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Strake

A strake is a longitudinal structural or aerodynamic element used in engineering applications. In shipbuilding, it consists of a continuous course of planking or plating along the hull from stem to stern, providing structural integrity.^[1]^[2] In aeronautics, a strake is a fixed aerodynamic surface, typically mounted on the fuselage, that modifies airflow to enhance stability, lift, or control characteristics.^[3]^[2] In civil engineering, helical strakes are spiral fins attached to cylindrical structures such as chimneys, stacks, or masts to disrupt vortex shedding and mitigate wind-induced vibrations.^[4]^[5]

Shipbuilding

Definition and purpose

In shipbuilding, a strake refers to a longitudinal course of planking or plating that forms part of the hull, typically running continuously from the stempost at the bow to the sternpost or transom at the stern.^[6] This element is fundamental to the vessel's outer shell, appearing in both wooden and metal constructions, where it consists of either overlapping or edge-joined planks in traditional builds or welded plates in modern ones.^[7] The primary purposes of strakes include forming the hull's watertight skin to prevent water ingress and providing structural integrity by distributing loads along the ship's length.^[8] They contribute to resisting hydrodynamic forces, such as wave impacts and pressure, while helping to maintain overall hull shape and buoyancy. Unlike transverse elements like frames or bulkheads, which provide sideways support and compartmentalization, strakes primarily handle longitudinal stresses from propulsion, cargo weight, and bending moments.^[7] The term "strake" originates from late Middle English usage in wooden boat building, dating back to around 1300–1350, where it described a row of planks stretched along the hull's length.^[1] This historical application underscores its evolution from early clinker or carvel planking techniques to contemporary steel fabrication, always emphasizing longitudinal continuity for strength and seaworthiness.

Types of strakes

In shipbuilding, strakes are specialized longitudinal rows of plating or planking that form the hull's outer skin, with distinct types serving specific structural and protective roles based on their positions.^[6] The garboard strake is the lowermost strake on each side of the hull, positioned immediately adjacent to the keel. It provides essential structural support to the hull bottom and contributes to overall watertightness, often featuring increased thickness to handle stresses in this critical area.^[6]^[8] The sheer strake forms the uppermost strake along the topsides, located just below the deck edge. It enhances the hull's longitudinal strength due to its distance from the neutral axis and connects directly to the deck structure, often requiring additional thickness for rigidity against bending forces.^[6]^[9] The rubbing strake is a thicker strake positioned below the sheer strake along the upper hull sides. Designed as a protective strip, it absorbs impacts and abrasion during contact with docks or other vessels, preventing damage to the main hull plating.^[10]^[11] The stealer strake is a shortened strake employed at the hull's ends, where the vessel's girth narrows toward the bow and stern. It facilitates a smooth taper in hull width by merging adjacent strakes into a single plate, optimizing the plating arrangement for structural efficiency without abrupt transitions.^[9]^[12] Other bottom and bilge strakes serve as intermediate layers between the garboard and sheer strakes, filling the spaces in the hull's lower and side structures. The bilge strake, specifically at the turn of the bilge, strengthens the curved transition between the flat bottom and vertical sides, with thickness varying by vessel size to accommodate differing hydrodynamic loads and bending stresses amidships. Bottom strakes between the garboard and bilge provide continuous support across the hull floor, their number and dimensions scaled according to the ship's overall proportions.^[6]^[9]^[13]

