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blackrose

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1 hour ago, blackrose said:

 

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.

 

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.

 

I made no mention of different methods of docking in my first post. I simply referred to situations where the stresses would likely be highest. A boat won't necessarily be on only two supports but it could be - because at times many are - so it represents a worst case in terms of how to calculate the stresses with which a narrowboat hull needs to cope. You wouldn't design it based upon what any particular yard does or any single owner prefers, but on what reasonably might happen.

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Been pondering this topic re narrowboats and I think the most likely thing that will happen when they are out of the water is that they will twist along their length fairly easily if the sopports are not absolutely level - hence those tiles falling off and doors jamming.  This is why it is important to do the final shaft alignment once the boat has been afloat at least overnight.

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9 hours ago, Captain Pegg said:


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.

 

 

Other than the perceived requirement to be able to walk around the boat on very narrow side decks I can:t really see a major problem with doing a narrow boat without gunnels. 

 

The old butty boats had no gunnel beside the cabin.

 

Not sure if anyone has done it but it could be interesting to build a narrow boat with no gunnels but a system of steps at bow and stern for reliable and safe access to the roof area. 

 

I wonder what the design philosophy was when it came to making motorised narrow boats and putting the side decks/gunnels on.

 

In modern elfin safety parlance they do look like quite a nasty risk. Much better to gain access to the craft from the cabin top. 

 

Perhaps it was a structural addition deemed necessary due to the craft being motorised.

 

 

 

Edited by magnetman
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1 minute ago, magnetman said:

Other than the perceived requirement to be able to walk around the boat on very narrow side decks I can:t really see a major problem with doing a narrow boat without gunnels. 

 

The old butty boats had no gunnel beside the cabin.

 

Not sure if anyone has done it but it could be interesting to build a narrow boat with no gunnels but a system of steps at bow and stern for reliable and safe access to the roof area. 

 

I wonder what the design philosophy was when it came to making motorised narrow boats and putting the side decks/gunnels on.

 

The gunwale provides stiffness along the boat.  Without it you'd have an oilcanning effect and the whole side would wobble.

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I did add a little bit to my post before I saw yours. 

 

Also I suppose the cabin height and bridges comes into it. 

 

ETA working boats with short cabins won't get the wobble problem. Good point though about a modern craft. Could be problematic. 

Edited by magnetman
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I was making the point that the change from butty boats to motor boats involved adding the gunnels and idly wondering why that would be. 

 

The craft performs the same basic function but has an engine in it rather than being pulled. 

 

Of course horse boats are another topic but it seems that butties copied horse boat cabins then when the motors came there was a rethink and side decks were added.

Edited by magnetman
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1 hour ago, magnetman said:

Other than the perceived requirement to be able to walk around the boat on very narrow side decks I can:t really see a major problem with doing a narrow boat without gunnels. 

 

The old butty boats had no gunnel beside the cabin.

 

Not sure if anyone has done it but it could be interesting to build a narrow boat with no gunnels but a system of steps at bow and stern for reliable and safe access to the roof area. 

 

I wonder what the design philosophy was when it came to making motorised narrow boats and putting the side decks/gunnels on.

 

In modern elfin safety parlance they do look like quite a nasty risk. Much better to gain access to the craft from the cabin top. 

 

Perhaps it was a structural addition deemed necessary due to the craft being motorised.

 

 

 

 

I suspect gunwales on early pleasure boats were provided for much the same reason as on working motor boats which is to provide access to the hold (and thence to further beyond) from the stern, remembering that on motor boats the engine room had solid bulkheads whereas on butty boats there was a door from the cabin into the back end of the hold.

 

The gunwales on historic working boats and other properly designed leisure boats are much wider than those on most modern leisure craft.

 

I think a traditional stern without gunwales would be very awkward. I wouldn't want one. It'd be a right pain to reach the front deck without them, involving clambering over furniture with my boots on. I don't use the gunwales to gain access to the roof either, even though there are footsteps for that purpose. I only use the footstep inside the hatch for that.

 

And without the gunwales I'd have to turn the boat around to clean down the cabin sides.

Edited by Captain Pegg
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7 hours ago, blackrose said:

 

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.

 

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.

The launch of a hull down a slip way is one of the more demanding structural situation. The ship rotates on the fore puppet until it fully enters the water. Stopping running into the opposite bank, if launched on a river like the Tyne, is fun as well.

