National 12 - find out more...

Tuning Guide

Here are some tip to get your 12 tuned to perfection. For loads more boat handling and sailing tips have a look at our Coaching Tips and Advanced techniques pages


Do you feel frustrated in your boat? Does sailing fail to bring the satisfaction that it used to? Has the sensual pleasure of yachting paled to a former shadow of its glorious past? Is there something missing? Boat speed? Boat speed is what you get from tuning or, conversely, tuning is how you make your existing boat go faster. So many articles and books have been published on tuning that anyone ought to be able to tune a boat.

The effect of boat speed on sailors is intoxicating. Racing becomes a doddle. Tactics fall into place naturally and the crew's ego travels in a large cloud above the burgee. Every situation is faced with confidence and a dominating ability to escape and continue to the head of the fleet. On the days when the boat is the wrong shape or the rig will not behave, when the boat speed has evaporated the intoxication becomes more like a hangover. Tactical situations invariably lead to an even worse position and the crew's spirits are somewhere about the level of the water in the bilge. Racing let alone winning, is a struggle.

Tuning is about getting many parts of the boat working in harmony. It is the combination of mast stiffness, rig tension, spreaders, sail shape, centre board position & section and rudder (to name but a few) that need to be set up to match each other. With so many variables (and we haven’t even got to the fickleness of the wind yet) it should come as no surprise that tuning is an iterative process. Whilst the coarse rig set up can be achieved fairly quickly, it can take a season to get it perfected as you need to sail in a fleet in all conditions from zephyr light breezes to honking gales.

The ability to tune and change the boat is one of the best features of the National 12. So the best plan is to read this guide, get a basic set up then go sailing. Make notes on the conditions and your performance, analyse the performance then make some changes. Then keep on iterating until it feels right.

There are many books on how sails work and how to get the most out of a boat in general. This guide is written to describe some of the subtleties of “getting the most” out of a National 12.

In the groove

Have you harnessed that fantastic feeling of boat speed?

How the rig works


Here are a few basic points on how the rig works and how it is controlled.

Sail Basics

The rig on any boat is the engine which drives the boat through the water. An understanding of how a sail works is necessary in order to get the best out of your tuning and sail setting. The sails work by generating aerodynamic forces much like the wing of an aeroplane. The lift that keeps the plane in the sky drives the boat forward. The air flow is deflected round the sails and in simplistic terms the greater the deflection (the fuller the sail camber) the greater the amount of drive generated.

If we transpose the pressures arrows on to a boat (figure 2) and note the direction in which we wish to travel, we see an interesting fact. When the sail is set on the centre line with no twist, the after part of the mainsail is not helping the boat’s progress forward and is in fact creating drag.

We need, therefore, to induce or regulate an amount of twist so that we minimize the drag from the sail whilst generating as much drive (deflection of airflow) as possible.

The optimum is to set the sail with enough twist so that the leech is as close to the centreline as possible (thereby obtaining the maximum deflection in the airflow) without the leech of the sail hooking round more (figure 3) than parallel to the centreline of the boat. The twist in the sail will increase as we look up the sail, close to the centreline at the foot and progressively more open. Below is a diagram showing the section of the sail near the foot and at mid height showing the wind direction and the desired leech position,

Note how much flatter the sail section needs to be near the foot and the fullness further forward. Higher in the sail the shape is fuller with the camber further aft (figure 4).

Figure 2 - Aerofoil on a boat

A diagrammatic representation of the drive generated by the wind passing round the sail.

Figure 3

Blue line shows the sail with the leech set parallel to the centreline of the boat

Figure 4 - Sail twist

Diagram shows how the twist in the sail increases from the bottom (green line) to the top (blue line)

Jib Slot

The jib slot is very important in accelerating the air round the leeward side of the mainsail and is paramount in delivering good boat speed. If the slot is too open, the air is not squeezed enough to obtain maximum energy. If the slot is closed too much, the air flow is constricted and the mainsail will backwind. In light /medium winds use the lower luff area of the mainsail as an indicator that the slot is set correctly, a very small amount of panting of the sail should be observed. (figure 5)

  1. Major back winding would indicate that the slot is too closed.
  2. Use the jib sheeting angle and sheet tension to achieve the optimum. Possibly inboard/ outboard adjustments may be necessary.
  3. Use the sheeting angle marks on the jib as below to calibrate your fairleads.

