Move Mass, Add Foils

The leeward float gained buoyant support by immersing farther from the main hull. Moving the crew to windward changed where weight acted instead. Both could contribute a restoring moment, but through different forces and connections.

A designer can make either arrangement more adjustable. Ballast can change position. An appendage can rise to clear shallow water, or change its orientation to the flow. A lifting surface can carry some of the weight previously supported by the immersed hull. Each movement needs its own explanation: what changes, what supplies the force, and what holds the loaded part to the boat?

Carry water, then decide where it belongs

Movable ballast is ballast whose position can change in use. We already met the sailing boat Annie, whose shifted ballast and centreboard did separate jobs. The ballast changed mass distribution; the board supplied adjustable underwater area. Changing either one did not automatically change the other.

Water ballast is intentionally carried water used for its weight. Its amount can change when water is taken aboard or discharged, and its position can change when water already aboard is transferred. These are different operations with different consequences for the whole yacht.

Fill a tank from the sea while the boat floats at rest. Its mass increases, so it must displace more water. Sinkage, trim, freeboard and the immersed hull shape can change. Where that extra mass is placed determines how it changes the combined centre of gravity. Water added low down may lower G, but its extra weight also changes the buoyant geometry; the earlier same-mass comparison no longer applies unchanged.

Transfer a fixed amount between tanks already aboard and total mass remains essentially the same. G moves towards the receiving tank. In a stated heeled condition, moving that mass towards windward can increase its restoring leverage. The yacht has gained no extra ballast weight during the transfer; it has changed where the existing weight acts.

Pen Duick V was built in aluminium for Éric Tabarly’s single-handed San Francisco–Tokyo race in 1969. The expected following conditions favoured a broad, light afterbody without excessive permanent ballast. In the yard participant’s account, Dominique Paulet describes lateral tanks holding 500 litres each and the difficulty of finding a suitable manual pump to fill or transfer that water.

The Cité de la Voile records about twenty minutes of manual pumping to fill the ballast. For the single-handed sailor, obtaining the useful weight position meant substantial work at the pump. Filling from the sea added mass; moving already-carried water to the other side changed its position. The benefit came with tanks, a water route and the effort to change the loading as conditions required. The published filling time does not time every transfer. Cité de la Voile, history of the yacht.

Even a central ballast tank needs a real route for both water and air. The BayRaider manual describes seawater entering an underfloor chamber through an opening accessible aft of the centreboard case. Its forward-facing self-bailer can act as an inlet in the selected arrangement. Air displaced from the chamber escapes through the opened hatch. Separate draining bailers beside the case provide outlets under the specified moving-water conditions. These are that boat’s fittings, not a reconstruction of Pen Duick V’s pipes.

A partly filled tank brings back the free-surface effect. As the boat heels, water can move towards the tank’s lower side, changing G and reducing restoring action compared with the same mass secured in place. A retained one-sided load can also produce a list after the sail’s heeling force disappears. List describes the persistent inclination; free surface describes the liquid’s freedom to redistribute. A central part-filled chamber can have the latter problem without a permanent one-sided load.

Hiking and trapezing remain useful comparisons. Both move crew mass outboard, but the boat’s side or hiking support carries one sailor while a harness and wire carry the other through the mast and hull. A ballast tank carries its water through its walls and supports. The common gravitational benefit does not give these arrangements interchangeable support systems.

Move the lead sideways

A fixed fin and bulb already put dense ballast below the hull. A canting keel lets that ballasted assembly rotate sideways about a broadly fore-and-aft axis. The bulb can move towards windward while remaining attached to the yacht. This differs from a centreboard swinging fore and aft around a transverse pin.

Hold the same yacht, carried ballast mass and chosen heel condition in the comparison. Canting changes the ballast’s position, including its height as it follows an arc. The whole-yacht G moves by less than the bulb itself. Its new position can increase restoring leverage, but the result depends on the complete geometry. There is no rule that an extra degree of cant supplies the same improvement at every heel.

Lead has not become heavier because it moved sideways. Nor has the fin stopped meeting the water. Cant changes the fin’s orientation and the components of its hydrodynamic force. A separate daggerboard may then carry more of the sideways-force job in a useful orientation. The canted fin, hull, rudders and other surfaces can still share the load. Experiments comparing canting-keel configurations investigated that continuing contribution; their result does not make a daggerboard compulsory on every such yacht.

