CatamaranHull Engineering
Technical Reference — Hydrodynamics, Geometry & Structural Systems
- OVERALL BEAM (B) — full width from outer edge of port hull to outer edge of stbd hull
- hull beam (b) — individual hull width, shown above both hulls
- DRAFT (T) — depth from waterline to bottom of hull, marked on the starboard side
- BRIDGEDECK — the cross-structure connecting the two hulls
- PORT HULL / STBD HULL — each labeled in their respective hull
- CLR (centre of lateral resistance) — marked with a crosshair symbol on each keel fin, with the label bridging both
The dashed blue line is the waterline, and the dashed vertical lines show each hull’s centerline. Let me know if you’d like to adjust any parameter positions, add more dimensions (e.g. freeboard, beam-to-length ratio annotations), or switch to a side/profile view.
Naval Architecture · Multihull Design
Fundamentals
A catamaran achieves its performance through the separation of two slender hulls, exploiting wave-making resistance far more efficiently than equivalent monohull displacement.
The defining characteristic of catamaran hull design is the extreme length-to-beam ratio of each individual hull — typically 8:1 to 15:1 — combined with a shallow, fine entry that minimises wave-making at speed. By distributing displacement across two hulls, each hull can operate at significantly reduced displacement and depth, keeping wetted surface area low while the wide overall beam delivers exceptional stability without the ballast required by monohulls.
The bridgedeck connecting the hulls is a critical structural and hydrodynamic element. Its clearance above the waterline governs slamming behaviour in waves, while its aerodynamic profile affects drag and accommodation volume. Racing cats may minimise this structure entirely; cruising designs exploit the space for living quarters.
Hull Form Taxonomy
Form Type · 01
Round Bilge
Smooth, continuous curvature from keel to topsides. Optimised for minimum wetted surface and laminar flow retention at displacement speeds. The preferred form for cruising designs prioritising low-resistance passage-making.
Wetted surfaceMinimal
Wave-makingLow
Build complexityHigh
Typical L/B9:1 – 12:1
Form Type · 02
Hard Chine
Angular transition between hull panels, enabling flat-panel construction in composite materials. Popular in performance cruisers and amateur builds. The chine creates additional lift at speed but slightly increases wetted area at rest.
Wetted surfaceModerate
Dynamic liftGood
Build complexityLow
Typical L/B8:1 – 11:1
Form Type · 03
Wave-Piercing
Extremely fine, plumb bow designed to penetrate rather than ride over wave crests. Dramatically reduces pitching motion and slamming. Standard on high-performance racing cats and fast ferry designs; increases structural loading on bow sections.
SeakeepingExcellent
Pitching motionVery Low
Structural loadHigh
Typical L/B12:1 – 18:1
Form Type · 04
Asymmetric
Hull sections that differ between the inboard and outboard faces. The inboard face is typically flat or concave to reduce interference between hulls; the outboard face is curved for hydrodynamic efficiency. Common in performance multihulls.
Channel flowOptimised
Interference dragLow
Design complexityHigh
Typical L/B10:1 – 14:1
Form Type · 05
Tunnel / Demi-hull
Hulls shaped to create an enclosed or semi-enclosed tunnel between them, generating aerodynamic and hydrodynamic lift from ram-air and Venturi effects. Used in high-speed powercat designs and offshore racing platforms.
High-speed liftExcellent
Low-speed dragModerate+
ApplicationRacing / Power
Typical L/B6:1 – 9:1
Form Type · 06
Foiling Platform
Hulls designed primarily as structural attachment points for hydrofoils. At flying speed the hulls clear the water entirely, leaving only foil struts submerged. Resistance drops by 70–90% compared to hull-borne mode. Requires active or passive flight control systems.
Foil-borne dragMinimal
Takeoff speed12–18 kts
ComplexityVery High
Top speed40–60+ kts
03
Hydrodynamic Performance
Resistance in a catamaran hull system is dominated by two components: frictional resistance — proportional to wetted surface area and flow velocity — and wave-making resistance, which increases sharply as the hull speed Froude number (Fn = V / √(g·L)) approaches and exceeds 0.4.
At Fn < 0.35, catamaran hulls operate in the displacement regime. Wave-making is low and frictional resistance dominates. Slender hulls with L/B > 10 achieve significant friction reduction by keeping cross-sections fine throughout the waterplane area.
As Fn approaches 0.4–0.55, the hull rides between its own bow and stern waves — the transition or “hump” region — where total resistance peaks dramatically. Many cruising catamarans are underpowered to push past this hump in light airs, limiting practical hull-borne speed.
Above Fn 0.55, semi-displacement behaviour emerges. The hull climbs its bow wave and resistance growth plateaus. This regime characterises performance cruising cats in brisk winds and is the operating envelope for which most modern designs are optimised.
