are flexing in and out with sun heating, how does the seven tonnes of epoxy stay in shape?

Edited By norm norton on 13/04/2021 10:11:12

Morning all

The RY Britannia was covered in a filler before she was launch in the mid 1950s by John Browns, the Clyde, to make her look good.

Early 50.s so not sure it was resin, but some thing else concocted to suit. The boat is still floating in Leith Docks after decommissioning about 1995 ish. Never been to it as I am not too impressed by the fine holidaying enjoyed. I did know a crewman near the end.. ...they still slept in hammocks, so two standards really; above and below deck.

The other matter found out in the Falklands for war ships was the use of the then modern vinyl slip-resistant flooring. It was not flame resistant so created terrible problems for burning and toxic fumes. I believe back to the old battleship lino made from sawdust, linseed oil and cement.

Keep safe everyone.

PS Woolen clothes were found were far better than modern synthetics. Wool doesnt flame.

PPS External Twin hulls with fuel tanks in between were also found wanting.

PPPS It is so easy to do reviews and loose sight of some of the sense and basic purpose.

I didnt intend to high jack the theme. All very interesting ...as always. Stay safe.

Referring back to Ady1s comment about them being big speedboats with guns I was fortunate to have a tour of one in the Falkland Islands back in 2009. They did a demonstration run at full power and you could certainly feel the acceleration and the centrifugal force as they turned at high speed. So I would say that his description is accurate.

Since big guns were rendered obsolete by aerial bombs, torpedoes and guided missiles, warships have all but abandoned armour in favour of high-speed, manoeuvrability, and automatic defences based on electronic counter-measures, anti-missile-missiles and chain guns etc. And as an anti-ship guided missile has considerably more punch than the largest 18" shells ever fired in anger, it's not unreasonable to hope a powerful missile might pass clean through the ship without exploding, which favours light-weight construction.

Warship hulls and superstructure are as light as possible consistent with remaining seaworthy and shaped to reduce radar reflections without regard to neatness. The hull is only faired as necessary to improve performance, and looking neat is well down the list of requirements.

If the indentations mattered, they could be fixed . A Type 26 Frigate costs about £10Bn so spending a few hundred million more on the hull isn't a problem. I suspect the indents either have no effect on performance, or there's a mildly positive benefit such as a reduced radar or acoustic signature, or maybe even a go-faster improvement as provided by the dimples on a golf-ball.

The cause is simple; distortion due to welding a thin plate to a frame. Merchant ships also have indented hull plating, but it's less obvious because cargo carriers are strongly built of thicker plate. Again, indents could be fixed if they caused a problem such as poor fuel economy, but I suspect they simply don't matter. It just looks untidy, and is only worth hiding on passenger ships.

Passenger ships have a long history of cosmetic engineering designed to attract customers. Classic liners built with four funnels rarely needed more than two or three. Four funnels created the impression of speed and power by imitating fast warships, even though the commercial vessel sailed at profitable speeds and, unlike a warship, was as comfortable inside as a good hotel.

Dave

At 1710 lb arriving at 1047 ft./sec., an Exocet doesn't quite compare to a WW1 - WW2 British 15" battleship round (1938 lb. at 1430 ft./sec @ 19,500 yds range), less still an 18" with 80%-odd more mass. The competitive Harpoon missile is lighter, slower, but with a bigger warhead. Both have larger warheads than the bursting charge of 15" HE shells, and much larger than AP, which formed the majority of capital ship ammunition outfits. These latter certainly did pass through lighter ships without exploding - eg. Warspite's fire at German destroyers at Narvik.

The missiles' kinetic energy is thus a good deal lower, but with current fuzing capabilities the prospect of passing through without exploding doesn't look realistic. The Exocet that hit HMS Sheffield may or may not have exploded - I can't imagine how the detonation of such a substantial warhead could be unclear - but the wrecking fire was caused by residual propellant. The missile body didn't pass through.