Design of Hydrofoil Small Craft

Why hydrofoils change the design problem

A hydrofoil craft is a boat with two operating modes: a low-speed hull-borne mode and a higher-speed foil-borne mode where underwater lifting surfaces support most of the craft weight.³ The engineering consequence is that the dominant resistances and loads shift with speed:

• Hull-borne: wave-making and wetted area dominate; structure is governed by “boat” loads (global bending, local slamming).
• Foil-borne: most wetted area is concentrated in foils/struts; lift distribution, induced drag, profile drag, and strut interference dominate; structure is governed by “aircraft-like” wing/strut bending and torsion in water.

Hydrofoils therefore reward a disciplined split between (a) hydrodynamics that create efficient lift and (b) structures that keep the lifting system stiff, aligned, and durable at minimum mass. Early hydrofoil handbooks explicitly treat design as a coupled hydrodynamic-structural-control problem rather than a hull-shape problem alone.¹

Configurations and control philosophy

Surface-piercing vs fully submerged

Two broad families of small-craft hydrofoils are common:

• Surface-piercing foils (often V-foils): the foil intersects the free surface. As the craft rises, less foil area remains submerged, which can provide a passive height-stabilizing effect. The design trade is susceptibility to ventilation and wave interaction near the surface.²
• Fully submerged foils (T-foils): the lifting surfaces operate below the free surface and generally require active control (flaps, incidence control, or both) to regulate ride height and trim. Many modern foiling craft use sensor-driven control loops; marine craft control references emphasize the importance of coupled hydrodynamics and motion control for high-performance craft.⁶

Foil system architectures

Small craft commonly use: (a) a main foil with an aft stabilizer, (b) a canard-like forward foil with an aft foil, or (c) twin main foils on multihulls. Choices are driven by packaging, pitch stability margins, and whether you want “aircraft-like” trim control (aft stabilizer) or “canard-like” characteristics. Early hydrofoil design literature catalogs multiple configurations and emphasizes selecting a configuration that matches stability and control requirements.¹

Hydrodynamics: lift, drag, and free-surface effects

Lift and induced drag (the core trade)

At a first approximation, hydrofoil lift can be treated like wing lift: L = 0.5 × rho × V^2 × S × C_L, where rho is water density, V speed, S planform area, and C_L lift coefficient.⁴ Because water density is high, hydrofoils can generate large lift at relatively small areas, but the craft becomes sensitive to small changes in angle of attack (AoA), immersion, and surface condition.

Induced drag falls with higher aspect ratio and cleaner lift distribution; however, higher aspect ratio increases bending moment and torsional demands, increasing structural mass and deflection sensitivity. Hydrofoil design is therefore a classic “aeroelastic” trade in a marine setting: more efficient foils push you toward slender, stiff structures and tighter manufacturing tolerances.¹

Profile drag and section selection

Section selection (thickness, camber, leading-edge radius, and pressure recovery) sets the achievable operating range before stall and influences cavitation margins. Designers often start with known families (e.g., NACA sections) because they have well-characterized lift, drag, and moment behavior over Reynolds number ranges, as summarized in classic airfoil compilations and test data sets.⁷⁸ In hydrofoils, you must further consider cavitation and ventilation, which effectively impose additional “stall-like” limits at high speed or near the surface.

Free-surface interaction is not optional

For small craft, the foil system often operates close enough to the free surface that wave encounter and ventilation change lift and drag dynamically. Hydrofoil handbooks treat surface proximity as a primary design driver because it impacts stability, ride comfort, and the probability of flow breakdown.²

Preliminary foil sizing and takeoff

Early sizing is largely about wing loading (weight per unit foil area) and the takeoff speed you can tolerate. Rearranging the lift relation gives a practical estimate:

The Hydrofoil Handbook stresses that preliminary performance calculations should be tied to configuration selection and that structural considerations (especially foil/strut loading) must be carried forward from the start, because “hydrodynamic optimum” solutions can be structurally impractical at small scale.¹

Planform and distribution choices

• Higher aspect ratio: lower induced drag and better efficiency, but higher bending moment and stiffness requirements.
• More sweep/dihedral: can aid stability or packaging, but increases coupled bending-torsion loads and complicates control.
• Foil count and spacing: affects pitch/roll authority and ride quality; also affects redundancy for partial lift in disturbance.

