IanD

So don't just suspect -- you know how big a boat is, how thick the hull is, and how much it weighs, the formulae are simple and out there, so go and work out how much it will twist by if (for example) all the weight is supported on diagonally opposite corners. Don't forget to allow for the bulkheads which act as panel stiffeners at each end and keep the hull "square" at that point. It's a standard undergraduate exam problem. If it turns out that the distortion will be big enough to cause damage -- as you seem to be claiming -- then I'll agree with you, should a boat ever find itself in this unfortunate (and unlikely) situation. Until then, my view is that it won't, backed by exactly the same hand-waving engineering estimate as yours. In other words until the numbers are crunched we're both just expressing opinions, and we all know how valid they are without facts to back them up... 😉 P.S. My suggestion to read the book was because your initial post suggested you didn't understood the subject, and also it's a damn good read for anybody with an enquiring mind 🙂

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

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... 😉

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... 😉

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.

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.