This could take a while...
No problem Geoff-this is a very common area of confusion, so let's clarify:
You may be confusing rake with BEND.
Rake is used to balance the center of effort (combined CE of main and headsail) with the Center of Gravity (in the keel). It has nothing to do with sail shape, since you could have the top of the mast in any fore and aft location, yet keep the same tension on the headstay and backstay, right?
For example if you wanted more forward rake you ease off on the backstay 3 turns and take up on the headstay 3 turns. You have kept the load the same as before, but moved the whole rig (and CE) forward).
This is rake as opposed to bend, and it works like this: with all other things being equal, if you move the CE forward of the CG (think for this discussion of the CG as a pivot point on a windvane), you will have the driving force of the sailplan FORWARD of the pivot point, which would "blow" the bow AWAY from the wind. This is felt as "lee helm", or the tendency of the boat to bear away when you let go of the wheel/tiller.
If you rake the mast AFT of the "pivot point", the driving force is now BEHIND it, and the STERN will be pushed AWAY from the wind (and the bow into the wind). This is experienced as "weather helm" or the tendency of the boat to head up into the wind when the wheel/tiller is let go. A small amount of weather helm is usually desireable for optimum upwind speed and pointing-as the boat will have a natural tendency to sail higher-which helps you point higher-and you can find a "groove" where the boat has that magic combination of speed and pointing. If you have a perfectly neutral helm (meaning the CG and CE are perfectly aligned), you will always be trying to force the boat higher, rather than finding a groove. You will have good speed, but not optimal VMG upwind. However, this neutral helm is very fast for reaching and running. We don't adjust rake to a large degree while racing (with some exceptions)for sail shape, but we DO change BEND. There are times that we DO adjust rake, but it is for balance-not sail shape.
The kicker to this picture is the CG (pivot point, CE of the keel, etc.) can be affected by moving weight around the boat. If the boat is at rest, empty, this point is somewhere in the middle of the keel. But when you are sailing, this point becomes the combination of the keel AND crew/gear weight.
If you move the weight way aft, the effective CG becomes a combination of the keel CG and the center of the weight on the boat (you and your crew, + gear), and the actual CG is now somewhere BEHIND the keel. If the mast were aligned perfectly with the original keel CG, you would now have the rig CE FORWARD of the boat's CG-creating LEE HELM. Move most of the weight forward and the reverse happens: WEATHER HELM. Have you noticed in photos of heavy air sailing the crew weight is in the back part of the boat, while in light air pictures the (good) boats have the weight forward-especially upwind? This is largely because in light air upwind, you are working to give the boat a bit more WEATHER HELM to help it point higher, and in heavy air, when there is naturally more WEATHER HELM, the goal is to reduce the WEATHER HELM and the excessive rudder drag that goes with it.
Weight forward= weather helm; Weight aft= lee helm
Rig rake forward= lee helm; Rig rake aft= weather helm
For rake tuning, just sail the boat in it's usual trim and adjust the rake until you have just bit of weather helm in anything over 4-5 knots of breeze-no more than 1/4 turn of the wheel. For rake adjustment, read on.
Now comes BEND. The purpose of mast bend (on masthead rigged boats) is 2-fold: Tensioning the backstay increases overall tension in the fore and aft plane, loading the headstay and reducing sag. It also has the effect of bending the mast (NOT raking it because you don't change the headstay!), which flattens the mainsail. Note the top of the rig moves aft while the middle moves forward, so the rig CE changes very, very little. This is a good thing because as the breeze builds, the headstay will sag more, and the mainsail will need less power. Adding backstay which bends the mast accomplishes both goals!
So, the idea of playing the backstay is to keep it softer in the lght air, add some in the puffs, ease in the lulls, etc. Or "season to taste" if conditions are steady.
For most boats these days, about the only rake adjustment we make while sailing is that we ease the backstay all the way when going downwind to allow the top of the rig to go forward, which will reduce WEATHER HELM-when sailing downwind you want to elminate any tendency of the boat to head up, since your destination is "down". There is an added benefit of getting the roach of the mainsail slightly higher in the air where the breeze is stronger and less affected by the rigging, other boats, etc. It may seem small, but there is a very real effect.
In the old days (70's), many race boats had adjustable headstays AND backstays, so that in addition to doing all the things mentioned above, they could easily adjust rake while sailing to more accurately balance the helm. In the trend towrds simpler boats these days, this is much less common, and many racing rules allow only one adjuster for fore and aft rigging-and most boats choose the backstay adjuster. So today, mostly we set up the rake at the dock, use the backstay for bend, and rake forward when going downwind as the only rake adjustment. It is not unusual for boats like the J105, and others to have several headstay length settings (rake adjustments) for different conditions, but it is changed only between races-not during.
Sorry to be so long-winded, but this is good stuff to understand. Thanks for your patience!
Cheers!
S