Construction techniques

In wooden shipbuilding, strakes are typically formed from planks selected for their length and curvature to follow the hull's lines. For small boats, each strake often consists of a single continuous plank, which simplifies assembly and provides inherent longitudinal strength without interruptions.^[14] In larger vessels, however, the required plank lengths exceed available timber, so strakes are composed of multiple shorter planks joined end-to-end using scarf joints—tapered overlaps glued and bolted for a gradual transition—or butt joints reinforced with internal butt blocks to back the seam and distribute loads.^[14] These methods ensure the strake maintains rigidity while accommodating the hull's expansion and contraction from moisture changes. Two principal planking styles define wooden strake assembly: carvel, in which planks are butted edge-to-edge and caulked to form a smooth exterior surface ideal for larger ocean-going vessels, and clinker (also known as lapstrake), where each strake overlaps the one below by about one inch, fastened with rivets or clinch nails through the lap for enhanced watertight integrity and hull flexibility in smaller craft.^[15] Carvel construction requires precise beveling of plank edges to fit tightly against frames, while clinker relies on the overlap to create a self-supporting shell before internal framing is added.^[16] Riveted steel construction, prevalent from the late 19th to mid-20th century, assembles strakes from rolled steel plates arranged in longitudinal runs. Adjacent plates are joined using lapped seams, where one plate overlaps the other and is secured with multiple rows of rivets, or joggled edges, in which the overlapping plate's edge is rebated to sit flush against frames for a tighter fit.^[17] For end-to-end connections, butt-strapped joints employ cover straps—flat steel bars—riveted over the abutting edges to create watertight seams and reinforce the joint against shear forces.^[18] These techniques, often executed in shipyards with pneumatic riveting guns, prioritized overlapping for redundancy in high-stress areas like the bilge strake. Modern welded steel construction has largely supplanted riveting, with strakes fabricated from high-strength mild steel plates butt-welded end-to-end to form continuous longitudinal courses without seams or overlaps.^[19] Plates are pre-cut using CNC machines based on 3D hull models, rolled to curvature, and assembled into sub-units with longitudinal stiffeners before full-penetration butt welds join them, often via automated submerged arc welding for precision and efficiency.^[19] This method reduces weight by eliminating rivet holes and straps while providing superior fatigue resistance. A critical aspect across all techniques is ensuring precise alignment of strakes during assembly, as misalignment can disrupt hydrodynamic efficiency by creating uneven flow and increased drag, or compromise structural continuity by introducing stress concentrations at joints.^[19] In practice, this involves using temporary fixtures, laser alignment tools, or digital modeling to verify plate positioning relative to the hull frame, maintaining fairness for optimal performance and longevity.^[20]

Terminology and labeling

In shipbuilding, strakes are systematically labeled using an alphabetic nomenclature starting from the keel outward to facilitate precise identification in design, construction, and maintenance documentation. The A-strake, also known as the garboard strake, is the first strake on each side immediately adjacent to the keel plate, providing the initial longitudinal plating along the hull bottom.^[6]^[21] Subsequent strakes follow sequentially: the B-strake and C-strake form the broad, flat bottom plating; the D-strake and E-strake cover the bilge and lower topside regions, transitioning the hull curvature; and higher letters (e.g., up to J or K-strake) denote the upper strakes, culminating in the sheer strake at the deck edge.^[8]^[20] The hood ends refer to the forward and aft extremities of each strake, where they are joined to the stem at the bow or the sternpost at the stern to ensure structural continuity and watertightness across the hull.^[20] These connections are critical in shell expansion plans, where strake alignments are detailed to prevent gaps or weaknesses. Related terms include the bilge strake, which specifically denotes the strake in the curved transition area between the flat bottom and the side shell, often requiring increased thickness due to concentrated stresses.^[6] A stringer, in this context, is an internal longitudinal girder or plate that provides support and ties into the shell strakes, enhancing the hull's overall rigidity, particularly in areas like the double bottom or sides.^[20] This labeling system is standardized in naval architecture drawings, such as midship sections and shell expansions, to denote plate positions, scantlings, and material grades for clarity during fabrication and inspection. Classification societies like Lloyd's Register, ABS, and DNV incorporate this nomenclature in their rules for hull construction, ensuring compliance with structural integrity requirements through designated notations and verification processes.^[20]

