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post #1 of Old 11-08-2008 Thread Starter
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Plimsoll Mark question

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 ?


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finished cruising after two years.
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post #2 of Old 11-08-2008
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Depends on the density of the lake water. We always measured with a hydrometer when loading in less than salt water.

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Toronto in summer, Bahamas in winter.

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It depends... if the harbor the ship was loaded in was of fairly low salinity, the change in draft may not be too significant... this can often occur if the harbor has a river and there have been very strong heavy rains for a significant period of time...since the river flow would be higher than normal—resulting in lower salinity in the harbor it feeds.

It also depends on the temperature of the water, since water gets denser as it gets colder... so going from a cold salt water port to a warm fresh water port would result in the greatest change in draft.

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Quote:
Originally Posted by tomaz_423 View Post
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 ?

Tomaz,
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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post #5 of Old 11-09-2008 Thread Starter
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Will !
Thanks a lot. Your answer was spot on.
I would give you some rep points, but sailnet wants me to spread some (You must spread some Reputation around before giving it to sailaway21 again).
I was sailing dinghies, windsurfs and sailboats (up to 47 feet) since I was a child, but I have no knowledge about ships.
On Wednesday I will take an exam to upgrade my licence.
I do it for fun and to learn something.
With my current licence I can operate vessels up to 24 m (79 feet) - non commercial. So that was "recreational" licence.
This new licence would allow me to operate ships up to 100 tons, an take up to 50 passengers national waters. It is a commercial licence.
This means I would be able to legally get paid for delivery or to take paying passengers on board...
I learned a few interesting things on the course I had no idea about: large ship engines, fuel, cargo, ...


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