The Set of your Sails

A recent contributor to this Bulletin commented that ‘cruising’ dinghy sailors often had badly rigged boats and/or were incompetent helmsmen. Although I assume that he was not referring to DCA members, it made me think that there are many reasons why dinghy sailors may not appear to be striving for the utmost efficiency when sailing.

1. The cruising sailor’s priority may be for an easily handled boat, so they often let the aerodynamic efficiency of their sails suffer in order to obtain some other advantage, e.g. to help their boat self-steer.

2. The cruiser is usually at the helm for a longer period than the racer. He prefers perhaps to admire the view and relax, to having to concentrate on taking advantage of every small windshift.

3. Pure laziness — and why not?

4. Pure ignorance — as Doctor Johnson would have said.

It is to these latter sailors that I address myself; with apologies perhaps for dispelling their ‘bliss’.

As whole books have been and continue to be written on this subject, I have been compelled to exclude much interesting discourse on technical detail in order to fit this article into a reasonable compass. I have also tried to avoid such things as ‘vector diagrams’ and algebraic formulae, as such things often deter me and so, I presume, others from reading a technical article.

HOW SAILS WORK

In the past it was customary to compare the sail with an aircraft wing in order to explain how it works. Not only is this rather like putting the cart before the horse, but the theory on how both devices worked was based on a misunderstanding of air flow. We will consider only the flow of air over a sail.

Diagram 1

The flow of air approaching the sail is repelled by the higher pressure caused by the angle of attack of the sail and bends to flow around the convex outer side. When it does this it is squeezed between the undisturbed flow to leeward and the curve of the sail. This causes it to accelerate. Such an increase of speed causes a decrease in pressure, a phenomenon discovered by Daniel Bernoulli in 1738 (Bernoulli’s Law). This depression increases the tendency for the approaching air to move to this side of the sail. There is thus less air flowing to the concave side and this flow slows down in relation to the free air stream causing an increase of pressure, by the same law. The negative pressure is the main driving force, sucking the sail to leeward, assisted by the positive pressure on the windward side. To make the most of these forces, the sail has to be angled to the wind so that it holds its shape and at the same time encourages the air to flow round the convex side adhering to its surface. This angle is the critical angle of attack and should be very precise. Beyond two or three degrees of the critical angle the aerodynamic forces deteriorate rapidly.

Diagram 2 Angles of attack.

Not only must the angle of attack be correct, but also the curvature of the sail. If it is too flat there will not be sufficient bending out of the air flow and therefore less squeezing and acceleration. If there is too much curve then the air will not remain attached to the surface of the sail and the air flow will become separated.

HOW THE SAIL MOVES THE BOAT

The magnitude of the negative pressure acting on the sail at any point depends on the relative velocity of the attached air stream at that point. The forces acting on a sail can be shown visually, as in the diagram below, by the length of the arrows, or you can combine them as one arrow to show the resulting direction of the total force.

Diagram 3

If we put this sail on a boat trimmed as shown, we see that most of the force is trying to heel the boat or move it sideways, and only a little to move it forwards. If we change it — as below:

Diagram 4

then there is much greater forward drive. We can see that when working to windward the sail is producing a greater force to move the boat sideways, or to heel it than to drive it forwards. The two things that resist this sideways force are the stability of the hull, produced by ballast and hull shape, and the resistance to sideways movement, produced mainly by the keel or centreboard. The result is that the hull has to move forward. Within reason, the greater the curvature of the sail the greater the drive of the sail: however in strong winds, the sideways force can become too great for the stability of the hull. This limit does not apply in light winds, which is why it is now generally accepted both in racing and cruising, that one has full sails for light airs and flatter sails for stronger winds. We will see how we can modify the sail shape later in this article.

INTERACTION OF TWO OR MORE SAILS

If we add a genoa jib that overlaps the mainsail, we are introducing a stronger leeward barrier to the stream of air passing round the convex side of the mainsail with the result that the airstream is accelerated. This produces a greater decrease in pressure which is the major force for movement. This constricted area is called the ‘slot’, and the desirable result is known as the ‘slot effect’. If the sails are too close the mainsail will be backwinded and will lose much of its drive. If the sails are too far apart then there will be no constriction and the air flow will not be accelerated to its maximum. When using a working jib there is, of course, no constriction of flow as there is little or no overlap. However, the jib directs the air flow towards the lee side of the mainsail in a similar way, increasing its velocity and thus the power produced, although not to the same extent as the genoa.

