Medium airs provide the best opportunity to go as fast as possible in an International Moth. On a reach in 25 knots you will probably be going faster, but around the course you will find that moderate conditions allow maximum velocity made good. The major distinction is that in heavy airs you will invariably have too much power in your rig and you will be flattening the sail out, whilst in moderate airs, depending on your body weight you can harness the full power of the rig. Depending on your physical fitness you may find that you are much more able to drive the boat hard in a force 3 or 4 than above.