Hi all, first post here.
I've dug around in my computer and found these comments from a tuning guide written by greg goodall, not sure when it was written, but probably around the beginning of mark 5s. Principles should still apply to the mark 4 and is consistent with Jakes comments. You also need a bit more rake for downwind in the Mark 4s and 5s. Exact tuning would depend on your mast, but its a good start. Jerry, PM me if you want a copy of the guide as it is no longer on the AHPC web site. There is also this quide on the Landenberger site, but I find it a bit hard to follow.http://www.landenberger-onedesign.com/spip.php?article21
Hope this helps
Extract from Greg Goodall A class tuning quide. Date ??
On our Auscat boats we set up the mast with about 5 deg. of aft rake for all conditions. We have found that it doesn't seem to make any significant difference if we change it for different wind conditions. Although we do increase the mast rake for rough water, i.e.. 1 meter+ waves.
On our boats we set the mast rake by using the trapeze wire. Measure the height of the trap ring off the deck at the front chainplate and then take the trap towards the stern. With normal mast rake the trap ring is the same height off the deck about 250mm behind the rear beam.
Rig tension on an A-Class doesn’t need to be tight when compared to other classes. The tension is about 50 –60 Kg.
Spreader Rake, Diamond Tension and Mast Prebend.
Spreader rake is the method of tuning the fore aft stiffness of the mast below the hounds. The ideal amount of spreader rake is dependent on the fore-aft stiffness of the mast and to a lesser extent, the amount of luff curve cut into the sail. Stiff masts require more rake to make the mast bend sufficiently, soft masts require less rake.
The spreader rake is measured by placing a straight edge or string-line between the diamond wires at the spreaders and measure the distance to the back of the mast. On the Saarberg and Australian masts we are currently using 50 – 60mm of spreader rake.
To answer how much is sufficient rake can only be determined by sailing the boat and knowing what to look for.
If you have excellent height, but lack boat speed up wind and the boat does not want to accelerate in the wind gusts, then you need more rake. This helps the mast bend fore and aft which allows the sail to flatten and the leech to open in the wind gusts.
If you are lacking height and "grunt" in light weather, then you have too much spreader rake.
This primarily controls the side bends of your mast. Loose diamonds allow the middle of the mast to bend to leeward and the top of the mast to hook to windward. This tends to cause the boat to heel very easily in wind gusts. Very tight diamonds do the opposite.
Downwind, tight diamonds keep the mast bent reducing camber and power. On our boats the diamond tension is set at 30 - 34 on the loose gauge.