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Disasters at Sea


blackrose

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23 minutes ago, blackrose said:

 

Yes the comparison may not be like for like. Similarly your scenarios may not be comparable either. Narrowboats aren't always lifted in a brutal way. Often they float into dry docks and are gently lowered. Some canal boats do encounter waves. I don't know how the <1m waves my boat has been though on the tidal Thames compare in terms of scale to an oil tanker at sea but I suppose any comparison could only ever be done on a case by case basis.


The method of docking doesn’t really matter. At some point the boat will be supported at only two locations and if it’s jacked up to re-position stocks - as boats routinely are - then one of those loads will be applied to a small area resulting in bending in two planes. I’m pretty sure the biggest loads my hull has ever experienced were in being docked by slipping last year.

 

I suspect ships have to be more carefully supported but that is because they have less redundancy in their design which was the premise of your argument.

 

We should also remember that narrowboats are constructed in lower grades of steel than sea going boats.

 

I can’t imagine what you experience on the Thames in a largely air-filled near rectangular box is anything like that experienced by a fully laden oil tanker on the high seas in terms of the extremes and contrasts in loads on the hull. A tanker in those conditions needs to act as a mega loading carrying structure in an ever changing range of support conditions where one second it’s almost like a bridge spanning its own length and the next it’s supported centrally and got a hundred thousand tonnes hanging off each end.

 

In reality I suspect canal boats are built to serviceability criteria and ships to strength criteria.

Edited by Captain Pegg
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Bloke I know said that the tiles in his shower room moved when his 50ft all steel widebeam was dry docked. 

 

Boat seemed to have twisted or bent somehow on uneven supports. 

 

This seems unlikely but it is possible I suppose. 

 

It would be quite interesting to do the maths and work out how much force would be needed from under the vessel to cause it to buckle lengthways. 

 

It is a sort of channel section in a way but the gunnels must introduce a weakness structurally. 

 

 

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I think the gunwales add strength.

 

Consider the hull sides as two C shaped main girders and the gunwales and a portion of the baseplate are the flanges to those girders.

 

The cabin top - which I’d ignore structurally - and the baseplate brace those girders.

Edited by Captain Pegg
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That makes sense but would it be stronger than if the whole thing was a box section? 

 

Maybe it would. Box section is not very strong is it. 

 

It is interesting to consider where the failure would occur if a narrow boat was secured front and stern and jacked up in the middle until it broke in half. 

 

 

 

 

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4 minutes ago, magnetman said:

That makes sense but would it be stronger than if the whole thing was a box section? 

 

Maybe it would. Box section is not very strong is it. 

 

It is interesting to consider where the failure would occur if a narrow boat was secured front and stern and jacked up in the middle until it broke in half. 

 

 

 

 


I think you were right first time - it would be stronger as a box with the roof as the top flange and baseplate as the bottom flange.
 

But that would be harder to model - for an old fashioned type like me - and crucially it brings into question whether you have actually designed a viable boat rather than a container.

 

I think a key criteria of any boat must be that the hull would be “seaworthy” in its own right.

 

That kind of brings us back again to the key differences between a sea going vessel and a floating metal boxed home.

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16 hours ago, Bee said:

Steel narrowboats are massively over engineered, can't think of one that has sunk or had really bad structural  damage unless it was already in poor condition

I think that the thickness for nbs is engineered with a view to corrosion rather than impact and certainly not in static strength.

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14 hours ago, blackrose said:

 

That's the case for any vessel, not just narrowboats surely? The least stress on any hull will be while it's being supported by water. I think it's the same principal for aircraft wings as long as they're not doing aerobatics. They're more stressed on the ground than being supported by fast flowing air.

Ships have to be designed for the situation in waves at sea which can create the analogous situation of being perched stem and stern on a wave with the middle on next to nothing! Of course, the real demands are in dynamic rather than static conditions. Most of this does not apply to canal craft. Double bottoms for ships were adopted for large ships more with a view to preventing pollution rather than structural strength.

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When considering how a NB hull will distort it is probably best to start with just the base plate and hull sides with no stiffening.  If you support the base in the middle and load down the two ends the top section of the hull sides will be in tension and if anything that tension will attempt to straighten any bows in the side so you would need to tension the hull sides until they started to tear. Now reverse that with the middle held down and the ends jacked up. In that case the top section of the hull side would go into compression and as steel will bend far easier than tear there would be easier for the hull sides to bow and bend, allowing the base plate to bend up at the ends.

