Trade Force for Distance

A halyard can lead to a convenient place and still demand more pull than a person can supply. A sheet controlling a loaded boom poses a similar problem along a different route. Securing the line in a clutch solves the problem of holding what has already been gained. The next problem is gaining it.

The rope loop in the trucker’s hitch gave us a first example of a purchase: an arrangement that exchanges greater movement at the hauling end for a greater force at the load. Turning wheels can make that exchange with less sliding friction. A handle and gears can make it in another form. Both arrangements still need the connections and supporting structure we have just followed.

What turns inside the block

Follow the halyard into one of its blocks. The shell is the enclosing body. The two sides, or cheeks, stand on either side of the grooved sheave. An axle, also called the sheave pin, crosses between them and supports the sheave’s rotation.

The rope presses into the groove; the sheave transmits that pressure through its bearing to the pin, cheeks and attachment. In a traditional stropped block, the shell passes the force into the strop embracing it. Other blocks connect their metal side plates more directly to a shackle or another attachment.

The bearing allows the sheave to rotate about the pin while carrying that contact load. Plain sliding bearings, balls or rollers provide different ways of supporting the movement. They influence resistance, size and load capacity; the presence of a wheel does not make its bearing friction vanish. The rope also bends as it enters and straightens as it leaves, so its construction and the sheave’s diameter affect the work needed to move it.

A small eye at one end of a block can secure the rope’s fixed end. This is its becket. The becket receives the pull of that attached end; the pin receives the forces transmitted through the turning sheave. Which block carries the becket becomes important when we count the forces supporting a moving load.

A single block contains one sheave. A double block contains two, commonly beside one another; a triple block, also called a treble block, contains three. A fiddle block puts two sheaves along its length, often with a smaller one above a larger one. This longer, narrower arrangement gives the returning rope separate passages while allowing room for it to approach the lower sheave.

There are other reasons to put two sheaves in one body. A historical sister block has two separated sheave openings in an elongated wooden shell. In the arrangement described by the nineteenth-century seaman William Brady, it is seized between shrouds and carries two different working lines, one through each sheave. They share a mounting, while keeping their routes separate. Counting two wheels therefore tells us neither that they work on the same rope nor that they multiply its pull.

Lead the rope where it is useful

A block fixed above a load can lead a rope up from that load, over the sheave and down towards the crew. Pulling down raises the other end. Neglect friction for the moment: the tension is the same on both sides, so the downward hand force equals the upward pull on the load. Each metre drawn down at the hand raises the load one metre. This fixed block has changed the direction of effort.

That change is useful in its own right. A turning block or lead block redirects a working line. A cheek block mounts one side of its body against a supporting surface, putting its sheave beside that surface. A foot block, often mounted near the foot of the mast or at another low turning point, leads a descending line towards the next part of its route. Those names can describe overlapping features: one block may be a cheek-mounted foot block used to turn a halyard aft.

Several lines running aft along the deck need separate passages. A deck organizer provides a row of sheaves in a common mounting, sometimes in stacked rows, keeping the lines apart as each is redirected. The mounting and the deck beneath it receive the combined reactions of the loaded lines.

A fairlead guides a line through a desired route. It may be a smooth opening, an eye or a fitting containing a moving guide. A roller fairlead uses rollers at the contact surfaces so the line can bear on a turning surface when its angle changes. Fairlead identifies the guiding job; roller identifies the way that particular guide handles contact.

A low-friction ring provides a compact, smoothly rounded surface for the rope to slide around or through. Its outer groove can receive a supporting strop, while the working line passes through the central opening. There is no rotating sheave in that arrangement. Its simplicity and small number of parts can suit a heavily loaded control that moves infrequently, but the rope must slide over the ring whenever it moves. The name does not promise less resistance than a suitable rolling block in every application.

Sometimes the rope is already connected at both ends. A snatch block has an opening cheek or another opening side arrangement that admits a bight without threading an end through the whole block. Closing and securing the opening restores the intended enclosure before the line is loaded. Its useful change is access to the rope route; its sheave still performs the ordinary turning job.

