Sail phenomena: Screwing into wind

I own four Severne Convert sails and I was happy with them until I came back from a vacation at Maui. I rented Hot sails Firelight and didn't notice anything special about them. Was so windy that I did not have much mental capacity left for assessing the sail performance. However, when I came back to my own sails I noticed how they "screwed" themselves into the wind during the gusts. I had to push with the front arm to keep the angle of attack, or else, the sail would "over open" and almost knock me into the water while all speed was killed. I am guessing the pressure center moves backwards in the gusts.

I was told this happens with lightweight sails with fewer battens. My 6.7 has 5 battens.

I therefore borrowed two stiffer 6.5 m2 sails: Severne Gator (5 battens) and Severne NCX (7 battens). I have them rigged with the correct downhaul and the correct outhaul. They both show the same tendency of unstability in the gusts as described above.

Is this happening to you guys?
Why is it happening?
What is the phenomena called?
Are there sails that are more robust against this unstability?

(My skill level: water starts most of the time. Hooked in and planing, but having a hard time planing upwind. Gybes in less than 7 m/s. Tacks are very good. Footstraps sometimes, but not comfortably. Still on my Starboard Go 195, but I am also enjoying my Fanatic Gecko 133 in over 8 m/s)

Harness lines too far forward? Enough downhaul? Never used the convert, but the NCX needs lots of downhaul.

Try backing off the outhaul a bit. I normally use the recommended downhaul or + 1 or 2 cms and use less 2 cm less outhaul. You need a bit of fullness in the sail for stability. Need to match sail size to board and experiment with the mast track a bit.

Are you positive it's just the sail(s)?
Different boards and different fins can make the entire thing feel or act different.
Were you using the same board(s), and fin(s) as at home?
Also your angle of attack may have been a bit different than what you usually do at home.

Not sure if you are describing the same thing, but when hit with a gust which is way beyond the sails range, a sail will tend to lock into an upwind position and be very difficult to sheet in or out, and it becomes difficult to bear away.
A gust within the sails range should not be an issue, and a correctly rigged sail on the right mast should have a relatively neutral feel.
Agree with Mastbender, Sails can work differently on different boards and fins.
Sailing with a severne Gt Turbo 8.1m on a starboard Atomiq 120l, the sail under performed and did not go up wind very well. Swapped to a Severne fox 120l board with same sail and same fin and the sail works perfectly . Nothing wrong with the AtomIQ, just doesn't like this sail fin combo.
I have 2017 Gators in 6.5 and 5.7 and they are awesome in my opinion, and i have not noticed any issues in gusts, other than the smile on my face broadens, and the urge to scream yeeha builds

The slower you sail, the less stable sails will feel in gusts. Since you are not comfortable in the footstraps, your speeds will be on the low side, which means you feel the gusts much more than better sailors. As you get better, your sails will start feeling more stable.

There are several things that cause this. One is the change in the apparent wind angle in gusts. Another one is related to sheeting in. Typically, intermediate windsurfers are slower, and their sails are not fully sheeted in. When a gust hits, the change in apparent wind angle causes a lot more pressure on the back hand. When you (automatically) sheet out, the front of the sail gets backwinded, and you need to push hard with the front arm. There's a detailed discussion with diagrams of this at boardsurfr.blogspot.com/2011/01/sheet-in.html

The bottom line is that this is a technique issue much more than a sail issue (assuming that you rig your sails decently). For someone learning to be comfortable in the foot straps, a sail may have a narrow range from 8 to 10 m/s. Below 8, he can't plane and/or get in the straps; above 10, and he feels overpowered. An expert sailor might be reasonably comfortable on the same sail over a much wide wind range, perhaps from 6 to 12 m/s. Not that he'd necessarily sail the same sail over the entire range, but he could.

So if you have money to spend on windsurfing, investing into lessons rather than new sails will probably give you more bang for the buck. Plenty of good instructors on Maui, including Matt Pritchard. If there are none in your area, plan your next windsurf trip around a windsurf clinic, or at a resort with decent instructors.

A 9 batten Maui race sail should fix the problem. Still need to rig them with enough down haul and outhaul if the winds up. The stiffness of the battens also plays a big part in the sail remaining stable. I once changed all the battens on a 5.7m Stuart Bell race sail with much,much stiffer battens from a 8.1m Tushingham Race sail and the difference in performance when it was windy was amazing. I cut the thin or flexible side of the battens to fit the smaller sail and it turned it into a flat frypan

Lighter guys do not have the weight or strength to make sails work when the sail is working above its capacity.

Technique and experience play a part too.

The old booms that had a plastic front end were useless too when the wind picked up.

Many good answers here! I definately beleive that going faster, being in the footstraps and sheeting in more will make Me less vulnerable to gusts as boardsurfr pointed out.

My weight is 78 kg.

Is this phenomena that I am talking about the same as being overpowered? In Norway, it is called "being screwed", as the sail wants to do a screw motion.

Backhanding is common on the modern no cam sails. The North e_type used to come with a sticker above the clew holes noting that they required more outhaul tension than regular sails. I found that 1 or 2cm more outhaul always fixed the backhanding.

Decreasing downhaul would have the same effect because it tightens the outhaul. However, you probably don't want more power in your sail when this is happening so I'd leave the downhaul alone.

I think it's cause by the luff inflating in the gust beyond the designed tolerance. Tightening the outhaul keeps that movement in check.

Im by far not an expert but I think Grich62 has a point. When this happens to me, or when loosing speed (which is when I usually fight most with gusts), moving my front hand forward usually solves the problem for me. Strapping in early is definitely a must to get comfortable, allows me to have a better grip when a strong gust hits me and also having my weight towards the back of the bord seems to counterreact the effect of a gust more. I am a very lightweight, 63kg and I sail a 6.5m normally... I have 2 boards, my 125L seems to jerk more with big gusts, but my 85L is so easy in stronger winds... I guess there is so much science behind sailing.