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1 hour ago, Captain Pegg said:

 

I suspect gunwales on early pleasure boats were provided for much the same reason as on working motor boats which is to provide access to the hold (and thence to further beyond) from the stern, remembering that on motor boats the engine room had solid bulkheads whereas on butty boats there was a door from the cabin into the back end of the hold.

 

The gunwales on historic working boats and other properly designed leisure boats are much wider than those on most modern leisure craft.

 

I think a traditional stern without gunwales would be very awkward. I wouldn't want one. It'd be a right pain to reach the front deck without them, involving clambering over furniture with my boots on. I don't use the gunwales to gain access to the roof either, even though there are footsteps for that purpose. I only use the footstep inside the hatch for that.

 

And without the gunwales I'd have to turn the boat around to clean down the cabin sides.

 

Agree gunwales were to definitely there on pleasure cruisers to allow people to get to the stern or bow .  Many had no other easy way of accessing them to jump off, with rope or to get access to the towpath to operate locks and the then more frequent lift or swing bridges ? 

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11 minutes ago, Stroudwater1 said:

 

Agree gunwales were to definitely there on pleasure cruisers to allow people to get to the stern or bow .  Many had no other easy way of accessing them to jump off, with rope or to get access to the towpath to operate locks and the then more frequent lift or swing bridges ? 

 

And of course the centre line wasn't commonly provided on early leisure boats.

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11 hours ago, magnetman said:

 

I wonder what the design philosophy was when it came to making motorised narrow boats and putting the side decks/gunnels on.

To provide access between the steering position and the engine room - not a requirement with a horse boat / butty.

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16 hours ago, blackrose said:

Also why will a canal boat be resting on only two supports?

 

The profile across the top of 3 or more supports will not be that same as the profile of the bottom of the boat. So the boat will only be in contact with all of those supports if it flexes enough to do so. Many boats will not be flexible enough to distort that much under their self weight.

 

15 hours ago, Alan de Enfield said:

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

 

But boats are only supported in two places when lifted by crane.

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I watched my boat be slipped on bogies on a tramway last year. The difficulties weren't in the way it was slipped or the way it was supported when in dock but the maneuvering required to extricate the bogies from underneath the boat once on land, which involved use of a jack. The moment pressure was applied to the jack the boat could only ever have been supported in two places.

 

It was hauled out on two bogies anyway.

Edited by Captain Pegg
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2 hours ago, David Mack said:

 

The profile across the top of 3 or more supports will not be that same as the profile of the bottom of the boat. So the boat will only be in contact with all of those supports if it flexes enough to do so. Many boats will not be flexible enough to distort that much under their self weight.

 

 

But boats are only supported in two places when lifted by crane.

Support in two places (e.g. crane slings) correctly positioned (around 1/5 and 4/5 of the way along?) shouldn't cause any damage to any narrowboat even a 72' one, assuming it has a cabin not a full-length open hold -- and even in this case it should be fine, which is why cranes do exactly this. A boat with a full-length steel cabin is actually quite strong because this forms a closed torsion tube with the hull.

 

But they do still flex noticeably under the strain, so if any internal fitting work is being done more supports are needed. Ricky at Finesse told me that when moving hulls around with two supports (or slings) you can see the curves in them if you look along the length -- like looking along a guitar neck to check for relief, the movement isn't massive but it's certainly there, and can be  enough to affect things like cupboards with wide doors.

Edited by IanD
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34 minutes ago, IanD said:

Support in two places (e.g. crane slings) correctly positioned (around 1/5 and 4/5 of the way along?) shouldn't cause any damage to any narrowboat even a 72' one, assuming it has a cabin not a full-length open hold -- and even in this case it should be fine, which is why cranes do exactly this. A boat with a full-length steel cabin is actually quite strong because this forms a closed torsion tube with the hull.

 

But they do still flex noticeably under the strain, so if any internal fitting work is being done more supports are needed. Ricky at Finesse told me that when moving hulls around with two supports (or slings) you can see the curves in them if you look along the length -- like looking along a guitar neck to check for relief, the movement isn't massive but it's certainly there, and can be  enough to affect things like cupboards with wide doors.

