Question About how to calculate Floatation of my design

Discussion in 'Boat Design' started by AdamOMahony, Sep 11, 2017.

  1. Heimfried
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    Heimfried Senior Member

    Why do you think it is less stable and which kind of hull are you compairing to?
     
  2. Nick.K
    Joined: May 2011
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    Nick.K Senior Member

    I agree.
    Perhaps we should ask Adam how much feedback he wants? This is a very small inland boat which he is designing and constructing for himself. Personally I like and respect that initiative even if the design and construction isn't what I'd choose for myself.
     
  3. AdamOMahony
    Joined: Sep 2017
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    AdamOMahony Junior Member

    More stable than a very rounded hull almost half a O was my first design, I was a little worried about rolling from it being a small enough boat that it could happen with the shifting weights of moving so much inside the boat, although maybe I was completely wrong with that assumption...

    I'm quite happy with the shape of the hull now and moving forward with the build, to answer Nicks question really any concerns and feedback has already been given and more so which has been amazing! :) my main concerns were an estimation of displacement/ flotation and about suggestions on bow design and I've gone ahead with the bow since starting the thread (going with almost a scow bow as someone had suggested) and my question on fibreglass was also put at ease :)

    I'll perhaps come across more questions as the build progresses. As Nick stated it's a small inland boat as such I'm not really looking for optimal performance just will it float and will it be safe and comfortable :)
     
    Last edited: Sep 15, 2017
  4. AdamOMahony
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    AdamOMahony Junior Member

    It seems I missed the last few comments on page 2 of the thread, as such here is the answers to those comments.

    The project is currently at this point (pictures below) and yes I looked at similar canal boats and am aware that my design is different than most standard canal designs although those boats are also a different construction method and completely different sizes to what I'm trying to achieve. Mainly steel hull and far longer than I would require. This combined led me to the design that I have shown.

    IMAG0002.jpg

    Yes it is no where near what canal boats in the area are like but none of those are wood and fibre construction either. the fact is I'd rather not build with steel (Lack of tools for such a construction) and really wouldn't need the 40 ft length most canal boats in the area have. I could have went with the flat bottom design or U and probably would have better displacement which Tansl had hinted at originally asking would I consider a mono hull.

    My original question was in regards to displacement and bow shape, construction had started because I was happy with the design for the main hull, my concern was I was not sure if it would be sufficient displacement.

    I still have everything that came off the ply wood and could join back the ends (although it would not be as strong anymore) making it a flat bottom but then i'd come to the issue of designing a swim at the stern for the new bottom.

    I understand that a lot of what I have found has proven to be wrong, and I will keep your concerns in mind, SamSam if you have suggestions to improve the current state of my construction I'll definitely give it a think over any suggestions are welcome, as you said generally with cloth it's 50/50 so I'll take that as what I need in weight ratio for the cloth, do you have any estimation for how much epoxy would be need for a single coat of epoxy without cloth?

    I've added some more photo's below, the cuttoffs are under the main cut piece's at the moment.
    IMAG0004.jpg IMAG0005.jpg

    Maybe I am a little insane for starting construction, although as I have said already that I am happy with the design, is it optimal? Probably not
     

    Attached Files:

    Last edited: Sep 15, 2017
  5. Heimfried
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    Heimfried Senior Member

    Given your hull form, the crucial factor to the stability is the height of the center of gravity (VCG) above the base line (keel). It schould be as low as possible. Place heavy parts low and keep the superstructure (cabin) light, especially the roof.
    It is not only a question of safety but also a question of comfort. E. g. weight = 4310 kg, fresh water displacement 4.3 m³, Draft 500 mm, two crew members (total 160 kg) moving 2000 mm transversal (from starbord to portside or vice versa) causing a heel of 7 degree (if the boat was upright before moving and the VCG = 1200 mm). Heel is only 5 deg., in the same circumstances, if the VCG = 1000 mm.
    AdamOFrame2D.jpg
     
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  6. AdamOMahony
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    AdamOMahony Junior Member

    So to get the best for comfort, stability and safety keep weight low and central, would adding extra weight in the form of more structure to the left and right lowest points help this? in terms of weight close to the keel I was planning on locating the water storage near there if that would help?

