How do Sailboat Blocks Work?
A block is an extraordinarily simple device. Fundamentally, a block is just a wheel inside a frame. A rope wraps around the wheel. When the rope moves, the wheel turns. There's very little friction inside of a block, and it doesn't matter which direction the rope moves or how much force it moves with.
All a pulley does is redirect the force of the rope. When blocks are used together, they work like a simple reduction gear system. The more blocks you use, the slower you'll lift an object. But the more you slow down the ropes, the easier it gets to lift the object.
Each additional block in a pulley system reduces the speed by one-half but multiplies the force by two. This is useful when lifting something heavy, like a spar or a topmast. Blocks work just as well against horizontal force, like the kind exerted on the mainsheet while under sail.
What are Sailboat Blocks Made Of?
Sailboat blocks are made of wood, metal, or plastic. Traditional sailboat blocks are large, heavy hardwood pulleys with wooden pegs and wooden wheels. A large metal shackle is attached to one end. It mounts to the deck or the object you need to exert force on.
Metal blocks are also quite common, and they're typically made of steel or various corrosion-resistant metals. Plastic blocks have become increasingly common over the years, and they're great for low-stress applications where the forces aren't too intense.
Block Sheaves
The sheave is the part of the block that rotates. It's a disc located within the frame of the block and secured by a pin. Sheave discs are grooved to encourage the rope to ride in the center, which keeps it from binding up between the parts.
Sheave is a term used to identify the number of discs in a block. The most basic block contains one sheave and is known as a "single sheave" block. Double and triple-sheave blocks are also common.
The force distribution of multi-sheave blocks works just like multiple single-sheave blocks. A set of two triple-sheave blocks accomplishes the same task as six single-sheave blocks, as long as they're connected to the load the same way.
Block Systems
There are four primary types of block systems that accomplish different tasks. You're likely to find at least three of the four common block systems on a typical cruising sailboat.
Fixed Block
A fixed block utilizes a single sheave block mounted to a fixed location, like the deck. Fixed blocks are often found in areas where ropes need to change direction, such as the base of the mast.
Moveable Block
A simple moveable block system is the opposite of a fixed block setup. Instead of mounting to a fixed spot, a moveable block mounts to an object that you need to lift. The sheave rides on top of the rope and manipulates the object attached to the block.
Compound Block
A compound block combines a fixed and moveable block in an "S" shape. The compound block reduces the force required to lift an object. Compound blocks are ideal for hoisting large objects. Remember, compound blocks reduce speed and increase your pulling power.
Block and Tackle
The simplest block and tackle requires two blocks. It's similar to a compound system, but with the blocks oriented directly below and above each other. The compound block line runs over the top block, down and around the bottom block, then to a fixed point at the base of the top block.
Types of Block and Tackle
Block and tackle are used on all kinds of sailboats for trimming the sail. Systems that use block and tackle include the mainsheet and the boom vang, among others. Here are the five primary types of block and tackle aboard a sailboat.
Gun Tackle
Gun tackle is the simplest block and tackle system. It utilizes two single sheave blocks. The line runs around the top of one block sheave then around the bottom of the opposite block sheave. The line returns and connects to a fixed point at the base of the original block.
Luff (Watch) Tackle
Luff tackle, which is sometimes called "watch tackle," utilizes three sections of line instead of two. Luff tackle uses a twin-sheave block and a single-sheave block. The end of the line affixes to the single-sheave block.
Two-Fold Purchase
Two-fold purchase tackle utilizes two twin-sheave blocks and four sections of working rope. Two-fold tackle applies more force than luff tackle at slower lifting speeds. The end of the line mounts to the top block, which is the entry point of the rope.
Double-Luff Tackle
Double-luff tackle has another multiple of two in the name, but it actually utilizes five sections of line. Double-luff tackle requires one twin-sheave block and one triple-sheave block. Double-luff tackle simulates, adding another block to the top and bottom of a luff tackle system.
Three-Fold Purchase
Generally speaking, the three-fold purchase is the most complex block and tackle system you'll find on a typical cruising sailboat. Three-fold tackle utilizes six working lines and two triple-sheave blocks. Three-fold purchase systems carry the most load with the least amount of effort.
Where to Find Blocks on a Sailboat
Blocks are located all over a typical sailboat. However, they're typically not used for stays and shrouds. Standing rigging is often tremendously tight and fixed, so there's little need for block and tackle.
Mast Blocks
Mast blocks are located on or around the mast. These blocks are involved in hoisting, lowering, and reefing the sails. Mast blocks reduce the force required to hoist and lower sails. Additionally, mast blocks are the points where halyards change direction from vertical to horizontal.
Vang Blocks
The vang applies a downward force to the boom. It connects the boom to the base of the mast and usually contains two or more blocks. The vang blocks on a sailboat make it easier to tighten down the boom and work against the force of the wind.
Traveler Blocks
Traveler blocks connect the mainsheet to a traveler, which is usually located towards the end of the cockpit. Travelers allow you to precisely adjust the trim of the sail using a set of pulleys and blocks.
Mainsheet Blocks
Blocks are an integral part of the mainsheet system on most cruising sailboats. Mainsheet blocks make it easier to trim the sail in heavy winds, and they reduce the load on sensitive deck hardware.
Choosing the Best Sailboat Blocks
Blocks are an essential part of sailboat rigging. They control everything from the speed of the boat to the tension of the mainsail luff. Blocks allow lines to move around corners, slide from side to side, and make it easier to manipulate them.
As you can see, it's probably not a good idea to cut corners when choosing blocks for your sailboat. The best quality blocks use strong corrosion-resistant materials, bronze or stainless steel. Wooden blocks are also an excellent choice for classic sailboats.
Bronze Blocks
Bronze blocks are costly and uncommon in the 21st century, though they're highly sought-after and make an excellent addition to any classic sailboat. Bronze blocks are available online and in second-hand marine stores.
Stainless Blocks
Stainless steel blocks are far more common and ideal for most modern sailboats. Well-made stainless blocks don't rust or corrode under normal conditions, which keeps them rolling smoothly for years.
The primary threat to stainless steel blocks is galvanic corrosion, which shouldn't occur unless stainless comes into direct contact with a dissimilar metal. However, inexpensive blocks made with inferior alloys can corrode rapidly on their own when exposed to saltwater.