The moving connection is substantial machinery. In the Cariboni arrangement, fore-and-aft bearings locate the keel’s rotational axis. Hydraulic rams connect the moving head to structure between bulkheads, with longitudinal plates and a supporting floor joining the installation into the hull.

Fluid pressure moves a piston and changes the ram’s length. The resulting push or pull turns the head about its bearings; those bearings and the ram attachments receive the reactions.

Bulb weight, fin water load and inertial loads when the moving assembly accelerates or decelerates still reach the hull. The central space around this particular head can contain seawater while rubber sealing elements separate it from the dry ram spaces. A seal controls that boundary; a bearing carries its assigned load while allowing rotation. Calling the whole assembly hydraulic does not explain either job away.

Movement also needs limits and retention. A mechanical stop meets a moving part to bound its travel; a lock restrains a chosen position. These differ from a controller merely ceasing to request movement. The IMOCA rule approved in April 2026 explicitly distinguishes mechanical cant stops from hydraulic or electrical cut-offs and requires a centre lock and manual operation from inside. Those dated class requirements describe separate functions, not one universal private circuit or a guarantee of recovery.

Canting keels were racing in the 1996–97 Vendée Globe. In 2013 IMOCA chose to standardise keels partly in response to reliability and cost concerns; its 2015 implementation account explicitly includes the canting system. The moving keel and its machinery now had a shared manufacturing and checking regime. Fixed ballast remained a rational alternative where the benefit did not repay the machinery, maintenance and operating work.

Change depth by a different motion

A shallow harbour asks for the lowest point to move upwards. A lifting keel is an arrangement that can raise its keel member to reduce draught. If that member carries ballast, it raises the mass as well as changing underwater area. The description identifies the useful change of depth; it does not by itself specify a sliding guide or a pivot.

Drop keel commonly describes an approximately vertical moving keel lowered from a housing. The name overlaps with lifting keel in actual usage. The builder and designer call the 2008 Nordic Falcon a drop-keel cruiser and describe its ballasted lifting keel as a way to reach shallow waters. Its remaining rudder depth limits the useful reduction. That named project establishes an application and overlapping terminology, without proving that every drop keel carries the same ballast or mechanism.

A keel box, or keel case, is the enclosing and supporting structure provided for a moving keel. The names often overlap. To understand a particular assembly, identify the surfaces that guide it, the contacts that carry load and the walls that manage water. A case is more than empty space into which the keel disappears.

Cariboni’s vertical lifting assembly makes those contacts visible. Its keel head slides in guides. A hydraulic cylinder is flanged to the moving head, with its other support at the top of the trunk. Changing the cylinder’s length raises or lowers the head and the fin below it. This moving-body arrangement differs from the fixed-foot steering cylinder in the rudder chapter.

At the deployed head, four lower pins provide specified contacts into the box. Guides restrain unwanted sideways movement; separate locking cylinders provide retention and constrain movement in the raised state. The cylinder performs the lifting job, while those contacts determine how other loads enter the enclosing structure. The manufacturer’s grounding diagram separates the reactions at pins and guides from the cylinder’s compression duty. It would be wrong to assign every impact and sailing load to the lifting ram alone.

An internal trunk needs height for the moving head and can occupy accommodation. That is a cost of the selected arrangement, rather than a universal consequence of retraction. Pino Koc’s analysed telescopic keel puts the fixed housing wholly below the hull. It is bolted to the hull structure, and a ballast-bearing portion slides within it on guides. A cylinder joins the fixed and moving portions.

In that particular mechanism, deployed cylinder compression presses the sliding assembly against a stop and closes the designed contacts. During lifting, the cylinder instead takes the moving assembly’s axial weight and inertial loads in tension. The housing remains outside the cabin, although it still needs a strong hull attachment and has its own depth and travel limits. A heavy-looking metal assembly has become a seawater-exposed mechanism with sliding surfaces, not an indestructible fixed casting.

Retractable keel describes the ability to withdraw the working appendage towards its stowed position. It does not specify that all of it enters the hull, nor that it moves in a straight line. Catalina’s 1987 manual calls its rotating arrangement retractable. That example is a swing keel: its after end follows an upward arc about a transverse pivot near its forward end.

The Catalina 22 drawing follows the actual hoisting connection. A crew-operated winch pulls a cable down through a reinforced hose. At the hull exit the cable changes direction around a turning ball and reaches an eyebolt towards the keel’s after end. Shortening that cable draws the after end up; the forward pivot remains held in its recessed shoe casting. Cable and winch move the member, while pivot and supporting structure locate its rotation.

