Winches are mechanical devices, which consist of a rotatable drum that carries a loaded line. The drum is rotated manually with a hand crank or electrically with a motor. This driving force is put on the drum through an integrated gear mechanism.

Winches use the principle of levers to increase force and torque. Modern winches use two types of lever principals: one internal, the other external. The external lever is the lever arm or the winch handle and the internal lever is the integrated gear reduction consisting of revolving sets of gear wheels. The combination of these two effects results in a power ratio, the theoretical mechanical advantage of the winch.

Self-tailing winches have a line stripper and a spring loaded disk that acts as a jam cleat on top of the drum. The self-tailing mechanism is designed to strip the non-loaded end of the line from the drum and to hold this part such that the line will not slip on the drum while the winch is operated. Self-tailing winches allow to have both hands free for grinding. Today, almost all installed winches on sailing yachts are self-tailing winches.

All self-tailing winches use some kind of mechanical brake or ratchets principle to prevent the drum from rotating in the wrong way if the driving load of the winch is removed. This means these wiches can be used to pull in the line under load but they have no direct arrangement to slacken the line.

In order to slacken the line, the "low-load" end has to removed from the self-tailing cleat and - while keeping sufficient load on the line - let it slip over the winch drum using the friction of the 3-4 turns on the drum to control the slackening. This procedure is a potential hazard: the load on the line my be to high to control the slackening with the danger of the line pulling the operators hand and fingers onto the winch.

• The standard handle length is 250 mm (10"). Shorter handles, which are used for faster winch speed are usually 200 mm (8").

• Drum radius can be measured or found from the specifications.

• Gear ratio can be determined from the winch specifications or it can be obtained by counting the number of handle rotations needed to rotate the drum over one full turn.

The "speed" of a winch, is the amount of line that is pulled for each revolution of the handle. The primary considerations in determining winch speed are the gear ratio and the drum circumference. Since the line is wrapped on the drum, each revolution of the winch will pull in an amount of line equal to the circumference of the drum (circumference = π x diameter).

The "load" a winch can generate is the force it can apply on the attached line with a certain amount of force on the winch handle. This is related to the power ratio of the winch.

Since a winch is a passive device, the mechanical power it delivers on sheets and halyards (which is equal to winch speed times winch load), must be supplied by the human operator and this power is limited:

  Winch_Power = Winch_Speed x Winch_Load

If the available power is constant, winch speed and winch load are inversely related. This is simply due to the energy-conservation principle of mechanical machines.

Fast trimming can only be obtained with lower loads and high loads will imply slow trimming. This is the reason that all moderate and large winches are offered with two or three speeds. For light loads, the fast speed gear can be used that doesn't offer much load, and as the load increases, you can shift to the lower speed gear and trim the last part at higher load.

A simple way to increase the speed of a winch is to use a shorter handle, typically a 200-mm (8") handle. The 8" handle is faster because it swings in a smaller arc and can be rotated faster. The same power problem is experienced, though, as handle length is a part of the power-ratio calculation and decreasing the handle length by 20% also reduces winch load by 20%. Still, the 8" handle is a very effective and inexpensive means of increasing winch speed in light and moderate conditions.

Faster trimming can also be obtained by using a wide-body winch with a large diameter drum. Unfortunately, since the drum diameter is also a function of the power ratio, the wide-body winch will offer less power for the same gear ratio. Faster winch speeds can also be achieved with three-and four-speed winches.

A typical smaller three-speed winch, offers direct drive in first speed. The drum turns one revolution for each revolution of the handle, so under low loads, sheets can be trimmed very fast. While this speed is not geared, there is only the slight mechanical advantage obtained from the relationship between the lever (handle) and the drum diameter.
Larger three-speed winches, typically have a geared first speed because for these winch sizes, even under low loads, some additional mechanical advantage is required.

Winches are further affected by how they are mounted on the boat. The most important factor is to ensure a proper line entry angle.
Lines must lead up to a winch at about a 5- to 10-degree angle to obtain correct winding and to prevent overrides of the sheet on the drum. Lead blocks should be positioned in an appropriate way to ensure this optimal entry angle up to the drum.
Alternatively, the winch must be raised or lowered slightly.

It is also important to mount the winch such that the loaded line is properly aligned to the position of the final drive gear pinion. This must be considered during the installation of the winch and it is important to know on which side (internal) the drum pinion is located. This position is normally marked on the the drilling cut-out template available from the manufacturer. The winch must be mounted on the deck so that the drum pinion is positioned on the same side at which the sheet enters and loads the winch drum.

Improper alignment will dramatically increases the load on the gear housing resulting in inefficient operations, and in severe cases, in winch failure.

On the left two extreme examples are shown:
one with the load aligned properly and one with the opposite situation. In case A, with the proper alignment, the load on the gear housing is minimized since Ls and Ldp are in opposite directions. In case B, where the load is improperly aligned, the load on the drive gear is maximized: in this case the loads are in the same direction and adding up.
Notice that the forces exercised on the deck are independent of this and consist of the line load (Ls) and a torque equal to line load times drum diameter (M = Ls x Ddrum). These forces must be delivered by the bolts connecting the winch to the deck.