Mast rake: explain please!

MM, you are correct regarding the forward sweep assuming a straight mast, but most dinghies run a reasonable amount of prebend. This has the effect of straightening up the 1/4 chordline from just below the spreaders to the tip.

On a boat like a 600 which runs a reasonable amount of rake with significant pre-bend, dropping the chain plates by a couple of holes probably increases the twist at the head by 5-10° which lowers the rolling moment significantly. In light airs the drag from such a swept configuration is larger than for a more upright, untwisted configuration hence why it always pays to go upright in the light.

It's got nothing to do with the structure of the mast, spreaders or shrouds. Sweeping the rig back induces a change in the local flow direction. So near the top of the rig the effective flow is actually rotated as if you were on a reach. As a result using kicker and cunningham, you should setup with more twist to ensure your tell tales are streaming. You will actually develop more power at the top of the rig but the loads will largely be forwards rather than lateral, hence why it is quicker than leaving your rig upright and blading off the top.

As for pre-bend on a 600, unfortunately it's achieved via halyard tension and the luff curve of the sail. As soon as you have enough wind to sit on the racks, the shape induced by the battens is enough to prevent the leech hooking and as such kicker can be applied aggressively to avoid too much twist. Cunningham significantly bends the mast as the wind increases and as the loads increase the head twists off to match the increased upwash. In theory you could overdo the kicker but in practice you run out of range before this occurs unless you've overdone the Cunningham to early.

Thanks all - an awful to take in and chew over there. I asked because I am indeed looking to depower my 600 even more. I'm also sailing a Fireball in our local Frostbites and the nuance involved in tuning that boat is more than a little intimidating. Everyone is constantly fiddling with rake and rig tension - these are entirely new inputs for me.

As Davidyacht says raking a Solo mast is effective in a blow. Solo rigging is fairly slack and without spreaders. But the Solo does have a oversized centreboard which provides too much resistance when it blows, so raising it is essential - up to 40% on a beat. So obviously the boat is a lot more balanced if the mast is raked to match. I think for this class it is as simple as that.

I feel it's got nothing to do with bend, because it works on boats where bend can be added separately, and in boats like the 600 where bend is not affected.
It's got nothing to do with raising the centreboard, because it works on keelboats too.
I suspect it's mostly about how the air flows up or down instead of straight back across the sail.

I just accept it TBH.