Boat & Marine Calculator

A collection of marine calculators for boat owners and marine enthusiasts

Boat & Marine Calculator - Introduction

HullMetric brings together fourteen free marine calculators that cover the questions boat owners run into most: how much fuel a trip will burn, how long a passage takes at a given speed in knots, how much anchor rode to pay out, how much engine horsepower a hull needs, whether a tow vehicle can handle the boat-and-trailer load, how many people a boat can safely carry, how much bottom paint a hull requires, and how a propeller's pitch, diameter, and slip translate into real-world speed. There are also tools for battery runtime at anchor, capsize screening (CSF) for offshore suitability, theoretical hull speed from waterline length, and fuel tank capacity from tank dimensions. Most results are shown in both imperial and metric units — gallons alongside liters, pounds alongside kilograms, knots alongside km/h — and several tools also accept metric input directly.

How It Works

Each calculator implements a published formula from naval architecture or established marine practice rather than a black-box model. Hull speed uses 1.34 × √LWL (ft) for displacement hulls and a 2.75 coefficient for semi-displacement hulls. Fuel burn is estimated from brake-specific fuel consumption — roughly 0.5 lb per horsepower-hour for gasoline engines and 0.3 for diesels. Anchor rode is (water depth + tidal range + bow freeboard) × a scope ratio that runs from 5:1 in calm water to 10:1 in storm conditions. Battery runtime applies each chemistry's usable depth of discharge — 50% for flooded lead-acid, 80% for AGM, 90% for lithium — plus the Peukert effect. Enter your boat's numbers and the result appears instantly, with unit conversions handled for you.

Usage Scenarios

  • Planning a weekend coastal trip: a skipper with a 200 hp gasoline outboard estimates roughly 15 GPH at 75% throttle (0.10 × 200 × 0.75), checks the navigation calculator for passage time at cruise speed, and confirms with the fuel tank calculator that the tank's capacity leaves an adequate reserve.
  • Anchoring overnight in 20 ft of water: with a 2 ft tidal range and 4 ft of bow freeboard, the anchor scope calculator recommends paying out (20 + 2 + 4) × 7 = 182 ft of rode at a moderate 7:1 scope — and 260 ft if conditions call for a 10:1 storm scope.
  • Diagnosing a slow boat: a propeller with 17-inch pitch turning 4,000 RPM through a 2.0:1 gearbox has a theoretical speed of about 28 knots; if GPS shows 24 knots, the slip calculator reports roughly 14% slip, which helps separate normal slip from a fouled hull or the wrong prop.
  • Comparing sailboats for offshore passages: the capsize screening calculator condenses beam and displacement into a single CSF number, while the hull speed tool shows that a 25-ft waterline yields 1.34 × √25 = 6.7 knots — both useful when shortlisting candidate boats on paper.

Frequently Asked Questions

Which formula does the hull speed calculator use?

It uses the classic displacement-hull relationship: hull speed in knots = 1.34 × √(waterline length in feet). A boat with a 36 ft waterline, for example, has a hull speed of 1.34 × 6 = 8.04 knots. For semi-displacement hulls the calculator applies a higher coefficient, 2.75 × √LWL, reflecting their ability to partially climb their own bow wave.

What does CSF mean in the capsize screening calculator?

CSF is the Capsize Screening Formula: beam in feet divided by the cube root of (displacement in pounds ÷ 64 for seawater, or ÷ 62.4 for freshwater). It is a quick screening ratio, not a full stability analysis. The calculator flags results under 2.0 as generally suitable for offshore work, 2.0–2.2 as marginal, and above 2.2 as better suited to protected waters.

How is propeller slip calculated?

Theoretical speed in knots is (pitch in inches × engine RPM) ÷ (gear ratio × 1215.2), and slip is the percentage gap between that and your actual GPS speed: (theoretical − actual) ÷ theoretical × 100. Some slip is always present — a propeller works by accelerating water, so it never advances its full pitch — but a sudden increase can point to fouling, prop damage, or ventilation.

Are these calculators a substitute for capacity plates or manufacturer specs?

No. They are planning estimates built on published formulas — for example, maximum persons ≈ (length × beam) ÷ 15 for a typical v-hull, or tank gallons = cubic inches ÷ 231. Your boat's capacity plate, engine manual, and manufacturer documentation remain the authoritative limits; the calculators are best used to sanity-check and plan around those figures.