Currently I use the "5 hand widths from the boom clamp" rule of thumb (geddit? rule of thumb!). But I know that this rule of thumb is dumb. It almost never feels right.
I know that you should basically set the harness lines at the centre of effort of the sail i.e. when both hands pull the same. I can't adjust them on the water - too busy struggling with the chop and I can barely hold the sail.
So do it on the beach in light wind?
Attach the rig to the board and bury the fin in the sand?
What should be the angle of the sail relative to the centre line of the board to be considered sheeted in (but not over-sheeted)?
Mark the sport on the sail or the boom?
Should the lines be a hand width apart or touching?
Why don't sail manufacturers just mark the centre of effort?
I always hold my boom with one hand after rigging on the beach, this will let you know when you have the right position. If it pulls toward the mast, put your harness lines a bit forward, pulls toward the clew, harness lines back. This will give you a good starting position. Occasionally you do have to adjust on the water, not easy but certainly do-able and if they are nearly right in the first instance, you won't have to move them far. I usually set mine about a fist width apart but I don't think it is super critical.
I move my harness lines by feel. I will do a short run out and back so you have a feel for both lines then adjust. If pulling too hard forwards I move the front of the line a bit forward. Then do another run. If good I would leave it. If still pulling forward I would move the back of the line forward. ( I am talking a cm or so each time). In reverse for pulling backwards. My lines are about a hand width apart, but that too is personal preference. 
Select to expand quoteIceman said..
I always hold my boom with one hand after rigging on the beach, this will let you know when you have the right position. If it pulls toward the mast, put your harness lines a bit forward, pulls toward the clew, harness lines back. This will give you a good starting position. Occasionally you do have to adjust on the water, not easy but certainly do-able and if they are nearly right in the first instance, you won't have to move them far. I usually set mine about a fist width apart but I don't think it is super critical.
You mean you hold the rig by the harness lines and see where it balances?
Are you sure your sail is trimmed correctly? If it is then you should definately feel the center of effort and balance things around that. I find it's almost impossible to set a good position for harness lines if the sail is not well trimmed.
And to answer why sail manufacturers don't mark the centre of effort is exactly that it changes with sail trim.
From my experience:
The back harness line should be positioned 1/3 rd of the way down the boom from the mast; the front harness line should be placed just over a fist's width in front of that.
Also, just before I start (in the water), I too grab the boom in the centre of the harness lines to make sure the rig is balanced (with the other hand on the mast for stability), I then mirror the harness line positioning on the other side. There maybe a bit of fine tuning after the first run but then I am set for the session.
I must give Mr Guy Cribb credit for this; it works amazingly well! 
Select to expand quoteSlickmezzie said..
From my experience:
The back harness line should be positioned 1/3 rd of the way down the boom from the mast; the front harness line should be placed just over a fist's width in front of that.
Also, just before I start (in the water), I too grab the boom in the centre of the harness lines to make sure the rig is balanced (with the other hand on the mast for stability), I then mirror the harness line positioning on the other side. There maybe a bit of fine tuning after the first run but then I am set for the session.
I must give Mr Guy Cribb credit for this; it works amazingly well!
There's nothing worse than having lines in the wrong spot, especially when it's blowing hard.
Agree with Cribb's 1/3 rule as a good starting point. 5 hands from the front of the boom is nowhere near far enough, unless you're using a 4.0m storm sail. Google Guy Cribb Harness Line.
I prefer to have my lines further back than too far forward, but I like a lot of front hand pull so when a big gust hits, I'm not tugging on the back hand (I hate that feeling). I also do a short first run to check my boom height and lines then I'll come back and make adjustments if necessary. I also make minor changes when out on the water, but I only do it when water-starting not while I'm sailing (I'd find that impossible!).
Once you find that sweet spot, make sure you mark where your lines are with electrical tape or something similar. You'll find the line position changes for each sail, so use a different colour tape respectively.
