Travel first, and it starts under the water
The travel a lift needs is not the height you want the boat at. It is the distance from where the cradle sits at rest, which is under the water, to where the hull ends up, which is above it. Thirty inches of cradle depth and twelve inches of clearance is forty-two inches of travel, and the first thirty of those inches produce nothing visible at all — the cradle is simply coming up to meet the hull.
Which also means the travel changes when the water does. A lake drawn down two feet in the autumn adds nothing to the depth of the cradle if the cradle sits on the bottom, and adds two feet of travel if the cradle hangs from the frame at a fixed height. Working out which of those you have is worth doing before ordering cable.
The multiplier, which is where the order goes wrong
Cable take-up is travel multiplied by however many parts of line are moving. Four corner cables all winding onto one drum take up four inches for every inch the cradle rises, so forty-two inches of travel is one hundred and sixty-eight inches — fourteen feet — of cable through the drum. A two part tackle takes two. A single direct line takes one.
Getting that number wrong scales everything downstream by the same factor: the cable you buy, the turns on the drum, the time for a raise and the speed the cradle moves. Count the parts on the actual machine rather than assuming, because arrangements that look alike from the dock reeve differently.
Why the turns are not a constant
Cable does not wind onto a drum at one diameter. The first layer sits on the barrel, so its pitch diameter is the barrel plus one cable diameter; the second layer sits on the first, adding two more cable diameters, and so on. On a four inch drum with 5/16 cable, the first layer runs at 4.3125 inches pitch diameter, which is 13.55 inches of cable per turn. The second layer runs at 4.9375 inches, or 15.51 inches per turn — fifteen percent more cable for the same rotation.
Work the default through. A four inch wide drum holds twelve wraps of 5/16 cable side by side. Three of those are dead wraps that never come off, leaving nine on the first layer, which is 121.9 inches of cable. The fourteen feet of take-up is 168 inches, so 46.1 inches spill onto the second layer at 15.51 inches a turn, which is 2.97 turns. The full raise is a shade under twelve turns of the drum, finishing two layers deep.
The practical consequence is that the cradle speeds up as it rises even though the drum turns at the same rate. On this example the last part of the lift is about fifteen percent faster than the first, and on a deeply wound drum the difference is much larger. It also means a drum turn is not a unit of height, so counting turns is not a way of setting a stop.
What this page will not tell you
Anything with a force in it. Cable size, cable type, drum, sheaves, reeving, terminations, brakes, motors and every working load figure belong to the lift manufacturer and to whoever services it. The arithmetic here is lengths and rotations, and it stays there deliberately.
The plain hazards, named and not managed. Working alone over water is how people drown quietly; nobody hears a splash from the parking area. A dock section, a float, a pile or a lift cradle is far heavier than it looks and it moves the instant it lets go, usually toward whoever is standing in the water beside it. Cold water takes strength and grip out of a swimmer in minutes, and shoulder season water is cold even where the air is not. And electricity near water is its own category: electric shock drowning around docks and lifts is a real and specific way people die, it leaves no mark, and swimmers in the water have no warning. Anything electrical at a dock belongs to a licensed electrician who works on marine installations, not to a calculator and not to a weekend.
Questions people ask
How much cable does a boat lift need?
Take-up plus dead wraps plus everything standing. Take-up is the lift travel multiplied by the number of cable parts that move, so forty-two inches of travel on a four cable arrangement winding onto one drum is 168 inches, or fourteen feet. Dead wraps are the turns the equipment maker wants left on the drum at full travel, and on a four inch drum with 5/16 cable three of them is about three and a half feet. Everything else is the runs to the sheaves, the dead ends and the terminations, which is a measurement of your own arrangement rather than a calculation. Add the three and buy the length, not the estimate.
Why does my lift speed up as the boat comes out of the water?
Because the cable is winding onto a larger diameter than it started on. The first layer sits on the drum barrel, the second sits on the first and is two cable diameters larger in pitch diameter, and each layer after that adds two more. On a four inch drum with 5/16 cable, one turn moves 13.55 inches of cable on the first layer and 15.51 on the second, which is fifteen percent more for the same rotation. The drum speed has not changed; the amount of cable it swallows per turn has. It is also why counting drum turns is not a reliable way to set a travel stop.
What does the reeving multiplier mean and how do I find mine?
It is how many inches of cable move through the drum for one inch of cradle travel, and you find it by watching what actually moves. If four corner cables all wind onto a single drum, all four are taking up at once and the multiplier is four. A two part tackle at each corner doubles again. A single line straight from the drum to the pick point is one. It is worth counting on the machine rather than assuming from the look of it, because the same figure scales the cable you buy, the drum turns, the raise time and the cradle speed all at once.
Does the lift travel change through the season?
It can, and it depends on what the cradle rests on. If the cradle sits on the bottom at rest, its depth below the surface tracks the water level directly, so a lake drawn down two feet gives you two feet less travel to the same clearance. If the cradle hangs from the frame at a fixed height instead, a falling water level increases the travel needed to get the same clearance above the water, because the starting point stayed where it was and the water went away. Work out which arrangement you have before sizing anything off a single summer measurement.
Can this tell me what size cable my lift needs?
No, and nothing on this page carries a force. Cable size, construction, drum and sheave sizing, reeving, terminations, brakes and every working load figure come from the lift manufacturer and from whoever services the equipment, and they depend on things arithmetic cannot see. What is here is lengths and rotations: travel, take-up, layers, turns and time. Treat it as an ordering and layout aid. Boat lifts are also frequently electrically powered at the end of a dock, and electric shock drowning around docks is a real and specific hazard — anything electrical there belongs to a licensed electrician who works on marine installations.