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

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Section I: Part H Wind/Boat Relationships - ‘How a Boat Sails’ Section I: Part H Wind/Boat Relationships - ‘How a boat sails’ © 2014 Advantage Boating Sail Canada Basic Cruising Standard Lesson Outline • Lesson Objectives • Introduction • Apparent wind, true wind and boat wind • Using line diagrams • Summary • Quiz © 2014 Advantage Boating 2 Sail Canada Basic Cruising Standard Lesson Objectives • 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 © 2014 Advantage Boating 3 Sail Canada Basic Cruising Standard Introduction • 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. © 2014 Advantage Boating 4 Sail Canada Basic Cruising Standard True and apparent wind – what’s the difference? • 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. © 2014 Advantage Boating 5 Sail Canada Basic Cruising Standard True and apparent wind – what’s the difference? • True-wind is the wind which you feel when you & your boat are not moving, when at anchor or standing on the dock. © 2014 Advantage Boating 6 Sail Canada Basic Cruising Standard True and apparent wind – what’s the difference? • 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 © 2014 Advantage Boating 7 Sail Canada Basic Cruising Standard True and apparent wind – what’s the difference? • 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. © 2014 Advantage Boating 8 Sail Canada Basic Cruising Standard The impact of apparent wind • 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. © 2014 Advantage Boating 9 Sail Canada Basic Cruising Standard The impact of sailing angles on apparent 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. © 2014 Advantage Boating 10 Sail Canada Basic Cruising Standard The impact of sailing angles on apparent 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 © 2014 Advantage Boating 11 Sail Canada Basic Cruising Standard The impact of sailing angles on apparent wind • 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 © 2014 Advantage Boating 12 Sail Canada Basic Cruising Standard The impact of apparent wind on the amount of sail • 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. © 2014 Advantage Boating 13 Sail Canada Basic Cruising Standard Impact of changes in boat speed on apparent wind direction • 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. © 2014 Advantage Boating 14 Sail Canada Basic Cruising Standard Impact of changes in boat speed on apparent wind direction • 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 © 2014 Advantage Boating 15 Sail Canada Basic Cruising Standard Impact of changes in boat speed on apparent wind direction • 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 © 2014 Advantage Boating 16 Sail Canada Basic Cruising Standard Impact of changes in true wind speed on apparent wind direction • 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! © 2014 Advantage Boating 17 Sail Canada Basic Cruising Standard Impact of changes in true wind speed on apparent wind direction • 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! © 2014 Advantage Boating 18 Sail Canada Basic Cruising Standard Predicting changes in apparent wind • 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 © 2014 Advantage Boating 19 Sail Canada Basic Cruising Standard Using line diagrams to understand wind changes • Simple line diagrams can help you understand how apparent wind changes with changes in true wind and boat wind. © 2014 Advantage Boating 20 Sail Canada Basic Cruising Standard How to use a line diagram ? 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 . © 2014 Advantage Boating 21 Sail Canada Basic Cruising Standard Since we have 3 different winds we represent them with different colours • Legend TRUE WIND – Knots: (Kn) BOAT SPEED – True Wind (TW) – Boat Speed (BS) APPARENT WIND – Apparent Wind (AW) © 2014 Advantage Boating 22 Sail Canada Basic Cruising Standard Example using lines to obtain apparent wind speed/direction • 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 © 2014 Advantagespeed Boating and direction. Knots 23 Sail Canada Basic Cruising Standard What happens if the boat speed increases? • 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 © 2014 Advantage Boating 24 Sail Canada Basic Cruising Standard What happens if true wind speed increases? • 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 © 2014 Advantage Boating bow 25 Sail Canada Basic Cruising Standard Summary • 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. © 2014 Advantage Boating 26 Sail Canada Basic Cruising Standard Next steps! • 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! © 2014 Advantage Boating 27 Sail Canada Basic Cruising Standard Let’s practice! • Complete the following quiz to test your knowledge.
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