Wave interference between the two hulls creates a complex pattern. When hull spacing (s/L) produces constructive wave interference, total resistance increases markedly. Optimal hull spacing minimises this interaction — typically s/L ≈ 0.25 to 0.35 for cruising designs.
Typical L/B Ratio
length / hull beam
10:1
Hull Speed (Fn 0.4)
theoretical limit
1.34√L
Wetted Area Ratio
vs monohull equiv.
~0.65×
Optimal Hull Spacing
s / waterline length
0.30
Stability Ratio (GZ)
righting lever advantage
3–4×
Bridgedeck Clearance
min. above DWL
600mm
04
Resistance Breakdown
Approximate percentage contribution to total drag at Fn = 0.40 (displacement / semi-displacement boundary)
05
Principal Forces & Equations
| Force / Parameter | Expression | Description | Design Impact |
|---|---|---|---|
| Frictional Resistance | Rf = ½ρV²·S·Cf | ITTC friction line coefficient applied to wetted surface S. Dominant at low Froude numbers. Minimised by reducing S and maintaining laminar flow. | Critical |
| Wave Resistance | Rw = f(Fn, L/B, Cp) | Dependent on Froude number and prismatic coefficient Cp. Fine entry (Cp ≈ 0.55–0.60) reduces wave-making at mid-range speeds. | Critical |
| Froude Number | Fn = V / √(g·Lwl) | Non-dimensional speed parameter. Determines resistance regime: <0.4 displacement, 0.4–0.55 transitional, >0.55 semi-displacement. | Governing |
| Displacement / Volume | ∇ = Δ / ρg | Displaced volume equal to vessel mass divided by seawater density (1025 kg/m³). Each hull carries approximately half total displacement in level trim. | Structural |
| Prismatic Coefficient | Cp = ∇ / (Ax·Lwl) | Ratio of displacement volume to circumscribed prism. For catamarans: Cp = 0.55 to 0.62 represents typical cruising compromise. | Form |
| Metacentric Height | GM = KB + BM − KG | Initial stability measure. Catamaran BM is dominated by the transverse separation of buoyancy centres (BM ≈ s²/12·∇) rather than hull form — inherently large, providing high initial stiffness. | Safety |
| Slamming Pressure | Ps = ½ρ·Cv²·V² | Dynamic impact pressure on bridgedeck from wave contact. Cv is the slam coefficient (1.5–3.5 for bridgedeck entry angle 10–20°). Primary structural load for bridgedeck scantlings. | Structural |
| Cross-Beam Torsion | T = F·(s/2)·sin(θ) | Torsional moment at hull-beam junction from differential heave of hulls in seaway. Governs cross-beam section modulus and joint design. Critical fatigue location. | Fatigue |
| Appendage Drag | Ra = ½ρV²·Af·Cd | Drag from keels, rudders, and shaft brackets. Twin rudders and twin keels multiply this term; integrated NACA sections minimise Cd to 0.005–0.012. | Secondary |
06
Construction Materials
GRP / FRP
Standard
Density1750 kg/m³
Tensile str.310 MPa
StiffnessModerate
Cost index1.0×
Fatigue lifeGood
RepairabilityExcellent
E-Glass / Epoxy
Standard+
Density1800 kg/m³
Tensile str.440 MPa
StiffnessModerate+
Cost index1.6×
Fatigue lifeVery Good
RepairabilityGood
Carbon Fibre
Premium
Density1550 kg/m³
Tensile str.1500 MPa
StiffnessVery High
Cost index8–15×
Fatigue lifeExcellent
RepairabilitySpecialist
Basalt Fibre
Advanced
Density2700 kg/m³
Tensile str.4840 MPa
StiffnessHigh
Cost index3–4×
Fatigue lifeExcellent
RepairabilityGood
07
Aluminium Construction
Aluminium alloy construction occupies a well-defined niche in catamaran building: heavier than composite but massively more repairable, weldable in any boatyard worldwide, and offering an indefinite structural lifespan when correctly specified and protected.
Marine-grade aluminium alloys — principally the 5000 and 6000 series — provide the corrosion resistance and weldability necessary for seagoing structures. The key trade-off versus composite is weight: an aluminium catamaran hull structure will typically run 20–35% heavier than an equivalent GRP/epoxy layup, and 50–80% heavier than a carbon-fibre design of the same stiffness. This weight penalty is most punishing in performance cruisers, less significant in live-aboard or expedition designs where structural resilience and long-term maintenance costs dominate the decision.