Diagram 5

You will see from the above that the air flow is also changed in direction, requiring the mainsail to be trimmed inboard. If you were carrying a mizzen you would find that you would have to trim this sail flatter still, as the change of direction of the air flow is cumulative as it passes from one sail to the next. Of course, the jib or genoa create driving forces of their own as well as increasing that of the main. They have the advantage too of not having a mast to disturb the air flow at the luff.

TRUE AND APPARENT WIND DIRECTION

In order to be able to trim your sails correctly, you must be aware of the differences in both direction and velocity of the true and apparent winds. Visualise a motor boat moored on a windless day. Assuming it has a burgee, this will hang vertically down the mast. As soon as it starts to move forward the burgee will fly aft. Imagine now that a wind is blowing from abeam. Whilst the craft is moored, the burgee will blow across to the opposite side of the craft, at right-angles to the fore and aft line, showing the direction of the true wind. As soon as the boat moves forward on a steady course, the burgee will be deflected and will fly at some angle between abeam and aft. This angle will depend upon the relative velocities of the boat and the true wind. If the wind has the higher velocity, then the burgee will fly nearer to the beam; if the speed of the boat is the greater, then the burgee will fly nearer to dead aft. If either the boat or wind speed changes as the boat is motoring along, then the angle of the burgee will reflect this change of what we call the ‘apparent wind direction’. In the above circumstance, not only will the direction be changed but the velocity of the apparent wind will be greater, as part of the boat’s speed will be added to that of the true wind. This is just as true if the craft were a sailing boat being propelled by the same wind. If the sailing boat were to sail with the wind from aft of the beam then the apparent wind would be less strong than the true wind.

When beating, in order to ensure that the sails are producing most power, they must be kept to as near the critical angle to the apparent wind as possible. We normally do this, assuming the sails are already correctly trimmed, by using the tiller to change the boat’s attitude to the wind. The wind is constantly changing in both strength and direction, and the art of sailing to windward is to appreciate and respond to these changes as quickly and accurately as possible. The inexperienced but observant helmsman will notice that most often when the wind changes direction, the change is accompanied by the wind increasing or decreasing in strength. In fact, the short-lived changes in the apparent wind are usually caused by the changes in velocity. Most sailing boats sail most efficiently to windward at about 50 degrees off the true wind; however, because of its speed, the fast boat will bring the apparent wind more ahead and its sails will have to be trimmed more inboard.

You will often notice that when you are sailing hard to windward, boats overtake you on the same courses that appear by the trim of their sails to be sailing closer to the wind than you. However, it their speed through the water that accounts for the fact that their sails are being pinned further in than yours.

If you are sailing steadily to windward and the wind velocity increases, it apparently changes to a direction more from abeam, or ‘frees’, as we say. One can therefore head up more to windward to take advantage of this shift. As the boat accelerates due to the increasing air pressure on the sails, however, the apparent wind heads you arid you have to bear away. When the wind strength fades away the reverse happens as the relative velocities of boat and wind are changed. The wind appears to head you and you have to bear away. As the boat slows up due to the slackening of the wind, the wind appears to free and you can then point up again.

SAIL TRIMMING

Before progressing to sail trimming we have to consider the way that the apparent wind direction varies according to the height above the water. As a broad generalisation it can be stated that due to the skin friction of the earth’s surface, whether water or land, the wind velocity will be less the lower the height. Put simply, the wind will be stronger at the top of the mast than at deck level.

As we have seen, the apparent wind direction is dependant upon the relative velocities of the boat and the true wind. On a constant windward course, if the wind strengthens it will appear to come from further aft or frees, allowing the helmsman to point the boat up further to windward. In a similar way the stronger wind at the top of the mast will appear to be freer than near the boom. This gradient of wind strength will vary according to meteorological conditions, but will always exist. Many people imagine that a sail should be tensioned down to the deck so that it is as near to a hinged door in appearance as possible but in fact, apart from when the wind is from aft of the beam, the natural sagging away of the top of the sail is desirable when controlled. Too much tension on mainsheet or kicking strap is detrimental to performance, as then, much of the sail will not be at the critical angle of attack to the airflow. The same can apply to the headsail if the sheet lead is in the wrong position.

