What does mast rake do?

Here's a funny thing. There is a keelboat class where 50% of helms are convinced that the way to handle the main upwind is to centre the traveller and ease main and kicker. The other 50% pull on maximum sheet and kicker and play the traveller. Results suggests both techniques are equally fast if done well.

My experience is that mast rake makes a boat go better upwind and its not just the opening of the leech of the jib that does it.  Too much mast rake really throws away power and for this reason we always rake the mast back on the 4000 by one click on the shrouds when conditions are at all difficult, this corresponds to bringing the mast tip about 80mm closer to the transom.  Why such a small change makes such a huge difference is a mystery.

As for letting the kicker off when its windy - it doesn't work.  More wind, more kicker.  The kicker makes the rig a lot less inclined to snatch in the gusts and the power more easy to modulate.  If the boat is difficult to handle try more kicker.  If you look at the more modern roachy type of rigs you'll find you can sail with the top of the sail absolutely flat when you apply lots of kicker.  That's exactly what you want in strong winds, not a full ragging mainsail causing masses of aerodynamic drag.

Here's something else to think about.  I have found that if I power up the rig (tighter lowers, less rake and use the mainsheet for leech tension) I can get the crew out on the wire at quite low windspeeds and the boat goes faster for it, but it has very poor gust response.  By which I mean it doesn't accelerate and it is often faster overal to sacrifice a bit of power and get the acceleration.  I can only liken it to driving a car slowly in 5th gear.

Increasing rake opens the slot (depowering) and move the Centre of effort aft. You can then lift the centreboard slightly moving the centre of lateral resistance aft.

Not really, because some boats can adjust mast bend and rake independently but still increase rake upwind as the breeze builds. I've never read a convincing account of why increased rake depowers the rig but it clearly does.