Cavitation, ventilation, and depth limits

Cavitation (vapor bubbles from low pressure)

Cavitation occurs when local pressure on the foil drops below the vapor pressure of water, producing vapor cavities that degrade lift and can damage surfaces. A standard nondimensional measure is the cavitation number: sigma = (p_infty − p_v) / (0.5 × rho × V^2). As speed increases, sigma decreases (worse cavitation margin) unless depth or ambient pressure increases. Hydrofoil component handbooks discuss cavitation behavior and the need to treat it as a design constraint rather than a rare anomaly.²

In practice, cavitation margin is managed with a mix of: foil section choice, limiting suction peaks (pressure recovery management), adequate submergence, smooth surfaces, and (if necessary) accepting larger foils operating at lower C_L for a given speed. Hoerner’s lift reference provides practical guidance on lift behavior and the penalty of operating near limits, useful for concept reasoning and trade studies.⁴

Ventilation (air ingestion from the free surface)

Ventilation is distinct from cavitation: air is pulled down from the surface along a strut or foil and disrupts the flow. Surface-piercing systems are especially sensitive, and even fully submerged foils can ventilate if the free surface is disturbed or the strut is poorly designed. The Hydrofoil Handbook treats ventilation as a key operational limit because it can cause abrupt lift loss and large transient loads on recovery.²

Speed, depth, and the “operating box”

For a given foil loading, you typically operate inside a box defined by (a) minimum speed (takeoff), (b) maximum speed (cavitation/structural margin), (c) minimum depth (ventilation/wave effects), and (d) maximum controllable ride height and trim range. Hydrofoil craft are successful when this box matches the intended operating environment rather than forcing the craft into continual limit avoidance.

Stability and control (static, dynamic, and “ride”)

Pitch stability is the first gate

Foil-borne craft often fail first in pitch: if the craft is not statically stable or if control authority is insufficient, small disturbances can lead to growing oscillations (porpoising) or sudden touchdown. The hydrofoil craft literature emphasizes balance and stability of foil systems as a central design topic, not an afterthought.¹

• Static stability: the craft must generate a restoring moment for small perturbations in ride height and pitch.
• Dynamic stability: damping (hydrodynamic and control) must dominate excitation from waves, steering inputs, and control lag.
• Control authority: flaps/incidence changes must produce the needed lift and moments without driving the foil into ventilation/cavitation limits.

Controls as a structural driver

If you use active control, the foil structure must be stiff enough that commanded flap/incidence changes actually translate to the intended AoA change. Structural flexibility can create phase lag and coupling (bend-twist), which may destabilize the closed-loop system. Research on composite hydrofoils highlights how stiffness and coupling characteristics change dynamic response and vibration behavior, reinforcing that “material choices” are also “control system choices.”¹⁰

Integration with hull, struts, and propulsion

Struts are often the drag and risk bottleneck

In many small craft, struts contribute a large share of total drag because they must be deep enough to keep foils submerged and stiff enough to carry high bending moments. They also act as “air conduits” for ventilation if their leading edges and intersections are not designed carefully. Hydrofoil component handbooks discuss strut/foil intersections and component drag as central to overall performance prediction.²

Propulsion placement and cavitation interaction

Powered hydrofoils must integrate propulsors (propellers, waterjets, or electric thrusters) without creating strong interaction problems: propulsor inflow can be disturbed by strut wakes; propulsor cavitation can interact with foil cavitation margins; and propulsion geometry often competes with foil span, depth, and retractability. For high-speed craft, broader high-speed marine vehicle references provide the contextual framework for these coupled phenomena.⁵

Transition mode (takeoff and landing)

Takeoff and landing are frequently the highest risk phases: loads can spike during partial ventilation, wave hits, or foil re-entry. Design should explicitly consider “partial lift” cases and ensure that touchdown does not create dangerous trim changes, spray ingestion, or structural overload.

Strength/weight factors and structural load paths

What loads govern small-craft hydrofoils

Hydrofoil craft loads include steady lift plus transient components from waves, maneuvers, control actions, and impacts. The Hydrofoil Handbook discusses structural loading conditions for hydrofoil craft and methods for structural design of foil-strut configurations and hull attachments.¹ For small craft, the following load cases are typically decisive:

• Vertical lift and wave encounter: lift fluctuates with submergence and speed; dynamic factors can exceed 1g-equivalent loads.
• Turning loads: lateral accelerations generate side force and torsion in struts and foil roots.
• Pitch/roll control loads: flap forces and hinge moments; actuator loads (including fail-safe conditions).
• Touchdown and re-entry: impact loads and rapid lift re-establishment; risk of local buckling or delamination in composites.
• Debris impact: floating logs, kelp, or submerged objects can impose short-duration, high-magnitude loads.