Figure 5 - Jib Slot

Jib slot setting, red line shows the main sail being back winded at the mast as a result of the jib sheeting being too tight

The influence of the spreaders.

The length and angle of the spreaders and, to a small extent, the height of the spreaders will all affect the mast. The spreaders should be long enough to keep the mast straight sideways from deck level to the hounds. If the mast bends to windward, pointing ability will be affected though the boat may be easier to hold upright. If the mast bends to leeward, this will close the slot, affect boat speed and make the boat more difficult to hold upright.

The length of the spreaders control the lateral stiffness and the fore and aft angle of the spreaders control the fore and aft bend. The mast is stiffened sideways by deflecting the natural line of the shroud outward: the further the deflection, the greater the stiffening effect.

The angle of the spreaders affects the mast stiffness. When the spreaders are angled forward and deflect the shroud forward of its natural line the mast is made stiffer fore and aft, when they are angled back the mast is made more flexible.

The objective is use different spreader lengths and angles to obtain the optimum bend characteristic for your crew weight and sail fullness etc. Mast rams / screws, etc, are used to restrict or control overall bend, particularly in the lower region.

Fore and aft bend

Fore aFore and aft bend controls the amount of camber in the mainsail and the tightness of the leech (and therefore the power). The actual amount and position of bend required for optimum performance will vary from boat to boat, also different sails and crew weight will also require different settings. Since the amount of power in the mainsail is controlled by fore and aft bend then it follows that if you are either under or over-powered in windier conditions then you may be able to remedy this with different settings. If the boat is difficult to hold upright (especially when compared with other boats with similar rigs and crew weights) then possibly the mast needs to be softer (move spreaders aft). If you feel under-powered, i.e. that you should be sitting out harder and earlier, or the boat lacks acceleration in puffy conditions, then the mast may need to be stiffer (move spreaders forward). When assessing mast bend it is important that it is done on the water in a sailing situation, in the dinghy park spreader settings don't have the same amount of effect.

Sideways bend

This can only be checked on the water from outside the boat (obviously by another person). When sailing upwind the mast should stand straight between deck and hounds. If the mast bows to windward, this de-powers the rig by opening the jib slot and freeing the mainsail leech, which makes the boat easier to hold up but also leads to lack of pointing ability. Remedy - longer spreaders.

Should the mast bend to leeward, the jib slot is closed and the mainsail leech is tightened. The light weather performance will be adversely affected and, in heavy weather, the boat very difficult to hold up. Remedy - shorter spreaders.

Having found a particular setting for the spreaders on your boat, record the information as

  1. A measurement from the mast to the spreader tip (spreader length) and
  2. The distance from the aft side of the mast to a straight line between each shroud where they pass through the spreader (spreader depth)

In this way it is easier to make accurate changes without having to find a long ladder or suitable roof since the adjustments can easily be checked with the boat on its side. Similarly mast rake should be checked by a measurement from the mast head to the top of the transom (on the centre line) or other suitable datum at the aft end of the boat. Any changes can then be easily done without the problems of trying to level the boat and accurately measure from the mast to a halyard blowing around in a force eight gale!

Jib sheeting position

The jib sheeting position affects pointing ability, the shape of the jib, the size of the slot and the tendency to backwind in the mainsail. Jib sheeting should be fairly steep, the line of the jib sheet if extended forwards and upwards would normally bisect the line of the luff at a point 40-50% up the luff from the tack (It can be useful to mark this jib sheeting angle on to the sail near the clew).

Deck level control

The mast should be restricted in its travel in the mast slot using either chocks, mast ram or strut etc. Having set the boat up with the required mast rake, pre-bend and rig tension the mast position at deck level should be marked and calibrated and the deck level control should hold the mast at the set up position. In strong winds when the rig needs to be de-powered the mast should be allowed to travel forward by around 15 – 20 mm at deck level by removing a chock or changing the ram / strut setting. For boats with struts which can induce pre-bend in very light winds it is usually helpful to increase the nominal pre-bend to flatten off the mainsail as much as possible without having to increase rig tension (Excessive rig tension can distort jib shape in very light winds).

Lowers should only be used to control the mast offwind. The use of lowers upwind can have a detrimental effect on boat speed especially in a large chop. They will keep the mast too stiff and not let the mast absorb the shock as the boat hits a wave, thus stopping the boat.