 

The 4-5mm plates welded at near right angles to one another that form the average narrowboat cabin don't really possess much torsional resistance. In any case when being lifted a boat is basically a beam, you are trying not to twist it. What the cabin - and particularly any ribs that run from gunwale to gunwale supporting the sides and roof - will do, is to brace the top edge of the hull to allow it bend without buckling. That is exactly what the chain and cross-plank arrangement on an open hold does, with a much greater degree of structural efficiency, that being the prime purpose of those members.

 

Edited by Captain Pegg
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13 minutes ago, Captain Pegg said:

 

The 4-5mm plates welded at near right angles to one another that from the average narrowboat cabin don't really possess much torsional resistance. In any case when being lifted a boat is basically a beam, you are trying not to twist it. What the cabin - and particularly any ribs that run from gunwale to gunwale supporting the sides and roof - will do, is to brace the top edge of the hull to allow it bend without buckling. That is exactly what the chain and cross-plank arrangement on an open hold does, with a much greater degree of structural efficiency, that being the prime purpose of those members.

 

Any closed tube (like a narrowboat hull with a continuous steel closed-ended cabin) is torsionally stiff, because all the bending stresses get turned into shear stresses in the walls, and as you say it's also pretty stiff as a beam like any square tube section.

 

The stringers (and gunwales) help to stop the hull buckling (when loaded as a beam) by stopping the sides creasing outwards or oilcanning, because the "tube" is quite thin-walled. As you say cross-chains do the same job more efficiently, but are somewhat inconvenient in a boat built for living on.

 

Either way, two-point lifting or support is extremely unlikely to cause any damage, though there will be a small amount of bending while it's happening.

Edited by IanD
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9 minutes ago, IanD said:

 

Any closed tube (like a narrowboat hull with a continuous steel closed-ended cabin) is torsionally stiff, because all the bending stresses get turned into shear stresses, and as you say it's also pretty stiff as a beam.

 

The stringers help to stop the hull buckling (when loaded as a beam) by stopping the sides creasing outwards or oilcanning, because the "tube" is quite thin-walled. As you say cross-chains do the same job more efficiently, but are somewhat inconvenient in a boat built for living on.

 

Either way, two-point lifting or support is extremely unlikely to cause any damage, though there will be a small amount of bending while it's happening.

 

Lifting at two points has never bothered me because there's so much metal in a narrowboat and a good deal of it is well dispersed although it is heavily weighted toward the bottom.

 

I think that if you tried to apply a serious twisting action to a narrowboat hull the roof plate would bend significantly, I don't think there's a whole lot of torsional stiffness there.

Edited by Captain Pegg
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7 minutes ago, Captain Pegg said:

 

Lifting at two points has never bothered me because there's so much metal in a narrowboat and a good deal of it is well dispersed although it is heavily weighted toward the bottom.

 

I think that if you tried to apply a serious twisting action to a narrowboat hull the roof plate would bend significantly, and I think the cabin side plates might do too, I don't think there's a whole lot of torsional stiffness there.

None of the bending stiffness of a hull comes from the thickness of even a thick bottom plate, these alone flex like paper in a 70' long boat.

 

Any closed tube is stiff torsionally and in bending (until they crease if the wall is thin, see Wagner fields), suggest you read a structures textbook 😉

 

"Structures" by J E Gordon is highly recommended and brilliantly written, it not only goes into all the conventional stuff but explains things like why bias-cut dresses work and you can't put a crease into a worm... 😉

Edited by IanD
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2 minutes ago, IanD said:

Any closed tube is stiff torsionally (until they crease if the wall is thin, see Wagner fields), suggest you read a structures textbook 😉

 

"Structures" by J E Gordon is highly recommended and brilliantly written, it not only goes into all the conventional stuff but explains things like why bias-cut dresses work and you can't put a crease into a worm... 😉

 

I have no idea what a bias-cut dress is but I think that's the point. I'm not at all sure that what Google tells me is the same thing you're referring to. Let's leave other people's work out of it since there's enough of that on the forum. I'm just working from what I think I know and if you know better, fine.

 

So assuming I'm wrong - since otherwise what's the point of your post? - what's the answer to the following questions:

 

Does the presence of thinner plates for the cabin structure compared to the hull make a narrowboat hull less torsionally stiff than if the whole thing were made of 10mm plate?

