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How many blocks on dry land?


Brucec

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Hi all,

I’ve just had my 58 foot narrowboat craned out of the water and put on blocks for maintenance - one pair of blocks about five feet from the prow and another pair around the same distance from the stern. Nothing at all in the middle.

 

I’m no expert, but surely it would want some support in the middle to stop the hull flexing unduly, wouldn’t it? What do people think - should I add more blocks? And if so, how frequently should they be spaced along the hull? Or am I worrying unnecessarily?

 

Does anyone know a quick rough and ready way to calculate how much support is needed and how to arrange it?

 

Thanks all,

Bruce

Edited by Brucec
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1 minute ago, Brucec said:

am I worrying unnecessarily?

Yes.  Nearly all boatyards do this, and have been doing it for decades.

 

A steel narrowboat is essentially a box girder, so you would be astounded how much force it would take to bend it significantly - the hull and cabin sides support the baseplate, making it much more rigid than just the flat plate thickness would suggest.

 

Did you ask the crane driver to use more than two straps when lifting it out?

  • Greenie 1
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Thank you for the reassurance. That’s one anxiety (among many, at present) I can put aside.

 

And no, I didn’t ask for more than two straps. Never occurred to me. Should I have?

 

Not sure they’d have appreciated it in the middle of an apocalyptic rainstorm, mind.

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1 minute ago, Brucec said:

And no, I didn’t ask for more than two straps. Never occurred to me. Should I have?

No, that's exactly my point.  If you were happy to let an experienced crane driver lift your boat on two straps, why worry about it being on two sleepers?

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Why indeed?

 

First, because I didn’t start thinking about it at all till I stared for a while at this great mass of steel sitting on those little piles of timber; and second, because somewhere in my head, rightly or wrongly, there seems to be a difference between dangling in the air from two supports for a few minutes and resting on two more widely spaced supports for weeks on end. I imagine the sustained load would cause the hull to flex more over time.

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2 minutes ago, Brucec said:

Why indeed?

 

First, because I didn’t start thinking about it at all till I stared for a while at this great mass of steel sitting on those little piles of timber; and second, because somewhere in my head, rightly or wrongly, there seems to be a difference between dangling in the air from two supports for a few minutes and resting on two more widely spaced supports for weeks on end. I imagine the sustained load would cause the hull to flex more over time.

No, the most likely time it would significantly flex (very rare and only on fragile/rotten boats) is at the initial lift as the baseplate clears the water, putting all the load on the straps.  The chaps with historic wooden boats are (rightly) paranoid about this sort of thing, but a steel boat is fine.

 

It's careless crane drivers dropping them you need to worry about - look on youtube to scare yourself silly :D

 

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It will flex. I once had a cracked kitchen carcass when the stern was pulled up a slipway with the bow still in the water to change my prop. Whether you consider the flexing significant or not  I guess will depend on whether there's any significant damage. 

Edited by blackrose
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10 minutes ago, nb Innisfree said:

Innisfree was blocked fore and aft for nearly 2 hrs during fit out, no flex observed in that time. 

Take it your fitter is fully booked for the next......ooh, half day? ?

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Should there not be more concern on the beams being level with each other to avoid twisting of the shell. Thay was my maon concern when i craned my 36 footer out. 

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A steel narrowboat shell is so rigid that unless the supports are exactly the same height and the bottom exactly flat the weight will only be carried on 2 supports anyway.

 

When Belfast was lifted on two beams at Circus Field last year the shell is so torsionally rigid that the front beam was only supporting the boat on one side - there was about a 1/8 inch gap at the other.

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8 minutes ago, W+T said:

Should there not be more concern on the beams being level with each other to avoid twisting of the shell. Thay was my maon concern when i craned my 36 footer out. 

The yard I used (pictures above) were very careful about that. once the dock is drained they check the levels and ensure the boat is sitting properly. You will see that they have differing 'spacers' fitted into the saddles to ensure everything is 'square'.

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2 hours ago, rusty69 said:

Wow, a quick fit out.Mine took years:D

It was full time mind. 

2 hours ago, rusty69 said:

I'm sure ours "moved" a bit when craned last year. Supported on railway sleepers,the front doors were difficult to shut. 

Most likely a bit of twist. 

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5 hours ago, nb Innisfree said:

Innisfree was blocked fore and aft for nearly 2 hrs during fit out, no flex observed in that time. 

Just spotted my '2hrs' typo, ha ha! Thanks for telling me NB Esk. 

I can be slow at times, hence 2year fitout. 

17 minutes ago, WotEver said:

Yeah, 2 hours is an absolute age...

So called time is very subjective! 

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16 hours ago, Brucec said:

Hi all,

I’ve just had my 58 foot narrowboat craned out of the water and put on blocks for maintenance - one pair of blocks about five feet from the prow and another pair around the same distance from the stern. Nothing at all in the middle.

 

I’m no expert, but surely it would want some support in the middle to stop the hull flexing unduly, wouldn’t it? What do people think - should I add more blocks? And if so, how frequently should they be spaced along the hull? Or am I worrying unnecessarily?

 

Does anyone know a quick rough and ready way to calculate how much support is needed and how to arrange it?

 

Thanks all,

Bruce

Many years ago the company Hancock & Lane started building a range of boats they called the "Norseman", and the demonstrator that went to all the boat shows was built unusually with a full length keel to avoid flexing when it was lifted out and blocked.  Granted this was a boat that spent quite a lot of time on the hard and I appreciate what others say but I'd put some more support under a 58 footer.  

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10 hours ago, W+T said:

Should there not be more concern on the beams being level with each other to avoid twisting of the shell. Thay was my maon concern when i craned my 36 footer out. 

I raised the bow of Innisfree with a bottle jack so that boat found its own level between jack and rear sleeper, front sleeper was then chocked up evenly to meet bottom plate & jack lowered. I wasn't concerned about it being level fore and aft. 

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