Aeronautics

Definition and aerodynamic principles

In aeronautics, a strake is defined as a fixed aerodynamic surface mounted on the fuselage, typically longer than it is wide, serving as a low-aspect-ratio lifting device to enhance aircraft performance at high angles of attack.^[22] Unlike winglets, which are vertical extensions at wingtips to mitigate induced drag, or canards, which are forward horizontal control surfaces, strakes are integrated into the fuselage to generate and manage vortices without primary control functions.^[22] The core aerodynamic principles of strakes revolve around vortex generation to control airflow separation, thereby enhancing lift and stability. Strakes create leading-edge vortices that persist over the wing and fuselage, re-energizing the boundary layer and delaying flow separation, which postpones stall and sustains lift at high angles of attack.^[22] These vortices also contribute to directional stability by producing side forces through asymmetric crossflow interactions, particularly on forebody strakes, where the leeward vortex induces yawing moments over the fuselage's long moment arm.^[23] Strakes differ from smaller vortex generators, which create localized turbulence to disrupt boundary layer separation; instead, strakes act as larger lifting surfaces that produce sustained, coherent vortices for global aerodynamic benefits, such as synergistic lift augmentation with the main wing.^[22] This vortex-lift mechanism fundamentally improves high-angle-of-attack handling by maintaining attached flow and reducing pitch-up tendencies through favorable pressure distribution.^[22]

Types and configurations

Strakes in aeronautics are categorized by their placement on the aircraft and the specific aerodynamic purposes they serve, such as enhancing stability, lift, or control through vortex generation. Common types include nose strakes, wing strakes, nacelle strakes, ventral strakes, and rear or anti-spin strakes, each designed to interact with airflow in targeted ways to improve handling characteristics, particularly at high angles of attack or low speeds.^[22]^[24] Nose strakes, mounted on the forward fuselage, primarily enhance yaw stability by generating controlled forebody vortices that provide directional control moments, especially beneficial for configurations like delta-wing aircraft operating at high angles of attack. These strakes manipulate asymmetric vortex shedding to produce yawing forces, improving maneuverability without relying solely on traditional control surfaces.^[25]^[26] Wing strakes, often configured as blended extensions from the wing leading edges, promote vortex lift by creating stable leading-edge vortices that delay flow separation and augment lift on highly swept wings, such as in double delta configurations. This design allows for increased lift at post-stall angles, contributing to overall aerodynamic efficiency in high-maneuver scenarios.^[22]^[24] Nacelle strakes, positioned on engine pods, mitigate interference drag and enhance wing effectiveness by generating streamwise vortices that counteract the adverse wake from the nacelle-pylon junction, thereby improving lift distribution across the wing at low speeds. These triangular or delta-shaped surfaces help maintain attached flow over the wing, reducing the impact of engine installation on overall performance.^[27]^[28] Ventral strakes, located under the fuselage, aid pitch control at low speeds by influencing pitching moments through vortex interactions that stabilize longitudinal dynamics and increase lift contributions from the fuselage. They help manage pitch-up tendencies during high-angle-of-attack maneuvers, providing additional control authority in configurations where traditional stabilizers may be less effective.^[29]^[30] Rear or anti-spin strakes, placed aft on the fuselage such as in dorsal or ventral positions, prevent spins by increasing damping in yaw and roll during rotary flow conditions, making recovery more predictable and reducing rotation rates. These strakes disrupt pro-spin vortices and enhance stability near the tail, particularly in aerobatic or high-agility designs.^[31]^[32] Key configuration factors for strakes include their angle of incidence, length, and leading-edge sweep, which are optimized to maximize vortex strength and stability while minimizing drag. For instance, higher sweep angles promote stronger, more persistent vortices for lift enhancement, whereas longer spans increase vortex area for greater control authority; these parameters are tailored based on the aircraft's mission to balance benefits like stall delay with overall aerodynamic integration.^[33]^[24]

