A single block carries one sheave and simply turns or redirects a line; double and triple blocks stack two or three sheaves in one shell so you can reeve a line back and forth and build a block-and-tackle that multiplies your pulling power. Choose the sheave count by how much mechanical advantage the control needs: a single for a turn or a lead, a double for a 2:1 to 4:1 purchase, and a triple for the higher-purchase mainsheet and vang systems where you have to tame real load by hand.
This guide is for the owner of a 28–45 ft cruiser or cruiser-racer laying out a mainsheet, vang, traveler, or backstay tackle and wondering whether to reach for a single, a double, or a triple. Get the count right and the system is light, low-friction, and easy to trim; get it wrong and you either cannot hold the load or you bolt on more block than the job needs. Below we explain how the sheaves combine, how load builds as you add them, and which real MAURIPRO blocks fit each stage of a purchase system.
Single, double & triple blocks at a glance
| Model | SKU | Sheaves | Max working load | Breaking load | Best for |
|---|---|---|---|---|---|
| Harken Carbo 40mm Single, Swivel | HAR2636 | 1 | 485 lb / 220 kg | 1,620 lb / 735 kg | Turning or redirecting a single line |
| Harken Carbo 40mm Double, Swivel | HAR2638 | 2 | 970 lb / 440 kg | 2,380 lb / 1,080 kg | 2:1–4:1 purchases, vangs, travelers |
| Harken Carbo 40mm Triple, Swivel | HAR2640 | 3 | 1,455 lb / 660 kg | 3,050 lb / 1,383 kg | Mainsheet & higher-purchase tackles |
| Lewmar 60mm HTX Double | LEW29196002 | 2 | 2,422 lb / 1,100 kg | Not published | High-load cruising systems, line to 12 mm |
| Harken Black Magic 57mm Double, Becket | HAR3218 | 2 + becket | 3,600 lb / 1,633 kg | 7,203 lb / 3,267 kg | High-load mainsheet/spinnaker with a dead-end |
Drop-ship note: availability is confirmed at checkout; the figures above are the manufacturers' published specifications.
What is the difference between single, double, and triple blocks?
The difference is the number of sheaves — the grooved wheels the line rides over — carried between the block's cheeks. A single block has one sheave and does one job: it changes the direction of a line, as a turning block at the mast base or a lead back to the cockpit. A double block carries two sheaves side by side, and a triple carries three. Those extra sheaves are what let a single length of line run back and forth between two blocks, and every pass, or “part,” that ends up pulling on the load adds to the mechanical advantage.
Notice that the sheave diameter can stay the same while the count changes. The Harken Carbo 40mm Single (HAR2636), Double (HAR2638), and Triple (HAR2640) all run a 40 mm sheave; what changes is how many lines the block can carry and how much load the shell is rated to take.
How do double and triple blocks build a purchase system?
A purchase system — a block and tackle — trades line for load. Reeve a line between a fixed block and a moving block and each part of line that pulls on the moving block shares the load, so pulling in three feet of tail might raise the load only one foot but at a third of the effort. Doubles and triples give you the sheaves to create those extra parts in a compact package instead of hanging a row of separate single blocks.
The count you need follows the purchase you want. A double against a single gives roughly a 3:1; a double against a double gives 4:1; a triple against a double or triple takes you to 5:1 and 6:1, the range most 30–45 ft mainsheets live in. For the full theory of simple versus compound tackles, see our guide to purchase systems for sailboats.
How much load does each added sheave carry?
Adding sheaves does not just add purchase — it raises the block's rated load, because more sheaves spread the total force across more of the shell and pin. The Harken Carbo 40mm family shows this cleanly: the single is rated to a 220 kg (485 lb) maximum working load, the double to 440 kg (970 lb), and the triple to 660 kg (1,455 lb) — each step up adds almost exactly one single-block's worth of capacity. Breaking loads follow the same climb, from 735 kg on the single to 1,383 kg on the triple.
The practical lesson: size the block to the load at the block, not to the load on the sail. In a 4:1 mainsheet the block nearest the boom sees far more force than the tail you pull, so the double or triple at the loaded end has to be rated for that peak. Always confirm the block's maximum working load clears your system's peak load with margin before you commit.
How many sheaves do I actually need?
Work back from the control and the boat. For a simple lead or a turning block where you only redirect a line, a single is all you need — anything more just adds weight and cost. For a vang, traveler, or a modest mainsheet on a boat up to the mid-30s, a double at each end builds a clean 3:1 or 4:1. For a bigger mainsheet, a stiff vang, or any control you trim hard by hand in a breeze, step up to a triple so you can reach 5:1 or 6:1 without stacking blocks.
Match the family to the load, too. The lightweight Carbo blocks suit control lines and mid-size mainsheets; for heavily loaded, frequently adjusted systems on a larger cruiser, a stouter block such as the Lewmar 60mm HTX Double (LEW29196002) — rated to 1,100 kg and built for line up to 12 mm — gives you more margin and takes a bigger rope.
What is a becket, and when do I need one?
A becket is a fixed attachment point — usually a small eye or strap at the bottom of the block — where you dead-end the standing part of your tackle. In an odd-purchase system such as 3:1 or 5:1, the line has to terminate somewhere, and a becket block gives it a clean, in-line anchor instead of a separate shackle. That is why one block in most tackles is a becket version, like the Harken Black Magic 57mm Double with Becket (HAR3218), a high-load block rated to 1,633 kg that suits mainsheet and spinnaker systems where the loads are serious.
You need a becket whenever your purchase ratio is odd, or whenever you simply want the dead-end to sit tidily with the rest of the system. Even-ratio tackles (2:1, 4:1, 6:1) can dead-end on the opposite block's head fitting instead, so a becket is optional there.
Which block should I choose?
Turning or leading one line: a single such as the Harken Carbo 40mm Single (HAR2636) — lowest weight, lowest cost, does the one job well.
A 3:1 or 4:1 vang, traveler, or mid-size mainsheet: pair doubles like the Carbo 40mm Double (HAR2638), or step up to the Lewmar 60mm HTX Double for higher loads and bigger line.
A 5:1 or 6:1 mainsheet or high-purchase control: a triple such as the Carbo 40mm Triple (HAR2640) at the working end.
The dead-end of an odd-purchase tackle: a becket block like the Black Magic 57mm Double, Becket (HAR3218).
Frequently asked questions
What is the difference between a single, double, and triple block?
The number of sheaves. A single block has one sheave and redirects a line; a double has two and a triple has three, letting you reeve a line back and forth to build a block-and-tackle that multiplies your pulling power.
Do I need a double or triple block for my mainsheet?
Most 30–45 ft mainsheets run a 4:1 to 6:1 purchase, which means doubles or triples at each end. A double-against-double gives 4:1; a triple in the system takes you to 5:1 or 6:1 for heavier boats and stronger winds.
How do more sheaves change the block's working load?
They raise it. In the Harken Carbo 40mm range the single is rated to 220 kg, the double to 440 kg, and the triple to 660 kg — each added sheave spreads the load across more of the shell, so the block carries more.
What is a becket on a block?
A becket is a fixed eye or strap, usually at the base of the block, where you dead-end the standing part of a tackle. Odd-ratio purchases (3:1, 5:1) need a becket to terminate the line; even-ratio systems can dead-end on the opposite block instead.
How do I size a block for a purchase system?
Size it to the load at the block, not the load on the sail. The block nearest the load sees the highest force, so confirm its maximum working load clears your system's peak with margin, and match the sheave to your line diameter.