I've built my own comverter, but this was easyer.

My batteries are said to be 12V, in real-life this is somewhere in the
13's
My laptop uses 15V, 4A, but the laptop-battaries are 10.8V
so, the input of the laptop should be somewhere between 15 and 10.8 V.

a direct link from the boat-batteries could be made, and for security I
added a fuse, diode (such a thing to let te power flow in one
direction), and a thing to catch any peak voltages (we call it a
"condensator")

this works well now for 2 years, third coming up. (laptop-batteries
should be removed, couse they are slowly getting empty)
the laptops it works with for me are a Toshiba 1800-100, toshiba
2140CDT and a real old IBM XXX. the IBM asked 18 volts too