Applications in aircraft design

In supersonic aircraft design, strakes have been employed to enhance stability during high-Mach operations. The Concorde utilized small nose strakes to improve directional stability, particularly by generating favorable vortex flows that augmented yaw control at various speeds, including transitions to supersonic regimes.^[34] These strakes contributed to the aircraft's overall handling by mitigating low-speed instabilities that could propagate during acceleration to Mach 2.^[34] Fighter jets have incorporated forebody and ventral strakes to boost maneuverability and control authority. Concepts for fixed forebody strakes, tested on the NASA F-18 High Alpha Research Vehicle (HARV), demonstrated generation of asymmetric vortices at high angles of attack, providing enhanced yaw power and enabling post-stall maneuvers up to 50 degrees angle of attack without reliance on thrust vectoring.^[35] This research significantly improved understanding of high-alpha agility for designs like the F/A-18 series, which uses leading edge extensions (LEX) to achieve similar vortex-lift benefits in dogfight scenarios by increasing sideslip control margins.^[36] Similarly, the Lockheed F-104 Starfighter featured a ventral fin—functioning as a strake-like surface—to increase static directional stability across the supersonic flight envelope, reducing Dutch roll tendencies and aiding high-speed control.^[37] In general aviation, ventral strakes address spin susceptibility in trainer and light aircraft. The SOCATA TB family, including the TB-10 Tobago, incorporates ventral strakes aft of the baggage compartment to meet spin recovery certification standards by enhancing directional stability and yaw damping during stalled flight.^[38] These features allow quicker pro-spin deceleration and recovery initiation, improving safety margins for student pilots.^[39] The de Havilland Canada DHC-1 Chipmunk employed anti-spin strakes on the rear fuselage to deter entry into flat spins, a modification introduced after early prototypes exhibited prolonged recovery times in testing.^[40] This addition ensured compliance with military trainer requirements, facilitating reliable spin avoidance and recovery within two turns.^[41] Experimental programs have leveraged strakes to stabilize unconventional configurations. The Grumman X-29 demonstrator integrated aft strake flaps as part of its three-surface control system, compensating for the inherent aeroelastic instabilities of its forward-swept wings by providing pitch authority and vortex management at high angles of attack up to 25 degrees.^[42] These strakes, combined with canards, enabled the aircraft to achieve relaxed static stability while maintaining positive handling qualities, validating forward-sweep benefits like reduced induced drag without compromising control.^[42] The aft strakes also assisted in takeoff by augmenting canard-induced pitching moments to reduce nose-wheel liftoff speeds.^[43] Strakes have demonstrated measurable performance benefits in operational contexts, particularly in high-alpha regimes and takeoff phases. In the F/A-18 series, forebody strake research extended controllable angle-of-attack limits in testing, enhancing maneuverability and reducing energy loss in aggressive turns.^[35] For the SOCATA TB and Chipmunk, ventral strakes improved spin recovery rates, contributing to lower stall/spin accident profiles in training fleets.^[38] On the X-29, aft strakes contributed to improved takeoff performance.^[43] Overall, these applications underscore strakes' role in balancing aerodynamic efficiency with safety across diverse aircraft types.

Civil engineering

Helical strakes for vibration control

Helical strakes are spiral-shaped aerodynamic devices consisting of fins or strips wrapped around cylindrical structures such as chimneys, stacks, or pipes to suppress vortex-induced vibrations (VIV).^[44] These passive flow control elements are particularly applied in civil engineering to protect tall, slender structures from wind-induced oscillations that could lead to fatigue or structural failure.^[45] The mechanism of helical strakes involves disrupting the organized vortex shedding process, where alternating vortices form in the wake of a structure under fluid flow, generating periodic forces that amplify vibrations at the structure's natural frequency—a phenomenon known as lock-in.^[44] By introducing artificial turbulence and altering the flow separation points along the structure's length, the strakes divide potential large-scale vortices into smaller, uncorrelated segments, thereby reducing the coherence and intensity of the wake.^[45] This "confuses" the incoming wind or water flow, preventing the buildup of resonant hydrodynamic forces that drive VIV.^[46] Design of helical strakes varies by application. For wind-exposed civil structures like chimneys and stacks, standards such as ASME STS-1 recommend a height of 0.1 times the structure's diameter (H = 0.1D), a helical pitch of 16 diameters (P = 16D), and a three-start configuration spaced 120 degrees apart. For subsea pipes and risers, parameters are often H ≈ 0.25D and P = 10–15D to account for underwater flow conditions, with one to three starts covering the full length or critical portions of the structure.^[46]^[47] These parameters balance suppression efficacy with added drag and manufacturing feasibility, though exact sizing may vary based on Reynolds number and flow conditions.^[48] In controlled wind tunnel and full-scale tests, helical strakes have demonstrated significant effectiveness, reducing VIV displacement amplitudes by up to 90% compared to bare cylinders, particularly in subcritical flow regimes relevant to many civil structures.^[49] For instance, configurations with H = 0.25D and P = 10D have suppressed oscillations to near-zero levels in tandem cylinder setups, though performance can degrade if strakes are applied only downstream due to wake interference.^[44] While they increase overall drag by 20–50%, this trade-off is often acceptable for vibration mitigation in static structures like chimneys.^[46]

