Agree with what marcus is saying above. I went off line to write this and it has turned out to be a very long post. This is how I do it and what people should consider IMO.

Here is my method for setting up a mast :

1, Spreader length. If you can change it, remember, longer spreaders increase the effects detailed below

2, Spreader Rake, the greater the rake, the less support the mast has laterally as any tension added to the wire(s) bends the mast forward (in the masts plane). Less rake provides more support and so less bend.

3, Spreader (diamond wire) Tension. This needs breaking into 2 parts

3.1 Well raked spreaders, the more diamond tension, the more (initial) forward bend induced by the diamond wires, you have a bent mast - Ergo flat sail (macro adjustment) you will not be able to add fullness without reducing tension in the diamonds, but this will allow the mast to bend more.

3.2 Flat (or nearly flat) spreaders, Again more tension induces more (initial) forward bend, but to a MUCH lesser degree. You also have a mast that will bend less as the diamonds provide more support for the mast (and the loads a sail will apply). Because there is little rake, you will (effectively) only be able to put a small amount of prebend into the mast before the loads on the wires and/or the spreaders cause a failure (I've stripped a bottle screw once).

So, in summary, 3.1 gives a bendy mast with slack(er) diamonds and a bent mast with tight diamonds. Depending on the bend characteristics of the actual mast section this may be what you want.

3.2 gives a stiffer mast that will bend less.

4, Downhaul, The control does 2 things

4.1 It stretches the sail cloth (to some extent) at the front of the sail and so moves the draft forward and opens the leach of the sail.
4.2 It also bends the mast which opens the leach more and slackens the rig tension as the hounds are (a very small amount) closer to the deck.


Consider this (I'd not suggest trying it). Make up a bit of wire (or old bolt rope) and attach your downhaul to it. You will be able to induce more bend into the bendy mast than the stiff one for the same tension on the downhaul, with the raked spreaders and diamond tension, the prebend "gives" you a little for free.

5, Now we need to consider the interaction between 2 different mast setups (bendy and not so bendy)

5.1. Bendy

The downhaul will be easier to pull down with prebend already induced into the mast by tight spreaders, thus (usually) tight spreaders on well raked spreaders in a big wind = a faster boat as it is more docile.

BUT, there is a corresponding loss of power when you need it in the lighter (marginal trap) winds as the mast is bending more and so loosing some power.

5.2. Stiff mast

The down haul will be harder to pull as there is less prebend in the mast, so for the same amount of mast bend, you need more downhaul tension

But, you get the power when you need it.

6, Mast shape

We all have elliptical masts, some are slimmer than others so we have a range of different fore-aft vs. Port-stbd bend characteristics.

Consider sailing up wind with a round mast Bend will be constant until you add the spreaders, then, what ever rotation setting you have, bend will be controlled by the spreader settings (and downhaul and mainsheet).

Consider a totally flat(fore-aft) mast (impossible I know). The fore aft bend (mast) will be small, lateral bend will be high. introduce the spreaders and these will only control bend between the top mounting(hounds) and the bottom mounting. The rotation setting will have a massive effect on mast bend. No rotation at all and no mast bend will be minimal, 90 deg rotation and lots of bend.

7, Traveller, this controls the exhaust of the sail (and the angle of attack on a large scale), Mainsheet controls the leach tension and angle of attack. Mainsheet will also move the shape of the sail as mentioned in previous posts. Wouter has talked about hooked sails (too much mainsheet).

8, Battens also (as said by others) effect sail shape - I subscribe to the stiff top batten group for "Fat head" sails and soft top battens (reversed if windy) for "pin top sails"

9, Sail cut - you can have a sail cut to be very full, or flat.

so we have a collection of different options we can use to control our mainsail shape and the effect of each can be listed in an order of changeability for the conditions



Least (or most difficult to change) first

1, Mast shape - we don't want a low and high wind mast section do we - it would be a pain changing them for each days forecast - if our class rules allowed.
2, Mainsail, you can have a full or flat sail, but we don't want to have a light, medium and heavy wind mainsail do we ?
3, Spreader rake - we can change it off the water by slackening the diamond wires, but it is not a quick job.
4, Diamond tension - we can change this on the water, but it is not a "gust by gust" trim option, it is a set and leave for the race trim.
5, Mast rotation - we can change this whilst sailing, but generally it will be set for the current leg and only changed because of a large wind strength or direction change.
6, Traveller - we will change this at times on the leg, but will not be used as a constant trim adjustment.
7, Downhaul - we will trim this regularly, up to max or min amount, it will be "played" in gusts
8, Mainsheet - we will trim this constantly.

so to set your mast and boat up you need to consider many things !

Finally I set my spreaders flat (usually approx level with the bvack of the mast track) and my prebend so the sail falls down easily (around 20mm on ny I17). And then go sailing.........