Sails, Tuning, Masts, Logic
The mysteries of sails are often best unravelled by thinking of them as simple and logical tools. We can go into details of flow, turbulence and aspect ratio, but it doesn’t really make the boat that we are trying to tune any faster. This blog is about understanding the goals of tuning masts and sails, and hopefully will give you some ideas on how to go faster.
One of the main challenges of sailing is to get the correct power from a sail. The power an individual may have will be entirely wrong for a person of different height and weight. This leads us to tuning for our own needs as we improve, and the fundamental advantage to this philosophy is that you can only sail a boat well when you know how to control the power, not when you are fighting the power of the rig, which can happen from around 8-10 knots of wind.
The normal way of thinking about controlling power is very Laser Dinghy based – we have the kicker, Cunningham and outhaul, which we try to use to control all the power. We may ignore, or use factory settings, for deflection, spreader settings, rake and mast step position. The rake and mast step position we will leave to another blog, but we will now show the Toplevel way of tuning, whether its a Laser or a 49er.
The mast is a lever. Yes, it supports the sail, but essentially the higher up the mast the wind hits, the more leverage is put on the boat to capsize, and the more righting moment (hiking or trapezing) has to be put on the boat to keep it upright. When we run out of righting moment we have to ease the sheets (negatively affecting airflow over the sails and losing speed), or drive high (pinching, again negatively affecting airflow) to compensate for our physical inability to keep the boat flat.
Looked at from the bow the effects of wind hitting the sail are these:
The illustration shows that the same amount of wind hitting near the top of the mast will have a much bigger heeling effect than the same strength of wind hitting half way up the mast. This means that when we run out of our own righting moment (hiking/trapping), we must move the place where the wind hits the sail DOWN the mast. Equally, if we can’t hike or trapeze, we must move the wind impact point UP the mast so we can hike, which is faster than sitting in underpowered. This is the basis of setting up sails and masts.
So how do we move the area of sail that takes the wind? It is by twisting the sail at the head, so that the wind slips by the head of the sail when we need to reduce power, and hits where the sail isn’t twisted.
From a perspective of the mast, if we have a mast with a soft top, it will bend and allow the sail to twist, not holding power. Now it will always hold some power unless the downhaul (cunningham) is pulled tight, but it certainly helps to control the power. Equally, if the mast is bent by shroud tension, it helps to allow the top of the mast to blade off in gusts. Tensioning the mast, and setting the spreaders profoundly alters our power control, and indeed the amount of power available.
The other thing to consider is this: A two sail displacement boat needs leech tension to get upwind. A planing boat doesn’t need leech tension, aside from in very light wind when it becomes a displacement boat. We consider this when finding the right combination of kicker and downhaul available.
Essentially the sails on a displacement boat (470 etc) can be thought of as the jib gives speed (yes, honestly!) and the main gives height. Try setting up a two sail boat based on that philosophy, and it’s incredible how much extra speed you can find.
On a planing boat, the sails need to work in harmony of a different sort. The lower part of the sails is a “big wing”, with the air flowing over the leeward side of the main and jib, creating a much bigger chord length. At the top of the sail, above the hounds (where the jib reaches to) is an area of the mainsail that must replicate the shape of the jib – however you have the jib trimmed. Why? Because both areas of the jib and the area of the sail above the hounds are meeting the wind in the same way, which means the shape should be similar to gain the maximum amount of forward drive.
Next week we’ll talk more about twist, and how different sails can deal with changes of velocity. At Toplevel Sailing, we have a structure to help sailors of all ages to understand the basic functions, whilst finding their own bespoke solutions to what previously had been very complex challenges.