Wetted surface is not a number you can eyeball
Deck area, sail area and displacement are all things owners know by heart. Wetted surface is not, because nothing else uses it. It is the skin of the hull below the water, plus the keel on both sides, plus the rudder on both faces, and the only cheap way to it is the product of waterline length and the sum of beam and draft, multiplied by a coefficient that carries all the shape information.
With the figures in the form — 32 ft on the waterline, 10.5 ft of beam, 5 ft of draft and a coefficient of 0.75 — that comes to 372 sq ft. Add 25 sq ft for the rudder and the strut and it is 397. The boot stripe adds another 18 on a waterline that measures 71 ft around, so 415 sq ft gets paint. Two coats at 400 sq ft per gallon, knocked back 12 percent for loss, is 2.36 gallons, and the extra coat on 60 sq ft of leading edges takes it to 2.53. Three cans.
The coefficient is where the uncertainty lives
Everything else on the form you can measure with a tape in ten minutes. The coefficient you cannot, and moving it from 0.70 to 0.80 swings the answer from 2.39 to 2.67 gallons on this boat. That spread is 0.28 of a gallon and it does not change the can count — all three readings round up to three cans, which is the honest result rather than a dramatic one. Put the same 10 percent uncertainty on a fifty-footer needing five gallons and it is most of a can, and there it does decide whether you finish the transom.
That is why the page prints the range rather than hiding it. A fin-keeled racer and a full-keeled cruiser with identical waterline dimensions have genuinely different wetted areas, because one of them has a keel that is a thin foil and the other has a keel that is most of the hull. If the builder ever published a wetted surface figure, use it and set the selector accordingly. If not, the girth method beats any coefficient: tape from the boot stripe down and under to the boot stripe on the other side at four stations, average, multiply by waterline length, and you have measured the thing instead of estimating it.
Why the extra-coat field exists
Bottom paint does not wear evenly. The waterline scrubs against surface debris, the leading edge of the keel and the rudder meet the water first, and both come back at the end of a season looking a coat thinner than everywhere else. Yards deal with this by putting an extra coat on those areas, which is why the calculation is not simply area times coats.
Sixty square feet is a plausible starting figure for that band on a boat this size and it is a starting figure only. If you have hauled the same boat three times you already know where yours goes bare, and that area is the one to type in.
What the arithmetic cannot tell you
It cannot tell you whether the paint you bought will stick to what is already there. Ablative over hard, hard over ablative, and any copper-free paint over anything at all are compatibility questions that get answered by the manufacturer technical sheet and by test patches, not by a coverage calculation. Nor can it know how much old paint the bottom is carrying: a hull with fifteen years of build-up may need less new paint and a lot more preparation than a hull that was stripped last spring.
And the loss allowance is doing more work than it looks. Twelve percent covers a normal roller job on sound paint. A freshly blasted or heavily sanded bottom drinks the first coat, and people who skip that allowance are the ones who run out with the transom left to do.
Questions people ask
How do I work out the wetted surface of my boat?
Multiply waterline length by the sum of waterline beam and draft, then multiply by a shape coefficient from the builder hull data. For the default boat that is 0.75 times 32 times 15.5, or 372 sq ft, before the rudder and strut are added. If you have no coefficient you trust, measure girths instead: tape from the boot stripe under the keel to the boot stripe on the other side at four stations, average them, and multiply by the waterline length.
How much bottom paint do I need for a 32 foot boat?
For the boat in the form, 2.53 gallons for two coats plus an extra coat on the leading edges, which means three one-gallon cans. That figure moves with the spread rate on your own can and with the coefficient. The page prints what a tenth of a point of coefficient is worth so you can see the size of the uncertainty before you buy.
Why does my paint label say a coverage my boat does not achieve?
Label coverage is a theoretical figure at a stated film thickness with nothing lost. Under a boat, paint stays in the tray, sits in the roller nap and soaks into anything porous. The loss allowance on the form is where that goes, and 12 percent is a starting point for sound existing paint. A bottom sanded or blasted back to gelcoat takes considerably more on the first coat.
Does the boot stripe get bottom paint?
That depends on how the boat sits and how you like it to look, which is why it is a field rather than an assumption. If you do paint it, the area is the waterline perimeter times the band height, and on the default boat a 3 inch band around a 71 ft perimeter is about 18 sq ft. Set the height to zero if the stripe is topside paint instead.
Is one coat of antifouling enough?
That is not a question a coverage calculator can answer, and it is not answered here. The number of coats, the film each one needs and whether a particular paint suits your water and your haul-out interval are set by the paint manufacturer technical sheet and by what the yard has seen on similar boats locally. The form takes coats as an input for that reason.