Raking Rigs

Let's think about the things that move the centre of effort forward in a sail as we get overpowered.
1. Cunningham. The Cunningham is designed to flatten and pull the centre of effort forward. Increased by blading off if the trailing edge higher up.
2. Mast bend. This again drags the chord and therefore centre of effort forward.
3. Easing the sails. Again this brings the centre of effort forward.

I am sure the canoe and several of the other boats Jim sails have the ability to adjust the rake to reset the centre of effort. Like I said, it is complicated. But a fast boat up wind is balanced. You have to re balance the boat with each major adjustment. This can also be done in various different ways. I have already alluded to one way in a two sailed boat which is to depower the jib by increasing twist or easing it.

I Have also suggested its a trial and error issue for each variation of boat and rig, but actually we are achieving similar results by doing different things.

To answer Rupert's question about rake and mast bend. This is a dynamic between rig tension and spreader angle, the amount of forward pressure by the vang or main sheet arrangement, the luff tension, and the low down mast support. As the mast gate is often fixed, or the lowers are often non adjustable, as the mast is raked the vang puts more relative forward motion on the lower mast. Merlin's for example have their lowers involved in the one string taking system to reduce this. Some classes such as the phantom can adjust lowers to get same effect. This makes a huge difference to the mast tip position. On a N12 mast you can get the mast tip to move a good 6 inches by playing with the lower mast supports, hence tuning numbers for initial rake set up are normally given with lowers and mast ram in an unloaded position. By manipulating these you can change where the bend occurs in the mast. For instance more lower tension and a lot of Cunningham encourages higher up bend and tip response. Release both and you get a very unresponsive flat sail with a tight leech. This can be good for pointing.

Bottom line is you have to experiment. But that is half the fun.

"As for the "one bad tack" a well set up boat will allow you to make a lot of bad tacks in my experience and the winners have good boat speed and don't tack too badly!" Why feel the need for sweeping "go practice" like comments? Yes on a gusty & shifty inland water expertise in boat handling can over come an imperfect rig set up. On a big sea course in a 505 with heavy steady winds correct setup would far outweigh an extra ten seconds a tack. There are a lot of different boats out there sailing in a huge range of conditions.

Sorry Oats but when on the water we are not half way up the beat removing clevis pins and adjusting the shrouds! The single handers are just easing the rig back by releasing forestay tension. Rig tension goes to nothing, then pull the kicker on till your ears pop. Generally, unless it's blowing dogs off chains this will be just up the beat or a tight fetch and then, downwind you would ease the kicker and slap some rig tension back on to bring it upright again. So mast bend doesn't come into it.

Originally posted by RS400atC Originally posted by mole It reduces the chord length so ...... Does it?Not sure that's true in a Merlin.

I'm with you 400atC. It amazes me how little people know, but how much they are willing to spout as if they knew it.

Raking the rig increases chord, fact. As to the effect this has, well it really depends on how the mast is supported and how you trim the sail out. Most classes will rake the rig on the shore by dropping the shrouds down a hole or two, and then applying normal rig tension. Lowers will need to be adjusted but differently on various classes due to geometry. Merlins have shrouds, forestay, lowers and kicker all connected, but most of us have to adjust all this stuff independently. It can be done on the water, but most will need to adjust for the day!

Umm, seems like lots of twaddle being spouted.  

What is clear is that moving the mast back a bit doesn't materially increase the chord.   Neither does it decrease weather helm or give vertical lift because the rig needs to be canted to windward fore this, windsurfer style.

It seems the only boats that practice rig raking are medium to high performance boats with swiveling centre boards.   As these boats begin to plane upwind, they need much less area of their boards in the water and the only way to reduce board area is to rake it back.  To re balance the boat, the mast is raked back plus the cunningham is applied to the main sail.  Now the  drive is much lower down and the C of E further back.

I'm not sure but when the cunningham is applied hard and the the top of the main is twisted off , does the C of E move backwards in the remaining, driving part of the rig ?

So, 5o5s, Contenders, Fireballs etc all benefit.  But low performance boats probably don't benefit as they never go quick enough upwind to start tripping over their board.

There is a secondary benefit of raking a mast as it opens loosens the jib leach making it twist but that can be achieved other ways without raking.   Tack height, jib cars etc

Boats with dagger boards are much simpler.  They just lift the board to reduce area and the Cof E just moves up and not backwards.    Therefore, the mast doesn't need raking, just the cunningham applying to to twist the main to lower it's C o E.    The jib is twisted off by lowering the tack. 

One of the great advantages of dagger board boats is the ability to change gear so easily.  As the breeze builds,  pull the dagger board up, pull the cunningham hard on and crack the jib off.    Job done.






Edited by boatshed - 04 Nov 13 at 11:46am