Thanks Guys,
The usual thing is to leave it on it's side stand and secure by strap over the seat. A tie on the brake lever helps. On this occasion it was the last sailing of the ship before a new ship took over the route, and to be honest the whole ship was a shambles. Restaurant and bar understocked, toilets not working etc.
This was the first time i had ever had my bike strapped to a shackled 5 inch chain on the deck. We were last on, (Bikes usually first or last) most people had left the deck, and the deck hand had limited English, he just made it plain what he wanted by pointing and saying 'Big Stand...Big stand' and left to continue rigging for departure.
I knew the bike was a lump to put on its main stand, ive done it thousands of times, but extra loaded with full panniers and wet greasy decks was enough to cause it to slip.
My point is; The most innocent of circumstances can catch the unwary, even if you think you know what you are doing.
A while back i saw a guy lose a gixer on the metal boarding ramp....just be carefull thats all.