Sailboat block types, explained

Blocks redirect lines and create mechanical advantage. They are sold as singles, doubles, triples and quadruples. A double block has two sheaves side by side, sharing one axle.^[1][2]

• Fiddle block

  Two sheaves of different sizes, one above the other in the same plane, instead of side by side on one axle like a double. The smaller sheave has a lower load capacity. Commonly used in mainsheet and boom vang (kicker) systems.^[2][3]

• Snatch block

  Its side plate opens, so the block can be added to a line that is already rigged without feeding the line through from one end. Used for temporary leads and for deflecting a line.^[4][5]

• Cheek block

  Mounts flush against a deck, mast or boom, horizontally or vertically, fixed with screws or bolts.^[6]

• Footblock

  Bolted to the deck, it redirects a line along the deck. A common job is turning a genoa sheet from the lead car to a winch.^[7][8]

• Mast-base block

  Sits at the foot of the mast and leads halyards and control lines aft to the cockpit. Harken lists these, not footblocks, for leading halyards aft.^[9][8]

• Stand-up block

  Sits on a spring or a flexible rubber or PVC boot that holds it upright when the line is not loaded, so it does not fall onto the deck.^[10]

• Soft-attach block

  Uses a fibre lashing or loop, often Dyneema, in place of a metal shackle. Harken warns that soft attachments wear, and recommends them only for regularly maintained race boats.^[2]

Beckets and ratchets are features rather than types. Ronstan sells the same block with and without a becket, and features combine: Harken's 57mm Ratchet Block also has a swivel and a becket.^[1][11]

• Ratchet

  The sheave turns only in the trimming direction, so the block grips the loaded sheet and you can hand-hold it with less effort. Common on mainsheets and spinnaker sheets on dinghies, sportsboats and smaller keelboats. The block must still be rated for the full sheet load.^[2][1]

• Becket

  A closed bail on the block that usually serves as the dead end of a line within a purchase system. It can be rated below the block itself: on Ronstan's Series 55 Orbit, load on the becket must not exceed half the block's.^[2][12]

Ball-bearing blocks give the lowest friction and suit lines that run under moderate, dynamic loads.^[13]

Roller-bearing blocks carry higher loads than ball-bearing blocks and are still low-friction, though ball bearings are lower-friction still.^[2]

Plain-bearing (sleeve or bushing) blocks have more friction than ball- or roller-bearing blocks, but handle very high and static loads.^[2]

Harken fits sleeve bearings to its cruising blocks to handle high static loads, and calls them ideal for halyards.^[14]

Some ranges come in more than one bearing. Ronstan's Series 40 Utility blocks are sold with ball-bearing sheaves, plain acetal sheaves, or a Nylatron sheave that also works with wire.^[15]

Block heads include fixed (non-swivel) shackles, swivel shackles, snap shackles, swivel eyes, soft lashings or loops, and bolt-down bases for cheek and foot blocks. Some small blocks also come with hooks.^[16]

Some shackle blocks can be locked in either of two directions, or left free to swivel so the line does not twist.^[17]

Harken sizes many of its blocks by sheave diameter in millimetres. Carbo blocks come in 29, 40, 57 and 75 mm sheave sizes, and the size is in the name: the 57mm Ratchet Block has a 57 mm sheave.^[11]

In the Ronstan ranges we checked — Series 20, 30, 40 and 55 — the series number matches the sheave diameter in millimetres. Treat that as a pattern, not a rule.^[12]

Which size of block you need depends on the diameter of your line and the load it carries. Those are safety figures, and we have not yet sourced a method for working out block loads that we are willing to publish. We will add a sizing guide when we have — until then, check the maker's own sizing tables for the block you are looking at.

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  Harken · Deck Hardware

  Harken Snatch Block

  The side plate opens, so it goes onto a line that is already rigged without feeding the line through from one end. Harken and Ronstan both describe the job the same way: temporary leads, and deflecting a line where it would otherwise chafe or lead badly.

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  Harken · Deck Hardware

  Harken Black Magic Air Block

  Harken builds these for big-boat sail controls that work hard — their own list is mainsheet, runner, halyard and spinnaker systems. They are made to be serviced: few parts, no loose balls or rollers to lose, and they come apart with a single hex wrench instead of being riveted.

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  Ronstan · Deck Hardware

  Ronstan Series 20 Ball Bearing Block

  A small ball-bearing block. Ball bearings give the lowest friction of any block bearing and suit lines that run under moderate, changing loads — the opposite job to a halyard block, which sits under heavy static load and is usually plain-bearing for that reason.

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