Head Affair: Squaretop vs. Pinhead Mainsail

28 Feb 16:16

The German magazine "YACHT" published an article in collaboration with SAPHIRE about the different types of mainsails. I like to follow up on this topic in English language, knowing that English is not my native language. The article is based on my own experience since we use both sails on our Saphire 27 and could gain some good impressions about the potential and differences.

The facts:

Squaretop mainsail, 29m2 on a Carbon mast, Pinhead mainsail 22m2 on an Aluminium mast. Same rig dimensions. Weight of the SPORT version 1341kg with electric engine. Weight of the CRUISE version 1540kg with outboard engine, both versions equipped with the performance keel:

http://www.saphire.swiss/keel

But first some theory

  • Aerodynamic propulsive force
    Is being created due to different static pressures as a consequence of the different flow rates (wing profile)
  • Parasitic resistance
    Form drag and frictional resistance are given facts due to the objects in our living environment (mast; sails, hull etc.)

  • Induced resistance
    Loss of kinetic energy as a result of the curling of an edge vortex at the trailing edge. Is caused by the pressure compensation in lee and windward (lateral forces)
    • The kinetic energy - also called velocity energy - is the energy that an object receives due to its velocity
    • The induced resistance is reducible in magnitude and can theoretically be brought to zero
    • The simplest way to reduce the induced resistance is to extend the wing span
  • Laminar flow
    Adherence of the flow, i.g. if the individual fluid particles move in an orderly manner, one recognizes the tell tails in the headsail and the leech on parallel paths

  • Turbulent flow
    The fluid particles whirl around without order and cohesion

  • To sail fast means:
    A maximum of propulsive force (surface), a minium of resistance (parasitic and induced) and a laminar flow of the wind on the sail.

The A-Catamaran is a open design class, founded in 1956 by the then YRU and has continued to evolve. Many famous sailors such as Dean Barker, Glen Ashby, Bill Slingsby and many others had been using the A-Class catamarans for many years as a training tool.

Before the year 2000, no Squaretop sails were used but the masts were built ever higher with classical sails. At that time over 10.5m high with Pinhead sails while the sails shown here use all masts around the 9.0m. However, the shape has evolved from a slightly flared to a more flared (top > 50% of the foot) and back again. The reason being the higher speed of the foiling boats, which is why the sails are cut down to the trampoline and a surf boom is being used to lower the centre of gravity.


Practice - how does a Squaretop mainsail work?

Trim of the mainsail at the A-Catamaran

A lot of cunningham is being applied, boom close to the centreline, sheet tension just as strong that the desired effect in the upper part of the sail arises, e.g. inverted or with open leech.

Result

In strong winds the upper part of the sail inverts and creates a righting moment!

The A-Catamaran sails 12.5 - 13kn upwinds and as soon as the sail inverts in the upper part, an enormous acceleration effect is created and the boat reaches 13.5 to 14kn upwind (without foililng).

The parasitic resistance remains the same, the induced resistance decreases because the flow is laminar and creates a righting moment from leewards. The entire sail area is used and the edge vortex arises only along the horizontal edge of the sail. If the upper part of the sail is only "emptied", more induced resistance arises, i.g. the edge vortex rolls up and pulls down the leech, the current becomes turbulent. In the following picture you can clearly see that the entire upper part of the sail is inverted and the tell tails are perfectly aligned (laminar flow).

With proper trim you can do without a reef. With the A-Catamaran, there is no reefing function. The helmsman stands already in trapeze at force 1.5 and can still manage to keep the boat with the same righting moment even at force 6 at much higher speed, even though he sails with the same body weight. This can only be achieved by trimming and precise control of the sail. If the sail had full pressure, the A-Catamaran would capsize immediately.

Upwind trim possibilities with a Squartop mainsail

In the picture on the right, there is more wind, the boom is slightly windward, the vang relatively loose and the sail twists enormously, means it opens up completely. There is not enough wind to invert it yet. However, with a small crew, very tight upwind angles can be sailed because the lift is produced by the centred boom position while the middle part of the sail generates propulsion. All tell tails along the leech point to the rear and the boat sails very fast.

