Plimsol

In plimsol marking there is an allowance of 1/48 given between summer and tropical sea water markings. What is the reason behind this allowance? why exactly 1/48th?

 

Perhaps because of the difference in specific gravity of seawater in cold water or warm water.

 

Sounds more like an allowance for icing.

 

Gonzo is sure to pounce on this ! Obviously there are differences according to salinity, but thermal expansion of water is tiny.

 

MY guess would be they want the ship lighter to take the far rougher winter seas.

 

I'm sure he would intervene. I look forward to getting feedback from him on this issue. He loves the technical issues.
What I can say is that the variation indicating the OP is the same as that between 1 kg / dm3 and 1,025 kg / dm3.
When I had to make a rigorous inclining test, among other measuring instruments, I used a hydrometer to check the specific weight of water at the time because these tini differences must be taken into account.

 

1/48 of what?

 

It is to account for the difference is water density. There are four basic marks: winter/summer/saltwater/freshwater.

 

You may have forgotten some basic mark.

 

Tansl,

the picture, you posted, answers my question.

1/48 of the distance keel summer load line.

 

Local salinity must have far more effect that thermal expansion, if I read this chart of water expansion Vs temperature correctly:

 

I suppose that in addition to the thermal expansion we must take into account that in warm seas the percentage of salt in the water is higher.

 

I don't think so. It is correct, that warmer water is able ti dissolve more salt than colder one, but there (surface near sea water) is no salt, which could be dissolved.

 

I do not know, I have my doubts. The salt content of the warm waters, as increasing evaporation, is greater. Therefore the density increases.

 

You are right, the evaporation increases remarkable with ascending temperatures. But if this would increase the density considerable, the draft of the ships would decrease. Does it?

Oceans are deeper than salt lakes.

Edit: You are right for coastal near sea water.