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Captain Pegg

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Everything posted by Captain Pegg

  1. That is what you need to do. You queried at the outset who you should trust but don't be in any doubt you have bought the boat from someone well qualified to know and who is as reputable as you're going to find. Over-plating is an emotive subject and speaking as the owner of an over-plated craft - where both the original and over plating is of some vintage - I consider a lot of well intended advice is unhelpful. During my ownership I've learnt quite a bit about over-plating and I know of a couple of places who seem to have developed good methods and are experienced in undertaking such work. I'm sure there are others but not all have the same capabilities. I bought my boat in 2015 and speaking to my surveyor during a BSS examination last Friday he suggested it probably has a value tending toward double what I bought it for. To be fair, realising that in the market is another thing because there is always a risk involved. It does have features that make it more desirable to certain buyers than Ramble On but I think it is in indication that an over plated boat is not automatically a basket case.
  2. 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.
  3. 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.
  4. 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?
  5. 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.
  6. 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.
  7. 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.
  8. And of course the centre line wasn't commonly provided on early leisure boats.
  9. 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.
  10. 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.
  11. No plates were punctured in the sinking of the Titanic. And what it mostly explains anyway is why ships are built in steel grades with yield stresses typically twice that of those used in narrowboat construction.
  12. Maintainability is what Tony was referring to but it isn't the same thing as a serviceability design criteria. That is about the ability of an object to fulfil it's user's requirements throughout it's service life.
  13. I was indeed merely referring to the structural elements but I do find that the business of attending to the vagaries of the non-boating sections of the boat detract from the pure enjoyment of owning a boat and going boating on it. I can understand the appeal of a simple life in a boatman's cabin if your objective is to go boating.
  14. Agreed, canal craft are built to serviceability criteria.
  15. 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. I think a key criteria of any boat must be that the hull would be “seaworthy” in its own right. That kind of brings us back again to the key differences between a sea going vessel and a floating metal boxed home.
  16. I think the gunwales add strength. Consider the hull sides as two C shaped main girders and the gunwales and a portion of the baseplate are the flanges to those girders. The cabin top - which I’d ignore structurally - and the baseplate brace those girders.
  17. The method of docking doesn’t really matter. At some point the boat will be supported at only two locations and if it’s jacked up to re-position stocks - as boats routinely are - then one of those loads will be applied to a small area resulting in bending in two planes. I’m pretty sure the biggest loads my hull has ever experienced were in being docked by slipping last year. I suspect ships have to be more carefully supported but that is because they have less redundancy in their design which was the premise of your argument. We should also remember that narrowboats are constructed in lower grades of steel than sea going boats. I can’t imagine what you experience on the Thames in a largely air-filled near rectangular box is anything like that experienced by a fully laden oil tanker on the high seas in terms of the extremes and contrasts in loads on the hull. A tanker in those conditions needs to act as a mega loading carrying structure in an ever changing range of support conditions where one second it’s almost like a bridge spanning its own length and the next it’s supported centrally and got a hundred thousand tonnes hanging off each end. In reality I suspect canal boats are built to serviceability criteria and ships to strength criteria.
  18. The forces associated with activation of the control surfaces on the wings must be significant too.
  19. I doubt it is because firstly ships are perhaps not lifted and docked in quite such a brutal way as narrowboats and also that they experience very different forces in service due to wave action than a narrowboat ever would, such that I think it's probable the highest stresses ever experienced by a sea going ship happen at sea and not in the dock. Hence the comparison may not be properly like for like. BTW the Costa Concordia didn't weigh 114,000 tonnes. 114,000 (unitless) was it's gross tonnage and it's displacement would have been much less, although the largest ships ever built exceed 600,000 tonnes displacement fully loaded. ETA - I'd also be very surprised if aircraft wings are more stressed under their own weight than they are by the dynamic forces of uplift and the effects of turbulence and changes in air density when flying.
  20. While the plate thicknesses of a narrowboat will be proportionally thicker than a sea going ship it is also the case that the steel in a narrowboat is most stressed when being lifted or sat on stocks.
  21. You're basically asking why people build boats with a hull that's more in the water than out of the water, something that I feel might only ever get asked on a canal forum. Canal boats don't require a lot of freeboard seeing as they are designed for still - or at least relatively still - waters. If you're not going to load a boat with 25 tonnes of coal and metal it doesn't need to sit high out of the water in it's natural state. I can only assume modern leisure boats are built with two thirds of the hull out of the water either to mimic the early leisure boats converted from working boats or more likely as a means of creating internal headroom without producing something that looks daft. My boat has 3' deep side plates and is designed to sit 24" in the water and 12" out with a full load and ballast, so it has very low freeboard. It tends to sit an inch or two above that naturally. The entire boat is only 6' 8" tall from baseplate to top of handrail and the internal headroom is 5' 10". That would be a bit low for many which emphasises why perhaps some boats have deeper hulls but I think there are many that don't have a particularly canal friendly profile either above or below the water as a result.
  22. Surely a walking challenge really needs to be independent from the boating challenge. The issue being to get enough interest to operate such an event in parallel to the boating challenge.
  23. This year we do have an eye on competing to win but I can't absolutely say the proposed route is the theoretical maximum score as I haven't computed to that level. It's an interesting route that we know will score well if executed. In any case I've never known the plan to run entirely as it was intended so it perhaps isn't that important anyway. To win you need both a plan that's competitive and events on the weekend to conspire in your favour. On half of my efforts to date we haven't had a competitive plan because we had other objectives.
  24. There's a lot more to it than just the weightings. The length of boat and number of crew determines whether a route biased toward locks or toward mileage is best for each specific entry. W&E is obviously good for mileage but it also links some high scoring lock flights. Even in trying to win the thing there is also an eye on covering as much of the BCN as possible, it's not just about the maximum points. Honest.
  25. I think most boats include the W&E in their itinerary including the three winning entries preceding you. There always seems to be a procession of boats somewhere on the W&E on Saturday evening. I've always included at least part of it on all four of the Challenges I've done, albeit once it wasn't actually planned.
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