Illustrated Sail & Rig Tuning E-Book

Ivar Dedekam

• Herausgeber: Fernhurst Books Limited
• Kategorie: Lebensstil
• Serie: Illustrated Nautical Manuals
• Sprache: Englisch

Seitenzahl: 140

Veröffentlichungsjahr: 2000

Beschreibung

A beautifully, uniquely illustrated nautical guide to how to trim your sails and rig for maximum performance. With a revolutionary approach, this guide tackles the practical and realistic elements of tuning your boat, plus the theory of sail aerodynamics. This new version has an improved layout and the eBook's functionality has been improved significantly.

Leseprobe

CONTENTS

GENERAL, GENOA & MAINSAIL TRIM

Aerodynamics

Apparent wind

Points of sailing

Sail shape

Trimming devices

Miscellaneous

Genoa trim

Stability

Helm balance

Mainsail trim

Interaction of the sails

Reaching & running

Marking

Miscellaneous

Trim examples

Reefing the main

Problems / Proposed solutions

Summary

Wind instrument & Windex

Setting sails

SPINNAKER & GENNAKER

Types of spinnakers

Equipment & terms

Preparation to set the spinnaker

Hoisting the spinnaker

Spinnaker pole angle

Spinnaker pole height

Spinnaker depth

Spinnaker draft position

Running

Close reaching

Broaching

Steering downwind in heavy airs

Gybing the spinnaker

Taking down the spinnaker

The gennaker

Bowsprit

Sock / Snuffer

RIG TUNING

Rig types

Sideways tuning

Fore & aft tuning

Backstay tension

Tensioning wire & rod

Tensioning the cap shrouds

Pre-bending the mast

Keel-stepped masts

Maximum mast bend

The fractional rig I

The fractional rig II

Tuning under sail

Straighten the mast sideways

Adjusting the lower shrouds

Rigs with multiple spreaders

Tuning diagrams

Miscellaneous

Why pre-tension the cap shrouds?

Index

The Fernhurst Books’ Knowledge Centre has lots of other information (including videos, book extracts & other resources) on sails & rigging – scan the QR code or visit www.fernhurstbooks.com & click on ‘Knowledge Centre’ to find out more

INTRODUCTION

This book will teach you how to trim your sails and rig in the best possible way. Aerodynamics constitutes the theoretical background for most books on sailing, but the reader should not be discouraged if this all seems a little daunting as it is not necessary to have an in-depth understanding of this subject in order to get good results.

Aerodynamics is a difficult scientific field of which few yachtsmen have in-depth knowledge. 'Experts' abound and try to explain sail and rig workings using the latest fashionable theories. Some of these theories are imprecise and do not stand the test of time. Frequently they apply to particular cases but cannot be applied in general. Lessons learnt from experience and observation are usually more valuable than the blind application of some advanced scientific theory that may have a poor link to what you are actually doing out at sea.

In this book I have tried to distil those theories and rules of thumb which are commonly agreed upon in the sailing community. Having said this, I feel strongly that careful observation and common sense are the best shipmates to have on board. What distinguishes this book from the majority is the short, focused text with illustrations. My aim has been to make it easy to find, understand and remember the information you are looking for.

This book has been inspired above all by North Sail's Fast CourseTM, but also by many other books on sail and rig trimming, too numerous to list. No doubt it can be improved and I will be most grateful to anyone who should be motivated to send me corrections or comments.

To the reader and sailor, I wish you good sailing and good luck!

Ivar Dedekam

GENERAL, GENOA & MAINSAIL TRIM

AERODYNAMICS

How can a boat possibly sail to windward? Well, it can’t sail directly into the wind, but it can be driven forwards with the wind 30-45° on the beam.

Hold a strip of paper close to your lower lip and blow along the strip’s surface (Fig.1).

The speed difference of the air on the two sides of the strip will create a suction that will lift the strip of paper upwards.

The same happens when air flows along a sail (or an aeroplane wing) (Fig.2). The shape of the sail forces the airflow on the leeward side to take a longer path than on the windward side. Therefore the air has to increase its speed on the leeward side of the sail, resulting in a lower pressure than on the windward side. (Bernoulli’s principle states that an increase of speed in a fluid flow gives a pressure decrease.)

In effect a sailboat may be 'sucked' through the water due to the low pressure on the leeward side. Conversely a slight increase in pressure will act on the windward side.

The total sail force may be split into two components, namely lift and drag as shown in Fig.3. The lift acts at right angles to the wind and the drag acts in the wind’s direction. Both lift and drag increase with windspeed but drag increases faster. As a consequence, different sail shapes have optimum lift / drag ratios at different wind speeds.

When sailing to windward (beating / close reaching) lift should be maximised and drag minimised. With the wind abaft the beam (broad reaching and running), however, drag works in the right direction and contributes to boatspeed.

