Part B

Ship designs

High Speed Craft Notes

Q.10 — Notes on High-Speed Craft: Catamaran, Planing Craft, Hovercraft, Air Cushion Vessel, SWATH, and Hydrofoil

Covering, for each craft type: (a) how high speed is achieved, (b) transverse and longitudinal stability, (c) seakeeping, and (d) propulsion arrangements.

1. Catamaran

(a) How high speed is achieved

As discussed in Q.9, displacement is shared between two slender hulls, each with a high length-to-beam ratio, drastically reducing wave-making resistance per unit displacement compared to an equivalent monohull, allowing higher speed for given power, particularly at higher Froude numbers.
Many high-speed catamarans also use hull forms optimized to minimize wave interference drag between the two demi-hulls, with hull separation (beam) tuned to the design speed range.

(b) Transverse and longitudinal stability

Transverse: Very high initial GM due to the wide effective waterplane created by the two widely-separated hulls (large parallel-axis moment of inertia contribution), giving strong resistance to heel — but with the “falls away sharply beyond critical angle” characteristic noted in Q.9 once one hull lifts clear or the wet-deck (bridging structure) contacts the sea.
Longitudinal: Generally good longitudinal stability due to the long waterline length of each slender hull, though the craft is susceptible to pitching and “slamming” of the wet-deck/bridging structure in head seas if freeboard beneath the bridging structure is insufficient for the sea state encountered.

(c) Seakeeping

Stiff, rapid roll motion (short roll period) as noted in Q.9 — while safe, this produces higher accelerations, which can affect passenger comfort and equipment.
Wet-deck slamming in a seaway is the principal seakeeping limitation — when wave height approaches the clearance under the bridging structure, slamming loads can be severe, limiting the catamaran’s effective operating sea state compared to a monohull of similar length.
Some designs incorporate a T-foil or trim-tab motion-control system at the bow/stern to actively reduce pitching and improve ride comfort in a seaway.

(d) Propulsion arrangements

Typically twin-screw (one propulsion train per hull), using waterjets (very common on high-speed catamaran ferries, since waterjets perform efficiently at high Froude number and avoid the cavitation/appendage drag issues of conventional propellers at high speed) or, for lower-speed/higher-displacement catamarans, conventional shaft/propeller arrangements, often driven by high-speed diesel engines or gas turbines for the largest fast-ferry applications.

2. Planing Craft

(a) How high speed is achieved

At low speed, a planing hull behaves in displacement mode, supported by buoyancy in the conventional sense.
As speed increases past a critical Froude number, the hull’s flat/warped-bottom aft sections generate dynamic (hydrodynamic) lift, causing the hull to rise and “plane” on top of the water surface rather than pushing through it — this dramatically reduces wetted surface area and eliminates most wave-making resistance, since the vessel is now largely supported by dynamic lift rather than displacement/buoyancy, allowing a steep reduction in resistance per unit speed increase beyond the “hump” speed.
The transition through the resistance “hump” (the speed range where the craft is neither fully displacement nor fully planing) requires a surplus of installed power to push through before the efficiency gain of full planing is realized.

(b) Transverse and longitudinal stability

Transverse: While planing, stability is provided largely by the dynamic lift distribution across the flat/warped bottom plus, at rest/low speed, conventional hydrostatic (waterplane) stability — a planing hull typically has a wide, flat afterbody giving good transverse stability once on the plane, but can be prone to chine-tripping or sudden roll instability if the dynamic lift distribution is disturbed (e.g., in a hard turn at speed).
Longitudinal: The fore-and-aft trim angle while planing is critical — too much bow-up trim reduces wetted area excessively and can cause porpoising (unstable pitching oscillation); too little trim (bow-down) increases resistance and risk of “digging in.” Trim tabs or interceptors at the transom are commonly used to actively control this running trim angle.

(c) Seakeeping

Planing craft seakeeping is generally poor in a seaway compared to displacement/multi-hull types — because the hull skims the surface at speed, encounters with waves produce severe slamming loads on the flat bottom sections, transmitting high accelerations to the structure, crew, and equipment; this is the dominant limitation on a planing craft’s usable speed in anything beyond calm/moderate sea states.
Deep-V hull forms (with significant deadrise angle) are used to reduce (though not eliminate) slamming severity compared to flatter-bottomed designs, at some cost in ultimate top-speed efficiency (a deep-V hull has somewhat higher resistance than a flatter hull at the same speed, due to the need to displace water sideways from the V-shape).

