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An anchor windlass within the forecastle on the main deck of the sailing ship Balclutha. The vertical shaft is rotated by a portion of the capstan above

An anchor windlass is a machine that restrains and manipulates the anchor chain and/or rope on a boat, allowing the anchor to be raised and lowered. A notched wheel engages the links of the chain or the rope. A brake is provided for control and the windlass is usually powered by an electric or hydraulic motor operating via a gear train.

Technically speaking, the term "windlass" refers only to horizontal winches. Vertical designs are correctly called capstans. Horizontal windlasses make use of an integral gearbox and motor assembly, all typically located above-deck, with a horizontal shaft through the unit and wheels for chain and/or rope on either side. Vertical capstans use a vertical shaft, with the motor and gearbox situated below the winch unit (usually below decks).

Horizontal windlasses offer several advantages. The unit tends to be more self contained, protecting the machinery from the corrosive environment found on boats. The dual wheels also allow two anchors on double rollers to be serviced. Vertical capstans, for their part, allow the machinery to be placed below decks, thus lowering the center of gravity (important on boats), and also allow a flexible angle of pull (which means rope or chain can be run out to different fairleads).

It tends to be the case that smaller boats use capstans, and larger boats have windlasses, although this is by no means a hard and fast rule.

The wheels on either a vertical or horizontal windlass provide for either chain or rope to be engaged. The wheel for rope is termed a gypsy, while a wildcat is for chain. Combination wildcat/gypsies are capable of handling both chain and line on the same wheel, although particular care must be taken with sizing and compatibility of rope, chain, and windlass, for this feature to work effectively.

It is important that the wildcat match the chain size exactly. Even a small difference in link size or consistency can cause undue wear on the wildcat and/or cause the chain to jump off the windlass when the winch is operating.

While most windlasses require power, many are manually driven in the same manner as most sailing boats' winches for sheets. In fact only modern boats have practical sources for power, and ships in the old days have always required manual power.

Powered solutions include steam (antiquated), hydraulics, and electrics. Electrics are convenient and relatively cheap, but hydraulics prove more efficient and powerful on all but small boats.

In general, windlasses and their power system should be capable of lifting the anchor and all its rode (chain and rope) if deployed so that it hangs suspended in deep water. This task should be within the windlass' rated working pull, not its maximum pull.