The manual separately describes a lock-down bolt acting by friction against the keel through the trunk’s side. That is its chosen restraint, not proof of a universal positive capsize lock. A different swing assembly can use a hydraulic ram and bearings within a watertight box, as Cariboni illustrates. Swing, vertical slide and telescope are different physical routes even when all reduce draught.

Haul a board without making its line the hinge

The same distinction applies to a relatively light board. A centreboard pennant, also written pendant, is the lifting connection used to raise the centreboard. It may be rope or wire in a particular installation. It is not the transverse pivot pin, and the name does not mean that the line supplies all of the board’s lateral support.

The Tanzer 22 guide names its stainless-wire pennant on the winch and follows it through cockpit-floor and hull passages towards the board. Pulling the connection raises the board about its separate pin. The case contains the board’s arc and keeps the water admitted around the board within the intended boundary. The published pivot drawing does not expose every hidden turn of the wire, so it cannot supply extra sheaves merely because another boat has them.

Compare the daggerboard. It slides along its case, with contact at separated heights restraining sideways motion and passing water loads into the hull. Raising it changes how much working blade is exposed below the bottom; it does not create a centreboard pivot. Any hoist or retaining line belongs to that sliding motion and must be distinguished from the case’s support job.

An external board has another arrangement. A leeboard tackle is a purchase used to raise or control the leeboard, leaving its support outside the hull. The board’s bracket and bearing contact with the boat carry its intended lateral reaction, while the lifting connection applies force at another point to turn it upwards.

Nexus Marine’s 25-foot Black Skimmer construction record shows a green lifting connection on the external board leading to a three-part purchase along the side deck. The hauling line reaches a rope clutch near the crew. Hauling shortens the working purchase and lifts the board; the clutch holds the line when the crew stops pulling. Easing the line allows the lowering movement to be controlled. The photographs identify these contacts without revealing every internal crossing of the purchase.

The board rests against its side support in use, and its lower end swings aft as it rises. A separate solid block at the after end of the support holds the raised board for trailering; it is not a pulley wheel. These particular leeboards also contain some lead. The external-board name describes location and support, not an absolute absence of weight, and that small embedded ballast does not make their jobs identical to a deep ballast keel.

The purchase applies the earlier force-for-distance exchange: reducing the crew’s required pull costs more hauling-line travel, while the board’s changing lever geometry still matters. A pennant wound onto a winch and a hydraulic ram use different transmissions to move their attachments through the required distance. The scale of the load changes the hardware needed; none of the three supplies movement without work or replaces the board’s pivot, the keel’s guides or their receiving structure.

Bring every moving load into the hull

Raising a ballast-bearing member changes more than the boat’s draught. G rises with the moved mass. The exposed span and area change, as can the way water load is distributed. A boat designed to use its lowered keel under sail cannot be assumed to retain the same restoring and lateral-force behaviour with it raised.

The hull also receives different local reactions in different states. In a fixed fin, loads entered through the root and its attachment. In a moving assembly they may arrive through pin bearings, guide faces, stops, lock contacts and actuator mounts at separated positions. Those contacts must still reach floors, girders, bulkheads and the joined shell. Keel attachment integrity now includes the moving contacts and their surrounding structure as well as the fin itself.

Grounding can impose a sharp load and bending demand at the lowest member. A designed pivot or retracting mechanism does not make every collision harmless: a lock may restrain movement, a guide can be loaded sideways, and the contact can occur away from the intended direction of travel. The useful question remains where this particular force goes, rather than whether the brochure says lifting.

The wider hull still bends under distributed weight and water support. Hogging and sagging do not disappear when the ballast acquires a ram. Shifting or supporting that ballast at another point changes the load entering the longitudinal structure; the wave-supported hull must carry it alongside its other loads. Off-centre reactions can also load the transverse framework towards racking. A rigid local box therefore needs a connection to the wider hull capable of sharing its reactions, not just enough wall thickness to hold a guide.

A foil can instead use flowing water to alter the forces supporting the boat, without moving ballast to a new position.

Give the foil a shape we can describe

A foil section is the profile seen in a cut across the foil’s span, along its working fore-and-aft direction. Its leading edge is the forward edge presented to the intended approaching flow; its trailing edge is the after edge where that flow leaves the section. The straight reference joining them is the chord. These manufactured edges keep their names when local flow separates or briefly reverses.