(d) Propulsion arrangements

Commonly high-speed diesel engines with surface-piercing propellers, stern drives (outdrives), or waterjets — surface-piercing propellers are specifically suited to planing craft since the propeller operates partially out of the water at speed, reducing appendage drag; waterjets are preferred where shallow draft and the elimination of exposed propeller/shaft appendage drag are priorities (also improving safety around swimmers/divers for recreational and rescue craft).

3. Hovercraft (Air Cushion Vehicle — fully amphibious, flexible skirt type)

(a) How high speed is achieved

A hovercraft is supported almost entirely by a cushion of air maintained under the hull at a pressure sufficient to lift the vessel’s weight, contained by a flexible rubber/fabric skirt around the periphery, essentially eliminating hull-water contact and hence both wave-making and frictional/skin resistance while cushion-borne.
With essentially no hydrodynamic resistance component, the primary resistance to forward motion is aerodynamic drag, allowing speeds substantially higher than a comparable-size, comparable-power waterborne displacement or even planing craft, and crucially, allowing true amphibious operation — transitioning directly from water onto land, ice, mudflats, or marsh with no change in operating mode.

(b) Transverse and longitudinal stability

Stability is fundamentally different from any waterborne hull — it derives from the differential pressure distribution within the air cushion, segmented in many designs into multiple cells to prevent the entire cushion pressure shifting/venting to one side during a heel or trim disturbance (an undivided single cushion would be inherently unstable, since cushion air would simply migrate to the low side under any disturbance, reducing pressure support exactly where it’s needed and worsening the tilt).
Segmented/compartmented cushions and the skirt’s own flexible geometry provide a self-correcting restoring effect, but hovercraft stability margins are generally more sensitive to load distribution and cushion pressure balance than a conventional hull’s hydrostatic stability, requiring careful design of cushion cell geometry and, in larger designs, active cushion pressure control systems.
Longitudinal trim is similarly managed via fore-aft cushion cell pressure balance and, dynamically, by aerodynamic control surfaces at speed.

(c) Seakeeping

Excellent capability for operating over very shallow water, mudflats, ice, and marginal terrain where no other marine craft can operate at all — a unique amphibious capability rather than a “seakeeping” advantage in the conventional wave-response sense.
In open water with waves, ride quality/seakeeping is limited by the skirt’s ability to follow the wave surface — in significant sea states, the skirt can lose full contact/sealing, causing cushion pressure loss, increased drag, and a harsher ride; hovercraft seakeeping in open seaway conditions is generally inferior to a true seagoing hull form and is why the type is favoured for sheltered water, ice, and amphibious/rescue applications rather than open-ocean high-sea-state transits.

(d) Propulsion arrangements

Aerodynamic propulsion — large air propellers (aircraft-type propellers) mounted above the hull, driven typically by gas turbine or high-power piston/turboshaft engines — since the craft is not in the water while cushion-borne, waterborne propulsors (propellers/jets) are not used for primary propulsion.
Separate lift and propulsion systems are common in larger designs: dedicated lift fans (centrifugal fans) maintain cushion pressure, while separate propulsion air propellers/ducted fans provide forward thrust, allowing independent control of lift and propulsion power — smaller hovercraft may use a single integrated fan/propeller system serving both functions.
Directional control is achieved via rudders in the propeller slipstream (aerodynamic rudders) and/or differential thrust, since conventional waterborne steering (rudders in water flow) is unavailable while fully cushion-borne.

4. Air Cushion Vessel / Surface Effect Ship (SES — rigid sidewall type, distinct from full hovercraft)

(Where the question intends the rigid-sidewall Surface Effect Ship variant, distinct from the fully-skirted amphibious hovercraft above — the specimen paper lists both “Hovercraft” and “Air Cushion Vessel” separately, suggesting this distinction is intended.)

