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Calculating stability

Discussion in 'Yacht Designers Discussion' started by tirekicker11, Mar 8, 2011.

  1. tirekicker11, Mar 8, 2011
    #1
    tirekicker11

    tirekicker11 Senior Member

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    Out of personal interest I am trying to make the calculations regarding the conversion of a 65' full displacement wooden ex-fishing trawler into a liveable trawler yacht. I am not intended to convert this trawler into a smooth, sleek looking trawler, just liveable and seaworthy enough for coastal cruising and the occasional 500NM passage.

    The dimensions of the boat are LWL 65', B 18', Draft 6'. Displacement is unknown. Hull is build in Yakal and superstructure is ply with glass. Engines are 2 Gardners LXB @ 150 HP.

    What I am trying to find out is how the extension of the existing lower deck superstructure and the addition of an upperdeck pilothouse/ saloon will affect the stability and so the seaworthiness of the boat. As this is a former fishing trawler there should be margin to install additional ballast.

    Thanks for idea's and suggestions.

    Attached Files:

  2. 84far, Mar 9, 2011
    #2
    84far

    84far Senior Member

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    Your interests is opening a big can of worms, at the end of the day if you want it done properly, you'll have to visit a Nav Arch.

    What info are you after, stability is a big tree...? Just the CB and Metacenter, which shouldn't be that hard if you have plans...? To find out the weight of the cabin, you may have to get core samples, weigh them, and multiply.

    Im about to do a 35' powerboat, but I have plans, which in turn will give me the displacement, and everything branches off from that. Cheers

    Far
  3. tirekicker11, Mar 9, 2011
    #3
    tirekicker11

    tirekicker11 Senior Member

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    Thanks Far,

    Basically what I would like to find out is whether this boat will still be sea-worthy after adding an amount of weight high up and increasing the windage at this height. Or what amount of ballast would I have to ad in order to maintain seakeeping characteristics. I can't get drawings of the boat and I can't get the displacement.
    What I can do is calculate the weight that is to be added and calculate the added windage area.

    I found a formula to calculate waterplane and I think there should be a formula to estimate displacement (knowing that it is a full displacement hull)but haven't been able to find it. If I know the displacement I should be able to calculate CB and M.

    I don't own this boat and have no access to it but like to find out whether it would be feasable to convert her into a liveaboard without losing seaworthiness.
  4. K1W1, Mar 9, 2011
    #4
    K1W1

    K1W1 Senior Member

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    Hi,

    Not even this can necessarily save you from disaster as some of the poor folks here in the link I have posted below could attest to.

    I am particularly referring to the Altair and Americus that are mentioned in this article.

    http://www.nytimes.com/books/first/d/dillon-sea.html

    I read the book a few years ago and it made for a very sobering read
  5. K1W1, Mar 9, 2011
    #5
    K1W1

    K1W1 Senior Member

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    Hi,

    I can't remember if it were in that book or another one I read that covered the Federal Investigation into those two sinkings.

    A very telling part of it was when the designer was shown a photo of one of them and asked if it was the boat he had designed, he first said no as the one he had designed had a row of portholes low down but above the water line.

    It transpired that these had been welded over and were well submerged by the time the vessels foundered in their grossly overloaded condition's.

    It is also my understanding that there was a change in the CFR's in regard to the inspection and operation of these boats as a result of the investigations findings.

    It wasn't simply a case of adding too much anti fouling, if the crew used to load X tons before why would they load Y tons after a paint job?
  6. tirekicker11, Mar 9, 2011
    #6
    tirekicker11

    tirekicker11 Senior Member

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    That's interesting.
    But hey there was a lot of money to be made...
    If I'm correct during the first seasons of the Discovery series they were fishing derby style in a season that lasted only a couple of weeks every year. One less trip up and down to the processing plant would obviously result in a lot more money.
    What aboat Load Lines? They're not required on fishing boats in the States?
  7. Marmot, Mar 9, 2011
    #7
    Marmot

    Marmot Senior Member

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    Uninspected fishing vessels under 79 feet may not require a loadling unless engaged in certain work or to meet class requirements, if classed. I believe new build boats 79 feet or more are now required to have a stability letter and loadline.

    It is safe to say that 99.9 percent are not required to have a load line.
  8. jhall767, Mar 9, 2011
    #8
    jhall767

    jhall767 Senior Member

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    I suggest you start with a book called "The nature of boats" by David Gerr. It will help you do the preliminary work on stability and line drawings. It has some basic guidelines or "rules of thumb" to keep things reasonable.

    For your particular boat I would be concerned with figuring out the original construction and how much ballast may have been taken out and how much topside weight has already added. For example at some point someone may have removed ballast to increase their cargo capacity. They might reason that because they always leave port with full fuel and water and return with catch in the hold this was acceptable. You will need to make sure your boat is stable with both 10% and full tank levels.
  9. tirekicker11, Mar 9, 2011
    #9
    tirekicker11

    tirekicker11 Senior Member

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    I have "the nature of boats" and hoped to be able to do these calculations with the formulas in the book as an excersize. It's a book I realy recommend for everyone with an interest in very basic Naval Architecture.

    Apart from that in order to get the boat properly insured I might need the (re-) calculations from a NA and this project would be to large and costly to take any uncalculated risks.

    In the near future many similar ex-fishing boats will come up for sale at very reasonable prices and that's why I used this boat as the subject of my excersize.
  10. travler, Mar 10, 2011
    #10
    travler

    travler Senior Member

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    most guy's don't look at the stabilty letter or the load line , if you want to live a long time you learn do do all of the above and learn the fuel plan and how to manage it on the old martinolic boats the decks were allways wet and you have to run with the galley door closed at all times or it would sink , some of those early boats could carry more than 50,000 us gallons of fuel ,the marco boats were allways dryer , a lot of the crab boats were sponcened witch made them much more stable , now the C/G checks a lot closer than then

    just some thoughts travler
  11. 84far, Mar 12, 2011
    #11
    84far

    84far Senior Member

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    tirekicker11, by the look of things your could really load the vessel up on weight with out much effect on the waterline, but the thing you will have to watch out for if you go ahead with the project is the Moments you will create if you load up the top cabin anymore. Anyways, good luck, Cheers

    Far
  12. tirekicker11, Mar 13, 2011
    #12
    tirekicker11

    tirekicker11 Senior Member

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    Thanks Far,
    As far as I can calculate the boat will sink 1 cm for every 1000 KG added.
    The top cabin will not weigh more than 2000 kg, roughly estimated.
    The top cabin itself won't be top-heavy, most of the added weight will be at around 2 meter above the existing deck (roof of existing superstructure and floor of the to be build cabin).

    It's the windage I am more concerned about.
    If I would extend the existing superstructure all the way aft, remove the existing wheelhouse and build a cabin on top of the existing superstructure and extending all to about 4 meter from stern railing (leave room for a terras), I would (roughly) double the windage of the superstructure and at twice the height from waterline. How this affect the stability I can not calculate. Again (I haven't been below deck but) I am sure there is plenty of room to add 1000 Liter water containers to act as ballast.

    When this boat after conversion was to sit in a marina or on a mooring and do short trips in sheltered waters I'd be pretty confident in it's sea keeping abilities (from a gut feeling) but in order to be confident about this boat making crossings I would want to see it in numbers.