In the picture on the left there is less wind, the boom is tighter and on the centreline, while the pressure point of the sail is shifted upwards. This allows you to produce as much pressure in less wind as in the picture on the right. In this case, the boat is not only fast, but reaches very tight upwind angles.

Inverted mainsail

Here, just after the tack, the profile is inverted from the top to the bottom, depending on how much sheet tension you give.

Advantage: the sail never flaps, is always still, the flow is not interrupted, no vibrations and a good righting moment in the top of the mast/sail

• In strong winds, the sail can be used in this way, instead of using a reef
• The induced resistance decreases with increased propulsion

Disadvantage: less sheet tension and thus less tension on the forestay - a drag can arise, because the forestay has too little tension. This could be corrected with backstays, but that makes the boat too complicated and the loss of performance is in the lowest percentage range.

Uwpind wind trim of different mainsails

Clear differences in trim can be seen in the following picture:

Squartop

The squaretop has a so-called straight open leech and can never be closed.

Trim: boom on the centreline, vang prety open, cunningham tight, mainsail open at the top

Pinhead

The Pinhead mainsail has a so-called round, closed leech and the biggest danger is to overlap or close it. In this case, the upper part of the sail closes and produces significant turbulence.

Trim: boom more leewards, vang tight, more sheet tension, leech closed

Comparison

The Squaretop mainsail allows you to sail closer upwind angles in these conditions, while from about 12 to 13kn of wind, the boats sail the same speed and angles. From about 16-18kn wind the Pinhead sails closer upwind angles and the Squartop a little faster. Reason: SQTOP generates less tension on the forestay and the jib starts producing some drag.

Both sails can be set on the carbon as on the aluminum rig. The weight difference of the masts is 12kg, the stiffness is similar. Of course you can tune the carbon masts a bit better in view of the characteristics of the square-top sail, but the differences are not significant and are only perceived in hard racing.

Vortex drag

(Graphic design from YACHT Germany, edition Nr. 21, 2016)

Downwind comparison

The Squartop mainsail is always faster - in all conditions - except in tight reaching angles and strong winds

The vang always remains prety lose and open so that the sail can twist a maximum.

The boom is to be kept not too far leewards. The sail twists in the upper part. The acceleration comes from the middle part of the sail and in the upper part you control the Vortex drag with the tell tails what can be seen on the picture.

Very important is to lose the vang when jibing otherwise you risk to broach.

Helming the boat with a squaretop mainsail

Wrong is the general statement that a Squaretop sail creates more rudder pressure: Due to the straight, open cut and the overall much flatter profile the Squaretop produces no more rudder pressure than the classic mainsail - there is one exception, however: At force 2 the sheet can be so close hauled and the pressure point of the sail so far moved up that in fact pressure on the tiller arises - wanted! Because with these conditions you can then put five people windward and the wind pressure drives the boat at a very tight upwind angle - much tighter than any classic mainsail. As soon as the wind rises, the mainsheet is realeased centimeter by centimeter and at the same time the rudder pressure is reduced and the propulsion increases - exactly what these sails like most!

Square Top sails require more attention of a helmsman. It has been shown that the maximum size of the top should be less than 50% of the foot, so that it can a) be cut well and b) have a good profile. In addition, the use of modern laminates or membran sails present a significant advantage, especially for larger areas.

The helmsman must always helm exactly at the edge of the wind, take advantage of every gust and work with the mainsheet and the traveler continuously. These sails offers a variety of options in all wind conditions, while the classic mainsail is easier to handle and can also be used without a mainshee traveler rail.

Of course, a square top sail brings more fun, but it is aimed at an ambitious sailor who always wants to get the maximum out in all wind conditions, while the classic mainsail forgives much - even steering errors - and thus clearly cruising is the right choice for this sail.