Richardw's Narrow Boat Project- PLATE THICKNESS

Discussion in 'Metal Boat Building' started by richardw66, Nov 27, 2013.

  1. richardw66
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    richardw66 Junior Member

    Preliminary Section


    In a way the primary objective of designing the boat myself is because I can. I have approached many builders but I find on the whole they follow the old school approach of chalking it out on the floor and building it by eye. Not to say that doesn't work but I doesn't give me the chance to explore the 'what if' alternatives.

    There was on builder who did use CAD but unfortunately they were also very expensive. However, they did email me a section which has served as a starting block and if I have done it properly you should be able to see them below. Their design was quite lean but I have managed to squeeze the pips a little and extract a few inches of internal space without increasing the beam.

    What are you initial thoughts to my tweaking?
     

    Attached Files:

  2. DCockey
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    DCockey Senior Member

    Rust (oxidation of iron) is one type of corrosion. Galvanic corrosion is another type of corrosion. Corrosion of all types should be of concern if it affects structural integrity.

    My understanding is that the salt in salt water accelerates the oxidation of steel by serving as a catalyst. Iron in pure water without dissolved oxygen rusts very slowly, if at all. A knowledgeable chemist or corrosion engineer could provide a more detailed explanation.
     
  3. richardw66
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    richardw66 Junior Member

    There are some PDFs on another response so you can the work-in-progress design, however this response subject leads on very nicely to my next post subject; Bilges and Ballast. I'd be grateful for any further detail input over there.

    Thank you.
     
  4. SamSam
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    SamSam Senior Member

    My understanding of narrow boats is that they bash and bang against each other a lot and if they are not made of steel like the majority of the others are, they are liable to get crushed or damaged.
     
  5. pdwiley
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    pdwiley Senior Member

    Rubbish, unless you're defining 'longevity' in terms of centuries.

    There have been a lot of boats built from 3mm plate, for example.

    The key to longevity is corrosion prevention far more than plate thickness.

    PDW
     
  6. DCockey
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    DCockey Senior Member

    A perusal of websites of narrow boat builders shows the usual plating thickness used on narrow boats is 10mm bottom, 6mm hull sides, and 4mm for the cabin. One builder said they use 12mm for the bottom.

    I saw a claim that it is common practice to extend the bottom an inch or so past the sides and that this is called a wet chine. The claimed benefits were it simplifies the outside weld at the chine and also protects against the chine from damage.

    I wonder if the bottom thickness is based on localized loads when stored out of the water on blocking rather than in the water loads.
     
  7. Ad Hoc
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    Ad Hoc Naval Architect

    I would refute that claim, wherever it came from. It is probably just an historical throw back of sorts.

    Chines always, or should, have a small amount of flat extending past the side plate, as shown:

    chine fillet.jpg

    usually the thickness or so of the chine plate. This allows fillet weld both sides. However if the chine plate extends any further than required, it is the opposite of that claim. It is prone to damage, since it is an unsupported plate extending away from the side of the hull, thus exposed to any object which the hull side is not and could under a side impact load, seriously affect the chine joints structural integrity.
     
  8. michael pierzga
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    michael pierzga Senior Member


    I repair plenty of steel boats !
    The thin plate ones suffer.

    Grind a weld, sandblast the bottom and you have already lost plate thickness.

    Fast forward twenty years..two additional sand blastings and a couple guys with grinders and...thin plate.

    Every time one of these goofy survey guys comes to sound the steel plate he breaks out his grinder and more steel goes.

    I cry when i watch those goofy surveyors ruin steel boats
     
  9. waikikin
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    waikikin Senior Member

    I love it when they do metal thickness testing... & seem to pretty much choose half way between frames & halfway between stringers, I love to point them towards the intersections & low points;)
     
  10. michael pierzga
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    michael pierzga Senior Member

    Its a scam
    Buyers demand a survey..... so they ruin the under water surface coating to discover corrosion issues that are inside in the bilge.

    Just use a flashlight and look
     
  11. SukiSolo
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    SukiSolo Senior Member

    Thanks Richard. I think that I would start from a basic hull form. Try and obtain drawings of existing boats and go through some of their basic properties. I see you have already got a good start from those PDFs'. A bit more information is required for the main hull form.

