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.

Boats that use the 7' axle: 
Hobie Wave

Boats that use the 8' axle:
Hobie: Getaway, 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
Dart
Sol Cat

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

The Tornado, Nacra Carbon 20, Carbon 20 FCS, and other boats wider than 8.5' use the 8'6" version with an axle extender.