The 41 cm fin is for beginners. It's a "trainer" fin.
If you are using sails > 7.0 m2, the 41 cm fin really is not very good.
> 7.0 you need the regular fin that normally comes with the GO 171.
What size rigs do you plan to use.
A 32-38 cm weed fin will normally replace a 40-48 cm vertical fin, but
sail size is the most important parameter here.