And the winner is: 2 of you !!!! but each of you have to send Don's Guide to the next in the list starting with and in order ...
1a. SeSmith ... bigger sail = bigger 'backwind', etc. in post #13, etc.
1b. Benny17441 ... better pointing ability with a smaller jib
Honorable mention: all you folks who participated!!!!!
Other Honorable mention:
1c. ScottT-bird ... FLOW ATTACHMENT better and easier to control with smaller sails !!!!!!! … even that he showed up late in the game.
Simple answers:
For light wind sailing, the smaller the LP and the flatter the draft the better the pointing ability.
Here’s the explanation.
Consider that air flowing across sails imparts or transfers energy to the sail and that energy can only be transferred if the air flow stays attached to the surface of the sail. In light and super light wind conditions the wind doesn’t have all that much energy, including energy to keep the flow attached to a sail … and the longer air has to travel while giving up its energy to the sail, the less energy it will have in the aft sections to stay attached … and not become a ‘separation stall’. Successful light wind sailing ‘starts’ with the prevention of ‘separation stalls’.
I don’t want to bore you with ‘aerodynamics’ but for wings and foils and sails, there is one very important fluid dynamics ‘law’ – the air/fluid flow leaving AT the trailing edge MUST be at equal velocity on both sides of the trailing edge, OR little to NO lift is created in the sail, etc. (called Kutta Condition) and that’s why we put lots of tell tales on the leeches of sails!!!!!!
So, If a BIG sail saps all the energy from the wind BEFORE it gets to the trailing edge and/or which causes a ‘separation’ of the flow … what is the energy imparted to the sail? Answer – nil to nada, the sail has a separation stall going on; and, if ‘kutta’ is not met – NO lift.
Lower air speeds causes a higher frequency of ‘separation stalls’ the longer distance the air has to travel along the lee side of a sail !!!!!! Therefore, to insure against such separation stalls: In lower energy wind, the choice towards the smaller the cord length (LP) sail is to better insure attached flow.
Also too, that’s why sails used in very light winds should be somewhat ‘flattish’ and with greatly reduced amount of draft (BIG outhaul tension). Low energy wind will not so easily ‘turn corners’ (such as too much draft in the sail) which increases the propensity for a ‘separation stall’.
In all cases set the amount of draft / outhaul tension by watching the speedo – the higher the speedo, the higher the ‘apparent wind’.
Also, and as noted by some posters, a smaller LP jib will usually promote better pointing (VMG) ….
Reason: with a BIG overlapping jib the flow velocity from the jib leech ‘may’ be too far aft onto or near the mainsail, as most all the ‘forward drive’ in a sail occurs mostly at ‘just aft of the luff’, tapering off in ‘driving force’ to where draft is at its maximum, and very little ‘forward’ after the point of maximum draft. When the LP of the jib is too large for a match with the mainsail, the jib will not correctly 'bootstrap' or 'dump its velocity' to the mainsail. With less mainsail efficiency due to a too large LP jib/genoa, there will be less ‘upwash’ for the jib to operate in with the result that the boat simply wont be able to point as well and will have to bear off a bit to get equal speed.
With sail interaction, one of the major functions of the mainsail is to create better ‘upwash’ for the jib to operate - 'upwash' is those flow stream lines well out in front of the oncoming boat/sails that are 'bending' and heading toward the leeside of the jib's leading edge.
Sail ‘interaction’ happens this way: the mainsail makes the jib more efficient and an efficient jib makes the mainsail more efficient.
FWIW – Mast headed boats were designed to use BIG jibs because you didn’t need to raise a troublesome spinnaker when going down or reaching/tacking downwind. Since there are usually no ‘reaching legs’ on most race courses nowadays, what usually wins is a fractional rig with a 100-120% jib, using an asymmetrical when going down wind – such a boat now has two available optimized points of sail: beating and reaching down.
Epilogue.
The ‘real’ answer for yourself and if you have ‘several’ sails is: take them out and compare the differences on YOUR BOAT and in all possible conditions, and write the stuff down … because your boat is different than everybody elses boat, even between two boats from the same manufacturer. Don’t ‘believe’ until you verify, in this way you may be quite surprised.
If racing is your game or future game, If you don’t have the developed reference data of many many 'trials' for your boat in various conditions, you better have a damn good memory, a memory better than most folks who cant remember what they had for dinner last Tuesday.
Postscripts:
"Fast Olsens" testimony came in a bit too late and after I did my write up. But his testimony will support the conclusions already given.
I really like Weinie’s sail shape in post #29
‘Protests’ about the selected winners, etc. can be filed in the race committee room until noontime, yesterday. ;-)
Hope you all enjoyed all the discussion.