(a) How high speed is achieved

An SES has rigid, catamaran-like sidewalls extending into the water, with a flexible seal only at bow and stern, and an air cushion trapped between the sidewalls supporting the majority (though not all) of the vessel’s weight — the sidewalls remain waterborne (unlike the fully-skirted hovercraft), providing directional stability and containing the cushion.
Because a large fraction of the weight is cushion-supported, hull resistance (from the sidewalls, which are now only lightly loaded) is much reduced compared to a conventional catamaran/monohull, enabling higher speed for given power, though not achieving the near-zero hydrodynamic resistance of a true fully-skirted, fully amphibious hovercraft, and hence an SES is not amphibious — it remains a waterborne craft, unlike the hovercraft.

(b) Transverse and longitudinal stability

Transverse stability benefits from the catamaran-like sidewall arrangement (wide effective waterplane from the separated sidewalls, similar in principle to Q.9’s catamaran discussion), generally giving good, predictable transverse stability compared to a fully-skirted hovercraft’s more cushion-pressure-dependent stability.
Longitudinal stability/trim is managed through cushion pressure distribution (bow/stern seal control) combined with the conventional waterplane effect of the sidewalls themselves.

(c) Seakeeping

Generally better open-water seakeeping than a fully-skirted hovercraft (since the rigid sidewalls provide more predictable directional stability and wave-following behaviour in the water), while still gaining a significant speed/resistance advantage over a conventional catamaran from the cushion lift effect — representing something of a middle ground between a full hovercraft and a conventional multi-hull in both performance and sea-state capability.
Bow/stern seal performance and durability in a seaway is a key maintenance and seakeeping-limiting factor, analogous to (though generally more robust than) a full hovercraft’s skirt.

(d) Propulsion arrangements

Since the sidewalls remain in the water, conventional waterborne propulsion — waterjets or propellers — can be used (unlike the fully-skirted hovercraft, which must use aerial propulsion), typically one unit per sidewall/hull, in addition to a separate lift-fan system to maintain cushion pressure between the sidewalls.

5. SWATH (Small Waterplane Area Twin Hull)

(a) How high speed is achieved

SWATH achieves its performance benefit not primarily through top-speed maximization (SWATH vessels are typically moderate rather than extreme-high-speed craft) but through exceptional seakeeping/motion stability at moderate-to-good speed, achieved by having the vessel’s buoyancy provided by fully submerged, torpedo-shaped lower hulls connected to the above-water structure by thin, minimal-waterplane-area struts.
Wave-making resistance is significantly reduced because the submerged hulls operate below the zone of maximum wave orbital motion, and the very small waterplane area (at the thin struts) minimizes the vessel’s response to surface wave action, though the deeply submerged hull form does carry a frictional resistance penalty.

(b) Transverse and longitudinal stability

Very small waterplane area (by design intent) means BM = I/∇ is inherently small (since I, dependent on waterplane geometry, is deliberately minimized), which would normally suggest poor stability — however, the wide transverse separation of the two submerged hulls (parallel-axis effect, as in the catamaran case) compensates, restoring adequate transverse GM despite the small local waterplane contribution from the struts alone.
The very small waterplane area is precisely the design feature that gives SWATH its outstanding motion stability in waves — since restoring forces (and hence wave-excited motions) in heave and pitch are proportional to waterplane area, a small waterplane area means the vessel is much less responsive to wave-induced heave and pitch excitation than a conventional surface-piercing hull, producing a remarkably steady, low-motion platform even in significant sea states.
Active fin/stabilizer systems on the submerged hulls are commonly fitted to further actively control pitch and roll, since the passive stability margins, while adequate, are typically less generous than a conventional wide monohull or catamaran.

(c) Seakeeping

SWATH’s principal advantage is outstanding seakeeping — the deeply submerged hulls and minimal waterplane area give dramatically reduced heave, pitch, and roll motion in a seaway compared to any conventional monohull or surface-piercing catamaran of similar size, making it exceptionally well suited to roles requiring a stable working/sensor platform (research vessels, some naval applications) even in moderate-to-rough sea states.
This superior seakeeping comes at the cost of greater structural complexity, deeper draft (limiting shallow-water/port access), and generally higher construction cost than a conventional hull form of similar displacement.

