Whenever I try to sail in 25KTs and above apparent wind close hauled my forestay shakes violently with a reefed headsail and main. At the recomendation of my sailmaker I installed a foam luff on the headsail and am getting the rig tuning checked.
Where do you usually place the headsail fairlead blocks when closehauled reefed and high winds? Do you put it way foward so it pulls down on the leech or way back so it pulls on the foot and then tighten on the leech line to minimize leech flutter?
Your sailmaker is correct !!!!!
Plain vanilla sails for 'cruising' are designed for usage in a 'target windspeed' of ~15kts.
Much beyond 15kts there will occur some stretch in the materials, and rigging, etc. If you want to sail in winds much above 15kts, then at least you should increase tension on the backstay (which in turn re-tensions the forestay)
Specifically a sail designed for the typical '15kts'. will have in-built certain shapes that apply for winds of that strength ... especially the 'head' or upper panels will reflect a certain amount of 'shoulder' - a typical 'fullness' and typical 'built-in' amount of 'twist'. Beyond much above 15 kts (20-25+) the amount of 'twist' isnt needed as the wind gradient quickly diminishes. So sailing a sail that was designed for 15 kts in 25kts (where the 'shoulder' shape is less important) can cause of a lot of sail shape distortions .... distortions that are difficult to correct by trim and 'normal shaping'. What a foam luff will do is allow you furl the sail 'a few turns' so that the 'shoulder' is now tightly furled and no longer subject to 'aerodynamic instabilies' of a 'too full shape' / flutter.
Several things could be happening:
1. forestay is 'stretching' and now too loose for the conditions ..... look for an extreme curve or sag to leeward developing in the forestay. A stretching or too loose forestay will GREATLY POWER-UP a sail. Remedy - increase backstay tension to get the forestay sag back to normal (for when beating).
2. Furl the headsail 'a few turns' on the foil ..... to reduce the amount of draft, the 'shoulder' or upper portions of the sail are the first to be 'reduced' inside the 'wrap'. Most sails will completely remove/flatten the 'shoulder' portion at the head of the sail at about '6 wraps' if the sail has a foam luff.
3. Make SURE that the mast is not PUMPING or excessively/visibly vibrating. Mast pump is a vibration that occurs when whats called the 'natural frequency of oscilation' is in 'sync' with an 'exiting frequency' .... caused by wind, etc. forces acting to the combo of sails, rig, mast shape, etc. etc. etc. IF the mast is pumping ....CHANGE the rig tensions, change the strain on the sheets, change the vang tension, fly smaller sails or REEF.
If the mast IS pumping, then consider (by yourself or by a rigger) to add 'pre-bend' to the mast. Pre-bend for a single spreader rig is about 3/4" forward bow or about 1/2" forward bow for each spreader. (Note: a sailmaker even when cutting a 'plain vanilla' or 'cruising' sail usually expects that the mast has the proper pre-bend .... a perfectly straight-up mast with no pre-bend will slightly 'power-up' (less speed, more 'punch') a mainsail.) A violently 'pumping' mast will quickly break rigging.
4. Dont 'over-do' moving the jib-fairleads aft. A sail that has its fairleads too far aft will soon SHRED its upper leach sections apart when operated in high winds. Better to reef/furl or change to a smaller sized sail.
From your brief description, my guess is
1. fairleads too far aft.
2. stretching / too-loose forestay; rig tension too loose for the conditions.
3. mast is beginning to 'pump'.
4. sail is tooooo big for the wind conditions.
5. If you are going to sail regularly in wind venues above 20-25 kts. consider a 'blade jib' (a smaller % jib, with greatly reduced/flattened 'shoulder').
