Mast and topmast Springiness.
The wind is never steady. Wind stronger than 6kts becomes a turbulent boundary layer and is always unsteady in the "fine grain" frequency range, ie. with a puff or lull every few seconds. This causes a sail on a springy mast to flex in constant motion around its static shape, particularly at the leech. If a mast is too stiff the boat will handle "woodenly" and sail slowly. As the mast is made more flexible the handling becomes more pleasant and speed increases. Beyond the optimum the mast yields to the gusts too easily and the rig loses power, handling becomes mushy and the boat sails more slowly.
The design flexibility of the Tasar mast/topmast is a rate of spring of 5.5lbs/in in the stiff direction, and 2.8lbs/in in the sideways direction. A simple way to check this is to put the boat on its side, put a support such as the back of a chair under the mast exactly at the height of the forestay shackle-pin, slacken the shrouds, spring the top diamond stay out of the spreader, and hang a weight from the tip of the topmast first with the major axis vertical, then with the major axis horizontal. Measure the height of the tip above the ground both unloaded and loaded. The difference is the deflection.
Example: I have just measured a stock mast and topmast. I clamped the butt to a bench, put a fulcrum under the extreme top of the lower mast (this is the design height of the hounds shackle-pin hole) slipped in a new topmast with sleeve and stiffener, and used a 30lb weight to load the tip. "F" means "fore and aft" ie. long dimension vertical in this test. "X" means "cross ship" ie. long dimension horizontal.
Height of tip unloaded 30.5 30.7ins
Height of tip loaded 25.0 19.9
Deflection (1 - 2) 5.5 10.8
Rate of spring:
Note. Do not be too surprised if your figures are different. There are some strange animals out there. In the discussion about topmast stiffeners, somebody quoted a topmast much heavier than design. Where did that one (those ones) come from? Heavier sections will necessarily be far too stiff, and slow. I understand that one pressing long ago was from soft alloy. (I didn't ear of that for years).
To understand how the rig is designed to work for you, let us follow a crew who start sailing to windward in say 7kts, with the wind speed varying every few seconds between 6kts and 8kts
Downhaul - No diagonal wrinkles
Rotation - 45 degrees
Outhaul - Closer to 1 than 2
Vang - Slack
Angles - (jib) Right in
Steer for both lower jib tufts streaming.
Tension mainsheet for upper leech ribbons "just popping in and out".
Trim traveller so boom is along centreline in 6kt lulls.
Sheet jib for upper windward tuft just breaking occasionally.
Let us assume that the wind surges to say 10kts, with puffs to 11 and lulls to 9kts.
Tighten the outhaul to about two dots. This will eliminate backwind from the jib.
It will be necessary to tighten the mainsheet to hold the upper leech in. If you don't, the upper leech ribbons will stream all the time, and that is slow. As you tighten the sheet, three things will happen. The topmast will bend back a bit, which flattens the upper mainsail to match your flattening of the lower mainsail with the outhaul. Diagonal wrinkles will appear downwards from the batten protectors.
Tighten the downhaul to eliminate them. This preserves mainsail shape. The forestay will become tighter, which flattens the jib to match your flattening of the mainsail. Note that the action of tightening the mainsheet has automatically flattened both the upper mainsail and the jib.
Trim jibsheet as before for upper windward tufts to stream with occasional break.
Now for the important bit.
By this time you will be hiking with say 70% total effort. If you look at your leech, it should be "shimmering" all the time with a movement of say plus and minus half an inch. The apparent wind in the brief lulls will be about 13kts, and 15 in the puffs. The heeling force will vary as the square of the apparent wind speed, so the difference between puff and lull will be about 33%.
To keep the boat steady and upright you will need to play the traveller for quick control as you move your bodies smoothly for coordinated control. As the traveller car moves to leeward, the design of the sheet system and the curve of the traveller track is such that the strop tension remains constant and the leech tension and forestay tension do not change, so the upper mainsail and the jib stay flat in the puff. This is fast and is the way the rig is designed to work.
Note. If you rig your traveller with the blocks high and the strop low, the tension will ease as the traveller car goes out. This way the upper mainsail and the jib will go fuller in each puff. This is not as fast.
In Stronger Winds
Let us assume that the wind increases further to about 13kts, with puffs to 15 and lull to 11kts. The critical change here is that the wind is now stronger than the design wind (which is about 12kts for the average crew.) At the design wind your whole object changes.
In winds lighter than the design wind the object has been to extract the maximum possible "power" without too much drag. Technically, the fastest trim is that which gives the greatest "power factor"; in the case if a glider this is the speed and flap setting which gives the lowest sinking speed. In winds stronger than the design wind, only one thing matters - minimum aerodynamic drag. This is the glider looking for greatest distance, which is achieved by flying absolutely clean and substantially faster than the speed for minimum sink.
As soon as you have more "power" than you can use - as soon as the heeling force is more than you can hold up without easing the sails - you are in the strong wind regime. Immediately flatten to extreme. Stay slides come right back to tighten the forestay and so flatten the jib. Outhaul is pulled tight (to 3 dots) to flatten the lower mainsail. In stronger winds the designed rig springiness gives you options with the upper mainsail.
In winds which are relatively smooth, you will sail fastest and point highest with a reasonably tight sheet and sufficient vang to flatten the upper mainsail.
In rough winds any tight sheet will cause the boat to stagger with each gust onslaught. You will sail faster with a slacker sheet and the traveller to windward and a much tighter vang. This will make the upper leech "springy". Set like this it will absorb the gust onslaughts by yielding elastically and driving the boat forward rather than pushing it over.
To sum up
In winds less than the design wind, if you trim to keep the upper leech ribbons just popping in and out, the mainsheet tension will automatically flatten the upper mainsail and jib to the fastest fullness. In stronger winds, you can alter the elasticity to match the roughness. This is how the rig was designed to work. Give factory settings a go - you too might win gold.