In a ratchet block, a one-way catch can prevent the sheave turning in the loaded direction while permitting it to turn in the hauling direction. The loaded rope then bears against a stationary, gripping sheave surface when it tries to run back. That friction helps the person holding the tail. It does not, by itself, secure the line or produce a force-multiplying purchase. The mechanism of the small one-way catch will become clearer inside the winch.

All these fittings change access, contact, direction or holding assistance. To gain a greater force from the same hand effort, we must change which parts move with the load.

Make the block move with the load

Fasten one end of a rope overhead. Lead it down around a block attached to the load, then up to the hauling end. The moving block is now pulled upwards by two rope lengths, one on each side of its sheave. With both lengths vertical and their tensions equal, each contributes half the force needed to support the load. A hand pulling with 100 newtons can ideally support a load pulling down with 200 newtons.

Follow the distance as well. When the moving block rises one metre, each of those supporting lengths becomes one metre shorter. Two metres of rope must be drawn out at the hauling end. Half the force accompanies twice the movement. In the ideal model, the work put in equals the work done at the load.

The mechanical advantage is the useful output force divided by the input force. Here it is two. The purchase ratio, written 2:1, expresses that force advantage; it also gives the corresponding ideal travel relationship. A real purchase needs extra input to overcome its losses, so a stated arrangement ratio describes geometry before it describes the effort a particular crew member will feel.

The connected rope and blocks form a tackle, or block and tackle. Its rope is the fall; the free length on which the crew pull is the hauling part. To reeve the rope is to pass it through the selected openings and around the sheaves. A rope already threaded through them is rove. Reeving determines which rope lengths shorten as the load moves and where their tensions act.

Our two-part arrangement has the crew hauling upwards. Put a fixed single block overhead and lead the hauling part over it, then down to the hand. The lower moving block still receives two upward tensions. The new upper sheave turns the hauling end without adding a third supporting length at the load. One fixed single block and one moving single block, rove in this manner, form a gun tackle with an ideal 2:1 purchase.

The upper attachments still carry substantial force. In the first fixed-block example, two downward rope tensions act on the overhead block. Its support must resist their combined pull, even though the hand has gained no force advantage. In the gun tackle, reactions also reach the overhead fixed end and upper block. Reducing the hand force does not reduce every attachment force by the same fraction.

These comparisons assume light blocks, a rope that does not appreciably stretch, equal tension throughout it and supporting parts aligned with the movement. Slanting rope parts contribute only the useful component of their pull. Friction makes the tensions differ when the rope moves. Real block mass adds to what must be lifted. The ideal model tells us what the geometry can do; those conditions tell us what must be checked before applying it to an installation.

Add parts, then follow every one

Replace the upper single block with a double and fasten the rope’s standing end to the lower moving block. Lead it up over the first upper sheave, down under the lower sheave, up over the second upper sheave, then down to the hauling end. Two rope lengths pull up on the lower sheave, and the fixed end pulls up on the lower block’s becket. The moving assembly receives three tensions. This is a luff tackle rove for an ideal 3:1 purchase: one metre of movement at the load requires three metres at the hand.

The becket accounts for the extra part. A count based only on the sheave in the moving block would miss it. Securing the standing end somewhere else, or choosing the other block as the moving one, changes the arrangement and can change the ratio.

For four supporting parts, use two double blocks. Fix the rope at the upper block, take it down around the first lower sheave, up around the first upper sheave, down around the second lower sheave and up around the second upper sheave. The free end leaves from above. Both lower sheaves now receive two upward tensions, making four altogether. If the load moves one metre towards the upper block, four rope lengths shorten by one metre each.

