Marine & Coastal Calculators
7 free marine & coastal calculators — standards-based, with formulas, worked examples, and no sign-up.
Two audiences share the water, and this category serves both. Boat owners get the small-craft canon: hull speed from the classic 1.34·√LWL displacement limit with speed-length regime bands and a Crouch planing estimate — the honest framing being that Dave Gerr's propeller and boat-design references treat these as regimes, not walls; propeller sizing as the four-way slip/pitch/RPM/speed solve with slip bands by hull type; anchor rode scope by conditions (5:1 day, 7:1 overnight, 10:1 storm — Chapman Piloting and USCG Auxiliary practice) with the taut-rode swing circle; and dock flotation from dead plus live loads with the float industry's 50% dead-load reserve rule and a freeboard estimate.
Coastal engineers get the design formulas: wave loads on piles by the Morison equation with Airy (linear) wave kinematics, split into inertia and drag components and integrated to base shear and overturning moment; breakwater armor by the Hudson formula with Kd stability coefficients per armor type, yielding required unit mass, cube size, and count; and ship stability via the full KB + BM − KG = GM chain, with a stability verdict and righting-arm GZ curve. The two halves genuinely differ: the small-craft tools encode customary practice, the coastal tools encode published theory — each page says which it is.
All Marine & Coastal Tools
Wave Load Calculator (Morison)
Maximum wave force on a vertical cylindrical pile via the Morison equation and linear Airy wave theory: velocity, acceleration, inertia and drag force, total force and overturning moment.
Metacentric Height Calculator
Floating box-barge stability: displacement, KB, BM, metacentric height GM = KB + BM - KG, stability verdict, and righting arm GZ at a chosen heel.
Breakwater Armor Unit Calculator
Hudson formula armor unit weight for rubble-mound breakwaters: relative density, equivalent cube size, layer thickness, and units per area with a Kd armor-type table.
Hull Speed Calculator (1.34·√LWL & Crouch)
Classic displacement hull speed V = 1.34·√LWL in knots and mph, the speed-length ratio with displacement / semi-displacement / planing guidance bands, and Crouch's empirical planing speed estimate V = C/√(lb/SHP) in knots per Gerr's Propeller Handbook with hull-type C presets 150/190/210 or a custom C.
Propeller Sizing Calculator (Prop Slip, Pitch, RPM & Speed)
Four-way prop slip solver: boat speed = (RPM ÷ gear ratio) × pitch × (1 − slip) ÷ 1056 in mph and knots, actual slip from a GPS speed with a faster-than-theory data-error check, or the pitch or engine RPM needed for a target speed, with typical-slip guidance bands by hull type (planing 10–15%, displacement 25–45%) and an RPM-vs-speed chart.
Dock Flotation Calculator (Float Count & Buoyancy)
How many floats a floating dock needs: dead + live loads from deck area (representative 10 / 25 psf defaults), per-float capacity from dimensions × water density (fresh 62.4 / salt 64.0 lb/ft³) or a manufacturer rating, the representative 50% dead-load submergence sizing practice, a freeboard estimate, and an adequate / marginal / overloaded verdict for an existing float count.
Anchor Rode Calculator (Scope & Swing Radius)
How much anchor line to let out from customary seamanship practice: required rode = scope × (depth + bow freeboard + tide allowance) with 5:1 day / 7:1 overnight / 10:1 storm presets or a custom ratio, the taut-rode swing radius and diameter including boat length with a top-view sketch, and an adequate / short check of the rode you carry.
Frequently Asked Questions
Is hull speed a hard limit on my boat?
No — it is the speed-length ratio (about 1.34) where wave-making resistance climbs steeply for displacement hulls. Semi-displacement hulls push past it with enough power, and planing hulls climb over their bow wave entirely (that is what the Crouch estimate covers). The tool reports your S/L regime rather than a wall: for a pure displacement hull, chasing speed past S/L ≈ 1.34 buys almost nothing per extra horsepower.
How do propeller slip and pitch relate, and which do I solve for?
Theoretical speed is pitch × RPM ÷ gear ratio; real boats fall short by the slip percentage, which varies by hull type (light planing hulls ~10–15%, heavy displacement ~25%+). The tool solves whichever variable you leave blank. The usual diagnosis workflow: measure actual top speed at known WOT RPM, solve for slip, and compare against the expected band — abnormally high slip suggests wrong pitch, fouling, or an overloaded boat.
Why does the dock calculator reserve 50% of flotation for dead load?
A dock must float level and safely with nobody on it, with enough reserve that live load (people, gear, snow) does not push the deck under. Float manufacturers' sizing practice reserves roughly half of total buoyancy for the structure itself, leaving the rest as live-load margin — the tool applies that rule and estimates resulting freeboard, so you can see how the dock will actually sit before you buy floats.