RichH
In a blow said:
Some time ago I tried out how much sail my boat could/should carry in stronger wind conditions.
On a beam reach in up to 28kts wind with shortened (furling) main and full 135 jib, she was still heeling a bit too much. My crew, an experienced sailor, suggested to let the jib sheet out in the gusts. I thought that a tight sheet would make for a flatter sail in those conditions, whilst letting the sheet out would make it fuller and make the boat heel further. (The fairlead was at its rearmost adjustment.) Unless he meant to let the sail almost flap? Unfortunately, we did not get around to discussing the subject further or try his suggestion.
According to your theory, we should have eased the sheets?
For me, there are several thing going on simultaneously when in high wind conditions that will dictate what to do and how all these factors 'converge' with respect to 'performance', etc.
1. Rig tension and wire 'stretch' -- especially the forestay
2. Sheeting angles of the Jib relative to its sail area.
3. The "hull speed" of the boat .... your Hull Speed is ~ 7kts.
4. Skid to leeward, especially noted when near or above a beam reach relative to the amount of heeling.
5. Sea state
Forestay stretch will cause the jib to 'power up' by the draft becoming more aft which tends to cause the leech to 'cup up' and which causes the slot to close.
(Disregarding adjusting the fairlead cars) - the more jib sheet tension, the greater effect on forestay sag and more 'power-up' shape changes. Easing the sheet will cause less forestay sag, allows the leech to 'open' or become more 'flat' and a more draft forward shape ... with the result of increased speed (up to the limit of hull speed) and less heeling. Generally on most boats the helm pressure will increase when the forestay is over-stretched and you will start to notice that the wake is no longer coming almost straight off the stern -- the boat is now 'skidding'. Easing the jib sheet will put less strain on the forestay, so your crew was probably correct, especially if he/she was watching the increased forestay sag and draft stripe on the jib.
The larger the jib, especially when the clew is waaaay outboard, will no longer be able to correctly be sheeted and will develop a 'large' smooth curve in the foot of the sail and the jib lower panels will be 'over trimmed' (leech in the lower panels again 'cupped up') causing artificially greater draft ..... causes more 'powered up' shape for more heel. The lower panel draft stripe will usually show if the jib is 'too big' or needs a 'jockey pole' or 'reaching strut' to hold the foot flatter and the clew 'out' --- the draft stripe 'should' be showing a 'flattish' leech section. Again, perhaps your crew saw this 'full' shape to the leech and therefore suggested less jib sheet pressure ... so the boat could 'stand up' a bit better with less heel / and less 'skid'.
Once you go much above 'hull speed' the bow and stern wave formation will tend to hold back the boat from going any faster although the boat may be intermittently 'surfing' or even beginning to 'plane'. However on a keel boat once you approach or exceed 'hull speed' the excess power required will simply cause more heeling, more forestay sag and increasing helm pressure, especially if the keel & rudder begin to lose their 'bite' because of the aggressive heel angle. A good indicator that you have too much sail area up when at or above hull speed is whats commonly known' as a 'death wobble' .... the beginnings of noticeable oscilations or 'yawing' of the boat that portend that increased speed can result in a broach ... or even 'tripping over' the keel (broaching to windward !!!) if the steering isnt quite precise so that the oscilations are 'dampened' by the helmsman. The yawing can be accompanied by noticeable stretching of the forestay wire during the yawing oscilations ... helping to adversely turn the boat.
I generally when encountering 'yawing' at above hull speed also check the action/direction of the stern/rudder wake and if starting to feel the oscilations as increasing/decreasing helm pressure .... I suspect 'death wobble' and then further shorten sail or ease the sheets, etc. and carry a fisherman's reef (luffing at the front of the sail and with the leeches still drawing), etc. etc. etc.
Sea state conditions have a great effect on how the forestay, etc. are performing. In relatively flat water (near shore) ~30 kts on a beam reach with 'full up' can be quite exhilerating .... known where I sail as a "Nantucket Sleighride". But if the waves arent perpendicular to the wind, arent 'regular' or coming at you 'from all over the map' and with wave on top of wave on top of swell ... then of course its time to shorten down because the mast top will be oscillating back and forth yielding variable output from the sails, etc.
So although I wasnt onboard, it seems that your crew probably noticed that large jib was overtrimmed at the foot, increased powered up full drafted shape (because of the forestay sag) and with a 'cupped to weather' leech, noticed the increased heel without increase in speed ... and simply suggested that the boat could probably go faster and more upright with slightly eased jib sheets.
FWIW ... in such conditions its usually better to sail with a 'blade jib' (high aspect, full hoist, short LP) so you dont have to 'overtwist' (can lead to sudden/surprise 'power-ups') to keep good control, have better sheeting angles (flatter and more constant jib shape), less forestay sag ... and can always 'blade out' the 'blade-jib' by simply easing the sheet. In high winds the 'blade' has an advantage because it keeps the 'so called' slot (between the leech of jib and the pt. of max. draft in the main) 'more open' / less 'choked'.
