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Resistance & Powering

Resistance is a sum of coefficients, every one of them riding on the same ½ρSV²:

C_TS = (1+k)·C_FS + ΔC_F + C_A + C_W + C_AA

Friction comes from the ITTC-1957 correlation line, the roughness and correlation allowances from the ITTC-1978 performance prediction method, and water density and viscosity from the ITTC property tables. The split between the terms is the useful output: at low speed a hull is a flat plate and friction is about wetted area; near hull speed the wave term takes over and that is about length. Which one you are fighting decides whether to chase a cleaner bottom or a longer boat.

C_W is an input, and it has to be. Nothing computes wave resistance from the main dimensions — it comes from a model test, a systematic series or CFD. Everything else on this page is either a published formula or a table, so the honest thing is to take the one number that is yours and ask for it.

Hull and condition

Speed (knots)

Waterline length L (m)

Wetted surface S (m²)

Water

Water temperature (°C)

Form factor (1+k)

Wave resistance C_W (×10⁻³)

Appendages C_APP (×10⁻³)

Hull roughness k_S (µm)

Windage area above water (m²)

Air drag coefficient C_DA

Correlation allowance C_A

Reynolds number

—

Froude number

—

ρ, ν used

—

C_T total

—

Total resistance

—

Effective power P_E

—

Where the drag is

From water to engine

Every coefficient below belongs to your hull and your propeller, so every one is an input — what this owns is how they multiply. η_D = η_O·η_H·η_R with η_H = (1−t)/(1−w), and the shafting takes its cut after that.

Open-water efficiency η_O

Wake fraction w

Thrust deduction t

Relative rotative η_R

Shafting η_S

Hull efficiency η_H

—

Quasi-propulsive η_D

—

Overall, P_E/P_S

—

Delivered power P_D

—

Shaft power P_S

—

Shaft power at +10% speed

—

A coefficient bookkeeping method, only as good as the C_W you put into it. It assumes a displacement hull whose wetted surface and attitude do not change with speed — which stops being true once a planing hull starts to lift, where wetted area, trim and the whole basis move together. The ITTC-1978 roughness and correlation allowances are ship-scale fits and are flagged where a small craft falls outside them. Nothing here covers added resistance in waves, shallow water, wind on the beam, or cavitation and the propeller itself. A helper, not a substitute for engineering judgment.