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Boat flexing


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What's an acceptable amount for a steel canal boat to flex when out of the water, or is any amount of flexing ok as long as it doesn't break? 

 

Depending on how a boat is supported when out of the water one normally sees things like cupboard doors that won't open or wooden joinery that opens up a bit.

 

When I was out the water this week my boat was only supported by 2 trolleys on the slipway about 12m apart and a massive crack appeared at the top of the tiled bathroom bulkhead. The stove flue pipe also dropped down by about an inch. I was quite surprised how much the boat flexed in the middle but I guess it was a lot of unsupported weight between the two trolleys. It's all popped back into position now with the boat back in the water (2nd set of pictures) but it seemed like quite a lot of flexing at the time. 

 

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Edited by blackrose
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1 hour ago, Alan de Enfield said:

Id say that the supports when we came out for blacking was around 25-16 feet spacing (boat 45 feet long)

1st support about 8' in from the bow, last support about 8' in from the stern.

 

 

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Yes, my boat could have done with one in the middle. 

 

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Edited by blackrose
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1 hour ago, Paul C said:

Who made the hull and what spec is it? Like 10/6/5/4 or whatever

 

10/6/5/5 Liverpool boat. 

32 minutes ago, MartynG said:

If it returned to normal no worries.

Next time more support required 

 

They only have two bogeys on the trolley, so next time I'll ask them to put them a bit closer together. 

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We have been out at blackthorn a number of times with no noticeable flexing. However when we were craned out on the GU a few years back there was a noticeable difference. Doors not closing etc. 

 

As we were supported on wooden blocks, I put it down to the possibility that they weren't level. 

Edited by rusty69
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1 minute ago, rusty69 said:

We have been out at blackthorn a number of times with no noticeable flexing. However when we were craned out on the GU a few years back there was a noticeable difference. Doors not closing etc. 

 

As we were supported on wooden blocks, I it down to the possibility that they weren't level. 

 

Which trolley were you on at Blackthorne? There's one for narrowboats that supports virtually the entire baseplate 

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An increase in floor to ceiling height, particularly if most pronounced in the middle of the beam, is suggestive of the baseplate sagging across the gunwales.

 

If so, repositioning the two stands closer to each other might help a bit, but does not really address the issue

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5 hours ago, Tracy D'arth said:

Are we talking Liverpool Boats here?  Narrow or fat boat?

The picture shows a fat boat unless it’s been photoshopped and he does mention Liverpool further up?  Perhaps the heat is playing up with the thread 🤔


Our 6’10 60 foot nb  was perched high up on 4 small supports 2 each side in order to grit blast and epoxy the base. I didn’t see such flexing. Perhaps it’s the additional widebeam  width that adds to the weight and flex? IIRC you were going to epoxy the base so it’s a trade off of little support,  possible flex and greater area you can epoxy. (Appreciate in the end you didn’t epoxy the base?)

 

Alans boat would have been difficult to grit blast or epoxy much underneath but looks like he was playing safe there for a change. 
 

 

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2 hours ago, Stroudwater1 said:

Alans boat would have been difficult to grit blast or epoxy much underneath but looks like he was playing safe there for a change. 

 

 

Many years ago I went into a boat yard not far away from Manchester and asked to see 'the boss man', I was told he was in the yard driving the 'big digger'.

 

Went into the yard to see a boat being lifted by said 'big digger' using a single strap - as the boat lifted, there was a 'bang' and both ends slid out of the sling - it had broken its back.

 

Guess you are correct - I must be pretty risk averse, I turned around and walked away having decided not to use that 'yard'.

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3 hours ago, Stroudwater1 said:

The picture shows a fat boat unless it’s been photoshopped and he does mention Liverpool further up?  Perhaps the heat is playing up with the thread 🤔


Our 6’10 60 foot nb  was perched high up on 4 small supports 2 each side in order to grit blast and epoxy the base. I didn’t see such flexing. Perhaps it’s the additional widebeam  width that adds to the weight and flex? IIRC you were going to epoxy the base so it’s a trade off of little support,  possible flex and greater area you can epoxy. (Appreciate in the end you didn’t epoxy the base?)

 

Alans boat would have been difficult to grit blast or epoxy much underneath but looks like he was playing safe there for a change. 
 

 

Painting is no excuse for inadequate supports. Paint what you can and then move supports to the already painted areas,adding extra supports as necessary.

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Funny things canal boats, common sense says that they should be really strong as they are a kind of hollow girder. However they will twist so if the supports are not both level in the same plane then the boat will twist as it settles, that will show up as movement inside even if they are made of huge thick sections. A narrow boat section has that good old principle of strength in depth but as boats get wider and therefore 'flatter' I reckon they will lose that strength. Would be interesting to squint along the roof and see if that sags as it settles, if it does then that is what is happening.

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Also internal steel beams, particularly cross beams welded to the base plate, or roof. A narrow boat might have good torsional rigidity, but a wide beam could need beefing up, or design changes to be similarly stiff in torsion. A mismatch in support across the width of the boat, front to rear will have a magnified effect on a fat boat.

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