Over the past few seasons it has become clear that many sailors struggling with speed or power simply don't know what their mast settings can do for them. The cunningham is obvious – you pull it to take pressure out of the sail as needed. But the mast is often left untouched, treated as an afterthought.
In reality, mast rotation is one of the most important speed controls on the boat. To use it well, you need to understand what it does and what sail shape you want in each condition. There are no universal numbers or angles that suit every mast and sail, so instead of a recipe, here are the key principles.
The four basic principles
- The more pre-bend you set in the mast, the more effect rotation has on sail shape.
- Rotating the mast further back: the spreaders let the lower section bend forward (toward the bow) while the top section becomes stiffer. Setting cunningham aside for the moment, the sail becomes flatter at the bottom and fuller at the top.
- Rotating the mast further forward: the spreaders make the lower section stiffer forward and the top softer aft. The sail becomes deeper at the bottom and flatter at the top.
- Cunningham flattens the sail, and more so at the top, because the unsupported masthead is freer to bend.
What shape do you want?
Once you understand these effects, the question becomes which shape works best in which conditions.
On flat water, aim for an even profile from top to bottom with good power up high. You want to be able to sheet hard without stalling the back of the sail – sheet on, and point high.
In waves, aim for good power down low with a more open, twisted top. The power drives you through the waves, and the twisted top lets the boat accelerate easily, so you can steer around and keep it moving. You generally can't point as high as on flat water, but the extra drive gives you a much better VMG (velocity made good).
The two most common mistakes
The first: as the wind builds, the sailor pulls on plenty of cunningham but never adjusts the rotation to match. The cunningham flattens the sail – mainly at the top – and opens the leech. The boat may feel fine, but the leech is too open and you can't point high enough. That can work in big waves, but on flat water you want the leech standing straighter, so rotate the mast further back. As in principle 2, this stiffens the top and lets the bottom bend more – powering up the top and flattening the bottom. Now you can sheet on hard and point high with good speed.
The second: in strong wind the sailor pulls the mast back in line with the boom and applies full cunningham. Because the rotation is too far back, the sail becomes too full at the top and too open at the bottom, with a lot of twist. Some sailors believe that much twist is good for strong wind, but once the masthead reaches its maximum stiffness aft, the top simply stays too full. The result is a sail that is twisted too far and carries too much depth for the conditions: the twist costs you height, and the depth adds drag. You end up slower and lower than boats trimmed correctly.
Pre-bend and spreader rake
Spreader rake is another major factor in setting up the rig, and it affects how strongly the rotation angle changes sail depth. It's a subject of its own, so just a few points here.
Think of pre-bend as controlling where the mast bends. The more pre-bend, the lower the mast wants to bend – and the straighter the top section stays. The flatter the pre-bend, the more the top wants to bend. The instinctive reaction is to add pre-bend for strong wind (to flatten the sail) and reduce it in light wind (to add power).
That's broadly right, but it only works when combined with the correct rotation to balance the sail. As with most things, too much or too little does harm. A common error is flattening the spreader angle so far that the mast is almost straight. In light wind this does more harm than good. Heavier sailors also often ask for more luff curve in search of power – but with more luff curve and flat spreaders, light-wind speed can disappear fast. The sail becomes very full down low, with a deep entry angle off the mast and a very closed leech. You may feel a burst of power in the first puff, but the boat wants to fly a hull rather than go forward – it won't point and won't accelerate.
Light wind: often do the opposite
In many cases the better move is the other way around. In very light wind, increase the pre-bend to open the lower sail and reduce the entry angle behind the mast. With the rotation set so the head stands up just the right amount, the boat can be very quick. Remember: flat is fast, deep is slow. Height and power come far more from the sail's entry and exit angles than from raw depth.
A final word on design
Among sailmakers there are many philosophies, and history shows there are many ways to build a fast sail. What matters most is that the sailor understands how the sail should work and manages it well. Class rules on materials and design also shape how a sail has to work.
We design our A-Class sails so the mast can be rotated well back. The mast itself creates a lot of profile across the rig, so as the wind builds it's essential to reduce its windage by rotating back – while the sail flattens rather than fills. That is the secret to success.
Good sailing.
