boat race

I am trying to track the position of boats using the xbee pro series 2.5 (50mw) on a track 2.5miles x 1.5miles.

I am tracking 20 boats.

What possible ways could I configure this network keeping in mind that the boats will be continuously moving around the track?

My main concern is how many xbees and what configuration on each xbee should I use?

Anybody have any ideas???

Is it wrong to think they will all be routers?

What will happen when these devices move out of range of another router will they automatically negotiate a new path to the coordinator?