My 5.3 black tip is my only sail to run on a 400cm mast and it annoys me pacl two masts i small car, how bad would it be to rig it on 370cm with a long extension ? I am 74kg

I hope this is in right section

In a pinch sure... In a surf zone you'd face the problem of weakened mast extension.

Also a 5.3 is a lighter wind sail so using a 400 is ok but 370 will definitely be too soft. May be ok for steady winds.

In stronger winds it'll flex and depower and may cause the center of effort to move. It'll take longer for the sail to respond.

What's your luff length?

Roy, I reckon try it. I run shorter masts in some of my sails + extender. However, I do it for tuning, not for convenience. I'm the same weight as you.

The HSM QU4D 5.2 is advertised as rigging on either but at 90kg I did not like it on the 370.

I always run my Naish boxer 6.2 with 42-44cm extension on 400 mast, and Chopper S and M often on 310cm mast with 35-42cm extension and they work absolutely fine. Chopper S and M on 310 feel super soft but if you consider the wind needed for Chopper S, an umbrella will pull you on to the plain. Softness helps a lot in gusts in these winds. Can Boxer be more efficient on 430cm, maybe, but fortunately/unfortunately windsurfing is very dynamic sport with wind speeds and water conditions fluctuating in real time.