Purchase and mechanical advantage explained (2:1 · 4:1 · 8:1)

A purchase multiplies the force you apply by running a single line back and forth several times between two sets of blocks or low-friction rings, so the ratio you hear quoted — 2:1, 4:1, 8:1 — simply tells you how many times your effort is multiplied. With a 4:1 purchase you pull with roughly a quarter of the load's weight, but you have to haul in four times as much line to move the load the same distance. That single sentence holds the whole idea and its price tag: mechanical advantage buys less effort at the cost of more travel and a little efficiency lost to friction. Understanding how ratio, travel and friction relate is what separates a control line that trims sweetly under load from one that feels heavy and slow. This guide covers what mechanical advantage is, how to count it, why friction quietly taxes every system, and how to pick the ratio that actually suits your boat rather than the biggest number you can rig.

What mechanical advantage really means

Mechanical advantage is nothing more than the ratio between the load you want to move and the force you must supply to move it. When a load is held by several parallel parts of the same line, that load is shared among them and each part carries only a fraction of the total. If four parts support a load, each sees a quarter of the weight, so the tail you grip has to overcome only that quarter rather than the whole. The purchase does not create energy or cheat physics; it redistributes the same work over a longer distance so the force at any instant is smaller. That is why sailors call it trading effort for travel: you spread the job out rather than shrink it.

The physics in plain language

Energy is conserved, which means the work you put in equals the work that comes out, minus whatever friction steals along the way. Work is force multiplied by distance, so if a purchase halves the force, the line must travel twice as far to keep the equation balanced. Lift a 100 kg load one metre through a perfect 4:1 and you apply about 25 kg of pull, but your hands travel four metres to do it. This is the same principle that lets a long lever shift a boulder or a low gear help a cyclist climb: a smaller force acts over a greater distance to do the same work. Once you internalise that symmetry, no purchase ratio can surprise you, because you always know exactly what you give up to gain what you gain.

The effort-versus-travel trade-off

This trade-off is the heart of every purchase decision, and it cuts both ways. A halyard you must tension quickly suits a low ratio and a winch, because you want speed and have mechanical help to supply the force. A backstay or vang you set once and then fine-tune suits a high ratio, because the extra line you haul is a small price for delicate, low-effort control. The art lies in matching the ratio to how the line is actually used: how often you adjust it, how much load it sees, and how much cordage you can tolerate at your feet. Choose well and the system disappears into the background; choose badly and every trim becomes a chore that discourages you from sailing the boat properly.

How a purchase is built

Every purchase is assembled from a handful of simple ingredients arranged so one line does the work of many. At its core sit two groups of turning points — traditionally blocks, increasingly low-friction rings — with a line reeved back and forth between them. One group is anchored to a fixed point such as a padeye or mast base, while the other is attached to the load you intend to move. The line runs from a dead end, around the turning points in sequence, and finally emerges as the tail you pull. How many of those spans pull on the moving group is what sets the ratio and therefore the mechanical advantage.

Fixed blocks, moving blocks, and standing parts

Naming the parts clearly makes counting the ratio almost automatic. The fixed block stays put relative to the deck and only redirects the line without adding to the lifting count. The moving block travels with the load, and it is the parts of line pulling on this block that you tally to find the ratio. The standing part is the end made fast to a fixed anchor, and the hauling part, or tail, is the end you handle. A useful habit is to trace the line from its dead end to your hand in your mind's eye, noting each place it turns; this walk-through reveals the whole system and stops you miscounting a lead that merely changes direction.

How to count the ratio

The most reliable way to read a purchase is to count the parts of line acting directly on the moving block, the one attached to the load. If four parts lift that block you have a 4:1; if two parts lift it you have a 2:1, and so on up the scale. A common trap is including the tail when it leaves the fixed block, because a line that turns at a fixed block on its way to your hand adds no lifting force and only redirects your pull. When the tail exits from the moving block it does add a part, which is why some systems gain an extra unit of advantage from where the hauling end emerges. Count carefully at the load, ignore any purely deck-level leads, and the number you reach is the honest geometric ratio before friction has its say.

The ratio table at a glance

The table below distils the common ratios into the numbers that matter: the force each demands against a round 100 kg load, the line you must haul to move that load one metre, and the jobs each ratio typically suits. Read the force column as a best-case, frictionless figure — a target the system approaches but never quite reaches once real hardware is involved. Read the travel column as a fixed, unavoidable cost that friction cannot change, because geometry sets it exactly. Together they show why the sweet spot for most sailboat controls sits in the middle of the range rather than at either extreme.

