Free Sailing
Every number, with the formula under it
Sail area, ratios, stability, speed and towing weight — for a real boat.
- 01 Sailboat calculator The comprehensive one. Search the measured hulls or type every figure in yourself — hull dimensions and the four rig triangles — and change anything you like. You get: fourteen figures at once — SA/D, D/L, ballast ratio, S#, comfort, capsize screening, hull speed, length/beam, the sail areas and forestay length — each with the formula it came from.
- 02 Ratio calculator Start typing a boat and its measured dimensions arrive already filled in — length, waterline, beam, draft, displacement, ballast, sail area. Then the standard naval-architecture ratios, with the formula printed under every one. You get: sail area/displacement, displacement/length, ballast ratio, comfort ratio, capsize screening and hull speed.
- 03 Sail-area calculator Reference areas for the standard production rig of each boat — classic, full-batten and furling mains, furling genoas, and both kinds of spinnaker. If a boat is not on the list, the rig triangles give the nominal areas from P, E, I and J. You get: each sail on the rig in square metres, the working sail area, and the reason a rating figure and a sailmaker's figure are not the same number.
- 04 Stability Ballast ratio, capsize screening, an estimate of the moment the ballast itself makes, and what the ISO design categories mean. You get: the figures the dimensions genuinely support. A true righting-arm curve needs the hull's offsets and centre of gravity — measurements most boats never had taken.
- 05 Wetted area and antifouling How much of the hull is underwater, by three different estimates shown side by side — then how many litres of antifouling that is, at your tin's own coverage rate and your own number of coats. You get: the answer to the question you are standing in the chandlery asking, plus all three estimates side by side.
- 06 Speed and passage time Hull speed, the speed/length ratio, and a drawn curve showing where a hull leaves displacement mode — with the boat's own D/L deciding which regime it realistically lives in. You get: distance and a realistic average turned into hours — passage speed sits well below hull speed.
- 07 Trailer weight check A boat's displacement is not its towing weight. Add the trailer, the fuel and water aboard and the gear you actually carry — every allowance an editable input with a stated default, none of it hidden in the total. You get: the total checked against UK, EU and US licence limits, and against the road-width rule that stops most people before any weight limit does.
- 08 Compare boats Two to four boats side by side — raw dimensions, a typical second-hand asking price for each one's kind and length, and derived ratios, with the widest gap in each row marked so the differences find you rather than the other way round. Switch it to compare classes instead of hulls. You get: a comparison you can send someone — the choice is in the link.
- 09 Boats like this one Name a boat you like and find the ones closest to it, across six dimensions and two ratios, normalised so displacement cannot drown out everything else. You get: the matches, and why each one matched — which dimensions agreed and how closely.
These are reference tools. The figures are indicative and worth confirming against the boat's own certificate, rig or plates before anything is cut, bought or towed on the strength of them.