Mast rake: explain please!

I can speak about this with some experience given the degree of rake we apply to racing sailboard rigs. We tend to slide the mast foot forward in a track then rake the top of the rig back which enables us to firstly depower the rig, why is it depowered? Logically I'd always attributed it to the fact that the chord of the sail is being moved to be more inefficient in it's lift property, same as the centreboard if you kick it back the water flow over the foil is changed.
Then with a sailboard rig which had cams (Cams and Mastrtacks appeared about the same time)the act of closing the slot once moving with apparent wind re established, introduced end plated effect so the combination of rake and end plate improved the efficiency once more. With a sailboard you get to 'feel' the quite significant effect, but then we moved our rigs a lot further than dinghies do. It was also noticeable when we use a smaller sail how the range of rake was shortened or the rig would be next to useless and sailmakers never really designed smaller sails that well to cope with the rake and the lack of end plate effect a raked small sail delivered, so we'd run smaller sails as vertical as possible. So Rake without end plate was not good.

Now with dinghies, I've never really had a boat with big rake control and the rig depowers differently with heel (we heel to weather in Raceboards) so my assumption was that the reason it works for boats with less rake (14" isn't much compared to what I was used to) seems to be mainly because the boats that employ rake all seem to be pin head sails so the foil area is lower down and the chord longer so the rake will effect the flow efficiency lower down as well. I don't know if this is deliberate but you don't seem to see boats with big fat head sails raking (Do 49ers and I14's rake?) it should work and they should massively depower with that higher percentage of the rig up high and the fat head sail is more efficient at twisting off than pin heads, so you only seem to hear of 505's and Merlins big in the rake department with lots of raking controls.
Obviously with the amount of rake we had on boards we rarely flattened the sail (it was pretty impossible anyway with our control set and the cams in place) but with a dinghy inevitable the flattening of the sail combined with the rake will reduce the foil efficiency with the resulting depower, the only thing that dinghies lack is the speed increase to reduce the loads that you get if you can actually plane to weather as a result of the various actions.
I've been trying to figure a one string answer like the Merlins have for my own boat, but every time I get a demonstration despite all the pictures I take, I still can't figure it out, it's quite complicated, it was why I was asking about that new raking chain plate Allen were advertising, I thought that might help.

Edited by iGRF - 28 Nov 14 at 9:45am

Originally posted by Oatsandbeans If you dont beleive that mast rake increases bend try it. Keep the same rig tension, a rig tension meter is useful . Drop the mast back incrementally, I normally keep the lowers at the same setting which has the secondary effect of increasing the lower bend as the rig moves back, as well as what is happenning higher up, tie the main haliyard down to the goosneck and look up the mast. The most bend will be at the spreaders. You can eyeball it using the mast depth as a gage. Ie. bend is 1mast width or 3/4 of a mast width ( the best way it to get up on a ladder and use a tape measure). Check the rake with a tape measure to the transom and rake it back 8" from your light wethaer setting. See how much more bend you get -i bet you will get 40mm more bend which is 1/2 a mast width on a nominal 80mm mast section.

Like most others I havn't a clue why raking works, but without doubt it does, I think that it must have something to do with the relationship of the centre of effort of the rig and the centre of lateral resistance of the hull and appendages.  In practice the boat (in my case a Solo) feels more balanced as you rake back as it gets breezy.

Raking rigs in two sail boats such as 505, 470 and 420's works because the centreboard can also be lifted to compensate for the centre of effort moving backwards. In a boat such as a 49er, musto skiff and Int 14 where the daggerboard cannot be moved back then increasing rake to a point where the load on the rudder becomes excessive is not useful. In the 420 with its fixed jib sheet points the raking of the rig also opens the leach where as boats such as a 470 and 505 can remove this effect by altering jib cars etc. There are also other things going on like getting crew weight a bit further back and lifting the bow which is again useful in boats that will plane upwind and are looking to transition from max waterline length to a planing situation where digging the bow in has a slowing effect.   

Bear in mind that with a vertical mast on a singlehander, the quarter-chord line is actually swept forward, so you need mast rake just to have in-effect an unswept wing.