Aluminium catamarans are fabricated by MIG or TIG welding pre-cut alloy plate and extrusions. The material is naturally self-supporting in stiffened-panel form, eliminating the sandwich core construction required in composites. Hull panels are typically 4–6 mm plate for cruising designs, with framing on 400–600 mm centres. Transverse frames at hull stations and longitudinal stringers combine to form a semi-monocoque structure with excellent energy absorption in grounding events.
Marine Alloy Specifications
5083
Al–Mg · Work Hardened
UTS (H116)305 MPa
Yield strength215 MPa
Density2660 kg/m³
Elongation10%
WeldabilityExcellent
Corrosion resist.Excellent
Typical useHull plating, frames
Primary Hull Plate
5086
Al–Mg · Higher Mg
UTS (H116)290 MPa
Yield strength207 MPa
Density2660 kg/m³
Elongation12%
WeldabilityExcellent
Corrosion resist.Very Good
Typical useDeck, superstructure
Deck Structure
6061
Al–Mg–Si · Heat Treated
UTS (T6)310 MPa
Yield strength276 MPa
Density2700 kg/m³
Elongation8%
WeldabilityGood*
Corrosion resist.Good
Typical useExtrusions, mast bases
Structural Extrusions
* 6061-T6 loses ~40% strength in HAZ post-weld; post-weld heat treatment or conservative design margins required at weld zones.
Fabrication & Design Considerations
| Topic | Typical Value / Standard | Notes |
|---|---|---|
| Hull plate thickness | 4 – 6 mm (5083) | Minimum 4 mm for oceanic cruising. Thicker in bow sections and keel areas subject to impact or grounding loads. |
| Frame spacing | 400 – 600 mm | Transverse frames at major hull stations; longitudinal stringers at 150–250 mm spacing on bottom plating. |
| Welding process | MIG (GMAW) / TIG (GTAW) | MIG with 5356 filler wire for high-speed production. TIG for precision joints and thin sections. Preheat not required for 5083. |
| HAZ strength loss | ~30–35% (5083), ~40% (6061-T6) | Heat-affected zone extends ~20–30 mm from weld bead. Design must account for reduced yield strength in this zone per DNV / ISO 12215. |
| Corrosion protection | Anodising + 2-part epoxy primer | Bare aluminium is naturally passive in saltwater but requires isolation from dissimilar metals (bronze fittings, stainless fasteners) to prevent galvanic attack. |
| Antifouling compatibility | Tin-free, no cuprous oxide | Copper-based antifoulants are incompatible with aluminium and cause rapid galvanic corrosion. Use vinyl or water-based antifoulants with epoxy barrier coat. |
| Bridgedeck scantlings | 5 – 8 mm plate + T-section frames | Bridgedeck underside is the highest-loaded structural panel. Slamming pressure governs; stiffener spacing reduced to 200–300 mm in slamming zone. |
| Fatigue life | Indefinite (properly designed) | Aluminium does not corrode to structural failure if protected. Unlike GRP, there is no osmotic degradation. Crack initiation at weld toes is the primary fatigue mode; design to DNV RP-C203 fatigue categories. |
| Classification standard | ISO 12215-5 / DNV Rules | ISO 12215-5:2019 covers scantling requirements for aluminium recreational craft. DNV Rules for Classification apply to commercial and charter vessels. |
Advantages & Limitations vs Composite Construction
Advantages
- Repairable anywhere in the world with basic welding equipment — no specialist laminating skills or vacuum infusion equipment required
- Indefinite structural lifespan with correct corrosion protection; no osmotic blistering or UV degradation of structural fibres
- High impact and grounding resistance; aluminium deforms rather than delaminating, absorbing energy progressively
- No health hazards associated with laminating resins or styrene emissions during construction
- Intrinsically fireproof hull structure — critical for charter certification and passenger vessel compliance
- Lower tooling cost for one-off and small-series builds; no moulds required, direct plate-and-frame fabrication from CAD files
- Consistent, inspectable weld quality verifiable by NDT; no hidden voids or dry glass as in hand-laminated composites
Limitations
- 20–40% heavier than equivalent GRP/epoxy structure; 50–80% heavier than carbon fibre — directly reduces sail-carrying ability and light-wind performance
- Galvanic corrosion risk wherever aluminium contacts dissimilar metals; requires careful isolation of all through-hull fittings, keel bolts, and shaft systems
- Incompatible with copper-based antifoulants; specialist antifouling regime adds cost and complexity compared to GRP hulls
- HAZ strength loss at welds requires conservative scantling allowances or post-weld heat treatment, increasing structural weight further
- Higher thermal conductivity than GRP — increased condensation inside hull; insulation of accommodation spaces is mandatory for comfort
- Significantly higher material cost than glass fibre; comparable to E-glass/epoxy but more expensive per structural unit than polyester GRP
- Electrolytic corrosion can occur rapidly in poorly maintained shore-power connections; stray-current protection systems are essential
