Windward and Leeward Side of Vessel Explained

In navigation, the role of the winds is very crucial. During the Age of Sail, which lasted almost 400 years, wind forces served as the primary means to power vessels, ranging from commercial to military. Over the years, though the wind action in sailing vessels as the basis for propulsion became supplanted by motorised propulsive systems, their effect remains relevant in terms of various aspects ranging from manoeuvring and seakeeping behaviour to resistance characteristics.

1. Understanding Winds and Wind Action

In the context of wind action, there is always an associated direction of wind flow. For instance, we say that it is a northern wind when the wind flows from the geographic north, and is an eastern wind, when the wind blows from the east side.

In practical real-world scenarios, at any particular location in space and time, wind mostly flows from random directions in varying degrees, thanks to the complex interplay of meteorological and physical factors, and the component of the wind force vector that has the highest intensity is mostly felt.

Thus, when we say that there is a ‘nice breeze blowing from the south,’ the component of wind forces wafting from the southern direction is most prevalent, whereas other wind force vectors are present in lesser magnitude.

Moreover, wind action is a very relative concept. In the instance above, we can experience the effects of this southern wind blowing only when we are directly facing the wind, that is, when we are in a position the impact of the wind forces is pronounced. Now suppose, in this case, there is a mountain or a cliff right behind us, and there is another person who is on the other side.

He or she is spared from the effects of the wind action that we experience. However, for instance, when after some time, he or she feels a gust of wind, it is northern wind for him or her, but nothing for us.

2. Windward and Leeward

The understanding of this is analogous to that of upstream and downstream. If a body is floating in a direction same as the water flow, it is said to be moving in a downstream direction, and if it is floating in a direction opposite to that of the water flow, it is said to be moving upstream.

On similar lines, when we move against the direction of the wind, we are said to be moving in an upwind direction, and when we are moving in the same direction as that of the wind, we are moving in a downwind direction.

Hence, coming to the physics of wind action, there are two very important definitions: 1) Windward and 2) Leeward. What are these? The windward side of a body is that side that ‘directly faces the wind.’

On the other hand, the leeward side is that side which faces away from the wind action. For any point of reference, the side that lies in the upwind direction is known as the windward side, and the other side in the downwind direction is known as the leeward side.

3. In Context of Vessels

In vessels, the side facing the direction of wind flow, or more accurately, the direction from where the wind originates is known as the windward side, and the other side that faces away from the wind action is known as leeward.

For instance, if a vessel is going in a north-south direction towards the north, and a gust of sea wind is blowing from the east, the starboard side of the vessel is the windward side, and the portside is the leeward side.

However, consider another vessel heading towards the south on the same route. For that vessel, the port side is the windward and the starboard side is the leeward.

Similarly, now consider that for a vessel, a gust of wind blows from the front side or against the direction of the vessel’s forward path. In this case, the bow region of the vessel is known as the windward and the aft region as the leeward.

Hence, if you stand on the main deck right in front, against the rails and above the prow region, you are essentially on the windward side, and if another person stands in the aftmost region facing the propeller wake of the vessel behind, he or she is well-positioned leeward.

In today’s age of motor ships, wind action on a vessel is only crucial when the magnitude is strong enough, as the air resistance component of the vessel increases on the exposed parts above the waterline. In very rough weather conditions, the ship often needs to change its course such that it wards away the maximum impact of the wind forces.

If the high-intensity wind forces are oriented in a strictly longitudinal sense, that is in a fore-aft orientation, the importance of windward and leeward differs from the situations when their components are at some other angle.

In the former case, if the windward side is in the bow region, the forward surge of the vessel is impacted due to mounting air resistance, and constructively, when the windward side is in the aft or stern region, then the propulsive efficiency of the vessel further increases due to this added force, depending on the intensity.

When any component of the wind force acts laterally or sideways, the course-keeping of the vessel often becomes critical up to some extent.

For example, if high degrees of gale winds act against the port side (port side being the windward and starboard being leeward) of a vessel moving in a definite path, the effects tend to somewhat veer the vessel towards the right. In this case, often course correction by applying rudder moments toward the starboard side needs to be done.

Often during heavy winds, crew and passengers on the weather deck are advised to stay away from the windward side of the ship to prevent injuries and accidents, and any cargo handling or onboard operations involving work and equipment on the windward side of the deck are halted.

From a transverse stability point of view, if high-magnitude wind forces act laterally on a vessel and tend to heel the vessel, the side that goes down with a higher draft is the leeward side. In such scenarios, bilge keels, stabilizers, and other anti-roll devices must work their way against the wind forces acting from the windward side.

3.1 Vessel and landmass

An interesting set of nomenclatures arises when a vessel is in the vicinity of a landmass or shore, based entirely on the physics of reference points. When we talk about a vessel close to a landmass, irrespective of the current time-varying weather conditions, the windward side of the vessel is always historically the side facing the sea or away from the shore. The other side of the vessel facing the land is the leeward side.

Now, to someone on the land at any point on the shore from which the vessel can be viewed, he or she is on the windward side as this side faces the sea. However, to someone on the vessel, the same person on the land is standing on the lee shore side as his position lies in the leeward side quadrant of the vessel’s point of reference!

Things get even more interesting and complicated when the situation in question is an island! Assuming the island as a circular mass for our simplicity, this nomenclature system then becomes partly dependent on the direction of the wind at that instant.

As with the above example, from the vessel’s reference point, the windward and leeward sides of the vessel remain the same. Moreover, the same side of the island facing the vessel remains the lee shore.

However, for the observer on the island viewing the vessel, it all depends on the nature of the wind at that time. If the wind blows from any other direction of the island that is not the same direction as that of the ship, he is standing on the leeward side of the island from the land’s point of reference!

You might also like to read-

• How Ships Sail Against The Wind – Impacts Of Wind Action On A Vessel
• What is Mediterranean Mooring of Ships?
• How Do Ships Survive Storms?
• 6 Common Mooring Methods Used For Ships

About Author

Subhodeep is a Naval Architecture and Ocean Engineering graduate. Interested in the intricacies of marine structures and goal-based design aspects, he is dedicated to sharing and propagation of common technical knowledge within this sector, which, at this very moment, requires a turnabout to flourish back to its old glory.

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