Stability Issues on my TUG - pls help!

Discussion in 'Stability' started by jfrech, Apr 3, 2014.

  1. jfrech
    Joined: Apr 2014
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    Location: ontario Canada

    jfrech Junior Member

    I am very new to boating and could use some help regarding my stability concerns on my new purchase.

    I just purchased a 1951 tug boat in which underwent some modifications to the hull making it taller, allowing for more head space in the belly of the ship. To make it more live able. Anyway I am very concerned that the ships stability has been jeopardized. (making it more top heavy) I sent a picture attached, it’s the one on the right (red). I was hoping you could guide me to the appropriate formulas to come to some conclusions to how stable this vessel should be and or if there is anything I can do to add to the beam? I intent do use vessel as a pleasure cruiser yet we I worry if I ever get caught in a storm on the great lake superior in Ontario Canada, that the stability concerns will be realized. Aswell many advanced sea goers tell me that the beam is way to narrow, yet cant give me any credible info.

    Dimensions, 13tons, 38 feet x 10.2” beam, 4 foot draft. After the hull was heightened (unsure how much it was added on to) they added a much larger engine, roughly 2500lbs in weight more than original in which they believed that would give the ship a lower center of gravity aiding in stability.

    Anyway, any help would be appreciated.

    Thank you,
     

    Attached Files:

  2. TANSL
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    TANSL Senior Member

    I would be happy to help you but to do some serious calculations is necessary to have a body lines plan, weight table with centers of gravity and tanks plan (very useful, also a GA).
    Tugs usually have high initial stability, especially if they have to work stretching over the side (maybe this is not the correct term, but I do not know to explain it otherwise). Therefore it is likely that, if you use she as pleasure boat has enough stability.
    In the picture you can see a line which may define the waterplane , which is very high: the freeboard is small, which is never good.
     
  3. DCockey
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    DCockey Senior Member

    You should consider talking to a naval architect who is close enough to have a look at the boat. One I know of is Steve Killing in Midland, Ontario http://stevekilling.com/ but there are others around.
     
  4. thudpucker
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    thudpucker Senior Member

    Test:
    Get some people on board, get into shallow water, and see if the people hanging onto the Top Rail, can tip the boat over 30 or 40 degrees.
    If they cannot get it over any further, it's probably OK, and just scares you badly.

    I did this with a 16' that somebody put a cabin on. It went right on over and swamped.
    The Cabin went immediately!
     
  5. JSL
    Joined: Nov 2012
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    JSL Senior Member

    I am involved with 2 reputedly 'top heavy' boats and to get things started, in both cases we are doing an inclining experiment to determine the GM - which should be in the region of 2'-3" to 2'-9" (pleasure boat) . One number you need is the displacement so rather than spend time lifting lines and doing calculations we just get the boat weighed. If there is 'hope', then we can proceed with the 'Lines' to determine the range of stability, etc.
    This is the first phase. The next is to assess other factors like freeboard, 'sail area' (tall boats will lay over in the wind), etc. As to 'girding' ("stretching over the side"), certainly an issue if you are towing in which case hull/deck integrity & the chance of downflooding must be closely examined.
    Adding ballast is not always good, and often bad. Yes, you lower the center of gravity (VCG) but it will reduce the freeboard, sometimes to minimal. For starter, I like to see a minimum of 10% of the beam which in your case is about 1'.
    Lastly, if you are a commercial tug (for business) you will be dealing with Transport Canada & that could be the trump card so to speak.
     
  6. TANSL
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    TANSL Senior Member

    JSL, by inclining experiment indeed the position of the center of gravity of the vessel is determined. What I do not understand is how you can do it if you have not previously calculated hydrostatic values of the hull. And that, my friend, you have to explain how you do it if you do not have the body lines plan.
     
  7. keysdisease
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    keysdisease Senior Member

    Its 60+ years old and looks like she's been around. She's had 2500 lbs added down low and has a reasonable length to beam ratio and only one deck. I doubt she's unsafe, and probably a lot more stable than a lot of "modern" designs.

    ;)
     
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    Definitely a matter for a professional to be consulted on, if you can follow the trail back through who did the alterations and what calculations and consultations were involved in the planning of it, all the better. Nagging doubts about the safety of the boat will ruin your enjoyment of it, it is worth it to spend some money to settle the matter one way or the other.
     
  9. rxcomposite
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    rxcomposite Senior Member

    If the boat has undergone extensive mofifications, an inclining stability test must be conducted by a Naval Architech. This is a small boat, the ISO method could apply.
     
  10. TeddyDiver
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    TeddyDiver Gollywobbler

    Wonder how long is the rolling period. Not too difficult to test.
     
  11. TANSL
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    TANSL Senior Member

    But if you do not know the weight (and therefore the draft), the rolling period is useless.
     
  12. jfrech
    Joined: Apr 2014
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    jfrech Junior Member

    Thank you very much I will contact him.!!!
     
  13. jfrech
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    jfrech Junior Member


    Here is the thing, there are no body line plans or weight table / tank plans before or after the modifciations. I have no blueprints what so ever.

    The black part of the hull sits above the water by 4". The draft is roughly 4 feet. I dont understand if you identify my freeboard being small .. y would that not be good? Pls explain.

    I simply want to feel confident (which comes with time i understand) with my boat by figuring out if there may be an issue before I place her in the water, that I may be able to fix prior to. Either by means of quick calulations with the info we have or by other means(less $1500).
     
  14. jfrech
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    jfrech Junior Member

    The boat is believed to be 8 tons dry and 13 tons loaded. (loaded seems quite high with 400L tank for diesel?)
    What is meant by Proceeding with LINES?
    I believe roughly looking at pictures of it being int the water that my freeboard is roughly 2.5 - 3 feet. I alwasy thought the lack of freeboard would assist with lower surface area for wind / waves to catch and or the abitlity to affect stability?
     

  15. jfrech
    Joined: Apr 2014
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    jfrech Junior Member

    Please define rolling period?

    Newbie question :)
     
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