Hello Francone,
your "problem" is as old as harness lines exist. Ideally the harness lines should be at the position of the sails pressure point. The whole idea is to not have to hold the sail power with your hands but have it all be held with the harness. This means both hands: so if you are having to sheet in actively with your back hand then the harness lines are too far forward.

When you are planing you will be leaning the clew close to the board and sheeting in. For this situation you might want to have the harness lines far back.

If you are not planing then you might not want the harness lines quite as far back since this will cause the sail to fall towards the back of the board.

If the wind is pretty gusty then you might also want to keep the harness lines a bit further forward and be a bit more active to sheet in to avoid catapulting.

As for the length of the harness lines, it really depends on your preference. For lighter winds you usually want longer harness lines to lean back further and really let your body sag into the harness. However, if you are not going to be planing then 24'' to 26'' might work for you. Add 2'' if you are using a seat harness rather than a waist harness.

I have written an article on this topic which you can check out here.
howtowindsurf101.com/how-to-set-up-the-harness-lines/

Ultimately you might be best served with some variable length harness lines to find your preference. This also lets you adjust to when you have planing or non-planing conditions. To set the correct position you can try it on your board on the beach without the fin on of course, or just standing next to it and not on it. You should be able to stand there, hooked in without holding on to the boom with your hands and it should stand stable. This is a bit more tricky with little wind.

For low wind (ie. non-planing) conditions you can just adjust them while you are sailing as you are in a close to balancing position already.

It ultimately comes down to how you feel comfortable and need the least energy to hold the sail.

I hope this helps.

Arne

Tune the sail correctly with downhaul and outhaul to suit the sail and wind conditions. Find the neutral point along the boom by moving the harness lines together and along the boom until the sail feels balanced. You can also do this using one hand. Mark this point on the boom and arrange your harness lines either side of this point about 150 to 250mm apart. I like a slight forward bias to my harness rope position so I then move both harness lines forward approx 25mm. This means you can sail with your front hand off the boom and control with your back hand

Joe, the original question involved a race carbon boom... give also that Francone's avatar says the location is WA **, we can only assume the discussion would involve sails built in this century. Since it sounds like you know more about what Francone is actually asking, maybe you can fill out all the missing details for the rest of us.

** given most of us here are Ozzies, we assume the 2ft snow comment was some jibe about how there isn't any wind at the moment due to it entering the doldrums.

In the meantime, the one item that hasn't really been identified -> Francone specifically said "non planing'... We know that when planing the draft moves backwards, when non-planing it is quite far forwards. In this case, the centre-of-effort could easily be 12 inches further forward on such a long boom.

joe windsurf said..
hey mathew don't disapprove

in light wind - no harness required for FUN n TOW, but should be there just in case !!!
half an hour on local river after work !! Fanatic Ultra CAT aka FUC board and TR-6 8.4

Thanks for your clarification on my behalf.
I have reached (and well passed!) the age where being “ tired and lazy”, usually, becomes part of the game ! I am not there yet, but I am getting ready!
I was wondering whether hooking on the harness, even in light winds, whenever possible, (which I hardly ever did) would perhaps avoid some unnecessary strain on the back. In fact towards the end of last season, probably while struggling with the boom or jibing I must have done some untoward stretching or pulling of the back muscles which caused some pain for about a month. I think the problem was more with posture and an unbalanced sail than with the force of the wind, but I want to cover base. Also, frequent uphauling with an 8.5 m2 could be one of the reasons. (This is why I want to stay away from sails larger than 6.0/6.5 m2 and longboarding becomes almost a necessity)
I know you still have to get off the harness when jibing or tacking, but I was thinking that keeping it on ( of course there is enough wind) would be safer. Any comments, anybody ?

Thanks

Francone

I have just started using adj harness lines as well; 28-34, and find the extra length useful in non Planning conditions, and also allows you to raise your boom height which helps in light winds.

To get the right harness line position get a peice of 1m long elastic, tie a knot or mark 1/3 the length, then stretch from clew to luff along the boom arm (knot to front). Put you rear harness line attachment where the knot is (this will be 1/3 from boom head) then the front attachment 1 hand width in front, you will find the position is almost perfect. A mate put me onto this and it is invaluable for harness line positioning.

In really light winds when cruising for fun (phantom raceboard & 7.8 slalom sail) I have been trying this; hold the boom with your hands each side of the harness lines , close, and let the sail find it's natural position with power, then relax and extend your arms with a thumbs over the top extended grip, lean back almost into the super 7 stance and let the harness hold you up. I think the longer lines let the sail be more upright which is more efficient and counter balances your weight.

I hope this helps, get out there and experiment, any time on the water is better than sitting at home or mowing the lawn