This little article first appeared in the Europe Dinghy-News. It looks at what the sideways bend of the mast does to your sail. The focus is on cat-rigged, unstayed masts, but I've added a paragraph about shroud supported rigs at the end. 

For an unstayed, cat-rig mast sideways bend means increasing the twist of the sail.

Side bend is an efficient way to depower a normal rig with shrouds, as well. In contrast to the cat-rig, the mainsail of a sloop twists mainly from the leech. Side bend at the luff accentuates the twist.

A mast that is soft sideways makes life a lot easier in a chop - the rig becomes more forgiving and the "dynamic response" to the waves seems to be better.  The same goes for softness fore-aft down low, just above (and even under) the deck.  

... and downs of side bend 

The nuisance with mast bend down low, and particularly side bend is that you loose some pointing ability, especially in mid-air and flat water. Very light air and very heavy air seem to be OK. Side bend at the tip of the mast may be the best solution - it tends to flatten the head without twisting off the sail as a whole.  

To help you to understand what side bend does to the sail, take a look at the mastcam photo with excessive mast bend illustrated on it. If you study the photo for a while, it should be easy to understand that side bend  for an unstayed, cat-rig mast is essentially the same as sail twist.  

While mainsails on rigs with spreaders & shrouds twist from the leech, cat-rigged mainsails like those of the Finn and the Europe twist mostly from the luff as a puff hits, or as the boat hits a wave, the leech of the sail remaining more or less in the same position. This is the "dynamic response" mentioned at the beginning.  

All this is a little more true for the Finn, whose sail has no roach. The Europe sail with its very large roach at the top batten also does exhibit some "normal" twist, as the top batten acts like a lever.  

Aerodynamically, there is no difference whether it is the luff moving to windward or the leech moving to leeward - the angle of attack of that part of the sail is decreased, which essentially is the definition of twist. 

Boats with shrouds & stays 

Side bend works much in the same way as for the cat-rig, but with spreaders you have more to play with. Shorter spreaders allow the mast to bend more easily to the side. As the middle of the mast moves to windward, the slot between the main and the jib is opened, depowering the rig efficiently. 

Many sloop rigged boats use the spreaders in light winds to create "negative" side bend, i.e. to push the middle of the mast to leeward. This introduces negative twist in the middle of the sail (the mid-sections are at a higher angle to the wind) and closes the slot - a very efficient way to add power in the light stuff.

Ice boaters have a very different approach to side bend and twist. They want to mast to bend "the wrong way" (see photo) as much as possible, and as light in a wind as possible. This makes a huge difference in boat speed and pointing for these skimmers that easily fly faster than 70 km/h.  

As pressure increases on the sail, the shrouds (no spreaders) coupled with the mainsheet put a tremendous compression on the mast. Under the compression the mast pops into its peculiar bend, and the iceboat is off. What's the reason for this? 

Killing twist with negative bend 

While sail twist is the friend of the salt water sailor, it is the enemy of the ice boater. With very little resistance between the skates and a good, slippery ice, all drag in the air is detrimental, and twist does no good to the lift/drag ratio of a sail. With the wind pressure at apparent wind speeds around 40+ knots, there is no way to keep the leech from twisting off. 

So, ice boaters use negative mast bend to eliminate twist from their sail (though they probably don't know that themselves!). The mast follows the curve of the sail leech, providing essentially zero twist. Seagulls, as birds in general, do the same - as the feathers at the trailing edge of the wing twist out under pressure, the seagull bends down its wing to eliminate the harmful twist.