Design standards and historical development

Helical strakes were invented in 1957 by Christopher Scruton and Denis E. J. Walshe at the National Physical Laboratory in the United Kingdom, initially to suppress airflow-induced oscillations on chimneys and similar cylindrical structures.^[50] Their design addressed vortex shedding by introducing helical protrusions that disrupt organized flow patterns, with early prototypes tested on full-scale stacks to verify reduced vibrations.^[46] This innovation marked a shift from passive damping methods to geometric flow control, quickly gaining adoption in industrial applications prone to wind-excited resonance. In the following decades, wind tunnel experiments from the 1960s through the 1980s refined strake parameters, focusing on pitch ratios of 12 to 20 times the cylinder diameter in early work, with coverage lengths often 50-70% of the exposed height to optimize suppression while minimizing drag penalties.^[51] Key studies, such as those by Woodgate and Maybrey in 1959 extending the original work, and later tests in the 1970s on marine analogs, demonstrated that triple-start configurations (three strakes spaced 120 degrees apart) provided the best balance of effectiveness and structural simplicity.^[52] These efforts culminated in the adaptation of strakes for underwater currents, particularly during the offshore oil boom of the 1970s, where they were applied to risers and pipelines to mitigate current-induced vibrations analogous to aerial vortex shedding.^[53] Recent studies as of 2025 continue to optimize coverage and geometry, showing 75% coverage effective for VIV reduction in flexible civil structures like high-mast towers.^[54] Codification into standards began with the American Society of Mechanical Engineers (ASME) STS-1 for steel stacks in the 1980s, specifying helical strakes for structures where natural frequency aligns with wind speeds, recommending three strakes with coverage over two-thirds of the height above the critical velocity zone, height of 0.1D, and pitch of 16D.^[47] The American Petroleum Institute (API) followed with guidelines in RP 2RD (1993, updated periodically) for design of risers and RP 2SIM for structural integrity, incorporating strakes as a primary VIV suppression method for offshore installations.^[55] Modern iterations of these standards, such as ASME STS-1-2021, continue to emphasize these parameters for steel stacks to ensure compatibility with finite element analysis for fatigue prediction.^[47] Contemporary designs emphasize durable materials suited to environments, including galvanized or stainless steel for atmospheric exposure on stacks and UV-resistant polymers like high-density polyethylene for subsea or splash-zone applications on risers, balancing corrosion resistance with installation ease.^[56] Over time, applications have evolved from early use on industrial smokestacks to comprehensive deployment on offshore platforms, subsea risers, and thermowells in petrochemical processes, where compact strakes prevent flow-induced failures in high-velocity lines.^[57] This progression reflects ongoing refinements driven by computational modeling and field data, enhancing reliability across civil engineering contexts.^[58]

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Helical strakes are a tried-and-true solution for suppressing vortex-induced vibration (VIV) of subsea tubulars such as risers, tendons, and pipelines.
[60]
Thermowell Helical Design Reduces Vortex Vibrations - WIKA blog
Engineers have used helical strakes for decades to reduce vortex-induced ... petrochemical processes, on/offshore applications, and plant construction ...
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[PDF] DEEPWATER RISER DESIGN, FATIGUE LIFE AND STANDARDS ...
A small number of reports of strake damage during offshore SCR installation have been received. Typically, the damage is surveyed and fatigue life re-evaluated ...