Hello Chez.

There is an other bug that bother me. Sometimes, when I'm in the ship battle mode at sea, the ship are extremely slow, almost stopped. 
They can  turn, but don't advance. After 1 or 2 minutes, all of a suden the sea seems to change (colors and animation) , and the boat advance normaly.

before and after:

Do you think it could be an engine issue?

I can send you a samegame where it is clear maybe?

Thanks in advance!

There is a modification made to Maelstrom, where the flag angle will drop down, or raise to near horizontal, depending on the wind speed (the original engine always had the same flag angle, no matter what).  This gives a hint to what you see.  In the left picture, the flags angles are down, which indicates the wind is very low.  If the wind is near zero, then your ship won't move and there are no ship tracks either.  When the weather changes at the next hour of the day, the wind changes, the sea/sky change and now your flags angles are more horizontal, and your ship moves.  My bet is that Immersion must allow very low, near zero wind.  Some mods actually disallow reaching zero.  I don't know if Immersion allows zero or not, but it's pretty clear in the left side of your picture, there is little to no wind, because the flag angle illustrates this.

Thanks for your enlightening answer!
Nice for the flags!
So I'll try to find in the INI something to change that.