Camber and thickness describe the section’s shape. They differ from the complete foil’s outline and from its angle to the local water. The profiles in Adam May’s Moth foiling guide include both symmetric and cambered examples. A familiar-looking section does not by itself determine the load: orientation, speed, surface condition and the surrounding flow still matter.

Foil span is the extent along the wing between its stated ends. For a simple transverse T wing, take the distance from one wingtip to the other, not the depth of the upright strut carrying it. For a curved board, distinguish length measured along the curve from its projected span in a chosen view. As with sail span, the endpoints and measurement convention belong to the comparison.

A foil flap is an adjustable part near the trailing edge. Turning it changes the aft portion of the working section and thus the pressure distribution and force. It can alter the load without rotating the entire wing or its supporting shaft. The flap needs a hinge or designed flexing connection and a physical control link; it does not move because a sensor has merely noticed the water.

A finite foil also has ends around which water can move. Pressure differences between its faces drive spanwise flow near a tip, producing a swirling tip vortex in the wake. This three-dimensional flow carries an induced-drag cost associated with producing lift. The same finite-span principle appeared in the tall-sail comparison. Under comparable total lift, speed and loading efficiency, more span can reduce that component; more span can also add structural demand, wetted surface and draught. It is not a promise of lower total resistance.

Let the curve decide what emerges from the case

Viewed from ahead, a C foil has a continuing curve along its supporting and immersed length. In the selected Carbonix comparison, its case follows a compatible curved path. As the board moves through that path, a different portion of the curve lies below the hull. Its exposed orientation and the shares of upward and sideways force can therefore change together.

A J foil has a straighter upper shaft leading into a more strongly curved lower hook. In the same comparison, partially raising it can leave the lower hook inclined while less of the shaft remains immersed. A C curve and a J hook do not present the same changing geometry simply because both can retract.

An L foil has a more distinct turn from its shaft into a lower lifting leg. The angle need not be exactly a right angle: an acute L can bring its inner tip higher than the outer junction. The shaft and leg can contribute different force components. Their local angles, immersion and interaction decide those contributions; the letter alone does not establish stable flight or a fixed upward-force percentage.

A T foil puts a transverse wing across the lower end of a supporting strut. On the documented Moth, one lies beneath the main board and another beneath the rudder. The transverse wings provide useful vertical components while the upright members continue to meet sideways flow. They have joined structural routes, but their areas and force directions remain distinguishable.

The hull opening is a concrete part of that route. Éric Houdas’s photograph, taken in January 2017, shows a curved foil of the 2015 IMOCA Hugo Boss meeting a rectangular opening in the hull side. The dated component view identifies outer foil and opening together. It does not reveal the hidden bearing arrangement or tell us its force in that dockside state.

In an actual retracting installation, the immersed foil’s distributed water load reaches its shaft or root and the designed supporting contacts, then the case and hull. The foil’s movement needs clearance and restraint, while the opening needs a managed water boundary. The IMOCA rule’s separate requirement for a watertight compartment around the foil casing makes that distinction explicit. A fair external opening is not evidence that the compartment and its supports can be omitted.

Share the weight between hull and wings

Foil-assisted sailing means that foil forces help support or control the boat while the hull continues to provide substantial buoyant support. For a selected steady-height comparison with no vertical aerodynamic contribution, upward buoyancy and the upward components of the other water forces together balance the same yacht weight. Increasing the foil’s upward share can reduce the immersed hull volume required. The yacht’s mass has not disappeared.

Foiling, or foil-borne sailing, describes the running state in which the lifting foils carry the hull clear of the water. The supporting struts remain immersed, and the yacht still needs forces and moments in balance. Take-off, steady height, pitch and roll control are different parts of achieving that state. A 2008 Moth study follows the transition from buoyant support towards flight in one narrow-hulled dinghy, with main and rudder foils and a crew whose position still matters.

This differs from planing. A planing hull receives substantial motion-dependent support on its own bottom while remaining in contact with water. Semi-planing describes an intermediate mix of that hull support and buoyancy. Foil assistance and full flight place additional lifting surfaces below or beside the hull. The hull’s changing support and the foil’s support can coexist, but they are not alternative names for one bottom shape.

The distinction also belongs to dates and particular boats. In March 2016, IMOCA described its new fleet generation as foil-assisted and less immersed, rather than fully flying. Designers were trading close-hauled side force, rudder workload, retrofit effort and expected gains. That describes a documented intermediate response; it cannot decide the operating state of every later ocean racer.

