Floating a boat around the world?

Discussion in 'Boat Design' started by AlaskaFisherman, Aug 14, 2005.

  1. AlaskaFisherman
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    AlaskaFisherman Junior Member

    I was telling a buddy about a boat floating higher in cold water than in warm and he this is new to him.
    Would some of you give up your knowledge as to a loaded ship having to take this serious as they travel to different parts of the world and maybe give the markings that would be found on the side of the hull for these different waters?
    Don't mean to be a bother but knowledge is always good, please share yours.
  2. Thunderhead19
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    Thunderhead19 Senior Member

    It should be easy to see for yourself at any deep sea port in alaska. All the big cargo carriers have two sets of load lines on the hull. I forget how many degrees latitude the "tropical" lines kick in though. Warmer sea water is less dense, and theres tables that correlate density to temperature. This might help.


    Interestingly enough during WWII the PT boats operating in the Aleutian Islands had to run smaller props than the ones in the south pacific because the water density change was that significant.
  3. AlaskaFisherman
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    AlaskaFisherman Junior Member

    Thanks for the attachment - just what I needed. That changing of the props on PT boats is sometihng I would not have figured on.
    Thanks again for the answer and markings.
  4. mmd
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    mmd Senior Member


    Water is certainly denser at 32 degrees F (unfrozen) than at 100 degrees F, but I don't know as it is enough to operationally effect draft. For example, given that water density at 32 degrees is 62.416 lbs per cu. ft., and at 100 degrees is 61.998 lbs per cu. ft., lets take a box-shaped barge with the dimensions of 100 ft LOA x 25 ft BOA x 6 ft D from Alaska to Yemen. In Alaska the draft is six feet, and the displacement is (100 x 25 x 6 x 62.416) 936,240 lbs. When it reaches Yemen, its mass is the same, but the water density is less, so it rides lower in the water - but by how much? Well, 936,240 lbs divided by the lower warm water density of 61.998 yields a volume of displacement of 15,101.13 cubic feet. Divide this number by the Length x Breadth of the barge and you will arrive at the new draft; 15,101.13 / (100 x 25) = 6.0405 feet. The barge has increased draft by not quite a half an inch due to the difference in density caused by temperature.

    Measurable, yes; enough to be concerned about? I don't think so.

    Does this affect propeller performance? Well, a change in density of 0.01% per degree I s'pose could have an effect on props that operate at the outer limits of their performance envelope in water temps that vary by fifty or one hundred degrees, but I have trouble imagining a vessel that would be expected to operate at this level of performance in waters so widely seperated that the ambient temperatures are disparate enough to have an effect on the propeller efficiency.

    But that is just my opinon.
  5. Skippy
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    Skippy Senior Member

    I'm not sure how salinity varies globally, but if you know that, this is x, where the density in g/ml is 1 + x/1000.


    From http://oceansjsu.com/105d/exped_briny/16.html

    "The salinity of sea water exhibits a variation of about 4% around an average value of 35.5 o/oo in the world's oceans"

    o/oo means parts per mil (.1%). That's the units for salinity in the graph.
  6. RThompson
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    RThompson Senior Member

    The two sets of loading marks on the hull are for fresh and salt water.


    edited: I just looked at the hyperlink - and realised my comment is obsolete...
  7. Herbert lotz
    Joined: Mar 2006
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    Herbert lotz Junior Member

    Hi guys
    I'm trying to get Rhino under control any one got tutorials to download, I have worked through most the on line stuff.
  8. Ari
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    Ari Patience s/o Genius

    There is no need to worry too much about loading capacity unless you are having a cargo ship. The marking on the port or starboard site hull of a ship is to guide the crew and loading masters so that those ship won't be overloaded for that particular journey and maximum loading ability are maintain. For a pleasure boat those character are not that critical I believe.
  9. waikikin
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    waikikin Senior Member

    I always thought the summer & winter marks related to weather/wave conditions for different seasons?Jeff.
  10. Ari
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    Ari Patience s/o Genius

    Summer/winter dead weight markings

    If that journey consist of a trip from winter to summer area or from summer to winter area than the summer dead weight / winter dead weight mark is very important. If that journey is limited to the same area than it is not that critical.Sea water salinity do vary between winter and summer,caused the specific gravity of the water to vary thus the ability to float the ship.

  11. tom28571
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    tom28571 Senior Member

    The Plimsoll lines were designed to set cargo loading in the water where the loading takes place so the boat would not be overdraft in other water around the world. This was a great step forward in preventing accidental groundings and allowed planing for dockage depth. Therefore density/salinity/temperature of the water must make a significant difference in hull draft. Cold and highly saline Arctic water will float a boat significantly higher than warm brackish tropical water.
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