The outhaulThe outhaul is an important control and therefore should have an efficient slider and purchase system. The normal minimum purchase would be 4:1. The outhaul should be pulled tight in all conditions when going upwind and should be eased considerably (maybe up to 6/7 inches) when sailing downwind, except for running when the outhaul should be pulled tight(ish) again to project maximum sail area.


The Cunningham is used to modify the sail camber and de-power the rig in strong winds. Very little Cunningham tension is used in light/medium conditions as maximum power is required. Once the boat is overpowered the Cunningham can be used to flatten the mainsail and open the leach. Remember that in most conditions you should see slight wrinkles running perpendicular to the mast at the luff. If you tension the Cunningham and remove these creases you will almost certainly have compromised your sail shape and power.


The kicking strap is an important means of controlling leach twist and mast bend and thereby adjusting the power in the rig. It is important that the type of purchase used enables the boom to lift in very light airs and also be capable of bending the mast as required in very strong winds. Many systems don’t give enough range of adjustment. In light winds with the kicker off you should be able to lift the boom around 6/10 inches at the outer end before the kicker pulls tight. In strong winds you should be able to bend the mast with the kicker so that the sail will just start to distort with creases running from the clew towards the spreader bracket (though in practice you may never actually apply this amount of kicker). So that the kicker is easily adjustable a purchase of around 16:1 should be used.


This is also used to control twist in the mainsail as well as controlling the angle of the boom. Remember that tensioning the kicker will bend the mast whereas pulling the mainsheet harder will close the leech without bending the mast to the same extent. Therefore when you need to power up the mainsail in light/medium conditions use the mainsheet in preference to the kicker.


Set up method


There are three stages to the set up process; first start with a coarse set up to get the rig roughly in the right position, then move on to fine tuning and finally optimise the dynamic performance so the rig responds as you want in gusts and various wind conditions.

Coarse set up

Once you have decided what figures to go for, this is how to set the rig up to those settings.

  1. Fix the mast heel position,
  2. Rig the boat and measure the forestay tension. Next increase the forestay tension to the figure decided. This tension is necessary because the jib will normally be cut with a 15mm luff hollow and under average conditions the forestay will sag 15mm if thus tensioned when the jib is sheeted in.
  3. Adjust the spreader aft deflection (this is best done from a garage roof or balcony) and clamp the spreaders in position with a couple of mole grips.
  4. Measure the static mast bend (NB. at this stage the mast must be unrestricted in the fore and aft plan at deck level). The easiest way to measure the static mast bend is to fix the end of the main haIyard to the aft of the mast at the gooseneck, apply enough tension to straighten the halyard and then measure the maximum deflection between the halyard and the mast.
  5. Check the mast rake by setting the boat up level to its water line (a spirit level on top of the plate case is normally good enough here) and using the main halyard as a plumb line measure the rake from the aft of the mast at the gooseneck to the main halyard. If the rake is not enough decrease the length of the shrouds, increase the length of the forestay and repeat the whole exercise again. NOTE: It is imperative that the forestay tension is measured with the mast chocked at deck level and that the spreader aft deflection, static mast bend and mast rake are measured with the chocks removed. It is more important to achieve the correct static bend than the correct spreader aft deflection but normally the two should coincide. Once the correct measurements have been achieved fix the spreaders and check everything again.
  6. Finally with the level chock / mast ram removed mark the neutral mast position at deck level and then arrange fittings so that you can lock the mast in this position for medium wind strengths. For light and strong winds arrange fittings so the mast can be held up to 10mm to 15mm forward of this neutral position to provide pre-bend as necessary.

Coarse set up - side view

Rig measurements: in profile Note: Pre-bend in the mast should be measured without the sail hoisted (as the halyard tension and sail weight will affect the bend)

Coarse set up - rear view

Rig measurements: from rear


Spread measurements: plan view A = Spreader length, measured from centre of wire at tip of spreader to the middle of the mast track. B = Spreader depth, measured from back of mast track to a line stretched between the tips of the spreaders

Fine Tuning - jib

The best way to see how the jib sets and how the slot between the jib and main sail looks is to set your rig in the normal positions then turn the boat on the side and look down the mast. The pictures below show what the jib and slot should look like aned also what it looks like if all it not right...

Perfect set up. Fully powered up good for strong winds on flat water

Correct sheeting - fully powered up for flat water

Front view showing that the jib leech follows a similar curve to the mainsail. At no point is the slot either chocked or too open.