Large yachts use more than one arrangement too. The 2019 Canova carries a transverse sliding Dynamic Stability System wing, deployed to leeward to obtain useful upward force and a righting contribution. The builder locates its cassette athwartships beneath the owner’s berth, with access for maintenance. The comfort-oriented cruising application has acquired a structural passage across the boat. Its particular trial reports do not establish a universal benefit for any yacht given a wing.

The 2023 Raven instead uses T foils on canting side arms. The builder’s technical account distinguishes adjustment of the arm’s cant from adjustment of the foil flap. Those settings change the dynamic support and righting contribution under sail. This retains two independently adjustable geometries rather than making foil assistance one standard installation.

In each case hydrodynamic lift remains a component relative to local flow, not another name for vertical force. Its upward component can help carry weight; another component may resist leeway, and drag opposes motion. An upward component acting at a leeward location can contribute a restoring moment through that offset. Ballast achieves its gravitational contribution even at rest; useful foil force depends on relative water motion.

Separate the lifting wing from the surface crossing

A fully submerged foil has its working lifting wing below the water surface. The Moth’s T wing can be fully submerged while the strut supporting it passes through that surface. Calling the strut surface-crossing does not make the entire lifting wing a surface-piercing one.

A surface-piercing foil has the lifting surface itself crossing the free surface. An inclined V arrangement makes the difference visible. As the hull rises, less of each inclined lifting surface remains immersed. Under suitable conditions the reduction of working area reduces support, contributing a response towards a lower height.

That geometric response does not guarantee stability of every surface-piercing shape. The loading can change non-uniformly, local water can move, and the foil’s orientation and exposed area need not vary in a helpful proportion. A submerged T wing does not acquire the same area reduction merely because its strut emerges farther. It can instead use a flap and a height-control connection.

Ride height is the boat’s vertical position above the water during foil-supported motion, measured between a stated hull point and the local surface. More height changes strut immersion, the room beneath the hull and the distance through which it can descend. It is not simply the foil’s installed length.

On a foiling catamaran, rising hulls increase the instantaneous gap beneath the spanning structure, connecting ride height with bridgedeck clearance. A crest or downward motion can still close that gap. Crossbeams must now share concentrated foil-support reactions as well as unequal hull buoyancy. A trampoline still carries people through tension to its edge attachments; its presence does not connect a loaded foil root to the other hull by itself.

Tilt about a named axis

Foil cant is sideways inclination about a stated, generally fore-and-aft axis. In the AC75 rule’s 2022 arm drawing, a hydraulic cylinder connects an inboard hull mounting to the upper foil arm near its longitudinal bearing axis. Changing cylinder length rotates the arm across the boat. The arm bearing and cylinder mount carry reactions into the hull while the foil changes position and inclination.

Foil rake is fore-and-aft inclination, involving rotation about a generally transverse axis. In side view, the foil tilts forwards or aft instead of swinging across the boat. This mounting angle differs from its instantaneous angle of attack to locally approaching water. Pitch, heave and waves can change that local flow even when the crew leaves the rake setting alone.

Nacra’s F20 Carbon FCS provides a mechanical rake-control example. A closed control line runs through lead rings near the shroud pins and eyes beneath the trampoline, around a worm-drive wheel, and back through a tensioning block. Hauling the appropriate part turns the drive and changes the board’s fore-and-aft inclination. A separate elastic length routed within the beam maintains tension in the control line.

The worm drive, its mounting and the board supports provide the actual adjustment and reaction. The line’s guides keep the crew’s pull on its intended route; they are not all foil bearings. The manual’s separate extension controls move the board through its case. Raising a foil, raking it and canting it are distinguishable movements, even when a particular system combines them.

Changing flap angle is different again. It changes the aft section shape rather than rotating the entire mount. Raven’s separate cant and flap settings and the Moth’s flap linkage use that distinction. A drawing must identify the axis or hinge that moves before a control name can predict what the water will meet.

Give the flap a measurement and a connection

A foil-control wand is a feeler whose contact with the water supplies information about the boat’s height. In Adam May’s documented Moth arrangement, a long light wand pivots near the bow. Elastic pulls it forwards; water acting on it pushes it back. Its angle therefore responds as the bow’s height above the water changes.