    In particular I would be very keen to examine the curve of areas and efficiency along with wave making properties. I believe that by softening the curve of areas you could achieve better efficiency ie less fuel more distance maybe even to the point of being part solar/electric driven. It won't be too radical either, so not too departed from the existing. In that sense stability will not be compromised and the hull may well be better on say the tidal Tames wher I have seen traditional boats making a pig ear of a job in a chop.

    A lot of the old late Victorian and Edwardian launches and large passenger vessels on the Thames were/are very good for low wash. They slip along very discreetly and are way nicer than modern V hull motor cruisers intended for 12kn up. Even the old Thames Water Authority boats (GRP) around 30' were much better than the black police launches yet they were also able to plane fast in smooth water. That quiet elegant displacement mode is what this narrow boat hull needs IMHO. It would also win you friends along the canal! It should have a lower environmental impact than a lot of the traditional boats, at least it ought to in my eyes.

    You will lose a little space in the ends but it is not super useable space. The trade off may well be significant over time in favour of a slightly different under the water form. I have only got some info on barges not canal boats in this respect but sources must be available. There is quite a bit of variation in barges BTW, despite them all having effectively a box midship section. They also handle a bit differently, so power unit and steering become important considerations.

    As David Cockey has correctly said there are limit parameters, effectively decided by the canal infrastructure itself. One other thing that does happen is full on grounding, when they drain a section of canal for maintenance so the craft has to be stiff enough. Not really a problem for essentially a tube with limited considered apertures.

    As long as there is no double curvature, I cannot see why all the panels could not be water cut (or laser) from the CAD files. The curved surfaces must be fully developable unless you intend a deliberate little 'wedge' cut out(s) to help create a form. Still a careful bit of rolling and maybe butt welding could result in some interesting forms. Sometimes you have to challenge the 'tradition', nothing wrong with that as long as you understand it and how to overcome the same problems.

    Once you have a direction for the form, then the interior needs planning and examining. Personally I'd get on as many boats as possible and keep notes as to what works and what does not. That will definitely show the limitations of beam and give a direction of a better longditudinal layout. Again different people will prefer one way over another, just a personal thing.

    I don't think the wasting anode is a real problem you just need to know about it. Even outboard motors have them.

    It still sounds like an interesting proposition as a new boat. Look forward to hearing about progress and seeing some of the ideas. My own backgound is more industrial design and design engineering so that might explain some of my take on it.
     
  12. richardw66
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    richardw66 Junior Member

    Very interesting!

    So at the moment I have a 15mm thick bottom (or chine plate?) and an 8mm thick hull side, I don't know why I just have; that's what tis was discussion was supposed to resolve!

    Currently I am showing an 18mm chine overhang because it was what was shown on the original section I was given that had a 6mm side and I just retained it. Could I reduce that to an 8mm overhang and thereby gain an extra 20mm of width on the inside?

    As the name suggests narrow boats are pretty narrow so to not maximise all the internal width would be almost criminal, every little helps as the saying goes.
     
  13. bregalad
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    bregalad Senior Member

    Meh. The sides of all the narrowboats I've looked at are plumb. The bottom could extend beyond the side plates by the width of the rub rails without infringing on the interior space. The rub rails I've seen in pictures appear to be ~18mm .... so that may be what is on the plans you have.
     
  14. richardw66
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    richardw66 Junior Member

    It evolved the sloping sides because of the value engineered 2m wide base plate that otherwise would have needed to be cut down from a 2.5 in order to make it 2.1m wide.

    However, I've taken on board what you said and made the sides straight by moving the plate bend down to form a chine. The side girth is still 1250mm wide and there are still two pressed bends. That does indeed give more room on the inside but there must be a pit fall because I've seen plenty of 'modern narrow boats' out on the water but none with a chine, (unless they have a 2.1m wide base plate?) traditional modelled ones tend be vertical but then they are also more expensive so perhaps that accounts for the material wastage.

    Anybody see any downside to the chine as drawing, if not on face value it seems like a good direction to go in. One thing I have noted is that it increases the cabin slop angle but that's only because of the value engineered 1.5m wide roof plate. If I cut that down from something a lot wider I could gain even more cabin volume. I have it 6mm thick at the moment as if to pay lip service to acoustics but if I went for a thinner section it could cover the wastage factor of cutting down a wider plate.

    Any thoughts?
     

  15. richardw66
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    richardw66 Junior Member

    I forgot to attach the PDF on the previous post
     

    Attached Files:

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