stability of two displacement hulls

Discussion in 'Stability' started by hori, Jan 24, 2018.

  1. hori
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    hori Junior Member

    Hello all, I have a questions regarding stability of displacement hulls. If one hull is 40 x 23 and had GT 14 and another is 45x 18 with GT 22 which hull would have better stability the longer one or wider? These are commercial lobster hulls and to ask in a forum of fisherman would draw a blank. All replies appreciated.
     
  2. TANSL
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    TANSL Senior Member

    It is not possible with so few data to know which boat is more stable.
    A certain ship is considered sufficiently stable when it complies with a series, 5 or 6, stability criteria. You must meet them all. It is possible that a ship has a huge initial stability but does not meet the other criteria which, although it would seem that the ship is extremely stable, the reality is that it is not "stable enough" in all circumstances that are required.
    Without knowing the shapes of your ships, their General Arrangement drawings and the loading conditions of each one, nothing can be affirmed.
    In general, for a similar displacement, a lower L / B ratio presupposes greater initial stability but, as I said, nothing can be guaranteed.
     
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  3. Tad
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    Tad Boat Designer

    A boat with 5' more beam will have greater low angle (initial) transverse stability, all else being equal. But that only applies to heel angles less than 10 degrees at most. Above 10 degrees other factors (freeboard, GM, etc) have increased effect and beam alone does not (as TANSL says above) tell the whole story.

    But of course things are never equal. The much higher tonnage of the longer boat suggests a higher deck/raised deck, or deeper deadrise, or maybe just a different guy doing the measuring. But probably a pretty different boat with different stability characteristics.

    The longer and narrower boat will be more comfortable at sea.
     
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  4. TANSL
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    TANSL Senior Member

    @hori is talking about two ships with different GTs but since the GTs are a measure of volume, not weight, it can happen that a boat with a very high GT value but low displacement is much more unstable than another one of few GT and a lot of displacement. A ship, with certain fixed GTs, will be more or less stable depending on the load condition being studied. GTs are not a determining variable, in principle, of stability.
     
  5. Tad
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    Tad Boat Designer

    Displacement is not mentioned, GT is. For the GT to be different, volume must be different. (Assuming the same measurement system) So we can assume the boats are of somewhat different configuration. But that says nothing about their comparative stability.
     
  6. Ad Hoc
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    Ad Hoc Naval Architect

    As TAD noted above, one cannot give proper advice without more data. One can generalise, as noted by the replies above, BUT..and it is an important BUT..since these are commercial vessels, their stability books will tell you the whole story. Since the GM in a light load may give a false impression of a "good stability" by passing known criteria. Yet when heavily loaded, may struggle to pass.
     
  7. hori
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    hori Junior Member

    Actually the 40x23 boat with 14 GT has higher freeboard, however... the keel does not extend fully to the stern. This is to make the GT calculation less to get it under 15 which in Canada means it will not have to go through mandatory 5 year inspection I will post pics of the hulls.
     
  8. hori
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    hori Junior Member

    see how the keel is shortened. The hull is 39`11'' there is a 6 foot extension mounted to the stern to give it more deck space and open stern.
     
  9. hori
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    hori Junior Member

    45x18 hull
     
  10. TANSL
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    TANSL Senior Member

    Frankly, I do not think that cutting the keel will substantially reduce the GTs. Probably, since it is a solid keel, do not reduce anything. Reducing the freeboard a bit will greatly decrease the GTs.
     
  11. Tad
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    Tad Boat Designer

    In Canada there are two options for GT measurement of small vessels. "Measured Tonnage" (L x b x depth) or "Assigned Tonnage", which is based solely on length, everything under 12m in length is assigned GT of 14.99. If you choose Measured Tonnage the depth measurement is to the bottom of the keel, taken midships. Which is why we now see boats with significant external keels suddenly develop a hole midships in the keel.
     
  12. TANSL
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    TANSL Senior Member

    I do not know the Canadian regulations but for boats like those in the pictures that formula, L * b * depth, is very harmful. It results in a value much greater than the actual volume of the hull. I would be surprised if the shipowners were satisfied with that calculation.
    In any case, it appears that the OP refers to ships of more than 12m. Although it would be necessary to check how that length should be measured.
     
  13. hori
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    hori Junior Member

    actually both of those boats fall under 15gt . The boat I was looking at is similar to the last one picture 44`11" but has a 5 foot extension and was measured at 22 GT where as same boat could be 14.9 ? Here`s a pic of the missing midship section of keel Tad was describing to get under the 15GT. Whats ones thought on that.

    [​IMG]
     
  14. Surfer Naval Architect
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    Surfer Naval Architect Naval Architect

    Dear Hori,

    If the vessel is larger and with higher free board+ long keel can have higher probability that the vessel has better stability. What give the
    certainty? The Metacentric height of course, and to calculate you really don't need more then couple of formula... If they complies or not to the criteria then we need to calculate the GZ for loading conditions. If you want you can contact me, please check our website www.sarc.nl, I can make easly a model for you running the calculations ;)

    Cheers,

    Raffaele
     

  15. TANSL
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    TANSL Senior Member

    You leave me totally open-mouthed. I thought it was something more complicated.
     
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