Broadly, in all conditions you want to keep the mast pre-bend suiting the mainsail. The controls at your disposal while sailing should allow you to change the shape slightly around this ‘neutral’ position, Flatter or fuller. However if the mast is allowed to bend too much or in the wrong places the mainsail will likely be distorting beyond its design range and become inefficient, unstable and difficult to control.
The objective is to reduce power in the top of the sails first, leaving the bottom sections to keep working. (So bend the top of the mast first). Similarly when it is very windy we are happy for the front of the mainsail to collapse but still have the back half (the leach) working. Otherwise the whole sail flogs and the boat becomes unbalanced and will create a lot of drag.
See this picture of Olympian & UK Olympic coach Penny and Russ Clark sailing a 5O5 in very windy conditions. The boat is very overpowered but the leach of the bottom of the main is still working to provide balance and the boat remains stable.
The top of the mainsail and the front part of the mainsail are clearly not doing anything.
VANG. In lighter conditions, before the boat becomes overpowered, most if not all of the mainsail leach twist is controlled by mainsheet load. The Vang is typically not loaded. (see separate article for end boom sheeting boats).
So before we ease the boom to de-power, the Vang needs to be pulled on to take over controlling leach twist.
Ideally you will have a mark on the Vang system so you know where to pull it to so its just right when you ease the main in the gusts.
When the gust eases and the main is pulled back in, its important to ease the Vang again to fully power the main back up.
You should also have a mark on your mainsheet so you can instantly pull it back to the right spot without having to look around too much. Leaving the Vang on in a lull is likely to cause the mast to bend too much up high which reduces overall power when you don’t need to and will tighten the leach too much so the mainsail stalls.
CUNNINGHAM. This will flatten the sail by bending the mast. Very effective initial power control. Quick to do and easy to reverse for when the wind lessens again.
SHROUD TENSION. Tightening the side stays makes the spreaders push harder and therefore bends the mast. Bending the mast flattens the mainsail. Note that shroud tension will also tighten teh jib luff so it does affect jib shape and may require a slight jib sheeting adjustment to compensate.
MAIN FOOT OUTHAUL. Arguably the least effective control. It does flatten the sail but really only at the very bottom.
MAST RAM. When overpowered, if you do not already have the ram down you should make a point of ramming down so that the mast is straight at deck level. The crew should be able to see this. Having a slight bend at deck level can be good when you need power in light conditions (explain) but when you are trying to de-power its important to get the mast straight at deck level.
RAKE. The above changes will be sufficient when wind speed has increased a few knots but eventually the wind will get to a point that these changes are insufficient. If the Cunningham and side stays are on tight and you cant pull any more Vang on without getting overbend creases in the mainsail or stalling the mainsail leach, then you need to rake the mast. Remember .
When you change rake you have to also change at least mast ram and shrouds to keep the mast prebend correct.
You will also likely have to change jib sheet, vang and sometimes shroud bases to keep everything in balance.
As wind increases further you will rake more and more. Try use Rake as a fairly Macro change. Don’t change it to often. Better to set it roughly right and focus on the easier adjustments . Consider just having, say, 4 rake positions (eg upright, 1/4 back, 1/2 back, 3/4 back and full rake). The Tuning Grids are ideal for this. Estimate the wind strength and then set the rake as per the Grid.
The jib should be sheeted outboard as wind increases and you have to ease the boom off the centre line.
The jib sheet should be on hard enough so the leach tell tale is about to stall or event stalls occasionally. All three luff tell tales should be flying.
In gusts when the boat is overpowered the jib sheet can be eased slightly to introduce more twist in the jib leach and compensate for the boom temporarily eased out board.
work in progress
STEERING TECHNIQUE. The above changes will flatten the sails and depower the rig . However the subtlety to the way you steer and combine rudder movement with mainsheet movement also makes a difference to keeping the bat flat and maximizing VMG (see section on steering technique for more information on this. Work in Progress)
CONTROLLING MAST BEND FOR MAINSAIL POWER
The shape and therefore power in the mainsail is controlled heavily by mast bend. Many things affect how and why masts bend The effect of each of these things depends on how the mast is rigged and differs between types of boat. However even within one Class such as the 5O5 where a lot of freedom is allowed in the rigging the effectiveness of the various controls on mast bend can differ between boats and the mainsail used.
All these things can affect mast bend:
- Spreader length and angle.
- Spreader height from deck level. (Some boats use more than one set of spreaders)
- Mast Ram
- Where the shroud bases are relative to the mast gate and the spreader tips.
- Hound height.
- Trapeze wire attachment height and crew weight,
- Whether you use centre boom or end boom sheeting.
- Where the mast is stepped.
- Whether the mast gate is such that the mast touches it when raked.
- Down wind when the spinnaker is up.
- The angle of the boom to the mast.
See Where Do I Start for more on mast bend an power control