Insel & Molland Catamaran Interference Resistance Calculator
Resistance, EHP, BHP, and propulsion-engine matching for catamaran hulls using an Insel & Molland-style interference structure
Inputs
Hull Dimensions
Coefficients
Catamaran Geometry
Leave WSA as 0 for automatic calculation via Insel & Molland formula
Hull Dimensions
Coefficients
Catamaran Geometry
Leave WSA as 0 for automatic calculation via Insel & Molland formula
Environmental & Calculation
Final calculated result
Total resistance RT
—
kN
Frictional resistance RF
—
kN
Residual resistance RR
—
kN
Interference resistance Ri
—
kN
EHP = RT × V
—
kW
DHP = EHP / ηP
—
kW
BHP = DHP / ηT
—
kW
Maximum required power
—
kW = BHP / margin
Froude Number Fn
—
V / √(gLWL)
Reynolds Number Re
—
×10⁶
Wetted surface area
—
m²
Spacing ratio S/L
—
ratio
CF
—
×10⁻³
CR
—
×10⁻³
CRi
—
×10⁻³
CT
—
×10⁻³
Status: —
Recommended Propulsion Engine(s)
Engine matching now uses the highest required power found inside the selected speed range, not only the current speed. The sizing power is calculated as BHP ÷ MCR service margin, split evenly across the selected number of engines, then matched against the reference marine diesel engine database. Figures are approximate — always verify against current manufacturer data sheets and classification-society requirements before final selection.
Sizing speed in range
—
knots
Sizing total required power
—
kW
Required power per engine
—
kW
Required power per engine
—
HP
Hull & Hydrostatic Properties
Waterline length
—
m
Beam (demihull)
—
m
Draft
—
m
Displacement
—
t
Wetted surface
—
m²
Hull spacing
—
m
Spacing ratio (S/L)
—
ratio
Displacement/Δ
—
m³
Charts
Speed vs EHP with displacement, transition, and high-speed catamaran regions
Green = displacement region, orange = transition region, blue = high-speed catamaran region. The red marker labels the EHP corresponding to the currently selected speed.
Maximum required power versus speed
Maximum required power = BHP / margin. Green = displacement region, orange = transition region, blue = high-speed catamaran region. The curve extends to the selected speed/range entered.
Resistance Coefficients vs Speed
Frictional (CF), residual (CR), interference (CRi), and total (CT) resistance coefficients. Values are plotted as ×10⁻³.
Speed Range Performance Table
Detailed resistance and power calculations across the requested speed range.
Insel & Molland Method Overview
FRICTIONAL RESISTANCE (ITTC 1957):
Re = V × LWL / ν
CF = 0.075 / (log₁₀(Re) − 2)²
RF = 0.5 × ρ × V² × WSA × (1+k) × CF
FROUDE NUMBER:
Fn = V / √(g × LWL)
RESIDUAL RESISTANCE:
The Insel & Molland method does not use one universal closed-form equation.
The residual resistance coefficient must be obtained from the published Insel &
Molland regression equations/coefficient tables as a function of:
Fn, L/B, B/T, CB, CP, and S/L.
CR = f(Fn, L/B, B/T, CB, CP, S/L)
RR = 0.5 × ρ × V² × WSA × CR
CATAMARAN INTERFERENCE:
Do not use Ri = Φ × RR.
The catamaran interference is represented as an additional coefficient:
CRi = f(Fn, S/L, L/B, B/T, CB, CP)
Ri = 0.5 × ρ × V² × WSA × CRi
TOTAL RESISTANCE COEFFICIENT:
CT = (1+k)CF + CR + CRi + ΔCF
TOTAL RESISTANCE:
RT = 0.5 × ρ × V² × WSA × CT
POWER CALCULATIONS:
EHP = RT × V / 1000 (kW)
DHP = EHP / ηP
BHP = DHP / ηT
Maximum required power = BHP / MCR service margin
ENGINE SELECTION (reference only):
Required MCR per engine = max(BHP / margin across selected speed range) / number of engines
Matched against a reference engine database (rated HP, RPM, weight).
Best match = smallest database engine with rated HP ≥ required HP per engine at the speed-range sizing point.
NOTE:
This version is structured correctly for the Insel & Molland method. The functions
inselMollandCR() and inselMollandCRi() are isolated so the exact published
regression coefficients can be inserted directly. The current fallback equations are
clearly marked as provisional and should not be used for final design validation.
The engine database is illustrative/reference only — verify against current
manufacturer data sheets before specifying an engine.