Most of the instructional websites tell us HOW to use the harness , but very little about the basic principles behind the rules and the interplay between the harness, the boom and the sail, in particular why it makes a difference setting the harness lines close( or closer) to the mast rather than farther away..
I can phrase my question simply by asking what this difference is and this will hopefully lead into a discussion about the principles behind it, which is the main reason of my post. .
I assume that the boom is equivalent to a lever, whose fulcrum is the point where the harness lines are positioned (or where we hold it with the hands). According to physics, the longer the lever, the higher the force at the fulcrum ( this is why thieves can cut ½ “ bicycle locks with 2 ft long cutters …)
On this assumption, I believe that keeping the lines close , or closer, to the mast will have a multiplier effect on the nominal wind force , with the risk of overpowering, because the lever ( the boom) , will have become longer and the force at the fulcrum will be at its maximum. . With strong enough winds this may mean difficulties in controlling the sail and ultimately overpowering.
If my reasoning is correct, then, to gain control in the same winds ( all things being equal, like the sail size) , we may have to shift the lines( or the hands) further back along the boom , in order to reduce the multiplier effect..
By the same token, keeping the lines close (or closer) to the mast, will be best in order to maximize the lever effect in light winds..
In other words, the rig (boom and sail) are a bit like a door: if we try to open or close a door by pushing it near the hinges, it takes much more force than when if we push it at a point farther away from the hinges, towards the outer edge.
Actually, if I apply the same strength towards the edge of the door rather than near the hinges, I may well slam it too strongly and possibly break the glass, if it is a glass door.
Finally, I know that, as a rule by thumb, the lines should be at approximately 1/3 of the boom length, but how is this distance calculated? On the curvature of the boom or the straight distance between clew and mast?).
I'd like to know if what I say is correct, based my lever analogy
Thanks
Francone
mm, mast isn't fixed like a door on one edge...
this link is pretty good info, by Peter Hart
www.windsurf.co.uk/hook-line-and-sinker/
The harness lines are not used as a lever but a pivot point for the sail.The sail pivots about one third back from the mast not at the mast . Because the sail is a triangle shape that is where the center of power comes from. This is where your harness lines should be. Then there is slight variations on preference and gusts.
Technically there are several pivot points in play here, but the two key ones are the sail's centre of effort (COE) and your harness hook. Both these points are the main center of rotation of the opposing forces (wind vs body wt). So the reason your 'recommended' harness position is one third of the boom length from the mast is because typically this is where the COE of the modern sail is. The sail has two levers to work on the pivot point, being the contact points at the front and back of the sail, where it is attached to the boom. Hence your harness lines is placed to be able to act on both front and back levers with your arms and body wt.
Agree with previous stuff, basically harness lines should be centered at the center of the sails lift, then everything is in balance and your arms aren't getting tired.
BUT, the COE of the sail will shift depending on the load, typically as the sail loads up the COE moves backwards. Sail-makers try their hardest to limit this effect, as it makes the sail unstable in gusty conditions. But on every sail I've used so far, it hasn't been completely eliminated.
If the harness lines are in front of the COE, then there will be more load on your back hand, not only will this make your back elbow/shoulder sore, it raises the chance of spin out, as the extra load on your back hand is transfered through to your back foot. And a gust will tend to sheet the sail out.
If the lines are further back than the sail COE then there's more load on the front arm, a gust will tend to sheet the sail in, risking over sheeting and stalling.
The situation is further complicated by your stance, if you lean backwards, then the lines need to be a little forward, and visa versa.
You notice this when swapping gear with other people, sometimes their stance is so different to yours that their setup is unsailable by you.
So start with the 30% position then adjust for minimum strain on front and back hand.
The lever analogy is not correct.
If you consider the whole shebang; you + board + rig as whole thing, then you can see that it is just the attitude of the sail to the board and the wind that will affect your lift and drive. It doesn't matter how the rig is held in that position.
The harness lines just have to be in the best position to make the rig easy to support and that is in line with the centre of lift in the sail.
The way you find that position is by feeling it in by holding the sail up in the wind and making any fine adjustments you need to on the water. The 1/3 way along the boom rule is just a rough guide. Most foils develop their resultant lift about there but it's not a hard and fast rule.
I reckon you ought not to over-analyze it, Francone.
The harness lines, if placed correctly, happen to be the "balancing" point between the forces on your boom. As you are well aware, if you are to take your hands off the boom while hooked in, the entire rig is balanced on just that one point, ie. the apex of the harness line. As for the rest of the forces such as the Centre Of Efforts, pressure on the board, etc., just assume they are there to work for you. Concentrate on your sailing, and worry about those mysterious forces later.
May the force be with you 