Too tight. If the jib sheeting angle is too close to the centre line, the when the jib is pulled in tight, it results in the slot between the jib and mainsail being closed off preventing the air travelling through. The jib will backwind the mainsail and the luff of the mainsail will lift

Too tight. The jib sheeting angle is too tight and is choking slot

Front view showing the slot being choked. There is very little gap between the top of the jib and the mainsail.

Jib sheeting angle too far aft. If the jib sheeting position is too far aft it results in the bottom of the jib being flattened too much, reducing power, and allows the top of the jib to twist off too much, spilling wind.

Too twisted. The top of the jib is dropping away and the bottom of the jib is too flat resulting in lost power

Jib too flat and twisted as a result of the jib sheeting position being too far back

Good set up for light winds The picture below shows a good jib sheeting angle for light winds. The jib sheeting position is forwards and the sheet is slightly eased to create a small amount of twist.

Nice slot for light winds. There is fullness in the bottom of the jib (note in the previous picture the bottom of the jib is very flat as a result of the jib sheeting position being too far back.

Fine Tuning - mailsail

As with fine tuning the jib, the best way to see how the main sets is to set your rig in the normal positions then turn the boat on the side and look down the mast. The pictures below show what the jib and slot should look like aned also what it looks like if all it not right...

Perfect set up. Enough pre-bend is set to create ideal main sail shape for medium winds. Mast bend matches the luff round of the main. Max draft 1/3 along sail

Main sail powered up for the reach. The outhaul has been let off to power up the foot, leeward shroud released to straighten the mast to power up the middle and top of the sail.

Fully depowered The cunningham has been pulled tight, shroud tension tight, kicker hard on. Sail is flattened off but retains enough power. Max draft about half way back.

Mast too straight, maximum draft too deep and too far forwards, mainsail will back wind, pointing may be affected.

Mast too bent, sail too flat, position of maximum sail depth has moved too far back towards the transom.

Dynamic tuning

Having got the static set up about right it is time to go sailing and check that it works in practise, see where you can improve and then iterate until you can go through the gears with ease and your speed feels good across the range of conditions.

It is always pleasIt is always pleasant to have pictures of your boat in action but photographs are also a useful tuning aid. If you have some influence over the photographer get them to take pictures of the boat - directly broadside from windward and leeward (so you can see the fore and aft mast bend, jib luff sag, mast rake, boat trim); directly behind (to see the sideways mast bend, mainsail leech tightness, boat trim); directly in front (for a view of the jib luff sag, sideways mast bend); and finally from the leeward quarter (to view the jib slot).

Boat tuning is often easier and more enjoyable if you can involve another boat and crew. On the water tuning is often more valuable if you have a pace maker to assess your changes against. Swapping boats and the discussion of the problems involved all adds to the enjoyment.

The two things that can be set up on the water without the aid of another boat or photographer are balance and gust response.

  1. Balance – when sailing flat there should be a small amount of weather helm (i.e you should have to pull the tiller towards you slightly). With the boat heeled slightly to windward (3-5 degrees) the helm should be neutral. The position of the mast foot, amount of rake and board position should be adjusted to achieve the neutral balance. If there is lee helm then the mast foot can be moved backwards, the mast raked further back or at a last resort the board moved further forwards.
  2. Gust response – the rig should de-power from the top of the sail down, so when it is windy and a gust hits the top sail should de-power first. This requires the right amount of twist in the sail and bend in the mast so the sail is flat.

Using tell tales to adjust sheeting angles

There is little that can be done to adjust the overall fullness of the jib or the angle of attack of the luff as these are cut into the sail by the sail maker, though minor sheeting adjustments will make sure that fullness and the angle of attack are balanced all the way up the sail. This balance can be checked using luff telltales which should be fitted to all jibs. Many articles have been written on how to sail with luff telltales but as far as balance up and down the sail is concerned just make sure that all the telltales on the windward side all do the same thing at the same time and the telltales on the leeward side likewise. This should happen anyway if the leech is set correctly. Getting all the telltales to stream parallel is then just a matter of pointing the boat in the right direction.

Remember patience is the watchword and only adjust one item at a time

Photo is a rear view sailing to windward, because of the mast rake the slot between the main and jib is open at the top. The wind is light so the clew outhaul is pulled tight to flatten the bottom of the mainsail. (in the medium winds the clew would be eased 25mm and then pulled tight as the wind strengthened). The mast is chocked 12mm forward of its neutral position giving a nice even mast bend. The overall result being maximum drive with no back winding of the main by the jib.