TELLTALES

As air is transparent we need some way of making the airflow visible so we can tell whether our sails are trimmed correctly. For the headsail we need some knitting wool of a contrasting colour to the sail. With a darning or sail needle we pass it through the sail material a few inches aft of the luff wire. We knot it close to the sail and cut it to allow four inches or so to hang either side. Two or three of these telltales spaced equidistantly up the luff will suffice. When the part of the sail at the height of the telltale is correctly trimmed the wool will stretch out horizontally along the surface of the sail on both sides. If the luff of the sail is pointing too close to the wind the airflow will break down on the windward (near) side, and the wool will hang down or even blow forwards. If the sail is at too great an angle to the wind, then the airflow separates from the lee (far) side of the sail and the wool will be affected in the same way.

For the mainsail we need telltales on the leech to show whether the air is flowing off the sail correctly. We can use dark coloured wool again but thin tape is preferable as they will be fluttering in free air and are therefore harder to see. They are also more vulnerable to damage when the sail is being furled. Three will do, sewn to the tabling (hem) of the leech. When the sail is trimmed correctly the telltales will stream out aft. The following diagram will hopefully make it all clear.

Diagram 6

SAIL TRIM — DRAFT CONTROL

Earlier I explained why it was desirable to modify the draft or depth of curvature of the sail to suit the wind strength when beating to windward. This can be important on other courses but it gradually becomes less important the further off the wind one is sailing. In this article we are always considering sail trim when beating unless otherwise indicated. In order to explain how we can do this I will briefly describe how the sailmaker puts the aerodynamic shape into the sail. There are two basic methods which can be used and he can use either or a combination of the two.

a) Rounding the luff and foot

If a mainsail is cut to the shape as shown, when it is put on the straight spars the surplus cloth will be put into the sail as draft and the wind will push it into a curved shape. In the case of a headsail the sag of the forestay or luff rope will affect the draft so the cutting of the cloth may he somewhat different as below.

Diagram 7

b) Tapering the Cloths

This is also known as broadseam, or, in America, seam dilation. The curved seams shown below will produce draft when sewn together.

Diagram 8

Normally the sail is cut so that the point of maximum draft of the foresail is some 40% from the leading edge. This is also true of mainsails meant to stand by themselves. When the latter are intended to be used with a genoa then the point of maximum draft is designed to be further back, even 50 or 55%, depending upon the overlap, because of the slot effect. As the amount of overlap of the genoa will diminish as one goes higher up the mast then the point of maximum draft should move forward. The small amount of overlap of a normal jib will not require so much modification to the draft of the mainsail. One can see from the above that a good sailmaker is worth his cost, especially when racing. One can also deduce that a mainsail cut for use with a jib will lose some of its efficiency when standing by itself. A tiny storm jib can be a good investment for when the situation gets tough. A couple more diagrams.

Diagram 9

Too much draft in the sail is a bad thing when the wind strengthens, for two reasons. As explained earlier it means that there is too much heeling effect; also the point of maximum draft in the sail gets blown aft and the sail loses much of its efficiency. Each sail is designed for a particular wind speed, in the case of headsails this does not matter too much as one normally changes the headsail to suit the conditions and the small amount of draft control available is sufficient. The mainsail however is not usually changed and probably is not reefed in a dinghy until force 4.

The only draft control we have on most dinghy headsails is sheet tension. In light winds the sail will produce more drive if we do not harden in the sheet too much. This will allow the sail to take up a deeper curve. In stronger winds the reverse is the case as we harden the sheet to flatten the sail. Some sails have a leach line. This should be used with great care: tightening it will produce more draft but overtightening it will hook the leach of the sail to windward. It should never be used to stop the leach fluttering; this is a matter for your sailmaker.