Ratio	Force needed (100 kg load, theoretical)	Line to haul per 1 m	Typical uses on board
1:1	100 kg	1 m	Simple lead, no multiplication
2:1	50 kg	2 m	Light vang, cunningham, outhaul
3:1	~33 kg	3 m	Vang, medium adjustments
4:1	25 kg	4 m	Vang, mainsail traveller, small-boat sheet
6:1	~17 kg	6 m	Main sheet, vang on mid-size boats
8:1	12.5 kg	8 m	Main sheet, backstay, heavy adjustments

These figures are theoretical, and the real world is less generous. Friction in the turning points always reduces the effective advantage, so the force you feel at the tail is higher than the ideal — sometimes noticeably so in a high-ratio system with many turns. The travel figures, by contrast, hold true regardless of hardware, because they are pure geometry. Keeping both truths in mind stops you over-crediting a purchase for its neat ratio and under-crediting the quality of its blocks and rings, which is where much of the felt difference actually lives.

Friction: the hidden tax on every purchase

Every point where a line changes direction bleeds off a little of the force you are trying to transmit, and in a purchase those losses stack up turn after turn. In an 8:1 the line may pass over seven or eight turning points before it reaches your hand, and each takes its cut, so the sum can devour a meaningful slice of the theoretical advantage. This is why two purchases with identical ratios can feel worlds apart at the tail: the geometry promises the same multiplication, but the hardware delivers it with very different honesty. Reducing friction is therefore not a luxury but a core part of designing a system that performs close to its paper rating. The higher the ratio you choose, the more the quality of your turning points matters, because you ask the line to negotiate more corners before it does any useful work.

Where friction comes from

Friction has several sources, and knowing them helps you attack the worst first. The tightness of the turn matters enormously, because a line bent sharply around a small radius loses far more than one sweeping gently around a generous one. The bearing surface matters too: a sheave on ball or roller bearings gives up very little, whereas a line dragged across a fixed surface gives up a great deal. Line diameter, cover hardness and even how salt-crusted the cordage has become all play their part, which is why maintenance is quietly part of performance. Misaligned leads that force the line to twist or rub against a cheek add a hidden penalty no ratio calculation will ever reveal, so fair, straight runs are worth chasing throughout the system.

Blocks versus low-friction rings

The choice between a traditional block and a low-friction ring is the single biggest hardware decision in most purchases, and neither answer is universally right. Ball-bearing and roller blocks offer very low friction under high, dynamic loads and are the natural home for a main sheet or any control trimmed constantly and hard, because the bearing keeps the sheave turning freely just when the line moves fast under tension. Low-friction rings, by contrast, have no moving parts at all: light, immensely strong for their weight, cheap to maintain and impervious to grit, which makes them superb on static or semi-static loads. For a deeper comparison of when each earns its place, see our guide on low-friction ring or block, and for the surprising breadth of jobs a simple ring can do, the roundup of low-friction ring uses. Modular systems such as Olli Flex let you build purchases at whatever ratio you need by combining rings, while Olli serves as the single base ring for individual leads.

Choosing the right ratio for the job

Picking a ratio is a balancing act between two competing constraints: the load you must overcome and the travel you can accommodate. Get the balance right and the control feels effortless without burying you in line; get it wrong and you either strain against too heavy a load or wrestle a tangle of cordage for every small adjustment. The good news is that the decision follows a simple, repeatable logic once you know the questions to ask. Start from the physics of the load, layer in the practical limits of your cockpit, and let the intended use break any remaining tie.

Start from the load

Begin with the maximum load the control will ever see, not the average, because a purchase comfortable in a breeze is comfortable everywhere. Estimate or measure the peak force and ask what ratio brings it down to something you can manage — by hand for a small boat, or through a winch on a larger one. A vang on a forty-footer works against far greater loads than the same control on a twenty-four-footer, so it naturally wants a higher ratio to stay tractable. If you plan to lead the tail to a winch, remember the winch itself supplies enormous advantage, so you can often run a lower purchase ratio and let the drum do the heavy lifting. Matching the purchase to the load first ensures the system is at least physically usable before you worry about anything else.

Check your travel and cockpit space

Once the load is tamed, confirm you actually have room to haul all the line the ratio demands, because travel is the cost you cannot escape. An 8:1 moving a control through half a metre asks you to pull four metres of tail, and that line has to go somewhere — through a clutch, onto a winch, or into a bag — without fouling crew or other controls. In a small cockpit a high ratio can pile up enough cordage to become a genuine nuisance, which quietly argues for a lower ratio even when the load might tolerate more. Consider too where the tail lands and whether a tired crew can gather and stow it cleanly after each adjustment. The most elegant purchase gives you the control you need with the least line left over to manage.

