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All the strength is in the gunwales?


Ajna
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From an engineering perspective, the angles create a pseudo box section which is significantly stronger than flat structures. Each opposing angle acts to support and counter bending, simulating a thicker structure. If you were a mega-super-anime robot capable of picking up a NB and bending it like a baguette then you would see that the creases would be in the slab sides of the cabin and hull. The bends would be resisted by wrapping around the gunwales!

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When we had the steel under cloth conversions on our motor and butty we wanted to do away with A frames. In order to achieve this and prevent the sides from spreading Graeme Pearce at WFBCo (now Stockton Dock) added extremely heavy weight box section gunwales.

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What do people think of this comment on hull strength?

I assume we are talking steel narrow boat here. What sort of hull "strength" ? If we look at the ways the hull could be loaded then:

 

Lateral and/or Longitudinal tension/compression- not much if any

Transverse Shear- hardly at all

Longitudinal Torsion - not really

Longitudinal Bending- to some extent. Much depends on what arrangements are made to restrain the hull from spreading or narrowing- a fully or mainly metal cabined hull could be well restrained without the step of a gunwale and a metal/composite working boat had/has chains and beams whilst a wooden one had chains, beams and stretchers. Many metal working boats had a metal engine room and bulkheads.

Lateral bending- not at all.

 

If you look at a ship that has real GUNwales then the gun ports are a serious loss of strength,( and potentially watertight integrity too), so the ship would have been stronger without them!

 

N

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I once nervously jacked up the front end of Innisfree so that it had no support between the jack at bows and a length of timber halfway along the swim, by looking along the edge of the baseplate I couldn't detect any sag whatsoever. Standard 10mm baseplate and 6mm sides (3mm cabin) Shows just how rigid they are.

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I once nervously jacked up the front end of Innisfree so that it had no support between the jack at bows and a length of timber halfway along the swim, by looking along the edge of the baseplate I couldn't detect any sag whatsoever. Standard 10mm baseplate and 6mm sides (3mm cabin) Shows just how rigid they are.

We always support the front end of boat over 60 feet long in the dock with a block of hardwood and wedge.

Existing boat stands are at 10 feet in, 25 feet and 45 feet- concrete sleepers.

Wouldn't like to see the first one that has some movement in a 70 footer.

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In a boat with a steel cabin and roof this together with the hull forms a roughly square-section tube, and any tube like this (a torsion box) is strong in both torsion and bending -- far stronger than a non-closed tube (e.g. an open-topped hull), but weaker than a circular tube.

 

But if you're then going to bash the sides in by ramming things they're much weaker because they're flat (round tube is much more resistant to this), the gunwales give a big increase in stiffness at gunwale level because they're horizontal.

 

Above and below this you've only got the stiffness of the material (cabin sides and hull) and any strengthening stringers attached to these -- but these only help if they are a lot stiffer than the underlying material, which depends heavily on their section (I or U are much better than traditional L) and thickness at right-angles to the hull.

 

Engineering lesson over...

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In most of the posts so far, you should delete the word "strong" and insert the word "stiff".

 

Since we're talking about steel, a material with high strength, manufactured to a shape with high stiffness (i.e. difficult to change shape or bend), the distinction between stiffness and strength isn't really relevant for the purposes of this discussion.

Edited by blackrose
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To quote J.E. Gordon:

 

"Strength and stiffness are not the same; steel is stiff and strong, nylon is flexible and strong, a biscuit is stiff and weak, jelly is flexible and weak..."

 

(but I agree with Blackrose, for a steel narrowboat hull either can be used)

Edited by IanD
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