 

Do the (near) right angled corners make it less torsionally stiff than if it were a circle of the same wall thickness and the same internal cross sectional area?

 

Do less torsionally stiff structures deform more under the same load than torsionally stiffer members?

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23 minutes ago, Captain Pegg said:

 

I have no idea what a bias-cut dress is but I think that's the point. I'm not at all sure that what Google tells me is the same thing you're referring to. Let's leave other people's work out of it since there's enough of that on the forum. I'm just working from what I think I know and if you know better, fine.

 

So assuming I'm wrong - since otherwise what's the point of your post? - what's the answer to the following questions:

 

Does the presence of thinner plates for the cabin structure compared to the hull make a narrowboat hull less torsionally stiff than if the whole thing were made of 10mm plate?

 

Do the (near) right angled corners make it less torsionally stiff than if it were a circle of the same wall thickness and the same internal cross sectional area?

 

Do less torsionally stiff structures deform more under the same load than torsionally stiffer members?

You could, of course, ask for a comparison with a solid round bar but that is 'silly'. In a way, your requested comparisons are equally silly, as any practical structure will contain sufficient material in the right places for the specific application. I doubt very much whether a formal stress analysis (dynamic as well as static) has been carried out for a narrowboat in its design phase! What happens, and happened for a long time with ships, is that 'rules' evolve to distil know practice of what has survived and what has not. This has the advantage that it takes into account loads that cannot currently be analysed (may even not be describable). It has the disadvantage that it does not encourage innovation and may lead to either insufficient or wasted material being used.

 

Early steel leisure narrowboats (which are a significantly different structural problem from working boats) followed a classical process of reducing the thickness of material sued until eventually some were found to be inadequate. As far as I know, the decisions to return to slightly thicker steels was not because of static structural strength (which is what is being talked about when discussing lifting a boat on two fixed points)

 

Sadly. my structural knowledge is now about as useful as the steel in a rusted out ancient working boat! Alas, I have never really re-plated that knowledge to extend its life  . . . 

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40 minutes ago, Captain Pegg said:

 

I have no idea what a bias-cut dress is but I think that's the point. I'm not at all sure that what Google tells me is the same thing you're referring to. Let's leave other people's work out of it since there's enough of that on the forum. I'm just working from what I think I know and if you know better, fine.

 

So assuming I'm wrong - since otherwise what's the point of your post? - what's the answer to the following questions:

 

Does the presence of thinner plates for the cabin structure compared to the hull make a narrowboat hull less torsionally stiff than if the whole thing were made of 10mm plate?

 

Do the (near) right angled corners make it less torsionally stiff than if it were a circle of the same wall thickness and the same internal cross sectional area?

 

Do less torsionally stiff structures deform more under the same load than torsionally stiffer members?

 

I'm sure you know the answer to all your questions is of course "yes", but also not relevant. The question is whether a typical narrowboat hull (with a steel cabin) is stiff in both torsion and bending, and the answer to this is also "yes", because it's a closed tube, and there's only a tiny difference between a circular and square tube of the same cross-sectional area -- look up structural moment of inertia.

 

If you cut a big hole in a closed tube or -- even worse -- put a lengthwise slit in it or -- much worse still -- cut the roof off, it becomes far weaker and less stiff (not the same thing). So big windows or roof hatches which remove a large part of the area do weaken a hull (and make it less stiff) because the stress has to find a way round them, and this changes the stress in the plates from shear stress (where they're strong/stiff) to bending/torsional stress (where they're weak).

 

A closed tube (hull with steel cabin) is actually a lot more efficient structurally than a spaceframe-type structure, especially one with massive holes in like a trad hull with chains and a top plank with bearers.

 

This might not fit with what you think (or "common sense") but it's correct. I suggested the Gordon book because it's one of the best popular science books ever written, it manages to make the subject interesting in an easily understandable way and is also amusingly written.

 

P.S. Yes I did structures as part of my engineering degree -- analysis, designing, building and testing -- and still remember a lot of it... 😉

Edited by IanD
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3 minutes ago, Mike Todd said:

You could, of course, ask for a comparison with a solid round bar but that is 'silly'. In a way, your requested comparisons are equally silly, as any practical structure will contain sufficient material in the right places for the specific application. I doubt very much whether a formal stress analysis (dynamic as well as static) has been carried out for a narrowboat in its design phase! What happens, and happened for a long time with ships, is that 'rules' evolve to distil know practice of what has survived and what has not. This has the advantage that it takes into account loads that cannot currently be analysed (may even not be describable). It has the disadvantage that it does not encourage innovation and may lead to either insufficient or wasted material being used.