Scaling: stiffness and bending moment grow quickly

A useful scaling intuition is that the primary bending moments at the foil root and strut base scale with load times lever arm. As span and depth increase to improve hydrodynamics or cavitation margin, bending moments rise, and the structure must stiffen accordingly. This is why “high aspect ratio” hydrofoils can become weight-limited: structural mass rises to preserve stiffness and prevent excessive twist.

Weight is not just mass; it is CG and inertia

Foil and strut mass typically sits low and outboard relative to the hull, changing roll inertia and impacting control responsiveness. Minimizing mass while maintaining stiffness is therefore doubly valuable: it reduces both total displacement and dynamic control burden. Composite hydrofoil research and case studies repeatedly emphasize stiffness-to-weight and strength-to-weight as primary material advantages, alongside the need to manage brittle failure modes under impact.¹¹¹²

Materials, stiffness, fatigue, and impact

Carbon composites vs metals

Many modern small craft foils use carbon composites because they offer high stiffness-to-weight and allow tailoring (bending-twist coupling) through layup design. The downside is that composites can fail abruptly under impact or poorly designed load introduction; “good” composite hydrofoils require careful root design, conservative knock-down factors, and manufacturing quality control. Case studies on composite hydrofoils for impact loading highlight design strategies intended to avoid hazardous brittle failure when striking objects.¹²

Metals (aluminum, stainless, titanium) offer ductility and damage tolerance but may incur weight and corrosion penalties; the optimal choice depends on craft size, production volume, and expected debris environment. For small craft, a common approach is a stiff composite foil with metallic inserts and robust corrosion-isolated joints.

Fatigue and vibration

Hydrofoils and struts see cyclic loading from waves and maneuvers. Vibration and hydroelastic response can couple with hydrodynamics and controls. Studies on composite hydrofoil dynamics show that stiffness choices affect forced response and can influence operational vibration behavior, which feeds back into fatigue and comfort.¹⁰

Surface finish and durability

Surface condition matters more than many boatbuilders expect: small changes in roughness can increase drag and shift cavitation inception. This drives both material choice (erosion resistance) and maintenance planning (coatings, inspections, repairability).

Verification: analysis, test, and sea trials

Hydrofoil craft design is unusually sensitive to coupled effects, so verification is a multi-step process:

1. Preliminary methods: planform sizing, section choice, takeoff speed estimates, and conservative load cases using handbook data.¹²
2. Higher-fidelity analysis: CFD for free-surface and ventilation risk, and FEA for foil/strut roots and load introductions (especially for composites).
3. Bench and component tests: proof loading, stiffness measurement (twist per load), and fatigue screening where feasible.
4. Tow or sea trials: controlled envelope expansion; validate takeoff/landing behavior, ride control margins, and structural temperatures/strain where instrumented.

A practical recommendation is to define “success criteria” for stability and control early (ride height error bands, pitch/roll rates, actuator limits, and safe reversion behavior) and to treat those criteria as constraints equal in importance to speed and efficiency.

Practical checklist

• Operating envelope: expected speed range, sea state, debris environment, and minimum depth.
• Configuration: surface-piercing vs fully submerged; number of foils; control strategy and fail-safe behavior.
• Foil sizing: wing loading and takeoff speed; conservative C_L,max assumptions for waves.
• Hydrodynamic margins: induced/profile drag estimates; strut drag; free-surface effects and ventilation risk.
• Cavitation margin: target sigma range at max speed; adequate submergence and section choice.
• Control authority: flap/incidence limits, actuator sizing, and stability margins including structural flexibility.
• Load cases: vertical + maneuver + touchdown + impact; include transient factors and recovery events.
• Structure: root and hull attachments; stiffness targets (twist/deflection) to protect control performance.
• Materials: corrosion and galvanic isolation; composite quality control; impact and repair strategy.
• Verification: benchmarks, component testing, and staged trials with documented limits.