(d) Propulsion arrangements

Propulsors (propellers, often in shrouded/ducted arrangements, or waterjets in some designs) are mounted on or near the submerged lower hulls, driven via shafting running up through the connecting struts to machinery located in the above-water cross-structure — the deep hull placement of the propulsors also contributes to reduced propeller ventilation/cavitation issues in a seaway compared to a surface-piercing hull’s propellers, which can be prone to emergence in heavy pitching.

6. Hydrofoil

(a) How high speed is achieved

At low speed, a hydrofoil craft operates as a conventional displacement hull, with underwater foils (wing-like lifting surfaces) mounted on struts below the hull.
As speed increases, the foils generate increasing hydrodynamic lift (exactly as an aircraft wing generates aerodynamic lift), until at a critical “take-off” speed the lift generated equals the vessel’s weight, and the entire hull rises clear of the water surface, leaving only the slender foils and their supporting struts in the water.
With the hull fully “flying” clear of the water, wave-making resistance and hull frictional resistance are almost entirely eliminated, leaving only the much smaller resistance of the thin foils/struts — this allows a dramatic increase in achievable speed for a given power compared to a conventional hull, representing one of the most resistance-efficient high-speed configurations at appropriate size/speed ranges.

(b) Transverse and longitudinal stability

While foil-borne, stability is provided by the hydrodynamic lift characteristics of the foil system rather than by conventional hydrostatics (the hull itself, providing no buoyancy contribution while flying, plays no role in stability at this stage).
Surface-piercing foil configurations (V-shaped or ladder foils that partly pierce the surface) provide self-stabilizing/self-damping roll and heave characteristics — as the craft heels or a foil begins to broach, the immersed foil area changes automatically, generating a self-correcting lift change (more foil area = more lift, restoring level attitude) — a passive stability mechanism requiring no active control system, though offering comparatively modest ride quality.
Fully-submerged foil configurations (used in the highest-performance/most seaworthy hydrofoils) provide superior ride quality and higher lift efficiency but are not inherently self-stabilizing — they require an active electronic control system with sensors (accelerometers, height/attitude sensors) continuously adjusting foil incidence angle (via flaps or foil pitch actuators) to maintain stable, level foilborne flight, analogous to an aircraft’s fly-by-wire stability augmentation system — a failure of this active control system is a critical vulnerability of the fully-submerged-foil type.
Longitudinal stability/trim while foilborne is managed by the relative lift balance between the forward and after foil sets (a hydrofoil typically has both a bow foil and a stern foil, analogous to a canard/tailplane arrangement on an aircraft), actively or passively adjusted to maintain level fore-aft trim as speed and loading vary.

(c) Seakeeping

While foilborne, a hydrofoil (particularly the fully-submerged, actively-controlled type) offers exceptionally smooth ride quality, since the foils, operating below the more turbulent near-surface wave zone, can actively maintain the hull at a constant height and attitude largely independent of the passing wave surface — a foilborne hydrofoil can “fly over” small-to-moderate waves with minimal hull motion transmitted to passengers/cargo.
This advantage is limited by wave height relative to foil submergence/strut length — in sea states exceeding the craft’s foil system design limit, the hull may begin to contact wave crests (hull-borne “touchdown” in a seaway) or the foils may broach, at which point the craft must revert to hull-borne (displacement) operation at much reduced speed, meaning hydrofoil seakeeping/speed advantage is strictly bounded by a maximum operable sea state, beyond which performance degrades sharply (unlike SWATH, whose motion advantage persists more gradually into higher sea states).

(d) Propulsion arrangements

Options include: surface-piercing or submerged propellers on shafts extending down from the hull through/near the foil struts, waterjets (increasingly preferred in modern designs, avoiding the mechanical complexity of a propeller shaft penetrating a lifting/retracting foil strut arrangement), or, in some older/specialized designs, an inverted “Z-drive” or podded propulsor mounted directly on the foil strut.
Since the hull itself rises clear of the water while foilborne, the propulsor and its shafting/strut arrangement must be specifically designed to remain effectively submerged and functional across both the hull-borne (low speed, propulsor deep) and foilborne (high speed, hull risen) conditions — a significant engineering challenge distinguishing hydrofoil propulsion design from simpler, single-condition hull forms.