The mainsail has two basic controls, foot tension and luff tension. The first is varied by means of the clew outhaul on the boom. This need not be very sophisticated; even a simple clew lashing can have its tightness altered. The luff tension is altered by means of the halyard tension or by hauling down the boom jaws or gooseneck on its slide. In the case of a class racing boat the tension of the luff is usually altered by hauling down the Cunningham hole, a cringle in the sail a short way up the luff, as the sail will usually be already hauled out to its allowed maximum size, indicated by black bands painted on the mast and boom. The effect of stretching the foot and luff of the sail is to flatten it and vice versa. In addition to the above, reefing the sail will usually have the effect of flattening it — more certain in the case of points or slab reefing, less so with roller reefing. The luff tension is the most important control to bring the draft forward again when it is being blown aft of its optimum position by a strong wind. When class rules allow it most racing skippers will consider installing a ‘bendy’ mast, as it is undoubtedly a very efficient method of controlling draft. When pressure is applied by backstay or mainsheet the mast bends forward in the middle thus flattening the mainsail. Great skill is demanded of spar maker and sailmaker when assembling a bendy rig if it is to be efficient in all wind strengths.

TUNING AND TRIMMING — GETTING STARTED

We will assume that the dinghy is afloat, the mast is erected, the standing rigging is correct in that the shrouds are of equal length and the mast is raked and the craft built as the designer intended. If the headsail is hanked to the forestay then the latter should be taut as the luff of the headsail should be as straight as possible. In any case the jib halyard should be nicely tensioned but it is most important if this sail is set flying that the halyard should be of wire (apart from the hauling part). However much tension you put into prestretched polyester it will continue to elongate until breaking point is reached, and the luff will sag into more of a curve than is desirable. There is no need to go to extremes however as sailmakers know that there is a limit to how straight you can get a dinghy’s luff and will cut the sail accordingly. The following are useful rules of thumb to get the draft of your sails approximately correct. When we rig the mainsail on the boom we pull out the clew until wrinkles along the foot of the sail smooth out. If the wind is light we leave it at that, but depending on the wind strength we may give it a little more tension which forms a flute in the cloth along the foot. If we have got it right this will smooth out from the pressure of the wind as soon as we start sailing. If not we can make another adjustment later. We apply the same principle when adjusting the luff of the sail, using halyard and/or downhaul.

SAILING TO WINDWARD

If we have only a mainsail we use the appearance of the luff of this sail to check whether we are sailing at the correct angle to the wind. We haul in the mainsheet until the end of the boom is about over the aft end of the gunwale and then use the tiller to keep the cloth of the luff just not lifting. The sail must be kept full of wind but it is easy to overdo this as the sail remains the same shape even if we sail at a greater angle to the wind. We can check by heading up into the wind a little too much from time to time when we will see the luff lift. The less tiller movement we need to get this effect the closer we were to the critical angle. The other check we have on this is the appearance of the telltales on the leech of this sail. If they fly gaily out behind then the air is flowing smoothly along the lee side of the sail. If they droop or fly forwards then a stall zone exists on the lee side and the boat should be steered more to windward. You can refer to Diagram 2. How much should I pull in the boom, you might ask. You will generally find that the life seems to go out of the boat if you overdo this. A dinghy is a comparatively slow sailing boat and therefore sails at a larger angle to its apparent wind than, say, a 12 metre, even if they are sailing the same course. Also, many expert helmsmen believe that you need not be too fussy about the angle the sails make with the boat. The extra speed you gain by sailing with the sheets eased a trifle compensates for the further distance you have to travel to get to your windward gaol. There is of course an optimum, but only experience or a sensitive speedometer combined with a computer will give you the exact answer. The important thing is to keep the sails at the critical angle to the wind using the tiller.

We now have to ensure that the twist in the mainsail is correct so that the sail is at the critical angle all the way from boom level up to the masthead. This is why we have more than one telltale. The ideal boom control consists of the mainsheet running down from the boom to a slider on the sheet horse which can be locked in any position. This enables one to control the angle the boom makes with the centreline of the boat independently of the downward pressure applied to remove twist. A reasonable alternative to this is a good kicking strap. Failing this sophistication, we have to juggle the tension on the mainsheet and the angle at which we sail the boat to the apparent wind to try to ensure all the telltales fly out aft of the leech.