 

Now add the gunnels and they will act as a stiffener and help to prevent the hull sides bowing and bending in the second case and also increase the amount of steel in tension in the first case. That should improve the resistance to the base bowing and hogging to a significant extent.

 

Finally we have the longitudinal strengtheners welded to the inside of the hull sides  and, depending upon they locations will add even resistance to the hull sides tearing or bowing, so a very strong section before we add the cabin sides and roof. As someone said, massively over engineered.

 

As It seems narrowboat hulls tend to go through the waves rather than ride them I suspect they may be subject to less stress than a similar length bat that  rides the waves until the waves get so large they would be likely to swamp the narrowboat.

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1 minute ago, Tony Brooks said:

 

Had to chuckle at that. I agree the hulls but when you start to consider the ease of routine service tasks on some boats you have to wonder.

 

I was indeed merely referring to the structural elements but I do find that the business of attending to the vagaries of the non-boating sections of the boat detract from the pure enjoyment of owning a boat and going boating on it. I can understand the appeal of a simple life in a boatman's cabin if your objective is to go boating.

  • Greenie 1
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14 minutes ago, Tony Brooks said:

 

Had to chuckle at that. I agree the hulls but when you start to consider the ease of routine service tasks on some boats you have to wonder.

It is called maintainability these days.  Regardless, I agree there seem to be few boats where it was even a minor design criterion.

 

N

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37 minutes ago, BEngo said:

 

It is called maintainability these days.  Regardless, I agree there seem to be few boats where it was even a minor design criterion.

 

N

 

Maintainability is what Tony was referring to but it isn't the same thing as a serviceability design criteria. That is about the ability of an object to fulfil it's user's requirements throughout it's service life.

 

Edited by Captain Pegg
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1 hour ago, Mike Todd said:

I think that the thickness for nbs is engineered with a view to corrosion rather than impact and certainly not in static strength.

When I first had a narrow boat and commented that it had 6mm sides, a colleague expressed surprise, saying he expected it to be little thicker than a car body!

Which would probably be fine in terms of static strength, but pretty useless in terms of impact or corrosion.

Edited by David Mack
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14 minutes ago, David Mack said:

When I first had a narrow boat and commented that it had 6mm sides, a colleague expressed surprise, saying he expected it to be little thicker than a car body!

 

A typical car body used to be 0.8mm, but more recently has been reduced to 0.6mm so your sides are 10x the thickness of a cars sides'

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19 hours ago, blackrose said:

I really enjoy watching this programme and also Aircrash Investigation. Not sure why? Maybe from morbid curiosity but I think it's more to do with the way the investigators systematically uncover the causes of the disasters, which often include a series or chain of events.

 

Anyway, I'm just watching one where a 166m vessel capsized as a result of going into shallow water and hitting rocks and it just made me think about how strong steel canal boats are - at least in terms of plate thickness. I don't think I could puncture the 6mm plate on the sides of my 29 tonne widebeam even if I deliberately ran against rocks at full speed. 

 

For example, at 114,000 tonnes, how thick would the plate on a vessel like the Concordia need to be to have bounced off those rocks rather than being sliced open? That's a rhetorical question, you don't need to do the calculation, but I dare say a few feet thick and completely impractical. I know some commercial hulls are double skinned instead.

Stiffness/strength of a panel -- like a hull -- goes up as the square of thickness, but mass/hull forces in a collision go up with the cube of size. So if you scaled up a narrowboat 10x in all dimensions to a ship 700' long and 70' wide drawing 20' or water weighing about 20000 tons, the hull would need to be built from 60mm plate to be as strong as a narrowboat built from 6mm plate.

 

Still wouldn't make it impossible to hole, but difficult. The hull of the Titanic (roughly the size above) was built with 48mm plate...

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26 minutes ago, IanD said:

The hull of the Titanic (roughly the size above) was built with 48mm plate...

 

The Haynes Manual (via Wikipedia) says:

 

The 2,000 hull plates were single pieces of rolled steel plate, mostly up to 6 feet (1.8 m) wide and 30 feet (9.1 m) long and weighing between 2.5 and 3 tons. Their thickness varied from 1 inch (2.5 cm) to 1.5 inches (3.8 cm).

 

https://en.wikipedia.org/wiki/Titanic#Construction,_launch_and_fitting-out

 

Hutchings, David F.; de Kerbrech, Richard P. (2011). RMS Titanic 1909–12 (Olympic Class): Owners' Workshop Manual. Sparkford, Yeovil: Haynes. ISBN 978-1-84425-662-4.

