Two measurements decide everything
Cover length is the hull plus whatever hangs past each end — 32 ft plus 3 ft at each end is 38 ft. Cover width is the girth over the ridge plus the skirt on both sides: 28 ft over the frame plus 2 ft each side is 32 ft. That is a 1,216 sq ft cover, and every other figure on the page comes out of those two.
The girth is the one people guess and should not. It is a tape measurement from the rubrail on one side, up over where the ridge pole will actually sit, and down to the rubrail on the other — so it already contains the frame height, the flare of the topsides and the shape of the cabin trunk. No formula reproduces that from a beam and a ridge height.
Bands, not a single sheet
Film comes 24 ft wide and boats are 38 ft long, so the cover is made of bands laid across the hull and overlapped along its length. Each band advances the coverage by the roll width minus the overlap — 22.5 ft on the default settings — so 38 ft of cover takes two bands, and each band pulls 32 ft of film off the roll. Sixty-four feet off a 149 ft roll.
Notice how coarse that count is. Thirty-eight feet of boat needs 1.69 bands and gets 2, so the second band is more than half wasted. Push the cover length to 45 ft and it is still two bands with nothing spare; push it to 46 and it becomes three. The step is 22.5 ft wide, which is why two boats of similar size can use quite different amounts of film.
The ridge height trade
Raising the ridge adds girth roughly foot for foot, and girth is what each band spends. Going from a 4 ft ridge to a 6 ft ridge on this boat takes the film off the roll from 64 ft to 68 ft — small, on one hull — while making the roof considerably better at getting snow and water off itself instead of ponding it in the middle.
The sweep on the page shows that trade in the only terms that matter, which is feet of film. What it cannot show is the other half: a cover that ponds water collects a load overnight that the frame was never meant to carry, and that is a structural question about the frame rather than a materials question about the film.
Vents are not optional and the arithmetic will not tell you so
A sealed cover over a boat with a damp bilge is a humidity chamber for six months. The water has to leave and the only route is a vent. The form takes a rate — one per 8 ft of cover length by default, so five on this boat — because that rate is a matter of local practice and of how wet the boat is, not of physics that a page can settle.
The rest of what makes a winter cover work is not a takeoff at all. It is how the frame is tied to the boat, how the perimeter is belted, and whether the whole thing survives the first gale. A materials list is the cheap part of the job, and the film itself is the cheap part of the materials list once the framing, the strapping and a day of somebody time are counted.
Questions people ask
How much shrink wrap do I need for a 32 foot boat?
For the default figures — 3 ft of overhang at each end, 28 ft of girth over the frame and a 2 ft skirt each side — the cover is 38 by 32 ft, which takes two bands off a 24 ft roll and 64 ft of film in total. That is well under half of a 149 ft roll, so the useful question is usually what to do with the rest rather than whether one roll is enough.
How do I measure the girth for a boat cover?
With a tape, from the rubrail on one side, up over the point where the ridge pole will sit, and down to the rubrail on the other side. It has to be taken over the frame as it will actually be built, because the measurement carries the ridge height, the topside flare and the cabin trunk all at once. Then add the skirt you want hanging below the rail on both sides.
How many bands of film does a boat take?
Cover length divided by the roll width minus the overlap, rounded up. At 24 ft wide with 1.5 ft of overlap each band advances 22.5 ft, so anything from 22.6 to 45 ft of cover length takes two bands. The step is coarse, which is why a boat a foot longer than a neighbour can need a whole extra band.
How much framing lumber does a shrink wrap frame take?
On the default settings, 38 ft of ridge plus 16 uprights at 4 ft, which is 102 ft in total. The station count is cover length divided by the spacing plus one for the far end, and the uprights per station is a field because a ridge carried on paired legs uses twice what a single centre post does.
How many vents does a shrink wrapped boat need?
The form takes it as a rate — one per 8 ft of cover length by default, giving five on a 38 ft cover — because it depends on local practice and on how much moisture the boat itself is carrying. No figure is stated here as correct. What is worth saying plainly is that condensation under an unvented cover routinely does more damage over a winter than anything the weather does to the outside.