When we have a headsail the trim of both sails should be adjusted so that the headsail luffs just before the main. Normally the angle the headsail foot makes to the centreline of the boat is preordained by the designer who specifies where the sheet lead should be located. One stage better from a sheet trim point of view is to have the lead on a track which runs in a fore and aft direction. Many racing sailboats have a track on a track which allows athwartship adjustments also. For simplicity we will ignore this last facility. One should also be aware that adjustable luff headsails exist where one can stretch it down the luff wire or stretch tape luffs which are adjusted by tension on the halyard. These luff arrangements of course allow draft control to be more independent of sheet tension. However we are not considering these either. The majority of headsails have only the sheet to adjust them by, using tension and fore and aft direction. The designer tries to position the sheet lead so that the maximum acceptable tension that the crew can apply to the sheet will give correct draft, and foot and leech tensions, in the strongest winds in which that sail will be used. The angle the sheet should make with the clew of the sail depends on the shape. The diagram below shows the principle.

Diagram 10

This does not guarantee that the whole length of the luff will be at the critical angle to the wind for all conditions, or even at all, as the designer cannot allow for all variations of sailmaking, rigging and boatbuilder. This is where the extra telltales come in. One should adjust the sheet lead on its track so that when one luffs, the telltales all fall in at the same time. If the telltale drops at the top first, move the fairlead forward: move it back if it drops at the tack first. If one does not have a track one can still tune the lead for average conditions by refastening the lead or by moving the sail up the forestay a little way by adding a short length of wire below the tack. This last is, of course, only a one way adjustment.

I now come to the combination of headsail and mainsail. They should be so trimmed that the headsail luffs just before the main as one points up towards the wind. This means that the foot of the mainsail is sheeted in to a narrower angle than that of the headsail. We should expect this in any case as explained earlier concerning sails used in combination. We then steer the boat to windward using the luff of the headsail as a reference, not the main. If we are using an overlapping genoa then we have to consider the slot. We have already seen that they should be as close as possible without the main being backwinded by the flow of air off the leech of the jib. We are generally in the hands of the designer and boatbuilder in this; however we can use a Barber Hauler, a device or just a bit of line, to haul the clew of the jib towards the centre of the boat in light winds and smooth water. This narrows the slot and the boom has generally to be hauled further amidships to keep it open: some boats will then point higher without loss of speed. When adjusting a combination of genoa and mainsail we sometimes find that however far we haul in the main boom, even to windward, the luff of the main is still backwinded. It can be for a number of reasons, but generally it is because the draft of the mainsail is too far forward. The sailmaker is usually to blame, if that is the right word, as he may not have realised that the main was going to be used with an overlapping sail. Anyway, one generally has to grin and bear it but check first that excessive tension in the luff has not dragged the draft forward, or that tension on the leech line of the headsail has not hooked the leech to windward thus directing the airflow onto the main. One often finds that the performance suffers very little, even if one can’t cure it.

SAILING OFF THE WIND

When we are beating to windward, we are by definition unable to lay a direct course to our destination. Accordingly we trim our sails using the tiller to make the best course we can. On any other course in relation to the wind we trim the sails using the sheets. Except when the apparent wind is aft of the beam we can use the appearance of the luff or the telltales in the same way as when beating. We may be surprised to find that we have to slacken the sheets to a surprising amount in order to keep the telltales flying horizontal. The most common fault when reaching is to keep the sheets pinned in too hard: racing fleets demonstrate this as clearly as tyro cruisers. The inexperienced helmsman is misled by the pressure of the sheets in his hand and the heel of the boat. One should let the sheets out until the luff lifts or windward telltale drops, and then haul in the sheet until the luff just fills or telltale flies. The sail will then be working at its best angle to produce forward movement.

This article represents only the tiny tip of a very large iceberg. I am well aware that I have not done the subject justice, in extent or efficiency, but I am also aware that it will bore the pants off a large number of people. It has therefore taken up more Bulletin space than is proper. For those who would like to dig deeper into the subject I recommend the following for further reading:-

Sail Power by Wallace Ross Sailing Theory and Practise by CA Marchaj Sails by Jeremy Howard-Williams