As mentioned above, the centre of effort will also move a lot with trim... especially with the amount of downhaul applied. If you don't have enough downhaul your sail will feel really twitchy and you'll get some serious pull on your back hand. Without enough downhaul, no mater where your lines are, it will always feel horrible and the centre of effort will shift around a lot with gusts.
Happy sailing!
Guy cribb sheet cheap and quick.You can make your own but Ijust use his sheet for each sail and fine tune if necessary on water..usually spot on.
Select to expand quoteMrCranky said..
Iceman said..
I always hold my boom with one hand after rigging on the beach, this will let you know when you have the right position. If it pulls toward the mast, put your harness lines a bit forward, pulls toward the clew, harness lines back. This will give you a good starting position. Occasionally you do have to adjust on the water, not easy but certainly do-able and if they are nearly right in the first instance, you won't have to move them far. I usually set mine about a fist width apart but I don't think it is super critical.
You mean you hold the rig by the harness lines and see where it balances?
No I grab the boom. You put your harness lines where your hand is.
Select to expand quotesboardcrazy said...
Guy cribb sheet cheap and quick.You can make your own but Ijust use his sheet for each sail and fine tune if necessary on water..usually spot on.
+ 1
I've used this for this method for the last year and its bang on target. The best tuning tip I've ever had. Basically it's just 1/3 the mast to clew length, just get a 1 m length if elastic and mark a 1/3 mark, stretch from clew to mast on your rig for the rear harness position. You may need to tweak a little based on boom height and wind strength etc. but this little tip puts your harness lines right in the ball park.
www.guycribb.com/windsurfing_technique_holiday0065v01.aspx?awvariantid=1
Select to expand quoteSlickmezzie said..
From my experience:
The back harness line should be positioned 1/3 rd of the way down the boom from the mast; the front harness line should be placed just over a fist's width in front of that.
Also, just before I start (in the water), I too grab the boom in the centre of the harness lines to make sure the rig is balanced (with the other hand on the mast for stability), I then mirror the harness line positioning on the other side. There maybe a bit of fine tuning after the first run but then I am set for the session.
I must give Mr Guy Cribb credit for this; it works amazingly well!
You mean the elastic line with the dot trick?
Yep or a knot..He has a sheet of laminated paper that the elastic comes from ( a hole in the middle) but just a small flat piece of plastic /wood etc would work.
You place the 'sheet under the clew hole and feed the elastic through the hole to the top and pull the front end along the boom to the mast.
You then put the rear (? forgotten..it says on the sheet) line at that point with the other a hands width or whatever you usually use in front..The paper/plastic/ wood just holds the line so it doesn't pull through the clew eyelet as you pull it tight..
The placement for my harness lines is different for each side by a few inches. I've tried sailing at different angles to the wind to see if it caused by heading upwind more on one tack than the other, but nothing really makes a difference. I'm totally clewless about why it is like this.
Select to expand quoteHarrow said..
The placement for my harness lines is different for each side by a few inches. I've tried sailing at different angles to the wind to see if it caused by heading upwind more on one tack than the other, but nothing really makes a difference. I'm totally clewless about why it is like this.
One arm is stronger than the other
+1 for the Cribby 1/3rd method
and once you have the right you will see the marks lefts on the boom foam by the lines. The marks for me are where 5.3 and 5.8 sit, so I know that 6.2 is back an inch and 5.0 is forward and inch and so on.
Mr Cranky, you can just adjust after each run when waterstarting just slide them up or down a bit as needed. I don't think anybody have their lines right until after doing 3 or 4 runs...?
Select to expand quoteHarrow said..
The placement for my harness lines is different for each side by a few inches. I've tried sailing at different angles to the wind to see if it caused by heading upwind more on one tack than the other, but nothing really makes a difference. I'm totally clewless about why it is like this.
i have the same thing , it feels like the sail is different each side but i think i probably just stand different each side .
Select to expand quote2035 said..There's nothing worse than having lines in the wrong spot, especially when it's blowing hard.
Agree with Cribb's 1/3 rule as a good starting point. 5 hands from the front of the boom is nowhere near far enough, unless you're using a 4.0m storm sail. Google Guy Cribb Harness Line.
