2.2 Wind Leeway & Calculating Water Track

2.2 Wind Leeway & Calculating Water Track

1. Aerodynamic and Hydrodynamic Principles of Leeway

Leeway is the angular displacement between the heading of the vessel (the direction the bow points along its fore-and-aft centerline) and the actual path the hull tracks through the water body (the Water Track).

For a symmetrical underwater foil (such as a fin keel or skeg) to produce hydrodynamic lift to counteract lateral wind force, water must flow across the foil at an angle of attack. This angle of attack is precisely the leeway angle ($\lambda$). Without slipping slightly sideways through the water, a symmetrical keel produces zero lift. Consequently, every vessel sailing or motoring across the wind experiences leeway.

       Wind Direction ────►
              \
               \   Heading (Centerline)
                \       ▲
                 \      │  \  Leeway Angle (λ)
                  \     │   ▼
                   \    │   / Water Track
                        │  /
                      [Hull]

2. Factors Determining Leeway Magnitude

Leeway varies with the yacht and conditions. Assess these factors rather than selecting an angle from a universal allowance table:

1. Point of Sail (Angle of Wind to Vessel)

• Close-Hauled: Strong sideways sail force and reduced speed in head seas can produce substantial leeway.
• Reaching: The balance of sideways force, speed and keel resistance changes as the wind moves aft.
• Dead Run: With the wind directly astern, little sideways wind force remains. A broad reach still has a sideways component.

2. Speed Through the Water (STW)

Keel hydrodynamic lift is proportional to the square of boat speed ($\text{Lift} \propto V^2$). If vessel speed drops by half, the keel generates only one-quarter the lateral lift for the same angle of attack. To generate sufficient resistance against wind pressure, the hull must slide sideways at a much larger angle. A yacht powering through chop at $6.5\text{ kt}$ may experience only $4^\circ$ leeway, whereas the same boat pinching at $2.5\text{ kt}$ may suffer $15^\circ$ or more.

3. Hull and Keel Profile

• Deep Fin Keels: Efficient underwater foils generally resist sideways motion well when the yacht has adequate speed.
• Long, Bilge or Shallow Keels: Underwater shape and draught affect resistance; the actual allowance must be established for the boat.
• High-Windage Motor Cruisers: Large topsides and little underwater lateral area can cause substantial sideways drift, particularly at low speed.

4. Wind Strength and Sea State

Aerodynamic force increases with the square of wind velocity. Steep head seas striking the windward bow repeatedly knock the vessel to leeward, further compounding slip.

3. Choosing a Leeway Allowance

Record observations in different conditions to improve the yacht's estimated allowance. Treat the result as an estimate and verify the actual track and clearance underway.

4. Determining the Direction of Leeway: Port vs. Starboard Wind

Wind displaces a vessel downwind toward leeward. The algebraic sign applied to True Heading depends on which side of the boat the wind strikes:

Wind on Port Side (Port Tack)

Wind blowing on the port side pushes the vessel to starboard (clockwise):

• Rule: ADD Leeway to True Heading. $\text{Water Track (True)} = \text{True Heading} + \text{Leeway Angle}$

Wind on Starboard Side (Starboard Tack)

Wind blowing on the starboard side pushes the vessel to port (counter-clockwise):

• Rule: SUBTRACT Leeway from True Heading. $\text{Water Track (True)} = \text{True Heading} - \text{Leeway Angle}$

5. Navigational Conversion Hierarchy

Leeway must only be applied to True Heading. Never apply leeway to compass or magnetic headings before correcting for deviation and variation:

$\text{Compass Course (CC)} \xrightarrow{\pm \text{ Dev}} \text{Magnetic Course (MC)} \xrightarrow{\pm \text{ Var}} \text{True Heading (TH)} \xrightarrow{\pm \text{ Leeway}} \text{True Water Track (WT)}$

Navigators can directly verify leeway underway by sighting the vessel's wake with a hand-bearing compass. The angular difference between the wake trailing toward the windward quarter and the reciprocal heading ($180^\circ + \text{Heading}$) equals the real-time leeway angle.

6. Worked Navigational Examples

Example 1: Wind on Port Bow (Port Tack)

• Heading: $068^\circ\text{C}$, Deviation $2^\circ\text{E}$, Variation $3^\circ\text{W}$.
• Wind: North-Northwest ($340^\circ$), blowing on port bow. Estimated leeway: $8^\circ$.

1. $\text{MC} = 068^\circ + 2^\circ\text{E} = 070^\circ\text{M}$.
2. $\text{TH} = 070^\circ - 3^\circ\text{W} = 067^\circ\text{T}$.
3. Wind on port pushes boat to starboard (clockwise $\rightarrow$ add): $\text{Water Track} = 067^\circ\text{T} + 8^\circ = 075^\circ\text{T}$

Example 2: Wind on Starboard Beam (Starboard Tack)

• Heading: $215^\circ\text{C}$, Deviation $4^\circ\text{W}$, Variation $1^\circ\text{W}$.
• Wind: From $300^\circ\text{T}$, blowing on the starboard beam. Estimated leeway: $4^\circ$.

1. $\text{MC} = 215^\circ - 4^\circ\text{W} = 211^\circ\text{M}$.
2. $\text{TH} = 211^\circ - 1^\circ\text{W} = 210^\circ\text{T}$.
3. Wind on starboard pushes boat to port (counter-clockwise $\rightarrow$ subtract): $\text{Water Track} = 210^\circ\text{T} - 4^\circ = 206^\circ\text{T}$

7. Common Exam Traps in Leeway Calculations

1. Applying Leeway to Compass Heading: Because deviation varies with heading, applying leeway early produces erroneous deviation corrections.
2. Confusing Wind Origin with Direction: A "Southwest wind" blows from $225^\circ$. If heading $135^\circ$, the wind blows onto your starboard beam, requiring subtraction.
3. Double Counting on Fixes: A position fix already reflects actual displacement. Allow for leeway when constructing an estimated position or finding a course to steer; do not add it to an observed fix or to the uncorrected DR plot.