Square Top Sails - Part 2: The Square Top Sail and SAIL TWIST

[Bill Roberts]

Part 2: The Square Top Sail and SAIL TWIST

There are other advantages to the square top sail planform not mentioned so far. These are “sail twist and sail twist control”. We sail on “Relative Wind” direction and speed. Relative wind is the vector sum of true wind speed and boat speed. The true wind speed has a significant speed gradient in it over the first 100 feet above the water. At low elevations near the water surface the true wind speed is slower. As we move up in height above the water the true wind speed increases significantly. Sailing to windward the relative wind direction or angle crossing the boat centerline is smaller near the foot of sail. As we move up the sail, the true wind speed is increasing with height above the water. When we vector sum the true wind speed with the boat speed we find that the wind direction crossing the boat is ever changing in a more abeam direction as we increase our height above the water. Marchaj suggests that a 22 degree incidence angle of wind on the sail is optimum for sailing to windward. In order to hold a constant incidence angle of the wind on the sail the sail leech must twist open as we move up in height above the water. The leech of the sail must twist open in concert with the twist in the relative wind direction as height above the water is increased. The square top sail can do this perfectly. The leech naturally twists open all the way to the tip top of the sail in a smooth continuous twist. There two control lines to affect the twist. Increasing mainsheet tension tends to reduce sail fullness and close the leech. Increasing luff downhaul tension tends to open the leech. In practice, mainsheet tension is used to set the fullness of the sail. For example if the leech is a little too tight, or closed, increasing downhaul tension will open the leech and set the sail twist to the desired position. All of this twist control is possible with the square top mainsail because the leech of the mainsail does not come back to the mast at the top of the sail.

The pinhead mainsail is whole other story with regards to sail twist. The fact that the sail has a pointed head is the primary cause of its sail twist problems. The leech of a pinhead sail will normally twist open from the foot of the sail, clew corner, up to about 60% of the overall height of the sail. This is the region of max mainsail twist for the pinhead sail. The relative wind vector is also twisting open in this region. So far so good, right? Here’s the problem, from this height of max twist to the top of the sail the leech returns back to the top of the mast. The top of the mast is located on the boats centerline which causes the top region of the pinhead leech to also come to boat centerline. This results in the sail twist decreasing while the relative wind vector is continuing to increase. Now the angle of incidence of the relative wind on the sail is increasing, not optimum anymore. Frequently the top telltale on the pinhead sail indicates stall while the rest of the sails leeward side telltales indicate flow attached and moving aft. The top of a pinhead sail always stalls first. On a pinhead sail increasing downhaul tension has a small tendency to increase sail twist but nothing like on a square top sail.

CONCLUSION: The square top mainsail can coordinate with the twist in the relative wind direction that enters the mainsail from the foot all the way to the top of the sail; the same cannot be said for the pinhead mainsail. The square top sail can operate at or very near the optimum angle of attack between the relative wind direction and the sail, for example 22 degrees, at all sail heights. This results in “maximum sail thrust” in a given wind strength. The pinhead sail cannot do this because it cannot be trimmed to match the relative wind angle entering the mainsail from its foot to its top. This results in less than peak thrust coefficients at all sail heights and less than max thrust in a given wind strength.

References: Aeronautical Engineering Text, AIRPLANE AERODYNAMICS by Dommasch, Sherby and Connolly. Also see Sailing Theory and Practice by C. A. Marchaj.

[Matt Haberman]

Here are a few examples of varying degrees of twist in a square top sail.

All photos were taken in 5-7MPH of wind over a period of a few minutes.

The first picture shows the sail with a very straight leech. This picture was made to show the extreme of leech position, minimum twist, rather than to show an excellent sail position and shape.

no twist.jpg

Picture number 2 was made to show very much leech twist. Nothice that the leech continues to twist open all the way to the very top of the sail. The relative wind vector, direction, does something very similar with increasing height of the wind above the water. A pin head sail cannot do this.

considerable twist.jpg

Picture number 3 shows the sail twisting off at the top. This is depower mode. It is achieved by increasing downhaul tension. In more wind strength the sail twist off at the top would be greater. This depower twist off can be varied with downhaul tension.

Depower twist at top.jpg