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Owner, Green Bay Packers
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This may be a very simple question for the professionals.

We recently discussed about Plimsoll mark (the load lines) - see the link for reference:

The Plimsoll mark

We all agree about different freeboard needed for different seas, but the question is this:

Fresh water is less dense then salt water, so a ship will sit deeper in the fresh water.
A ship loaded to S (Summer) enters a lake (no change in cargo or fuel).
Will they submerge to F (Fresh) ? Or less? Or more? Or it depends ?
Yes, she will submerge to her 'F' or fresh water marks. The difference between the summer load line and the fresh water load line is known as the ship's "freshwater allowance". On most ships it averages about 10 inches.

Tidal estuaries will have brackish water and a hygrometer is used to calculate the allowance in those conditions. ( Water temperature differences have little practical effect on the ships draft.) Salt water has a specific gravity of 1.026 while freshwater is 1.000. You can calculate the change in draft from either salt to brackish, or brackish to fresh, by multiplying the freshwater allowance by the difference in specific gravities between the two bodies of water.

As a side note...this explains the lack of cargo shipped from the Great Lakes eastward to the North Atlantic trade routes; a trade you'd expect to be heavily utilized. The Welland Canal has, if memory serves, a controlling depth of 28' fresh water. If you transit eastward at the controlling depth, somewhere along the St. Lawrence Seaway, when the water changes from brackish to salt, you're going to be at a draft of about 27'-02". That is a lot of tonnage "lost", in addition to the fact that you were already forced to load light for the canal to begin with. Most freighters draw between 32-42 feet load draft today, thus the punitive economics.
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