Block Heads

Sail shape

Over the past 50 years sailing boats have probably evolved more than in the previous several centuries.  Drag has been greatly reduced, planing has been understood and rig efficiency and drag has been researched.  The wing is in, square head sails the norm, carbon masts are common.  Let’s consider some of our own observations reference these rapidly evolving rig features of sailing boats, and what makes the features genuinely faster, both in theory and reality.

Significantly the advent of windsurfing appears to have triggered a new dimension as to what is possible on the water with a genuine sail driven craft.  Before this a huge breakthrough was made by the Flying Dutchman design – the first sailing boat that was acknowledged to be designed to plane to windward.  This made development of foils critical to furthering performance on all boats, and possibly led to windsurfing designers having the base knowledge and prior information to enable them to achieve extraordinary speeds around a course.  We are going to ignore multi-hulls in this blog, but they themselves have some extraordinary performance, often for slightly different reasons – the main ones being the superior leverage that can be applied to the rig and the hull drag that can be lost from fine entry hulls.  So this blog is concerning our understanding of mono-hull evolution, sail technology and why it can all come together and create the massive performance increases we see in everything from dinghies, through skiffs to IMOCA and Volvo round the world mono-hull boats.

The big breakthrough for windsurfing came in the 90s, when sail designers discovered that mast bend is desirable to sail with, but that in bending the mast destroyed a lot of sail shape.  The “loose leech” was invented, which went totally against all conventional knowledge where the leech tension dictates the height that the boat can sail.  What is overlooked in this conventional knowledge is that once planing the hull becomes largely irrelevant and the foils take over totally in terms of the relationship between center of effort of the sail and center of lateral resistance of the boat.  Convert this to dinghy (critical centerboard position, fully powered rig that you can drive against the foils by trapeze)  from windsurf (raked mast, single fin at rear, feet pushing against fin) – it is a relatively obvious transformation.

So why do dinghies and skiffs still have the ability to make their leeches tight?  Because before planing happens, you need conventional wisdom in sail design in order to gain light wind performance to windward.  Generally we can say that planing happens on a dinghy when the crew is easily able to extend on the wire.  Varies, but it’s a reasonable generalization.

Now let’s look at the rig.  Leech roach is the area of the sail that is aft of the line between the head of the sail and the clew of the sail.  Anywhere outside the triangle formed by that line, the mast and the boom, is called leech roach.  When a sail is designed, how is this roach designed, and what parameters control how it is supported?

First the mast – this controls leech tension in ALL sails.  A softer mast reduces tension on the leech sooner than a harder mast IF the mast is allowed to bend towards the back of the boat.  However, most masts bend sideways a lot as well as backwards – check out the corrupted oval cross section that you see in most masts.  This ensures that fore and aft stiffness is increased as far as possible – a relic from the need for leech tension to go upwind in all weathers.  Skiff masts have begun to be more uniform than this so flexion is in both planes – sideways and backwards as load increases.  With carbon technology we are able to introduce much tighter controls on crew weight and effective mast bends, and have masts made to very precise flex figures.

The other factor involved in leech control of block head sails is the battens.  Because the battens are the only component which supports leech roach, the load they can take before bending away and reducing sail area is a primary factor in tuning of a block head sail.  The majority of modern sails have many full battens.  Not only is the stiffness of the batten critical to control of the leech, but the characteristics of WHERE the batten bends is totally critical.  If the batten is softer at the front than the rear, the leech will remain stiffer for longer.  If the tail is soft and the middle hard, we can begin to not only control leech roach, but also sail shape.

Let’s now look at sail functions.  The primary function is to drive the boat forward.  We can do this by sheeting to the wind, so the entry of the sail is always at the optimum angle.  However, we know this is impossible because velocity and angle vary, and we cannot always predict how, or even sense when this happens.  In the first days of windsurfing, they sheeted the sails out and in as much or more as conventional dinghies.  Suddenly the loose leech meant they could clamp the foot of the sail to the board and rarely have to sheet out – they were generating large amounts of apparent wind and could effectively sheet to this wind by steering with their feet.  The sail did not move significantly compared to the board and rider, and this was found to be the most effective speed tool.  Again – let’s convert this to dinghy sailing.  Active main-sheet was always looked on as being fast, while we find “the groove”.  We need to micro trim, get fast, recognize a change and do everything over again – what we refer to as the cycle of tasks.  Now imagine a sail that can go to “auto trim” mode, so that main-sheet travel is reduced to a minimum.  This leads to greater concentration on other tasks and the knowledge that the sail knows when to spill wind means that full extension of levers (trapeze technique) is possible on a more consistent basis.  On good days it feels easy, because it IS easy, and the sail is in tune with the sailors’ needs.

The implications of this “auto trimming” are massive – on a short handed sailing boat the ability of the rig to deal with its own micro trimming gives a huge return in terms of speed around a course.  The challenges arise in how to set up the sail, especially the head, and how to accommodate different crew weights on what are essentially one design sails such as 49er and 49erFX – obviously alongside windsurfing!  The other aspect – the transition from displacement sail to planing sail, may be more obvious but requires more action and understanding than most sailors currently apply to it.  At Toplevel Sailing we believe that unless the sailor has a deep and meaningful understanding of rig evolution allied to the confidence to respond to the needs of the boat, they will always be vulnerable compared to the sailors with the knowledge, whether this be through experience or hard learning at an early stage of their career.

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