 

Early steel leisure narrowboats (which are a significantly different structural problem from working boats) followed a classical process of reducing the thickness of material sued until eventually some were found to be inadequate. As far as I know, the decisions to return to slightly thicker steels was not because of static structural strength (which is what is being talked about when discussing lifting a boat on two fixed points)

 

Sadly. my structural knowledge is now about as useful as the steel in a rusted out ancient working boat! Alas, I have never really re-plated that knowledge to extend its life  . . . 

 

I totally agree and my knowledge is similar in that while I was once a fully fledged structural designer it was a long time ago.

 

All I said was that I didn't think a narrowboat would withstand being twisted very well - despite the fact that box sections do have intrinsically good torsional capabilities in comparison to other shapes - and that it isn't the prime consideration in any case.

 

Mr D seems to think I'm missing something. He's just posted so let's see.

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12 minutes ago, Captain Pegg said:

 

I totally agree and my knowledge is similar in that while I was once a fully fledged structural designer it was a long time ago.

 

All I said was that I didn't think a narrowboat would withstand being twisted very well - despite the fact that box sections do have intrinsically good torsional capabilities in comparison to other shapes - and that it isn't the prime consideration in any case.

 

Mr D seems to think I'm missing something. He's just posted so let's see.

Then you understand that a box section -- round or square -- with closed ends is actually the most efficient way to carry bending or torsional stresses over any distance?

 

Neither type of stress is likely to pose any problem whatsoever for a narrowboat as built today. If the plates were much thinner then the hull would still be strong so long as it has stringers at regular internals to prevent buckling, which hulls also do.

 

Torsional or bending stiffness of a square tube is a standard formula which you could look up if you wanted to work out the actual stiffness, but I'm sure the result would be "adequate" 😉

 

Also square and circular tubes with the same moment of inertia have very similar properties in both torsion and bending:

 

https://bbs.homeshopmachinist.net/forum/general/42265-round-tube-versus-square-tube-stiffness#post827381

Edited by IanD
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22 minutes ago, IanD said:

 

I'm sure you know the answer to all your questions is of course "yes", but also not relevant. The question is whether a typical narrowboat hull (with a steel cabin) is stiff in both torsion and bending, and the answer to this is also "yes", because it's a closed tube, and there's only a tiny difference between a circular and square tube of the same cross-sectional area -- look up structural moment of inertia.

 

If you cut a big hole in a closed tube or -- even worse -- put a lengthwise slit in it or -- much worse still -- cut the roof off, it becomes far weaker and less stiff (not the same thing). So big windows or roof hatches which remove a large part of the area do weaken a hull (and make it less stiff) because the stress has to find a way round them, and this changes the stress in the plates from shear stress (where they're strong/stiff) to bending/torsional stress (where they're weak).

 

A closed tube (hull with steel cabin) is actually a lot more efficient structurally than a spaceframe-type structure, especially one with massive holes in like a trad hull with chains and a top plank with bearers.

 

This might not for with what you think (or "common sense") but it's correct. I suggested the Gordon book because it's one of the best popular science books ever written, it manages to make the subject interesting in an easily understandable way and is also amusingly written.

 

But those questions were the premise of the point you queried and the suggestion to go read a book without even bothering to address the actual point I raised I thought was pretty condescending. As it happens I've studied and practiced structural engineering to a high degree so I don't need to be told that box sections are intrinsically good in torsion. I'd far sooner debate from knowledge than from reference material, I want to hear what people really know and understand, warts and all. Save reading up for afterward when you can go and check up on something you realise you maybe didn't know as well as you thought.

 

I still though suspect that if you induced a twist into a narrowboat accidently it wouldn't take that much to cause cabin distortion that could cause internal damage to linings or fittings, and that would be a result of the thin cabin sides and profile. I'd agree it wouldn't get any near failing but as is common in inhabited spaces serviceability criteria take precedence over structural criteria in design. As boxes go they aren't that great.

 

 

 

Edited by Captain Pegg
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