The Yamaha 30 began life as an IOR era race boat. As such they were intended to carry a lot of crew weight on the rail for optimal performance. IOR boats of that era were designed to carry a comparatively large sail inventory and do comparatively frequent sail changes. A boat like the Yamaha would have been raced with a crew of 7 or so and would have carried a Mainsail, a # 1 (155% and perhaps even a heavy and light #1), #2 (135%-140%) and #3 genoa (110%) and a blade
jib (95%) plus two
spinnakers (star cut reacher and radial downwind) and maybe a blooper. As I said, in changable conditions there were frequent sail changes.
So what do you do when you don't have six gorillas hung out on the rail......First of all you depower. Depowering is a process of flattening your sails, moving the sail's camber forward and reducing the angle of attack of the sail. It is a step by step process.
In order, as the wind builds, you tighten the backstay adjuster, tension halyards, increase outhaul tension, lower the traveller to leeward, increase mainsheet tension, move the genoa sheet lead aft a couple inches, and increase genoa sheet tension. Once you have done all of that, if you are still over powered then the next step is to tie in a flattening reef, which does not remove much sail area but greatly flattend the mainsail. If you are still over powered you really tighten the
boom vang and then ease the mainsheet so that the mainsail is carrying a bit of a luff. On a boat like the Yamaha 30 most of your drive comes from the genoa and so even if you have to 'flag the mainsail', in other words allow the mainsail to luff, you still may be better off.
Your boat speeds are actually quite good for an IOR era 30 footer. It may also be time for new sails as older blown out sails will produce heavier weather helm and more heel angle.
Jeff