Playing as Britain I badly need sea trade routes.

I've eventually found how to build 'Brigantines' (Merchant Vessels)! Finding them seemed obscure enough but now I have this trade vessel swimming in the English Channel and can do nothing with it!

I cannot move it to an adjacent sea zone.

I cannot drop it into a nearby harbour that lights up when I attempt to move it.

I cannot drop it on an unlit sea route marker.

I cannot build another trade vessel 'until the first one is on an active sea route'.

I cannot build a land trade building in any of the ports that light up.

I cannot think of any other way of activating it.

I need help. Anyone know the answer please?


*** UPDATE *** It seems in my case I've found a solution. I must have the Commercial View fully zoomed out and then very carefully manoeuvre the trade vessel over a lit up port until the vessel also lights up at a very specific juxtaposition to the port symbol. A bit fiddly but doable at least!