Any buoyancy mathematician's around ?

Discussion in 'Multihulls' started by Crashed, Aug 6, 2020.

  1. Crashed
    Joined: Aug 2020
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    Crashed Junior Member

    I have a 10m displacement Catamaran workboat which weighs 10 ton and is supposed to be good to carry 3.5 ton cargo .

    Long story short she is stern heavy by accident and design which is of no great concern until she is carrying load which is a daily occurrence and up until now not much of a worry , however circumstances mean the load weight is set to increase considerably .

    I intend to alter the hull above the waterline so when she sinks surface water is inclined to lift her but that's neither here nor there , ideally i don't want that modification to touch the water other than in heavy sea's .

    So here's the plan , what if i make (have made) a couple of foam filled grp torpedo's , slice them lengthways and have them glued inside and out to both grp keels ?

    I'm thinking it adds volume under the water (90cm draft) which should increase displacement without slowing the boat and while i doubt it would alter the waterline i could lift that maybe 5 inches so she sits tidy on it unloaded in dock .

    While looking at different pvc pipes and guttering which might be used to create the grp mold and thinking 6 metre of of 4" diameter pipe might give me the same increase as 2m of 12" diameter pipe . I would assume the more streamlined the better but hey i'm a fisherman and no engineer so here i am hoping someone more qualified might offer some calculations to guide my guesswork .
     
  2. Crashed
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    Crashed Junior Member

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    Last edited: Aug 9, 2020
  3. TANSL
    Joined: Sep 2011
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    TANSL Senior Member

    Welcome to the forum.
    Any calculation will probably have to be based on the body lines plan of the ship. Do you have that information?
     
  4. Crashed
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    Crashed Junior Member

    I'm afraid not the man who built it and many others of the same mold claims at the time sea trials were evidence enough for a seafish certificate . My personal belief is the error i'm trying to correct originates from the fact the mold was designed to be an 8m boat , they simply made it longer in accordance with popular demand . She's a tank no need to look at the weather forecast .
     
  5. Crashed
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    Crashed Junior Member

    What if i simplified my question by asking what weight a 6m length of 200mm diameter sealed pipe would support if it was submerged in salt water at 80cm ?

    Thats 19.5 ft of 8" pipe submerged at 31.5 inches in old money because an inch and a bit is as accurate as i will ever get :)
     
  6. bajansailor
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    bajansailor Marine Surveyor

    Re your 'daily load carrying', how much load is this typically?
    And re 'circumstances mean the load weight is set to increase considerably', how much will this increase be approximately?

    If you add two 6 metre lengths of 4" diameter pipe to the hulls, the increase in buoyancy is approx 48 kg per pipe - and you would still have to subtract the weight of the pipe from this. So you will gain maybe 50 kg of extra buoyancy in total, which is simply not worth it.

    Would it be feasible to extend the sterns by perhaps 1.0 - 1..5 metres?
    Doing this would help towards your stern heavy trim problem, and add a lot more reserve buoyancy than half-round pipes attached to the keels.

    Edit - re the buoyancy of 8" diameter pipes - each pipe completely immersed will have a buoyancy of approx 193 kg in salt water.
     
  7. Crashed
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    Crashed Junior Member

    Ah ok i need like half a ton as is though it copes fine with its daily load , i was hoping to correct the problem now so an extra half ton load might do nothing more than reintroduce the problem .

    I can think of many ways to sort this but they run into the realms of a 100k full refit notwithstanding the old adage "time is money" . As for making it longer some muppet in the EU thinks the length of a boat has some magical effect on what fish it catches so i'm legally obligated to drown rather than extend my hull by a few inches .
     
  8. bajansailor
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    bajansailor Marine Surveyor

    Oh dear re the 10 metre rule on fishing vessels in the EU - that does throw a spanner in the works, because the easiest way of achieving your objective would be by simply stretching her (a bit more?).
    I guess that your boat is already a slightly stretched version of the Sutton 7..95 m. boat as per the link below?
    Sutton Workboats http://suttonworkboats.co.uk/cat795.html
     
  9. Crashed
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    Crashed Junior Member

    Yep , he sold the 10m mold and i'm not sure anyone makes the original 8m any more .
     
  10. Crashed
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    Crashed Junior Member

    Hull tapers down to the stern so it can be put right , just not in a week and there's a lot of material costs .
     
  11. Crashed
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    Crashed Junior Member

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    Last edited: Aug 9, 2020
  12. Crashed
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    Crashed Junior Member

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    Last edited: Aug 9, 2020
  13. bajansailor
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    bajansailor Marine Surveyor

    I think that you are proposing filling in the sides with a sponson (for want of a better term) that is triangular in section, and 8' long?
    I am guessing that the depth of the triangle might be about 16" / 400 mm?
    If this is the case, then the total buoyancy of the enclosed space created would be about 400 kg (200 kg each side).
    But note that this is only reserve buoyancy, in addition to the (considerably more) reserve buoyancy in the hulls.
    The hull resistance / drag will certainly increase as a result of doing a modification like this, and I am not so sure that the increased fuel bill will be offset by the very minor gain in buoyancy that you will have, if say the draft increases by 6" or 8".
     
    BlueBell likes this.
  14. BlueBell
    Joined: May 2017
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    BlueBell Ahhhhh...

    A bit of a predicament you're in Crashed.
    Any way you can remove unnecessary weight from the boat?
    I would consider partially filling in the tunnel (as you say).
    That way, when it squats, for whatever reason, reserve buoyancy will keep it in trim.

    Question: Is the bottom of the transom in the water when you're in this overload situation?
     

  15. Crashed
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    Crashed Junior Member

    Thank you bajan to my astonishment you understood me correctly . Originally this idea was reserved to run a few inches above the water only coming into play when she dipped in heavy seas but an estimated 400kg at the stern given weight is on and off (pot fishing) increasing with catch stored forward and to the sides as the day goes on that might just balance things out . Fuel is an unavoidable expense when i want it at sea 12 hours a day every day and there's always going to be some compromise .

    Again that's not my boat above , my hulls are squared off at the stern with someone else's commonly adopted attempt at correction with a glassed on hydrofoil which i personally think adds to rather than subtracts from the problem because in my mind its sucks the stern down at speed .
     
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