The wand has a physical route to the main foil. Its hinged connection moves a cable or carbon rod running aft along the deck to a lever at the board head. That lever acts on a pushrod inside the supporting blade. The pushrod reaches the trailing-edge flap on the submerged wing. The bow mounting, link supports, board head and flap connection each constrain the intended movement.

In this selected linkage, a low hull puts more of the wand into the water and pushes it back. The link and board-head lever drive the pushrod so the flap moves down, increasing the demanded lifting action. As the hull rises, the wand can move forwards under its elastic return; the linkage reduces that flap demand. The changed water force moves the hull, and the changed hull height returns to the wand. That closes a mechanical feedback loop.

The wand does not lift the boat by its own little water load. It changes the larger foil’s setting; the water flowing over that foil supplies the useful supporting force. Nor does it settle every control problem. Crew position and the aft foil’s setting affect pitch and the distribution of support. May’s separate rudder control runs from a twisting tiller extension through a worm and lengthwise rod in the tiller, then a small lever and vertical pushrod to the rudder foil’s flap.

An electronic loop separates these jobs in another way. The TF35 class account describes GPS, ultrasonic height sensors and gyros supplying information to a central computer. Crew choose height and pitch targets. The controller commands electrically driven foil and rudder flaps, with batteries supplying the actuator energy. Coarse whole-foil rake also has its own manual worm adjustment.

This is a documented racing system, not evidence that the crew can stop sailing. Sensors report conditions, the controller chooses corrections within its remit, and actuators move surfaces against load. A sensor supplies information rather than lifting energy. The larger questions of power supply, degraded control and independent fallback will return with control machinery. Here the mechanical wand and electronic system show why a useful new freedom creates another connection that must work.

Keep the water in the comparison

Less immersed hull can reduce hull resistance, but foils and struts add their own wetted surfaces and pressure forces. Skin-friction resistance still develops through their boundary layers. Section shape and separation contribute form drag, while finite loaded wings generate induced drag. The hull’s altered wave-making and the disturbances around surface-crossing supports also remain parts of the total. Removing some hull contact does not remove resistance from the boat.

The laminar-versus-turbulent distinction applies locally to these surfaces too. A smooth-looking foil is not proof that its boundary layer is laminar everywhere. Transition, surface condition and pressure recovery influence both friction and separation. A turbulent layer can have greater wall shear in a comparable attached-flow case while resisting separation better; that is the same two-sided consequence established at the hull.

Reynolds number needs an appropriate length for the question. At a hypothetical water speed of 3 metres per second and kinematic viscosity of 0.000001 square metres per second, a foil chord of 0.20 metres gives a Reynolds number of 600,000. The earlier 10-metre hull length at that same speed gave 30 million. Neither number alone diagnoses the complete local flow; they show why a hull-scale number cannot simply be assigned to its foil section.

A Froude number likewise needs its stated characteristic length. A hull-waterline comparison concerns the gravity-wave scale associated with that length. A submerged-foil study may instead use immersion depth for a free-surface question. Those differently defined ratios must not be compared as though their denominators were identical. Conventional hull speed remains a waterline-based reference, not a ceiling that foiling has somehow broken by suspending physics.

The surface introduces another limit. Ventilation brings air or another gas into the water flow around the foil. In the natural surface-air case, a path opens from the free surface into a lower-pressure separated region beside the immersed member. The pressure distribution and useful load can change abruptly. Surface proximity, waves, geometry and the path’s persistence matter; a wing can be below water while air reaches it along its support.

Cavitation has a different origin: sufficiently low local pressure allows liquid water to form vapour. The vapour can later collapse as it reaches higher pressure. It is a phase change within the fluid, rather than air merely entering from above. Formation and collapse can alter loads, create noise and damage surfaces under relevant conditions.

The distinction does not make the two mutually exclusive. Young and colleagues’ experiments show how a low-pressure vapour cavity can help ventilation develop if an air path opens. No single yacht speed identifies their onset across different depths, sections and conditions. Names identify different causes, so the explanation must follow where the gas came from before attributing a sudden change of foil force.

Fixed ballast, moving ballast, retracting boards and lifting foils therefore remain different answers to changing needs. One changes the gravitational lever, another the accessible depth, another the water force or the share of weight carried by the hull. Combining them adds contacts, controls and states that the boat must accommodate.

The connection ends at the hull, but the work does not. Crew must reach lines and mechanisms, move around the boat, close openings and manage water that arrives where it is unwanted. Those working surfaces and boundaries are the next part of making the complete sailing arrangement usable.