Take a look at Fig.4 where total sail force is once more split into two components, in this case drive force aligned with the direction of movement and heeling force aligned with the boat’s beam. The heeling force will tend to push the boat sideways.

To optimise performance sailing upwind, driving force should be maximised while heeling force is kept below certain limits.

An efficient keel is a major factor in a boat’s ability to point high into the wind. This is because it resists leeway (sideways movement through the water). The keel is also weighted to resist heeling.

The keel (and the rudder) act in the water as the sail does in the air. The water stream flows across the keel at an angle due to the boat’s leeway. A lift will therefore be generated. This lift works against and reduces the leeway. On a reach or a run, with little or no leeway, the lift of the keel disappears.

Wind crossing the sail should nearly align with the leading edge at a small angle of incidence. Too large an angle of incidence will cause the air to separate from the sail creating large vortices.

If the point of separation moves too far forward, the sail will lose its lift completely – it will be stalled. The boat will rapidly lose its speed. A turbulent trail of air will then minimise drive and increase heeling force.

If the boat is pointing too high into the wind (small angle of incidence) the sail 'back winds' and may flap in the area near the luff – it will be luffing.

A good sail setting test is to ease the sheet until the sail luffs and then pull it in again until it just stops.

Note that it is more difficult to identify a stalled sail than one that is luffing as the appearance of the sail does not change. Stalling a sail is a common beginner’s mistake.

Generally we may say that the sails are most efficient when they are on the verge of luffing.

Fig.6 shows how any single force can be described by two component forces within a parallelogram. Two component forces can also be combined into a resultant force. This principle is also valid for wind velocities (Fig.7).

You may split a force (or velocity) in any direction you wish so long as the parallelogram principle is followed, as shown in Fig.6 (you don’t need to understand why). It can be very useful to show a force as two component forces. You may then see how this force acts in specific directions (e.g. the split into a driving and a heeling force in Fig.4).

If you are motoring ahead at 10 knots on a dead calm day, you’ll feel a wind of 10 knots (c. 5m/s) in your face. This apparent wind will be equal in strength but opposite in direction to the movement of the boat. The real wind, which we call true wind, in this example is 0 knots.

Note that due to boatspeed the apparent wind comes more from ahead than the true wind.

Exceptions are when heading directly into true wind (motoring) or directly away from it (on a run). Note the large difference in apparent wind strength when beating upwind compared to running (Fig.8). This is the reason why you may feel comfortable when lightly dressed on a boat on a run, while people on another boat going upwind at the same time and place are encountering quite a chill!

APPARENT WIND

True Wind – Boatspeed Wind – Apparent Wind

Note: Boatspeed wind direction is the opposite of boatspeed direction.

Note that examples H, G & F are ordered in complementary symmetry to examples B, C & D (i.e. mirrored). Note also the large variations in strength of the apparent wind, which is the on-board wind you feel in your face. It varies from 0 to 10m/s even if all the boats are sailing at the same speed in the same true wind strength.

POINTS OF SAILING

Apparent wind

True wind

Beat

20-40°

35-55°

Reach

40-135°

55-150°

Run

135-180°

150-180°

The sails must be constantly trimmed to the correct angle of attack. When you change course or encounter a windshift, the sails must be adjusted accordingly.

When bearing away, the sheets must be eased so that the sails’ angle relative to the boat’s centreline increases.

We say bear awayorpoint lower when the boat 'turns away' from the wind. Alternatively we are luffing when turning into the wind. We may also say point higher (head higher).

NB! As we bear away, a beat changes into close reach, reach, broad reach, run and finally a dead run.

Bearing away through a broad reach, you’ll end up running before the wind. The sheets are let further out until it is impossible to keep the jib filled.

You can then either take the jib down or set it to windward by using a whisker pole or a spinnaker pole.

If you bear away until the wind is coming in straight over the stern, you are on a dead run. The main should now be as far out as possible. If the boat is turned further in the same direction, the wind will finally enter the leeward side of the main which should now be led over to the other side of the boat – you gybe. This can be a difficult manoeuvre in heavy winds.

SAIL SHAPE

It is difficult to describe the correct sail shape, but the three most important things to adjust are:

☐ Sail draft (fullness of the sail)

☐ Draft position

☐ Twist (controlled by vang and leech tension)

Sail chord depth or draft identifies the fullness of a sail. An imaginary line from luff to leech is called the chord. Chord depth can then be expressed as the ratio percentage between the maximum draft (d) and chord length (c). Draft stripes or seams in the sail can be used to estimate the depth. It is quite difficult to measure so the cruising yachtsman uses his eyes and experience to estimate draft.