Photo is taken from the front in medium airs, note that the kicker has been pulled on to achieve a parallel top thus giving maximum power. The boom is eased to give drive and the boat is being sailed dead flat with the transom just immersed to reduce drag.

Photo is taken from the front in strong winds. The jib leech is still tight although it is starting to open up a little at the top. The mainsail is flat at the top and developing twist, this reduces the drag in the sail and also the power.

With heavy weights it should not be necessary to chock the mast forward of the neutral position in strong winds so for medium and strong winds chock the mast neutral and pre-bend by 15mm in light airs.



Choosing your settings


So far the guide has talked about how the rig works and how to set up the rig in a repeatable. Now comes the time for you to decide your settings. You can always copy someone else’s but there are probably some subtleties – your combined crew weight, your sail cut, the bend properties of your mast and the power that your hull design can take. You can start using some settings from the table but it is then best to move on and collect some data and measurements for yourself as described below

Collect some figures

The measurement points below are very useful to help you understand you best settings

  1. Go sailing in the conditions where you are just starting to de-power. Pull on the shrouds so the lee-ward shroud is just starting to pant (i.e just starting to take up the tension). When you come ashore measure the rig tension. This is a critical rig tension as up to this point you are searching for power and after this point you are trying to lose power. So it is a good tension to use as your benchmark.
  2. Perform the tool box test on your mast to understand how bendy it is (see description later)
  3. Measure the luff round in your mainsail.
  4. Adjust the settings to make your mainsail look right across the wind conditions

Standard settings used by various people


P&B Design 8

Dave Peacock’s Baggy


Graham & Zoë




Stevie Sallis


Crew weight (Stone)




18 stone


19 stone

Mast type

Aluminium Proctor Kappa

Aluminium Proctor C


Carbon Angell

Carbon SuperSpar

Aluminium SuperSpar M7

Mast to transom (mm)







Centreboard bolt transom  (mm)







Rake Light – Windy (mm)

200 – 400






Spreader Length (A) – (mm)



320 - 360




Spreaders Depth (B) – (mm)







Shroud deflection outwards (mm)







Shroud deflection forwards (mm)



25 (according to pre-bend)




Spreader height

(mm measured from shear line)







Mast pre-bend  – (mm)


15 – 20

30  (according to sail luff round)




Rig Tension; Light – Windy (Lbs)

350-400 lbs on shrouds

250lB on forestay

200-250lbs on forestay

150-400 lbs on shrouds


enough to keep the shrouds taught when sailing.

Jib sheet (mm from transom)

1800 (to back of track)






Jib sheet (mm from centre line)







[1] Pre-bend should be set to suit the sails, in particular the luff round.
[2] Rig tension is best measured on the jib luff (rather than the shrouds) this is because the shroud tension tends to vary more with different shroud bases (width of boat and distance back from the mast)
[3] Note: The jib sheet position is further forwards on double bottom boats as the floor is higher.

The idea (like most good ones) is blindingly simple. The pole slides up and down the mast on a piece of string that is tied from the spreaders to the jibstick eye or gooseneck. It is launched with a line that runs from the clew of the jib through the length of the pole and then down to a cleat on the deck. Retrieval is simply done by a piece of elastic dead ended at the top of the pole round the spreaders and down the mast to the goose neck area. Velcro on the bottom end of the pole is sufficient to keep it snug to the mast up the beats.

The dangly pole certainly increases offwind boatspeed. The crew needs to adopt a new technique as they are using both the launch string and the jib sheet to control the twist and shape of the jib. (see the technique guide)

Rigging a dangly pole schematic

The dangly pole in action



The key properties of the centreboard are lift (i.e. prevent leeway) whilst minimising drag. To achieve this the area of the board, profile (i.e shape when looking from the side) and the section can all be selected. There are also options for gybing boards, laminar flow boards or standard NACA sections.

The theory of laminar flow foils. Laminar flow foils attempt to prevent the flow going turbulent therefore reducing drag. To achieve this laminar flow boards generally have a finer entry and the maximum width further back (say at 40% of the chord rather than 33% as per the standard NACA sections). Laminar flow sections can operate at a very small angle of attack (<4 degrees) so a National 12 is just in this range with leeway less than 4 degrees. Starting from stationary can be a problem as the flow stalls more easily.