I prefer to have my lines further back than too far forward, but I like a lot of front hand pull so when a big gust hits, I'm not tugging on the back hand (I hate that feeling). I also do a short first run to check my boom height and lines then I'll come back and make adjustments if necessary. I also make minor changes when out on the water, but I only do it when water-starting not while I'm sailing (I'd find that impossible!).
Once you find that sweet spot, make sure you mark where your lines are with electrical tape or something similar. You'll find the line position changes for each sail, so use a different colour tape respectively.
As mentioned above, the centre of effort will also move a lot with trim... especially with the amount of downhaul applied. If you don't have enough downhaul your sail will feel really twitchy and you'll get some serious pull on your back hand. Without enough downhaul, no mater where your lines are, it will always feel horrible and the centre of effort will shift around a lot with gusts.
Happy sailing!
Thanks for the comprehensive reply. Thanks to all who replied in the thread, in fact. But what is the optimal angle of the sail relative to centre-line of the board - ie. sheeting angle when on a reach? It seems like you like to sail over-sheeted? Doesn't that mean you lose power by default? I thought the best setting is when no hands need to be used (equal pull).
Select to expand quoteIceman said..MrCranky said..
Iceman said..
I always hold my boom with one hand after rigging on the beach, this will let you know when you have the right position. If it pulls toward the mast, put your harness lines a bit forward, pulls toward the clew, harness lines back. This will give you a good starting position. Occasionally you do have to adjust on the water, not easy but certainly do-able and if they are nearly right in the first instance, you won't have to move them far. I usually set mine about a fist width apart but I don't think it is super critical.
You mean you hold the rig by the harness lines and see where it balances?
No I grab the boom. You put your harness lines where your hand is.
Matt Pritchard taught me this one aswell. It workes!
Key points:
i use one finger.
Try to get out of the wind. If not possible, point the base of the mast into the wind.
Once you you get the right position, mark it with a permanent maker.
Select to expand quoteYuppy said..Iceman said..MrCranky said..
Iceman said..
I always hold my boom with one hand after rigging on the beach, this will let you know when you have the right position. If it pulls toward the mast, put your harness lines a bit forward, pulls toward the clew, harness lines back. This will give you a good starting position. Occasionally you do have to adjust on the water, not easy but certainly do-able and if they are nearly right in the first instance, you won't have to move them far. I usually set mine about a fist width apart but I don't think it is super critical.
You mean you hold the rig by the harness lines and see where it balances?
No I grab the boom. You put your harness lines where your hand is.
Matt Pritchard taught me this one aswell. It workes!
Key points:
i use one finger.
Try to get out of the wind. If not possible, point the base of the mast into the wind.
Once you you get the right position, mark it with a permanent maker.
Wouldn't that be centre of mass rather than the centre of effort?
Guys you are really over complicating this topic! You can't simply mark the centre of effort on the sail as there are too many other factors that change the harness line position. Stance, boom height, harness position, sailor height, mast foot position, foot strap position.
The only way is to set on the beach first, just do it by feel, I.e. where the sail is balanced and can be held by the harness alone, or by using 1 hand in the same position.
Then fine tune on the water.
Getting someone else to do it, or using weird tricks like counting hands won't work well, as they don't account for the factors above.
Select to expand quoteinsight said..
The faster you sail the more you sheet in.
LOL. That makes perfect sense, since I lean back more when there is more pull from the sail.
Select to expand quoteinsight said..
The faster you sail the more you sheet in.
Ah huh... something like this
North sails come with a recommended harness line position marked on the sail:
The coe is a fuzzy point on the sail. Ideally it stays in the same place always but in fact it moves up and down a bit and fore/aft a bit. There is a neutral line from the mast foot through the coe. You put the centre of your harness lines somewhere along that line if you have neutral fore/aft balance. The higher the booms the further back along the boom the balance point is and vice versa.
Select to expand quoteMrCranky said...
Currently I use the "5 hand widths from the boom clamp" rule of thumb (geddit? rule of thumb!). But I know that this rule of thumb is dumb. It almost never feels right.
