Pascalou31:
What kind of sensor could be suitable to measure boat speed with a 0,1 knot accurency ?
I reckon you need a GPS for any sort of accurate speed measurements and I don't know if it could be that accurate or if it would work at all over the short distances moved by a model sailboat.
I speak from the experience of money wasted on a paddle-type speed log for a real sailboat.
I guess a model sailboat has the advantage that you could easily clean a speed-detecting paddle. You would probably get better info about speed measurement on a model boat website.
Separately, if you want to transmit data directly to a smartphone or PC you will either need Bluetooth or WiFi. I don't know if Bluetooth would have the range you require.
AFAIK if you want to use XBee you will need an XBee receiver to relay the data to your PC - but I have no experience of XBees.
If you are content to have a shore-based receiver then I reckon using a pair of nRF24L01+ transceivers would be a lot cheaper than XBees. If you need the range the shore-based module could be the high-power type (with the external antenna). This Simple nRF24L01+ Tutorial may be of interest.
...R