    Edit:

    Just received some information via email about resin as follows:

    "If you are purely looking to protect the boat against moisture 3-4 coats of
    Epoxy applied in one day should be adequate. When your first coat goes
    tacky, similar to what sellotape feels like, the next coat is applied. This
    will save washing the entire boat and sanding it between coats will still
    giving excellent adhesion.

    If you're going to be using fibreglass cloth as well the amount of resin you
    will need will depend on the weight of the cloth

    To work out the coverage we can use some of the numbers below.
    Since your first coat will be going over wood which obviously is a porous
    surface coverage will be a little less. 1kg of your mixed resin and hardener
    will cover 6.5 - 7.5 sq. m.

    Next layer is your cloth. If you go with 200g per sq. m. and your covering
    37m that would weight 7.40kg so in turn we would need 7.4kg of mixed resin.
    If however you use 450g cloth to give some additional strength the same area
    would require 16.65kg.
    So total cloth weight is what we need to budget for resin. I always like to
    add 10% to account for wastage through brushes and rollers etc.

    Before final fairing you will also need to fill the weave of the cloth. I'm
    afraid for this I have no magic formula as it will really depend on the
    application. I would allow for 3 thin coats though (you don't want to sand
    into your fibreglass). Because at this stage the wood is sealed we get a bit
    more coverage out of the epoxy. About 8.5 - 9.5 sq. m. per mixed kg."
     
    Last edited: Sep 15, 2017
  7. Heimfried
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    Heimfried Senior Member

    Yes, water, waste water tanks, batteries, fuel ...
     
  8. AdamOMahony
    Joined: Sep 2017
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    AdamOMahony Junior Member

    Great thanks a million :) I'll try find tanks suitable to place them within the structure.

    I'll make a separate thread for the build progress and place a link to it here in-case any one would like to see what I'm up to with it.
     
  9. SamSam
    Joined: Feb 2005
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    SamSam Senior Member

    A flat bottomed scow hull seems to be the most practical shape for a displacement canal boat. Easy to build, it gives the most volume with a lower center of gravity. It gives the most static stability. I bet it would only have half the draft of what you are doing. As far as dynamic stability I can't see where much is needed in a canal. You could haul it around on any flatbed trailer as opposed to a custom built one suitable for only one purpose. You probably won't be able to get very much usable space in those v shaped pontoon sections.

    About the op, the bow needs work. It's too blunt and I've no idea how you would bring it all together with the deeper catamarans and raised v hull between them, and waves will pound against it I imagine.
    Did anyone ever answer about the displacement?
     
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  10. AdamOMahony
    Joined: Sep 2017
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    AdamOMahony Junior Member

    Thanks for the input Sam :) yes tansl answered about the displacement initially and then Heimfield elaborated on it, I believe I could flatten the bottom although then I may have to bring the floor level of the home further into the extended hull, I am wondering if it is really worth the work involved I understand the transport may be beneficial as well although my uncle is a boating enthusiast and has several size's of boat trailers for transport. He had originally suggested that we look for a boat that was second had with two keels to deal with the chop on the Shannon.

    My main worry is that changing the hull shape at this stage may end up with a sub par structure in terms of strength since the pieces cut will have to be added on to meaning not solid pieces.

    I am quite inexperienced with boat's but I know timber well enough and right now the pieces cut are solid and all in line (only a slight variation where the jigsaw went awry )
     
  11. TANSL
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    TANSL Senior Member

  12. AdamOMahony
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    AdamOMahony Junior Member

    You are a legend Tansl =D

    I'll draw it up on sketch up tonight and see if I can figure something out for the swim. or is this really needed for an outboard engine? I know most canal boats have this but also have an inboard.
     
  13. TANSL
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    TANSL Senior Member

    But before cutting anything, make some preliminary calculations to check that the boat floats properly, with sufficient stability, and that there is space in the hull for everything you need.
     
  14. AdamOMahony
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    AdamOMahony Junior Member

    Yeah tomorrow is raining anyway so I'll be stuck inside, I'll try have a play around with the site heim linked and see if it will be okay :)
     

  15. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    I know what the OP posted. He is cutting stations on a boat that is not yet designed.
     
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