H. J. Dana and W. A. Pearl’s 1922 account calls this two-double-block arrangement a double-luff tackle. Their illustration adds a separate fixed turning block to bring the hauling part towards the worker. Its ideal purchase remains 4:1. Some other seamanship conventions use double luff for a double-and-triple-block arrangement and call two doubles a twofold purchase. The name needs its accompanying layout; the described rope route makes the comparison unambiguous.

A handy billy is a portable tackle brought to a job that needs extra pull, rather than a permanent control assigned to one sail. Its hooks or other end attachments let it connect to suitable strong points. It may use a luff arrangement, but portability does not establish its ratio or capacity. The crew still need to identify the moving end, the reeving and the structure taking each reaction.

Extra supporting parts reduce the ideal hauling force while increasing the length that must be handled. They also add bends, bearings and room for the blocks to approach one another. Once the blocks meet, that purchase has exhausted its available travel. A useful choice must fit the required movement as well as the force.

Make one purchase haul another

A cascade, or cascade purchase, connects purchases in stages. Suppose a first 2:1 purchase acts on a control point. Instead of pulling its hauling end directly by hand, attach that end to the moving block of a second 2:1 purchase. The second stage now hauls the first stage’s fall.

If the control point resists with 400 newtons, the first stage ideally needs 200 newtons on its hauling end. The second stage supplies those 200 newtons with 100 newtons at the hand. The force advantages multiply: two times two gives four. Moving the control point one centimetre requires two centimetres of movement at the first stage’s hauling end, and therefore four centimetres at the final hand end.

The arrangement can divide a large adjustment into a short, heavily loaded stage and a longer, more lightly loaded final control. Each stage still needs enough movement, correctly supported blocks and a rope appropriate to its own tension. Friction and unequal alignment accumulate through the actual route. The phrase 4:1 alone cannot tell us whether that advantage comes from a single four-part tackle or two connected stages.

There is another way to gain a small movement from a loaded line. To sweat a halyard is to take up tension by pulling sideways on a suitable exposed span, then retain the gain at its hauling end. Deflecting the middle draws extra rope into the two slanting legs. Their sideways force components oppose the hand, while their larger lengthwise components transmit tension along the line. A companion can take in and secure the resulting slack as the span straightens.

That geometry can produce a large line tension from a modest sideways effort when the deflection is small. Its travel is limited, and the forces change markedly with the angle. The explanation is a reason to understand the loaded span and its attachments; it supplies neither a universal multiplication factor nor an instruction to seize any accessible rope under load.

Put the travel into a handle

A winch turns a drum to haul a rope. On an ordinary manual sailing winch, the working line takes wraps around the winch drum and leaves again as a tail. The loaded side bears against the drum; friction at the wraps allows the rotating surface to transfer force into the line. The crew turn the drum through a winch handle, inserted into a socket connected to the drive.

The handle applies the lever principle to rotation. A longer distance from its axis to the hand gives more turning effect for the same hand force. The line acts at the smaller drum radius. Imagine a direct drive with a handle radius of 250 millimetres and a drum radius of 50 millimetres. Neglect losses and tail tension for this simple comparison: the ratio of the radii is five, so a 40-newton tangential hand force can produce a 200-newton line pull. The hand travels five times as far around its circle as the drum surface travels around its own.

The handle need not drive the drum directly. A toothed wheel in the drive between them is a winch gear; its teeth mesh with those of another wheel to transmit rotation. A small wheel driving a larger one turns the larger wheel more slowly while increasing its turning effect, subject to losses. Several stages or alternative gear paths can provide different ratios. Selecting a slower, stronger drive means making more handle movement for the same amount of rope brought in.

At the same ideal 200-newton line load, the four-part tackle required 50 newtons and four times the load’s travel. Our direct-drive winch example requires 40 newtons and five times the line’s travel at the hand. The geometry differs, but both exchange force for distance. A manufacturer’s force or power-ratio label must be interpreted for that machine; it does not mean that the mechanism creates extra energy.

