Give the Water Something to Push Against

The sail pulls forwards and sideways. The hull moves through water, which acts on its immersed surfaces. Useful sailing depends on both parts of that exchange.

A boat can point towards a destination without moving directly towards it. Part of the difference may come from slipping sideways through the water; another part may come from the water moving too.

Where the boat points and where it goes

Heading tells us where the bow points. Track through the water is the path the boat actually follows through its surrounding water. Sideways movement relative to the heading, usually towards the lee side, is leeway. A boat can therefore sail forwards while making some leeway, with its water-relative path angled away from the line of its bow.

Current adds another movement. Course over ground, COG, is the direction travelled relative to the Earth. Speed over ground, SOG, measures the rate of that ground-relative travel. Neither necessarily agrees with heading or speed through water.

Take an illustrative boat pointing five degrees west of north. It makes enough leeway to travel due north through the water at four knots. Now let that water move east at three knots. In one hour, the boat moves four nautical miles north relative to the water, while the water carries it three nautical miles east. Its movement over the ground is the diagonal of those two distances: five nautical miles, about 37 degrees east of north.

Its heading is still five degrees west of north. Its track through water runs due north. Its COG lies about 37 degrees east of north, with a SOG of five knots. Leeway accounts for the first directional difference; current accounts for the second.

The wake, the disturbed trail left in the water, can help us notice the boat’s recent movement through it. That disturbed water spreads and moves with the current, so the trail gives only an approximate water-relative track.

The other moving fluid

Water is much denser than air. A relatively small immersed surface can therefore develop a substantial force when water flows around it. A shaped surface used to obtain useful force from water is a hydrofoil. The force can act mainly sideways to resist leeway, or upwards to support the boat. A near-vertical fore-and-aft foil can supply mainly sideways lift; a suitably angled, more nearly horizontal foil can supply upward lift. The surface orientation and incoming flow determine which direction that useful force takes.

Relative to the moving boat, undisturbed water approaches in the opposite direction to the boat’s motion through that water. The sail’s aerofoil has already supplied the basic model. An immersed surface meeting the water at a useful angle deflects flow and experiences a force. Resolve that force relative to the incoming water: its perpendicular component is hydrodynamic lift, and its along-flow component is drag. Resolve it relative to the boat and part of it can oppose the sail’s side force.

For a symmetrical underwater surface aligned fore and aft, a little leeway usually supplies the angle between its chord and the approaching water. It can then push the water towards the lee side while the water pushes it windward. An angled or asymmetrical surface can obtain a useful angle or force differently. Leeway is not a universal requirement for every possible foil arrangement.

The sideways movement and the resisting force can coexist. The blade does not have to stop every trace of leeway to contribute useful support. As a boat settles into steady sailing, the sideways actions of sail, hull and immersed surfaces can balance while the boat continues on an angled track through the water.

That makes travel towards the wind possible on two oblique headings. Sail close-hauled with wind on one side, then turn so that the bow passes through the wind and the wind reaches the other side. The manoeuvre is tacking. The following stretch sailed with wind on one side is a tack, or leg. Alternating those close-hauled legs to make progress towards the wind is beating, also called working to windward.

During the turn the sail’s useful force changes, and after it the underwater surfaces must supply the corresponding sideways response on the new side. The boat needs enough motion and a workable arrangement to complete the change. The sail-corner tack stays attached while the boat tacks onto another tack. Context now gives all four uses a physical meaning.

Give the water force a surface

The hull’s own sides contribute to sideways resistance. Viewed from the side, the projected underwater area is called the lateral plane. That outline helps locate available surfaces, but their forces depend on depth, shape and flow as well as projected area.

A keel can name the main lengthwise backbone along the bottom centreline of a traditionally built hull. Frames, crosswise supporting ribs, and the hull shell connect to that backbone, which helps tie the structure together. In another common use, it names an underwater appendage projecting below the hull and developing lateral force. Such a keel may also carry heavy material to help the boat stay upright. Structure, water force and the use of low weight are three jobs; one component can combine them without making them the same job.

A fixed appendage is always available in its working position, but it contributes to the boat’s draught. A movable board allows some underwater area to be lowered for sailing and raised for shallower water. The choice brings moving supports and changes in the available surface as the board is raised.