The theory of gybing boards. The gybing board works by the sideways pressure of the water pushing the front edge of the board to windward, twisting in the centre-board case. The theory is that the water increases its angle of attack on the foil and therefore more lift is produced. Typical leeway is 2 to 4 degrees so a gybing board may reduce this. The gybing board is most effective in light to medium air and flattish water. In windy conditions you do not want the centreboard to gybe. Generally the centreboards are designed so that by bringing them up very slightly, part of the aerofoil blade goes inside the centreboard trunk, and jams at full width, stopping the centreboard from gybing.

Aspect ratio. Generally drag on foils is caused by the turbulence at the tip, so for a given area then the longer the foil, the smaller the impact of the end turbulence will be. Higher aspect ratio is therefore better - but you are limited by the class rules.


Standard board

Laminar flow

Gybing board

Rudder section NACA


Max width 40% of chord


Length (below waterline)









25 – 32mm








The key properties of the rudder are to produce a little bit of lift but mainly to turn the water without stalling whilst also minimising drag in a straight line. As with the centreboard the key parameters are area, profile, section (generally NACA). There are options for buoyant rudders which provide static lift or winged rudders which provide lift (or downwards force) at speed.

Buoyant rudders are hollow and can provide lift whilst static in the region of ~10KG. This provides lift at the transom and can be a benefit in light winds preventing the stern dragging. The only downside is the bigger cross section and increased overall area which will increase drag slightly.


Standard rudder

Slim line rudder

Buoyant rudder

Rudder section naca




Approx area





(below waterline)









25 – 32mm







Mast stiffness comparisons and how to measure

The key properties are weight, stiffness and springiness (how quickly the mast drops off in a gust and how quickly it returns). The mast can be well controlled with the spreaders so while the mast section has a certain stiffness the overall stiffness is very much influenced by the whole rig set up.


Carbon is lighter and springier, which gives better gust response than aluminium. Aluminium is stiffer and cheaper. Carbon masts are now as durable as aluminium ones. Production carbon masts are available from Seldon. There are three common aluminium masts available at present. Proctor "C" and SuperSpar Ml sections of medium stiffness and the Proctor Kappa, slightly stiffer for more power downwind, but harder work upwind

  1. The shrouds and rig tension along with ffness all combine to make the perfect rig set up. Some people sail with very stiff masts and do well, others sail with more floppy masts and do well whether sailing with heavy or light crew weights.
  2. The mast must be a tight fit sideways at deck level. We recommend a mast ram or strut at deck level is used to control fore and aft bend upwind.
  3. Limited swing spreaders are a must for maximum speed downwind. The outer tips should move around 40 mm forward when sailing downwind, which in turn will keep the mast straighter.

Table of mast comparisons

Mast bend comparisons

Load = One sailors tool box (18Kg)

Fore-aft (mm)

Side (mm)

 Weight (Kg)

Carbon Original Proctor




Carbon Superspar




Carbon Angel




Carbon (Mike Cooke’s N3489)




Aluminium Proctor Kappa




Aluminium Superspar M1




Aluminium Superspar M7




Note: There is no easy way to measure springiness hence no figures in the table.

The tool box test

The above measurements were taken using the "tool box test" as follows:

  1. Support the mast at its tip and also at the gooseneck
  2. Find a tool box that weighs 18KG
  3. Hang the tool box at the spreaders
  4. Pull the halyard tight from the tip to the gooseneck
  5. Measure the maximum deflection of the mast and measure the position of this relative to the gooseneck.
  6. Take lots of other measurements along the mast and plot this relative to the luff round of the main sail.

The toolbox test in action

Sail Construction

All sails are designed as three dimensional shapes and aren't just flat areas of sailcloth that just "blow into shape". Most of the panels in the sail are designed to give a specified shape and fullness to the finished product.

When you lay a mainsail on the ground it will lay fairly flat with a noticeable convex curve at the luff. You may choose to measure the amount of curve (as shown in the photo). This gives you an idea of how full the sail is (bigger curve = more sail camber) but do remember that this curve is influenced by the bolt rope (in a used sail the bolt rope will gather and cause wrinkling along the luff tape and change the apparent curve). This curve should be called the total fullness curve, as it represents the point at which the mast bend would pull the sail flat and destroy the designed shape.

In reality this curve is the combination of the sail's built in fullness and the actual luff round cut into the sail. In a sail that shows around 150mm of total fullness curve, the luff round would in fact will be far less than this (dependent on the cut it could 50% less) approx 80mm.

