Lightship weight and CoG creep over time

Discussion in 'Boat Design' started by RThompson, Oct 17, 2010.

  1. RThompson
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    RThompson Senior Member

    Hi,

    Can anyone point me in the direction of a paper, discussion, method or anything related to estimating the change in lightship mass and CoG of an old ship as it ages (due to various undocumented installation changes, GA changes etc).
    In particular: I am looking for something that discusses what happens to a large fishing vessels' lightship weight and CoG after so many years of use (maybe some kind of empirical study?). The ship I am pondering is about 80m and 35-40 years old. I am hoping that someone somewhere has studied this problem (published or not).
    I had thought to simply increase mass by, say, 5% of 'as built' and leave CoG where it was. I can then play with the 'increase percentage' and see how much it effects final hydrostatic results (which will give me an idea of uncertainty).

    Thanks,
    Rob
     
  2. MikeJohns
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    MikeJohns Senior Member

    There's no hard and fast guide and it can go both ways, a trawler for example can end up with greater stability just as easily as less as gear is replaced with better and often lighter equipment. The permanent changes are usually small relative to the operational variables.

    For example I just saw a ships two stage air compressors replaced from 250kg each to 120kg for the same rating. The radome went from 70kg to 11kg. Paint mass accrues. But consider that if you have 50 tonnes of fish in the hold and a similar amount of bunkered fuel then it's all pretty insignificant in the scheme of things.
     
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  3. Ad Hoc
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    Ad Hoc Naval Architect

    Concur with Mike

    If you're serious about purchasing the 80m, it wont be cheap. Therefore worth doing a proper inclining expt before purchase to get the exact values. If she is in class/flag, this should be done after any major work done. After 35~40 years, im guessing major refits etc would have taken place, hence there should be some guidance there too.

    But the main golden rule, CoG of boats rarely improve with age.

    As Mike notes while the displacment may be slightly better, better modern eqpt, the effect of lighter low down compressors changing, for example, is that the Lightship CoG rises!
     
  4. RThompson
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    RThompson Senior Member

    Thanks for the replies.

    Good point re whether or not the actual mass increases or decreases.
    Yes, also the actual loading at any time changes things a lot, however I can account for loading.

    So, whether lightship mass increases or decreases over time is probably unknown and/or a fairly minor change hence could be made constant for the time being.

    However there seems to be the consensus in this and other conversations that CoG goes up over time. 'How much' is the question.
    "But the main golden rule, CoG of boats rarely improve with age." -if I could find a study to demonstrate this then all would be well in the world. :) Or even if I could find the data and draw my own conclusions.

    For the record I'm not looking to purchase a ship, this is more about stability of old fishing ships in general (and one in particular). I am not able to incline or weigh it.
    Thanks again,
    Rob
     
  5. Perm Stress
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    Perm Stress Senior Member

    As I am told by shipowner of cargo ship (~120m, ~2000t cargo capacity ), a ship in 20 years can grow 100-200t of extra weight: extra spare parts, old spare parts kept "just in case" by bosun and chief engineer, sludge in ballast tanks, paint accumulation... .

    Nothing will beat (or even challenge) an inclining test after 40 years of hard life of fishing vessel. Too many unpredictable factors involved.
     
  6. MikeJohns
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    MikeJohns Senior Member

    Inclining test is simple but it takes a day with suitable weather and a couple of people. We perform inclining tests after most vessel modifications so there will be data on file somewhere (Presuming she was under USL the data will be with the state maritime authority) .

    If it's local I might know the vessel.

    PS
    Designers should allow for a certain amount of accrued spares in initial stability work.
     
    Last edited: Oct 23, 2010
  7. RThompson
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    RThompson Senior Member

    Hi Mike, PM'd you.
    Rob
     
  8. wardd
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    wardd Senior Member

    when I was in the service we used to weigh the aircraft once a year to check weight gain

    one CH-37 had a weight gain of over 360 lbs
     
  9. Ad Hoc
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    Ad Hoc Naval Architect

    If the vessel is in Class too, any modification that will result in an increase in lightship displacment of more than 2% and/or a change in LCG of 1% requires an inclining expt to establish the 'new': displacement, VCG and LCG.
     
  10. RThompson
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    RThompson Senior Member

    So, here's what I found so far:

    The investigation into the sinking of the RORO vessel Herald of Free Enterprise (1) found an unaccounted for increase in lightship mass of 0.3% per annum (across the class of vessel). According to Biran (2) warships must allow for a 0.65% per annum increase in lightship mass and a 0.45% per annum increase in KG, although Hoskins (3) found that weight growth was actually ~0.1% per annum for aircraft carriers.

    Unfortunatley there are subtle differences between RORO's and aircraft carriers to fishing ships...

    1. (UK), Department of Transport. Report of Court No. 8074 (MV Herald of Free Enterprise). s.l. : Her Majesty's Stationary Office, 1988.
    2. Biran, Adrian. Ship hydrostatics and stability. Burlington : Butterworth -Heineman, 2003.
    3. Hoskins, Clinton. CVN 68 Class displacement concerns. Monterey : Naval Post Gradutate School, 2009.
     
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  11. rxcomposite
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    rxcomposite Senior Member

    Even newlybuilt ships undergoes inclining experiment. No matter how detailed and careful the NA is in his calculations, the ship mass will deviate somehow from theoretical to actual.
     
  12. Commuter Boats
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    Commuter Boats Commuter Boats

    My experience is with considerably smaller boats but it's been my experience that owners are willing to make huge modifications to small commercial fishing boats (10-20 meters) without updating stability calculations and quite often insurance companies are willing to ignore that until there is a loss.
     

  13. wardd
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    wardd Senior Member

    in las vegas you can gamble under 18, you just can't win if you're under 18

    the meaning is insurance companies will take your money without much checking but when it comes time to pay they check with a fine tooth comb
     
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