Plimsol

Salinity of oceanic waters does show considerable variation, in the ballpark of 1% density difference by the look of some figures. The Baltic is much "fresher" than the Med.

 

Mr. E

The salt content of the world’s oceans is almost the same, 32~37gl-1, including 30gl-1 of NaCl, or ‘salt’. However, in coastal areas this varies according to tide and time of year. The Baltic Sea, for example, has a total salinity of 8gl-1, the Black Sea 22gl-1, the Mediterranean 41gl-1 and the Persian Gulf 57gl-1.

 

Just so you need different plimsoll marks because, for the same weight, depending on the water the boat is sailing, the draft is different.

 

The Plimsoll mark shows an increasing draft for rising temperatures.
The (sea water side) summer load mark is above the one for the winter and the tropic mark is above the summer mark.

If the (because rising temperatures) increasing salinity would govern the density, the summer load line should be under the winter load line.

 

Density vs Difficulty/Danger?

Correct me if I am wrong, but more than just density variations are considered in the determination of Plimsol markings. I was under the impression that nominal weather conditions, such as rough sea conditions, also has an influence on the criteria and placement of Plimsoll marks?

 

Not much ice in summer, nor in tropical water latitudes.

 

I think, I got the solution of the interrelation.

I found a script of the Heidelberg University on limnology. The appendix shows an international state equation of sea water, published 1980 by the Unesco.
http://www.iup.uni-heidelberg.de/institut/studium/lehre/AquaPhys/docMVEnv3_12/AqSysSkript_Kap2.pdf

rho(S, T, p = 0) = A + B · S + C · S^(3/2) + D· S^2

(where A, B, C and D are polynomial expressions up to 5th grade)

This formula tells, that water with the salinity of sea water changes its density with temperature a few times stronger than pure water. )***

The results of the formula (I only was calculating for hydrostatic p = 0, at water surface):

Temperature, Salinity, Density

0 °C ; 35 g/l ; 1028.11 kg/m³

10 °C ; 35 g/l; 1026.95 kg/m³

20 °C ; 35 g/l ; 1024.76 kg/m³

30 °C ; 35 g/l ; 1021.73 kg/m³

Please, pay attention to the fact, that salinity stays the same.

(Excel file with coefficients and formula attached, blue fields to fill in temperature in deg. Celsius and salinity in g/l (= kg/ m³). Reddish fields
results: Density in kg/m³.)


)*** Edit: Sorry, a second view shows me, that salt water density changes not a few times stronger with increasing temperature, but only slightly stronger.

 

I think, at least regarding "winter north atlantic" you must be right.

 

Hence the different line placement, no need to allow for ice, but hey, just a thought bubble ! One thing I don't give any credence to is thermal expansion of water, which would appear to be about 1/5000th; over a range 20 degrees C. And that doesn't factor the thermal expansion of the hull !

 

I agree, that's way out there.

There are several things about LL assignment which are not immediately clear. The development of the FB tables for Type A and Type B vessels, for instance.

I do recall a colleague of mine had a document which I think went into some discussion of the technical sessions that led to the ICLL 66, but I can't find any reference to this on the web. This colleague worked for LR before he came to the states, and he had all sorts of obscure publications in his files.