The luff round is the key to determining how much prebend your sail will need.

A jib, when laid on the ground would show an S bend convex in the lower half (approx) and concave higher up towards the head. In reality the sail has no convex curve at all cut into the luff, in fact it will have a luff hollow which would normally be around 15-20mm. The apparent lower convexity is the built in fullness distorting what you are observing.

Measuring the fullness of your main sail

Comparing the luff round relative to the mast bend

The purple curve shows mainsail luff round, green curve shows fore aft bend for an Angel mast, both against length

This diagram illustrates the panel shaping, fullness curve with sail laid flat and the true luff round with sail flaked to remove the influence of the built in shape.







Woven Polyester fibre with resin coating to increase cloth stability (resistance to stretch especially on the diagonal / bias).




Contender Polykote;

Bainbridge NYT (now discontinued);

Dimension HTTP +. 

Laminate sailcloth is usually made up of 2 layers of Polyester film, approx 1mm thick, with a scrim of woven fibres sandwiched between.  The weights commonly used Polyester laminate: all the scrim fibres are Polyester.


Pentex Laminate: the scrim fibres are a mix of Polyester and Pentex (Pentex is a modified polyester with very low stretch)


Kevlar laminate: The scrim fibres are a mix of Polyester Kevlar and Spectra


All the laminates rely substantially on the film for the basic stability of the cloth. The scrim adds tear strength to the cloth and supports the film as the loads on the cloth increase. Kevlar gives the highest resistance to stretch, though the fibre does have issues of durability.



Usually 3.9 or 4.46 oz or similar

Twelve sails range from 2.8 to 3.4 oz.


Shape & life

Sail shape changes slightly initially, and later in life more pronounced changes take place and bring about a decline in performance as the finish of the cloth degrades. The power and pointing ability of the boat is reduced as the sail flexes and stretches in stronger winds.


Dacron sails may lose their shape in a relatively short time, but they have a pretty long physical life.

All laminate sails, apart from being lighter in weight, are more consistent in their shape retention during their racing life, but suffer from the film fatigue and delamination, due to constant flexing and fluttering. Consequently their physical life can be much shorter than Dacron.


Ratios and Dimensions

  • Most current Ratios are between 2.2 sq mtr jib - 6.2 sqmtr main or 2.4 sq mtr - 6.0 sq mtrs.
  • Mainsail luff lengths range from 5400 to 5486mm.
  • Jib luff lengths are pretty standardised at 3800mm.
  • Relationship with boom Under the current Class rules the bottom mast band is permitted to be above the boom(check with the current rules). This gives some margin for optimising the rig. A reduction in the luff dimension = a potential increase in the foot length and consequential wider cross width allowances for the mainsail. Having the band above the boom does also enable the loose foot to flip from one side of the boom to the other when gybing etc.




Fit the usual leech telltales below each batten pocket (others can be fitted where you wish, but are not necessary and beware of getting into a ?telltale watching? syndrome) You can mark the boom and the mast with number strips for different outhaul and cunningham settings.

The top batten should be set at one tension ( reasonably tight) for most conditions. If the top batten becomes difficult to gybe in very light winds, ease the tension to allow 3mm of slackness in the pocket and this problem should ease. You can mark the correct tension for the batten with a felt pen on the batten corresponding with the leech.

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Telltales should be fitted about 120/180mm behind th luff (3 sets required) A single telltale can be fitted about a third down the leech where it is visible through the mainsail window (if fitted).

Mark the three sheeting angle lines on the clew ( as shown in fig.6). The fore and aft sheeting position in the boat should be capable of achieving any of these angles (particularly the 40% and 50% angles). Any adjustments should be made with reference to these lines.

You should start with the sheeting at about 45% of the luff. Then check that all the luff telltales react in unison. If the top telltale lifts before the lower ones move the jib sheeting forward (steeper angle towards the 50 % mark) and vice versa.

Using the sheeting angle marks on the jib rather than the jib track position etc is important because, if you choose the change the mast position/rake, the jib clew will rise or fall and throw your Using the sheeting angle marks on the jib rather than the jib track position etc is important because, if you choose the change the mast position/rake, the jib clew will rise or fall and throw your jib track reference out. Also if you are comparing track positions with another boat it is irrelevant if the jib size and geometry is different. If you are experimenting with mast rake you should re-adjust your jib sheeting. Otherwise your change in performance may be due to the sheeting angle change and not the difference in rake.