A centreboard, also spelt centerboard, usually swings down from inside a centreboard case, or trunk. The case is a box with watertight sides where it passes through the hull; its interior admits the board and water without admitting that water freely into the boat. The centreboard pivot is the axis about which the board swings. A centreboard pin passes through the supporting arrangement at that pivot.

The pin supports and retains the board, while its contact with the case or other bearings can also carry lateral load. The exact share depends on construction. Follow the whole route: water acts on board, board loads its supports and case, and those structures pass force into the hull.

A daggerboard slides approximately vertically instead of swinging. Its daggerboard case guides it and carries contact loads into the hull. The difference becomes intelligible when thinking about movement: a centreboard needs room for its arc, while a daggerboard needs room along its sliding path. Both lose working area below the hull when raised.

A leeboard places the board outside the hull’s side. Its leeboard bracket attaches and supports it there, carrying load into the boat. Paired examples commonly work with the leeward board lowered; particular single-board arrangements can work on either tack. The external mounting can leave the interior free of a central board case, while putting the support and moving board outside the side.

These arrangements remain alternatives. The nineteenth-century sailing boat Annie, preserved by the Mystic Seaport Museum, combines a centreboard with movable ballast. The board provides adjustable underwater area; shifting heavy material changes the weight’s position. They are separately identifiable jobs on one boat. Later craft continue to use fixed keels, sliding boards or external boards where their water depth, structure and handling needs suit them.

Friction has a surface to act on

The immersed hull and any included appendages have a wetted surface, the area actually in contact with water. Fluid-surface friction acts over that area as the boat moves. At the same speed, with comparable surface condition and local flow, adding wetted area generally adds frictional resistance.

That is one part of the total water resistance. Hull shape also changes waves and pressure, and a board or rudder can add other forms of drag. Two boats cannot be ranked by wetted area alone without considering those effects and the loading condition. A shallow body may gain access to a useful harbour while requiring more immersed surface than another shape carrying the same load.

The resistance explains why the sail must keep supplying forward force even after the boat has reached a steady speed.

A blade that can turn the boat

The steering oar already gave a blade the job of changing direction. Fix a movable blade to the hull so it can turn about a defined axis, and it becomes a rudder. The rudder blade is the surface working in the water. A quarter rudder is mounted near one side of the stern; many other rudders work on the centreline. Position and support change how the steering force enters the boat.

Take a small transom-mounted example. A projecting hinge pin, the pintle, fits into a receiving eye, the gudgeon. In this selected arrangement the pintles are carried on the rudder and the gudgeons on the boat. Aligned pairs support the rudder while allowing it to turn. Water force reaches the transom through those fittings.

The upper part receiving the steering connection is the rudder head. A tiller is the lever moved by the person steering. Its connecting fitting is the tiller head. Turning the tiller turns the rudder about its supported axis, just as moving an oar handle turned the blade about its bearing.

With a forward-pointing tiller and the boat moving forwards through the water, push the handle to port. The rudder’s trailing portion turns to starboard. It deflects water towards starboard; the water’s reaction pushes the stern towards port and turns the bow towards starboard. The boat therefore turns opposite the direction in which the tiller’s forward end was pushed. Reversing the water flow changes the working situation, and large rudder angles can disrupt useful flow rather than keep increasing control.

Another arrangement has a blade beneath the hull carried on a rudder stock, also called a rudder shaft. The stock transmits turning movement and load between the steering connection and blade. It passes through the hull within a rudder tube, or trunk, with bearings supporting its movement. The tube provides a housing around that passage; its construction and the fittings at the openings determine how water is kept out.

Rudder post is another term encountered for the rudder’s shaft. In that sense it belongs to the moving rudder, distinct from the stationary sternpost in the hull’s structure.

A helm can mean the steering equipment, the steering station, or the person operating it, as in “the helm asked for less sail”. “At the helm” can locate someone at the operating station; “take the helm” asks them to take control of the steering equipment. Those meanings follow the thing or action being discussed.

The rudder works through water-relative flow. A boat held still over the ground in a current may have useful flow past its blade. A boat drifting with the surrounding water may have very little, even while it moves over the ground. The chosen reference frame matters to steering as much as it did to the wind.

Sail, hull and underwater surfaces now form one working boat. Their forces act at different places: the sail above the water, the boards and rudder below, and the crew wherever they choose to sit. Those positions explain the leaning and turning that a simple list of force sizes would miss.