Control the twist with
1. the mainsheet tension and
2. the downhaul and all sorts of cool things will happen!
Close the leech the hull will fly. Open the leech the hull will lower…
To explain twist.
Speed is all about twist, get the twist right and the cat will move like never before! Too little twist for the prevailing conditions and you’ll feel a very edgy boat. Too much twist will feel like you’re under-powered!
Rig the main up and stand directly behind your boat and a few metres back. Using the mast as a straight edge, sheet the main and off and sight the sail at the same time. You will notice that the top region of the sail moves laterally a lot more than the bottom of the main sail. This is twist and it’s controlled primarily by the mainsheet.
An ‘open leech’ is one that is “twisted” to leeward.
A ‘closed leech’ is one that lines up with the mast.
A ‘hooked leech’ will curl up to the windward side.
Think of it as a valve
If the top of the main sail, twists to leeward it will spill the air pressure and keep the boat level. If the valve closes the pressure will be retained and basically, the hull will fly.
Attaining high boat speed is a matter of the right twist for the prevailing conditions. With the modern squarehead sail things are much simpler. The squarehead wants to twist automatically because the squarehead has created so much sail area high up. In light winds the common problem with sailors new to the squarehead is, over sheeting and creating the ‘hooked leech’ syndrome. It is imperative that you keep an eye on the telltales in this area. If the telltales aren’t flowing nicely, you’ve stalled the top and then you’ll suffer the penalty of drag.
DOWNHAUL and twist
If you have your mast properly set up i.e. with the right degree of prebend etc…when you pull the downhaul tighter you effectively bend the mast ion the longitudinal axis (front to back). This brings the head of the mast closer to the boom. When you let the downhaul go the mast will want to spring to its ‘straight’ profile. This helps to create a loose leech (open) or tight leech (closed).