Check the slot between the jib leech and the mainsail. It should be parallel from top to bottom and the twist/curve in the jib leech should match the mainsail curve. When the setup seems ok on the land, mark the jib sheet as an indicator for the correct sheet tension. The settings should be checked on the water and adjusted as necessary and additional marks used if required for angle/ tension changes.



Black bands


As you know the 12 has a maximum total measured sail area of 8.4 m2. This may be divided however you choose between the main and the jib. This means that you can have any size of mainsail or jib you like provided that their combined areas total 8.4 m2 or less. The way that we measure the area of the sails is also very simple. As you will remember from way back when you were at school (or not so way back) the area of a triangle is half of the length of the base, multiplied by the height. Now, for even the most mathematically challenged, using a calculator to multiply two numbers together and dividing the result by two is not a problem. When we measure the rig all we do is pretend that the main and jib are both simple triangles, find the size of each of them, add them together and voilá.

So where do we get the numbers that we need to multiply together to find the size of these two triangles I hear you all asking; obviously this must be the complex bit, but no…

For the jib:

  1. The length of material measured along the luff of the jib.
  2. The shortest distance from the clew to the luff (usually a point about two feet up from the tack.

For the main - we don't actually measure the sail, we measure the mast and boom bands and insist that the sail must stay within these bands. The bands allow everyone to see at a glance that a boat is being sailed legally.

  • The distance from the bottom of the top band on the mast to the top of the bottom band on the mast.
  • The distance from the inboard edge of the boom band to the mast.

You should look at the rules or ask your friendly local measurer to see exactly how to take these measurements.

Black band calcuations.

This gives you two areas to add together to find out how big your rig measures.

As you can see once you've got a suit of sails the jib area is fairly fixed (but it's amazing what you can do to a jib's area by taking a sharp knife to the head and tack). Because we measure the bands, not the sail, moving the bands on the spars can easily alter the measured mainsail area. When people talk about making the sails "fit the boat" they are moving these bands on the spars.

You can't quite put the bands wherever you want to get the correct mainsail size to match your jib, there are two things to remember.

  • The sail must not be capable of being pulled beyond any band. This could be because the mainsail is not big enough to go that far, or because there is some form of stopper on the spar or in the control line pulling it.
  • The cross widths of the actual sail used must not be bigger than those allowed by the length of the foot (i.e. where you put the boom band).

The cross widths are used to control what we call "unmeasured area" and give the mainsail the profile/aesthetics that it has. As you will remember we measure both sails assuming that they are perfect straight-sided triangles. Obviously they are not, as the foot of the main and jib curve down, some of the jibs have "fat heads" and the leech of the mainsail sticks out in a large roach supported by the top batten. All of these bits combine to make up the unmeasured area. The measured area and the unmeasured area added together give us the actual sail area of approximately 11 m2, which we go sailing with.

To provide some control over the amount of unmeasured area in the mainsail we have something called "allowable cross widths". These govern how big the roach can be and are measured across the mainsail ¼, ½, and ¾?s of the way up the leech. Again look at the rules or ask a measurer to see exactly how to take these measurements. The maximum allowable cross widths on your boat depends on your boom band measurement, the longer the measurement the bigger the allowable cross widths. The actual allowable cross widths for a given boom measurement can be calculated from the formulae in the rules or taken from a table that all the measurers have. So now you can see why it is important that you have marked the bands on your spars, actually it's the bands that govern how big your mainsail is seen to be not the sail. When we measure the mainsail we're only making sure that it can be used with the bands that are marked on your boat.

You can check what the measurements on your boat should be by looking at your certificate. The max luff length, max foot length (both between the bands on the spars) and consequent allowable max cross widths (on the sail) and the resultant maximum jib area are all shown. If you don't have a certificate a new copy for your boat can be obtained by contacting Kevan Bloor on the telephone number in the yearbook.

If you want to alter the numbers on your current certificate (foot, luff, max jib, or allowable cross widths) you need to have it signed off by a maintenance measurer. Remember, it's up to you to ensure that your boat is legal and meets the certificate at all times?

Thanks to:

Tom Stewart

Dave Peacock

Kevan Bloor

Graham Camm

Zoë Ballantyne

Alan Bax

John Sears

Patrick Elcombe

Jon Brown