The Boat Mill

Boat calculators, with the working shown

The Boat Mill is a set of 43 calculators for the numbers that come up on a boat: how far off that headland is, how much chain to let out, whether the fuel will last, how long the batteries will carry the fridge, and how much resin to mix. Each one shows its formula and a worked example, so you can check the answer before you rely on it.

What you will find here

Every page has a calculator that updates as you type, the formula written out in plain words, a worked example with real numbers, and a list of what the math does not account for. Twenty of the pages (marked ★ below) are full guides: they explain when the method is the right one, walk through two or three realistic cases, include a reference table generated from the same formula, and list the mistakes people actually make with it.

Nothing you type leaves your device. The calculators run in the browser, and there is no account to create.

Start with a guide

Underway: Running fix / doubling the angle, Distance off by vertical angle, Course to steer for current, Set and drift, Apparent wind and VHF radio horizon.

At anchor: Anchor rode length and Anchor chain catenary.

Fuel and power: Fuel range / hours, Battery bank runtime and Battery cable voltage drop.

In the yard: Deck seam caulk, Fiberglass layup resin, Epoxy fillet volume, Dropboard stack height, Cabin sole hatch cutout area, Pounds per inch immersion and Inclining test GM.

On the ramp and at the prop shop: Launch ramp backing distance and Prop pitch / RPM.

How to get an answer you can trust

Measure on the boat. A tape measurement, a reading off the tank gauge after a known fill, or the amp draw on your battery monitor beats a brochure figure every time.
Check the units on each field. Most wrong answers come from metres typed as feet, speed over ground used where speed through the water belongs, or fabric weight per square yard entered as per square foot.
Run the worked example first. Put the example numbers in and confirm you get the same result. Then change one input at a time to your own values.
Keep a margin. Fuel reserves, under-keel clearance and voltage-drop limits exist because the real boat is never as tidy as the formula. The guides say how much margin is usual and why.

These are planning numbers. Charts, tide tables, the engine and battery makers’ manuals, and local knowledge come first when they disagree with a calculator.

All 43 calculators by job

Filter tools

Navigation & pilotage

Position fixes, current, clearance and radio range worked from numbers you can take on deck or off the chart.

Running fix / doubling the angle ★ — Two bearings on one landmark and the run between them give your distance off and passing distance.
Distance off by vertical angle ★ — Sextant angle to a light or cliff of charted height gives distance off.
Course to steer for current ★ — Heading to hold a track across a known current, with speed made good and passage time.
Set and drift ★ — Back out the current from your course and speed through the water versus over the ground.
Apparent wind ★ — Combine true wind and boat speed into the wind the sails actually feel.
VHF radio horizon ★ — Line-of-sight VHF range from both antenna heights.
Under-keel clearance — Charted depth plus tide, minus draft, squat, waves and your margin.
Bridge clearance — Charted vertical clearance corrected for the tide against your air draft.
Tide window planner — Tide height you need over a bar or shoal, checked against high and low water.

Anchoring & docking

Rode, scope, chain shape and dock-line sizing for the boat you have and the water you are in.

Anchor chain catenary ★ — How much all-chain rode lifts off the bottom at a given wind load.
Anchor rode length ★ — Rode to let out for depth, bow height, tide rise and the scope you want.
Anchor scope / horizontal — Actual scope and horizontal reach of the rode you have out.
Anchor rode nylon stretch — How far a nylon rode stretches under load and what that does to scope.
Dock line length — Bow, stern, breast and spring line lengths from boat length.
Spring line length — Spring and breast lines sized to boat length plus cleat-to-cleat extra.
Cleat size — Line diameter for the boat and the cleat length that suits it.
Fender spacing — Even fender spacing along the side you put against the dock.

Fuel, power & water

Range, runtime and wiring numbers that answer the question every trip starts with: is there enough?

Fuel range / hours ★ — Hours and nautical miles left after holding back a reserve.
Fuel weight — Weight of gasoline, diesel and water aboard.
Propane cylinder runtime — Hours and days a propane cylinder lasts at your burn rate.
Battery bank runtime ★ — Hours a house bank can carry a steady load, by battery chemistry.
Battery series / parallel — Bank voltage, amp-hours and usable watt-hours from how batteries are wired.
Battery cable voltage drop ★ — Voltage lost in a DC cable run, by wire gauge, length and current.
Shore power amps — AC load in amps against a 30 A or 50 A shore cord.
Solar harvest Ah — Daily amp-hours from panel watts and peak sun hours.
Watermaker runtime — Watermaker hours, and the amp-hours they cost, to make a day’s water.

Hull, refit & stability

Material takeoffs for yard work, plus how weight and a refit change the way the boat floats.

Deck seam caulk ★ — Seam compound and cartridges for a laid teak deck.
Fiberglass layup resin ★ — Glass weight and resin needed for a hand layup.
Epoxy fillet volume ★ — Thickened epoxy for cove and straight fillets.
Antifouling paint area — Bottom area and gallons of antifouling for the coats you plan.
Dropboard stack height ★ — Face height of each dropboard with laps, and the stowed thickness of the set.
Cabin sole hatch cutout area ★ — Total area of the openings you plan to cut in a cabin sole.
Pounds per inch immersion ★ — How far the boat settles per pound of fuel, water or gear.
Inclining test GM ★ — Metacentric height from a simple weight-shift heel test.
Displacement from dimensions — Displacement from waterline length, beam, draft and block coefficient.

Trailering & launching

Getting the boat on and off the trailer and down the road without drama.

Launch ramp backing distance ★ — How far down a ramp of a given grade the trailer must go to float the hull.
Trailer tongue weight — Target tongue weight for a loaded boat trailer.
Trailer bunk spacing — Support spacing along each bunk.
Trailer winch strap length — Winch strap length including the wraps on the drum.

Props, engine & sails

Propeller swaps, sail plan ratios and running-rigging lengths.

Prop pitch / RPM ★ — Expected wide-open-throttle RPM after a change of pitch.
Prop tip clearance — Tip clearance in the aperture and the biggest prop that keeps it.
Sail design ratios — Displacement/length, comfort ratio and capsize screening value.
Genoa sheet length — Genoa sheet length from the lead route, foot and winch wraps.