Choosing Axel Width: The best way to know which axle you need is to measure the width of your boat.  

Some boats are more fragile than others. Most Daggerboard or Centerboard boats should have cradles or bunks added if you're not selecting the Cat Trax with Cradles product. 

Boats that use the 7' axle:

Wave

Boats that use the 8' axle:
Hobie: Getaway, Wave, 14, 16, 17, 18
Prindle: 15, 16, 18
Nacra: 4.5, 5.0, 5.2, 5.5SL, 5.7, 5.8, 460, N15
G-Cat: 5.0

Boats the use the 8'6" axle:
Hobie: 20, Tiger, Wildcat,
Nacra: 6.0, 6.0NA, F18INF, F18, Inter 17, 18, 20, N17 MKII Olympic
Supercat: 17, 20

Boats such as Tornado, Nacra Carbon 20, Carbon 20 FCS, and other boats wider than 8.5' would use an 8'6" axle with an extender.