Section I: Part H Wind/Boat Relationships - ‘How a Boat Sails’

Section I: Part H Wind/Boat Relationships - ‘How a boat sails’

• Lesson Objectives • Introduction • Apparent wind, true wind and boat wind • Using line diagrams • Summary • Quiz

• In this lesson you will learn the difference between true wind, boat wind and apparent wind. • You will learn how wind speed changes as you move through the sailing circle • You will learn about line diagrams, and how to interpret them

• In the last few lessons you have learned about how the wind and boat work together in order to move the boat through the water. You also learned how to turn the boat and change direction.

• In this lesson you will learn about apparent wind - what it is and why it is so important to learn how it effects boat speed. Without an understanding of apparent wind you cannot develop into a polished sailor.

• First let’s define some terms:

• Boat Wind is generated by the movement of the boat…consider a very still and windless day…hot and humid. If you get in a convertible and drive away…you feel wind in your face. This is ‘boat’ wind caused by the movement of you and the boat through still air.

• True-wind is the wind which you feel when you & your boat are not moving, when at anchor or standing on the dock.

• Apparent-wind is the wind which you feel when you & your boat are moving, and the wind (true wind) is also blowing. • The apparent wind is the combination of the true wind and the boat wind

• Determining the apparent-wind direction, as opposed to true-wind direction, is a skill all sailors should have.

• The reason? You need to know how and why the apparent wind changes to execute proper steering (helming) and proper sail trim.

• In sailing, the sails are usually trimmed for maximum drive or power. • As the boat picks up speed, the strength and direction of the wind on the sails changes because of the impact of the apparent wind. • The effect of apparent wind increases as the speed of the boat increases in relation to the true wind.

Close Reach • When the true wind is Boat speed: abeam, or forward of the 7 knots beam, the combination of boat speed and true-wind velocity produces an True wind: 12 knots apparent wind that has a greater velocity than the true wind and it is forward of the true wind.

• Key Point: As true wind goes aft of the beam apparent wind decreases and is less than true wind but it is still forward of the true wind direction

• As the true wind moves aft, the (APPARENT) wind the boat "feels" is decreased by the speed of the boat. • This decrease in apparent wind is one reason why, under normal conditions, boat speed decreases downwind unless a good deal more sail area is added.

• Check web references to help explain apparent wind:

• http://m.youtube.com/watch?v=yqwb4HIrORM&feature=r elated# • http://www.youtube.com/watch?v=CvSRuer2V_k

• Apparent wind has a significant impact for the amount of sail that can be carried on different points of sail since you trim to the direction and velocity of apparent wind! • Increasing winds cause excessive heel unless sail is reduced in size. • Decreasing winds cause less heel and can slow the boat down too much unless sail size is increased.

•  When the boat speed increases, but the true wind remains the same, the apparent wind moves forward and increases in velocity.

•  If boat speed decreases, and the true wind remains the same, the apparent wind decreases in velocity and shifts aft.

• As boat speed increases, apparent wind increases and moves forward.

• There are two courses of action to take: – trim sails to change in apparent wind and hold course or – alter course to leeward while maintaining trim

• As boat speed decreases, apparent wind decreases and moves aft.

• There are two courses of action to take: – trim sails to change in apparent wind and hold course or – alter course to windward while maintaining trim

• As true wind velocity increases, apparent wind increases and moves aft. There are two courses of action here;

– Trim sails to change in apparent wind and hold course or – Alter course to WINDWARD while maintaining trim – called a lift!

• As true wind velocity decreases, apparent wind decreases and moves forward. There are two courses of action here;

– Trim sails to change in apparent wind and hold course or – Alter course to LEEWARD while maintaining trim – called a knock or a header!

• Changes in the apparent wind can be detected/expected by noticeable changes in true wind by:

– Feeling - a detectable change in velocity or direction on your body

– Observation - 'cat paws' on the water, other boats and their relative pointing ability (boat heading) on the same point of sail

• Simple line diagrams can help you understand how apparent wind changes with changes in true wind and boat wind.

0 ⁰ • In this line diagram we have drawn a line to represent wind direction and velocity.

• In the image at the right – the thick black line is just the vertical axis…the green arrow represents the wind .

• Legend

TRUE WIND – Knots: (Kn) BOAT SPEED – True Wind (TW)

– Boat Speed (BS) APPARENT WIND – Apparent Wind (AW)

• Draw a 5 units (1 unit = 1 knot) long line to represent boat speed (boat 3 wind) of 5 knots 90 ⁰

• Draw a line 5 units (knots) long to 4 3 represent true wind speed of 5 knots 1

• Draw parallel lines for both boat and 7.1 true wind speeds. This creates a 2 90 ⁰ parallelogram • Draw a line joining the corners of the parallelogram and measure its length. Apparent wind This represents the apparent wind speed = 7.1

• Extend red line #1 by 2 units 6 to 7 knots to represent the 5 increase in boat speed (boat 6 wind). 3 90 ⁰ • Make new parallel lines for both boat and true winds. 1 7 4 3 • Draw a new line joining the 8.6 corners of the newly created big parallelogram and measure its length. 90 ⁰ Represents new apparent 2 wind direction and wind speed Apparent wind speed = 8.6 knots and moves towards the bow

• Extend green line #2 by 5 units to 10 knots to 3 9 represent the increase 90 ⁰ in true wind speed. 3 10 4 9 • Make new parallel lines 1 for both boat and true wind speeds. ⁰ 8 2 90 • Draw a new line joining 5 units the corners of the parallelogram and Apparent wind speed = 11.2 measure its length. knots and moves away from

• In this lesson, you learned about boat, true and apparent wind • You also learned how boat and wind speed changes as your sailing angle changes • With the introduction to line diagrams, you also know that changes to boat speed and true wind speed change apparent wind speed and direction.

• In the next lesson, Section 2, Part A, you will focus on the importance of safety gear and equipment.

• Understanding safety issues and how to avoid calamities at sea is a critical part of sailing!

• Complete the following quiz to test your knowledge.

• How can you detect changes in the apparent wind by noticeable changes in true wind? • Fill in the blanks: As true wind velocity ______, apparent wind ______and moves forward. • Fill in the blanks: As boat speed ______, apparent wind ______and moves forward