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1 hour ago, TheBiscuits said:

 

The Haynes Manual (via Wikipedia) says:

 

The 2,000 hull plates were single pieces of rolled steel plate, mostly up to 6 feet (1.8 m) wide and 30 feet (9.1 m) long and weighing between 2.5 and 3 tons. Their thickness varied from 1 inch (2.5 cm) to 1.5 inches (3.8 cm).

 

https://en.wikipedia.org/wiki/Titanic#Construction,_launch_and_fitting-out

 

Hutchings, David F.; de Kerbrech, Richard P. (2011). RMS Titanic 1909–12 (Olympic Class): Owners' Workshop Manual. Sparkford, Yeovil: Haynes. ISBN 978-1-84425-662-4.

Oops, misread cm for inches and then did the conversion... 😞

 

1" amidships seems to be the accepted figure, so that's equivalent to 2.5mm for a narrowboat. Explains a lot....

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3 minutes ago, IanD said:

Oops, misread cm for inches and then did the conversion... 😞

 

1" amidships seems to be the accepted figure, so that's equivalent to 2.5mm for a narrowboat. Explains a lot....

 

No plates were punctured in the sinking of the Titanic.

 

And what it mostly explains anyway is why ships are built in steel grades with yield stresses typically twice that of those used in narrowboat construction.

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1 hour ago, Captain Pegg said:

 

No plates were punctured in the sinking of the Titanic.

 

And what it mostly explains anyway is why ships are built in steel grades with yield stresses typically twice that of those used in narrowboat construction.

And there are lots of debates about whether the Titanic steel was brittle at cold water temperatures, whether the rivet material was bad and so on.

 

But to answer the OPs question, the hull of a narrowboat made from 6mm plate is a *lot* stronger than that of a typical ship (or the Titanic), to be similar it would need about 2.5mm plate.

 

Which is why narrowboats are robust, even when bashed into things harder than icebergs... 😉

Edited by IanD
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4 hours ago, Captain Pegg said:


The method of docking doesn’t really matter. At some point the boat will be supported at only two locations and if it’s jacked up to re-position stocks - as boats routinely are - then one of those loads will be applied to a small area resulting in bending in two planes. I’m pretty sure the biggest loads my hull has ever experienced were in being docked by slipping last year.

 

I suspect ships have to be more carefully supported but that is because they have less redundancy in their design which was the premise of your argument.

 

We should also remember that narrowboats are constructed in lower grades of steel than sea going boats.

 

I can’t imagine what you experience on the Thames in a largely air-filled near rectangular box is anything like that experienced by a fully laden oil tanker on the high seas in terms of the extremes and contrasts in loads on the hull. A tanker in those conditions needs to act as a mega loading carrying structure in an ever changing range of support conditions where one second it’s almost like a bridge spanning its own length and the next it’s supported centrally and got a hundred thousand tonnes hanging off each end.

 

In reality I suspect canal boats are built to serviceability criteria and ships to strength criteria.

 

Ok. The premise of your original argument was about the different methods of docking, but now apparently it doesn't matter? Also why will a canal boat be resting on only two supports? Again that would depend on the particular situation and particular dry dock. Yes, I'm sure ships do have to be more carefully supported because they are proportionally weaker. If they weren't carefully supported they would break under their own weight because they aren't designed to support their own weight out of the water. You're probably correct that ships are under more stress being battered by waves, but the fact they have to be so carefully supported out of the water really backs up what I was saying. The rest of what you say also sounds about right.

5 hours ago, magnetman said:

Bloke I know said that the tiles in his shower room moved when his 50ft all steel widebeam was dry docked. 

 

Boat seemed to have twisted or bent somehow on uneven supports. 

 

This seems unlikely but it is possible I suppose. 

 

It would be quite interesting to do the maths and work out how much force would be needed from under the vessel to cause it to buckle lengthways. 

 

It is a sort of channel section in a way but the gunnels must introduce a weakness structurally. 

 

 

 

I've had kitchen cupboard doors that wouldn't open properly when my stern was pulled up a slipway to change my prop. When the boat went back into the water everything went back to normal.

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58 minutes ago, blackrose said:

Also why will a canal boat be resting on only two supports? Again that would depend on the particular situation and particular dry dock.

 

 

At 45 foot long we had 3-supports, the 60 footer next to us had 5-supports. Any yard supporting only bow and stern would not get my custom.

 

 

CAM00042.jpg

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