Standard Minix GX2180S

Bought this marine-radio(on a second-hand-site), but without any manual or so, looking for a way to add channels like channel 31 or just looking for a manual of this Marine vhf fm radio, thanks in advance,
Walther (The Netherlands)

You may need to contact the manufacturer for a manual. Unfortunately, according to a a quick Google search, it seems the original company, known as Minix, Hannover was sold years ago and may no longer be in business.

I am not familiar with the laws and regulations regarding marine radios in The Netherlands, but in the US this marine radio would probably not meet the new FCC requirements. If you are having trouble finding a manual for it or are unable to program it to the marine frequencies needed, you may be better off purchasing a newer, current make and model.