Draft position

The distance (l) from the luff to where you find the maximum draft in the sail is called the draft position. Draft forward gives a lower lift / drag ratio and you can’t point as high as with the draft aft. But it is easier to steer (more forgiving shape) giving a wider groove*.

Therefore draft forward suits rougher conditions and / or a less experienced helmsman. Draft aft gives a better lift / drag ratio than draft forward and you will be able to point higher. The sail will stall more easily if the boat is not steered correctly. Draft aft sail shape is therefore best in easy conditions – medium winds and flat sea.

☐ Draft forward in rough conditions

☐ Draft aft in medium winds and flat seas

The shape of the sail’s entry can be critical especially for the genoa (jib) which has no mast in front of it to affect the airflow. A round entry reduces the pointing ability but is less affected by changes in angle of incidence making it easier to steer. A fine entry allows higher pointing but is less tolerant to changes in angle of attack which makes steering more demanding.

The forestay and halyard tension also affect the fullness of the sail entry as shown on p.21.

*The groove is a narrow course range determined by a combination of your sail trim, boatspeed and pointing ability. Once 'in the groove' your boat comes alive and travels at maximum efficiency.

☐ Round entry – Wide groove, easy steering

☐ Fine entry – Narrow groove, difficult to steer

The groove may be easier to find if it is widened but the flat water pointing ability will be reduced.

Twist

True wind speed increases with height, so the higher above deck level we measure, the stronger it gets (Fig.17). As the boatspeed generated wind is constant with height then vector addition shows that the resultant apparent wind shifts aft and increases with height.

You therefore have to trim the sail in such a way that the wind’s angle of incidence to the sail will be constant from the foot to the head of the sail. This is done by letting the sail fall more out to leeward at the top than in the lower parts, i.e. twisting the sail.

Mainsail twist is primarily adjusted by the kicking strap (kicker or vang), the mainsheet and the position of the traveller on the track (Fig.21).

The twist of the genoa (jib) is adjusted by moving the sail’s sheeting point (Fig.24). It is also affected by sheet tension. (Note: There are other, more subtle, reasons why you may wish to twist the sails which are not discussed in this book.)

Vertical sail shape

The sail should have a little more fullness in the upper parts to be more efficient. This can be difficult to judge and is of more interest to the keen racing yachtsman. Most cruising yachtsmen and many racers don’t try to fine-tune the sails vertically beyond the curvature the sailmaker has built into the sails. But in strong winds the upper parts of the sails ought to be flattened in order to avoid excessive heeling forces. This is a common problem for most yachtsmen and usually more twist will be employed.

Sails

We will not look in detail into how sails are made, but only indicate what gives the sail its shape and depth. The sailmaker gives the luff of the sail a curved shape and / or sews (or glues) the sail together from panels with curved edges.

When you set a sail on a straight mast or headstay, a certain fullness is forced into the sail. The rest of the shape is for you to adjust! The edges of the sails are named luff, leech and foot. The luff of the genoa is usually attached to the headstay and the luff of the mainsail is attached to the mast track. The foot of the main is normally attached to the boom track. Both genoa (jib) and mainsail are always attached to the boat by their three corners:

☐Tack (to the deck or boom)

☐Head (where the halyard is attached)

☐Clew (where sheet / boom outhaul are attached)

NB! Nautical terms can be a bit confusing and often there are two or more words to describe the same thing. Both beating and tacking describe the same activity.

TRIMMING DEVICES

The most common devices used to control the shape of the sails are listed below. On the next page these devices are shown in more detail.

Mainsail

1. Mainsail sheet (mainsheet)

2. Traveller

3. Kicking strap (vang)

4. Outhaul

5. Backstay

6. Mainsail halyard (main halyard)

Genoa / Jib

7. Genoa / Jib halyard

8. Genoa / Jib sheet

9. Traveller (sheet lead, genoa car)

10. Forestay (headstay)

The mainsheet (1) interacts with the traveller (2) to control the boom’s angle to the centreline and the twist of the mainsail. On a broad reach / run the twist must be controlled by the kicking strap (vang) (3) which governs the boom’s vertical angle.

The main halyard (6) and the genoa halyard (7) are used to hoist, keep in place and lower the sails. The halyard tension also affects the position of the maximum draft in the sails (Fig.15). The more you tighten the halyard, the more the draft will move forwards.

The outhaul (4) controls the chord depth in the lower part of the mainsail. The more the outhaul is tensioned the flatter it becomes. The backstay (5) most noticeably bends the upper mast which draws out and flattens the upper and middle sail panels.

The genoa sheet (8) controls the sail’s angle to the boat’s centreline (it also affects the twist of the sail). Genoa twist is most affected by fore and aft adjustments of the genoa traveller (9). The tension of the forestay (10) greatly affects the shape of the genoa and is often altered by adjusting the backstay (5).

MISCELLANEOUS

How sails are made