The pawl, called a winch pawl here, helps preserve the gain between movements. It is a small pivoting catch that engages teeth on a rotating member. In the allowed direction, successive teeth lift it and let it fall into the next space. Reverse movement presses a tooth against the engaged catch, which transmits the reaction into its support. A spring encourages engagement; the catch and tooth carry the principal resisting force. Gears transmit the drive at a chosen ratio, while pawls permit or restrain movement in selected directions.

The ratchet block used the same one-way idea at its sheave. The winch combines that directional restraint with a handle, a drive and a drum that actively brings in rope. Its base then transmits the machine’s reactions into the deck. The small effort felt at the handle is only one force in that assembly.

The drum still needs a tail

While the handle turns, the rope leaving the drum still needs attention. The wraps need contact pressure to transmit force through friction, and pull on the outgoing tail helps keep the rope seated against the drum. To tail a winch is to maintain that outgoing tension and take away the rope as it emerges. The loaded side can then carry substantially more tension than the tail, within the limits of the rope, surface and wrap arrangement.

A self-tailer performs that outgoing grip mechanically. On the familiar sailing design, jaws above the drum grip the departing line and carry it round until a fixed guide strips it out towards the free tail. The handle and gears still turn the drum, and the wraps still transmit the main line force. The upper jaws take over the person’s tailing task. Rope size and the jaw arrangement must match for that grip to work as intended.

The wraps also need a clear route. An override occurs when one turn crosses and traps another on the drum. Load can squeeze the crossing so tightly that the rope will neither feed nor ease normally. The incoming lead and the way successive wraps lie therefore matter as much as the presence of a strong drive. A trapped turn is a change in the contact path, not simply a shortage of handle effort.

After hauling, a suitable clutch, jammer or cleat can receive the load and free the winch. To resume controlled movement, take the load back on the hauling arrangement before releasing the holder. The rope then has a continuous loaded route during the handover. The same principle applies at a pinrail: a separate rope stopper can hold the line while its end is belayed, after which the wraps on the pin pass the pull into the rail and its supports. We have changed who holds the gain; further hauling still needs the winch or tackle.

The same distinction explains the capstan. In its traditional form, crew walk round a vertical-axis drum, pushing long bars fitted into its head. Their large walking circle supplies leverage relative to the drum radius, while rope wraps and a managed tail transmit the pull. Powered capstans can drive that vertical drum by machinery. The principle is still a rotating hauling surface with a load path into its foundation. The familiar deck sheet winch is a compact related machine with a different drive arrangement.

Our ordinary sheet winch passes rope around and off the drum. A winding drum can instead collect the hauled rope on itself. Holding, hauling and storage can be combined in equipment, but they remain different functions. We will need that distinction again when larger yachts put power and stored rope below deck.

An anchor windlass performs the related job of hauling the anchor’s rope or chain. Its gripping and holding arrangements will need their own explanation when we connect the yacht to the seabed.

More pull changes the crew’s job

Greater mechanical advantage can bring a heavy control within human strength while leaving substantial work to do. Crew workload includes the effort, distance, time and coordination demanded by the arrangement. A long fall must be hauled and kept clear. A winch may need one person turning and another tailing. Increasing the force ratio can slow the response unless more movement or more people supply the required work.

The 1903 racing yacht Reliance combined tackle and drum. In a later eyewitness account, A. Griswold Herreshoff described a six-part mainsheet, about 305 metres of rope and three crew winding a large drum below deck. The arrangement placed both mechanical advantage and rope handling between the sail and the crew. Its scale also required communication between people who could not all see the same equipment. Tackle and winch served together; one had not simply displaced the other.

On a smaller contemporary yacht, a tackle can provide a sensitive local adjustment while a geared winch handles a longer or heavier pull. The useful development is a choice of mechanisms fitted to force, travel and the crew’s working position. The ropes and machines still pass every reaction into their attachments.

Many of those attachments pull on or near the mast. A slender spar has to receive the sail’s load, the downward pull from its supporting rigging and the reactions from the lines that the crew have hauled tight. We can now follow that connected structure from the top of the mast into the hull.