The length of the vessel is not so much a factor when choosing the size of your Boatcatch, the weight of the boat is more critical. It is suggested that the small coupling is used on boat with on water weight from 300kgs up to 1.5 tonne. Any greater than this and you should use the large boatcatch
The small coupling comes with a galvanized mounting arm that bolts around trailer bow posts that are between 50mm and 75mm wide, and the large boatcatch fits on bow posts between 60mm and 100mm wide. If your post is different just ask us and we may have something to help.
Note: Occasionally people choose to disregard the mounting components and adapt the coupling to the trailer by other means. Should you wish to make a custom mounting, our coupling has a flat 4 bolt flange that is very simple to mount onto.