I know that you should basically set the harness lines at the centre of effort of the sail i.e. when both hands pull the same. I can't adjust them on the water - too busy struggling with the chop and I can barely hold the sail.
So do it on the beach in light wind?
Attach the rig to the board and bury the fin in the sand?
What should be the angle of the sail relative to the centre line of the board to be considered sheeted in (but not over-sheeted)?
Mark the sport on the sail or the boom?
Should the lines be a hand width apart or touching?
Why don't sail manufacturers just mark the centre of effort?
I am using 6.7 hand width for my 6.7 and 5.5 hand width for my 5.5..I have the same question as you many months ago...now I have sorted it out..
It is all to do with your stance...the distance between your front hand and the harness lines should be longer than the back hand with the harness lines..
Before planing, front hand should feel much heavier..upon increasing of speed..front hand will feel lighter..I have an exercise for you to feel.. put the back hand at the back harness lines..only move your front hand backward on increasing board speed..don't move your back hand..feel the pressure..
Your body should face forwars, not facing the rig. Putting the back end farther back will force the body to face the rig, ruining your stance.
If the gust is hitting you..you don't pull the sail towards you using your back hand..you step backward using your body weight to pull the sail..you can get to the straps straight away..
I set the lines touching, the feel of pressure is very precise.
For details.. please Google guy cribb early planing article..have many answers you are looking for..
Select to expand quoteIceman said..
I always hold my boom with one hand after rigging on the beach, this will let you know when you have the right position. If it pulls toward the mast, put your harness lines a bit forward, pulls toward the clew, harness lines back. This will give you a good starting position. Occasionally you do have to adjust on the water, not easy but certainly do-able and if they are nearly right in the first instance, you won't have to move them far. I usually set mine about a fist width apart but I don't think it is super critical.
Thanks for this tip. Tried it yesterday and moved my lines back about 2"'s and it made a big difference To seemingly have it right for the first time.
The ideal sheeting angle varies with apparent wind angle, which varies with point of sail, speed, shape of sail foil, strength of winds and probably lots of other things as well. Not easy to give a simple definitive answer.
It is something you learn by feel and feedback. It is just as detrimental to oversheet as undersheet. I guess this is one of those things that you heard from experience and makes you a better sailor.
I almost always sail with the harness lines just loose enough to adjust while I sail. I am constantly making small teaks as conditions, wind strength and sail trim changes. (I don't like to wrestle with my sail - always find the easiest way to do something.)
One of my pet peeves is that most modern harness lines designs are crap. They over tighten when is use so can't be easily moved as you sail. They are also impossible to remove and replace without taking the boom apart. Ropes and tubes are sewn in and this is nuts! They should be infinitely replaceable - just a money grabbing strategy for manufacturers and retailers?
I also use tube plastic that has a very long life and replace all the new plastic with this stuff before I even use them. I am trying to track down a new source of the plastic tube and I think it is Polyethylene. It's used for drink dispenser tube in pubs and restaurants, but not all of this tubing is the good stuff, there are different types.
I don't get why so many sailors now have their harness lines set so close together. I run mine at least three fists apart and sometimes much wider. Two advantages:
1. It spreads out the load on alloy booms (well all booms actually)
2. Most importantly, it allows me to move my hook a little forward or back and change the centre of pull on the sail without having to move the line attachment points on the boom all the time and when I change angles of sailing.
OK, rant over. Resume normal transmission, keep calm and struggle on..................
Select to expand quoteinsight said..
The faster you sail the more you sheet in.
Apparent wind moves forward the faster you go.
Select to expand quoteinsight said..
The faster you sail the more you sheet in.
Well that's the way it works on a raceboard any way. Presumably short boards function similarly.
Select to expand quoteinsight said..insight said..
The faster you sail the more you sheet in.
Well that's the way it works on a raceboard any way. Presumably short boards function similarly.
Well yeah. The apparent wind moves forwards on anything, your car your bicycle and your boards.
