Dedekam Illustrated Sail & Rig Tuning

Genoa & mainsail trim, spinnaker & gennaker, rig tuning
1. Auflage 2000
ISBN: 978-1-118-35155-0
Verlag: Fernhurst Books Limited
Format: EPUB
Kopierschutz: 6 - ePub Watermark

E-Book, Englisch, Band 1, 84 Seiten

Reihe: Illustrated Nautical Manuals

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.

Ivar Dedekam, of Dedekam Design, specialises in uniquely illustrated nautical manuals which are easy to understand and remember. Ivar has been sailing for over 30 years and has logged more than 50,000 nautical miles, including three Atlantic crossings. His book Illustrated Sail & Rig Tuning is an international success and has been published in 24 languages.

Autoren/Hrsg.

Dedekam, Ivar

Leseproben

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 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.

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.

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.

Apparent wind (relative wind) is a combination (i.e. the vector sum) of true wind and boatspeed. You can find the apparent wind in this way: Draw the true wind and boatspeed with correct direction and speed (scale used in Fig.7 is 1cm = 1m/s) and draw the parallels as shown. Apparent wind will then be the diagonal in the parallelogram.

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

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

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 away point 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).

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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...