Common ratios by application

Experience across cruising and racing boats has settled on a rough consensus that makes a sensible starting point. Light-load trims such as a cunningham or outhaul are usually well served by 2:1, because they need modest force and benefit from quick, direct action. A vang typically lives between 4:1 and a cascaded 8:1 or more, depending on boat size, since it must hold real load yet reward fine tuning. Main sheets on mid-size cruisers commonly settle at 4:1 or 6:1, a compromise that keeps the sheet manageable in a gust without drowning the cockpit in line, while backstays and other high-load, set-and-forget controls justify the highest ratios precisely because their slow use makes the extra travel irrelevant. When a control terminates at a winch, the winch's own gearing folds into the calculation, and our guide to choosing sailing winches is worth reading alongside this one.

Rigging a purchase the right way

A well-chosen ratio can still disappoint if the purchase is reeved carelessly, because how the line runs matters almost as much as how many parts it has. The goal throughout is fair, friction-light leads that let each part pull cleanly in its intended direction without rubbing, twisting or crossing its neighbours. Attention at the rigging stage pays back every time the control is used, which over a season is a great many times. Reeve the line so the parts stay parallel and never chafe against one another or the block cheeks, because crossed parts add friction that no hardware can fully offset. Keep the moving block free to align itself with the load, anchor the standing part to a point strong enough for the full load the purchase can generate, and give each low-friction ring a generous wrap angle rather than a sharp one.

Frequently asked questions

How do I count a purchase ratio?

Count the parts of line that pull directly on the moving block — the block attached to the load you want to move. Four parts acting on that block give you a 4:1, two parts give a 2:1, and so on, with the number of load-bearing parts always setting the ratio. The one subtlety to watch is the hauling tail: if it leaves the moving block it counts as a part and adds to the ratio, but if it leaves a fixed block it merely redirects your pull and adds nothing. A simple way to avoid mistakes is to trace the line in your mind from its dead end to your hand, tallying only the spans that lift the moving block. Ignore any purely deck-level leads that change direction without contributing to the lift, and the number you land on is the true geometric ratio.

Why shouldn't I always use the highest ratio?

Because every extra unit of ratio costs you proportionally more line to haul and adds another turning point that bleeds off force to friction. An 8:1 that moves a control through half a metre makes you pull four metres of tail, which is slow to gather, awkward to stow and prone to cluttering a small cockpit. On top of the travel penalty, the additional turns pile up friction, so the effort you actually feel at the tail can be considerably higher than the tidy ratio suggests. For most vangs and main sheets on cruising boats, a 4:1 or 6:1 delivers the best balance of manageable force, tolerable travel and honest efficiency. Reach for the highest ratios only on genuinely high-load, set-and-forget controls where the slow adjustment simply does not matter.

Does friction really change the result that much?

Yes, and it grows more significant the more turning points the line has to negotiate. In a low-ratio system with one or two turns the loss is small, but in an 8:1 with many turns the accumulated friction can swallow a large share of the theoretical advantage. Good hardware recovers most of that loss: ball-bearing or roller blocks and well-designed low-friction rings keep the effective advantage close to the number on paper. Poor or neglected hardware does the opposite, so a salt-crusted sheave, a hard-worn line or a badly angled lead can make even a modest purchase feel heavy. This is precisely why the quality and cleanliness of your turning points deserve as much attention as the ratio itself.

Should I use low-friction rings or blocks?

It depends on how the control is loaded and how often it moves. Blocks with ball or roller bearings excel under high, dynamic loads and constant trimming, which makes them the natural choice for main sheets and other lines you handle hard and often. Low-friction rings shine on static or semi-static loads where the line moves little once set, offering light weight, great strength, low maintenance and complete indifference to grit and salt. Many boats end up using both, matching each turning point to its duty rather than committing to one type everywhere. If you want to weigh the two properly for a specific job, our comparison of low-friction ring or block lays out the decision in detail.

Does a winch change how I should size the purchase?

It does, because a winch is itself a powerful source of mechanical advantage that stacks on top of whatever the purchase provides. When a tail leads to a winch, you can often run a lower purchase ratio and let the drum supply the extra force, which keeps the system simpler, lighter and less cluttered with line. The winch also changes the ergonomics: instead of hauling a long tail hand over hand, you grind a handle, so a bit more travel becomes far less of a burden. That said, the purchase still matters for the initial hand-tensioning before the winch takes over, and for any fine adjustment you prefer to make without grinding. Sizing the two together, rather than treating the purchase in isolation, gives the smoothest overall control.