Flexing Narrowboats ?

[John Orentas]

Several times in the past few weeks I have read about narrowboats 'flexing' and even in one case, about the possibility of one breaking it's back if left high and dry.

 

I have hit solid obstructions very hard, never have I ever witnessed any kind of flexing, steel narrowboats are very tough, ridgid structures. The only circumstances in which my hull ever changes shape in some way is when through the action of very strong sunshine on a hot summers day, one side of the boat gets very hot when the bottom half is imersed in cold water.

 

To suggest that a hull can flex sufficiently to have any effect on the structure or internal fittings of the boat is surely misguided.

 

The one exception that I can think of is composite working boats, they can be a bit bendy.

 

John Squeers

Edited October 24, 2004 by John Orentas

[David Schweizer]

The only time I have experienced any hull "flexing" was when my boat was in dry dock and I noticed that a pair of doors were not lined up and were not closing properly. This rectified itself as soon as the dock was re-flooded, so I assume that it was something to do with the beams in the dock not being quite level.

[timboharticus]

I've just done a very rough calc on the forces involved in a low speed collision

 

These come out at about 1-5 tonnes force which a narrowboat hull should withstand comfortably, although the nasty dent in my roof where I clipped a low bridge is no consolation.

 

Most boats are relatively thin skinned structures and when they are evenly supported by floating there is no problem.

 

It's when the boat is out of the water that there is a danger of deflection/buckling.

Luckily the wave height on a canal is not sufficient for this to occur.

 

The key thing is to properly support the hull when lifting or chocking up.

[Gary Stacey]

It is quite common for boats to flex when taken out of the water and for doors etc to then not fit correctly. A boat is very strong when hit form the fornt or back but has much less strength when hit from the sides say by another narrow boat and will 'flex' in ths situation. Old working boats sometimes had to be 'pulled in' at their mid section in order to fit through locks etc.

 

Gary

[Gary Peacock]

A good hull only flexes a little. A cheap hull, well that can be a very different story and any problems can always be blamed on unskilled lifting or dry docking!

 

Gary

[Martin]

When Octarine was craned out, the boatyard supported her on just two narrow wooden spars - each about 15' from each end.

This was never going to be sufficient to support a 28 tonne 72' boat!

Sure enough, doors moved and the parquet floor in the hallway also started to move.

Unfortunately, the crane-out was done while I was at work and it was the following morning when I noticed what they had used by way of support.

A boat on the bank WILL flex to varying degrees but you can minimise the likelihood by sorting out regularly spaced supports.

[John Orentas]

Martin.

 

Octarine isn't an ex working boat is it, if so, yes the riveted contruction of a working boat is another matter.

 

If no, we will have to agree to differ on this one, your description is so far away from my own experience.

 

John Squeers

Edited October 26, 2004 by John Orentas

[Martin]

No - it's not an ex working boat.

It is also not made of plate of any less thickness than today's recognised standard.

AND we emptied the water tank before it was craned.

It's just that if only two supports are used - and they are too near the centre of the boat, then each end will tend to sag.

The sagging was only slight of course, but enough to do a little damage.

She was on the bank for a total of about 6 weeks - we were promised 10 days, but that's boatyards for you!

Although we added more supports and the sagging ceased, she was back in the water for about three weeks before everything finally lined back up again.

[Big COL]

This is not recommended, but most of us will have done it by accident, when running aground and the boat has risen onto the mud at the bow but only half way across the base plate. Have a look at the line of the roof down its length it's surprising just how much flex there is in it.