Have you married each sailboat block on your small cruising boat to the right marine rope "mate"? How can you know if they will be compatible with each other for years to come? Use this easy guide to save big $$$s and keep your sailing rigging in tip-top shape.

How would you match the line diameter to the sheave diameter on these single blocks? This secret will help you prevent chafe between the sailboat block and marine rope.

Block Anatomy 101

All blocks have three basic parts: cheeks form the outside of the block like the bread on a sandwich. The sheave or pulley (the meat of the sandwich) lies between the sheaves. Metal straps and pins clamp the sheaves together to complete the assembly.

More sheaves give more mechanical advantage (pulling power), but the trade off is more friction, or resistance when pulling on a line. Count on 10% friction for each sheave in a block.

Some blocks have a u-shaped fitting on one or both sides called a becket. Splice or tie a nylon or dacron line to the becket, or attach it to the boom or sailboat mast.

Other blocks have a snap shackle on one side. Pull the plunger pin and the snap shackle pops open for fast attachment to a deck pad eye or stemhead fitting.

Blocks used on boom vangs and mainsheets might have a spring loaded cam-cleat on one end.

When hauling on a line, the cam cleat opens up. When you release the line the cam cleat clamps down on the line to hold it in place.

Single, double or triple blocks

Count the number of sheaves between the cheeks of a block to determine its generic name. Headsails use a single block combined with a winch. Large mainsails require a combination of two single, double or triple blocks to handle their loads.

Mainsheets and boom vangs often have fiddle blocks, with two single blocks in a row instead of side-by-side. This allows you to reeve (thread) the line over each block to increase the mechanical advantage of the assembly. A cam cleat often attaches to the lower fiddle block for easy in and out adjustment.

Turning blocks (cheek blocks, foot blocks)

Turning blocks lie flat against the sailboat mast, boom or on deck. They change the direction (or fairlead) of a line. Increase turning block size as the line direction becomes more acute.

For example, a small cheek block on the sailboat boom might change the lead of a line by 90 degrees. But you might need a larger deck mounted spinnaker foot block to redirect a spinnaker sheet 180 degrees!

Lead Blocks (jib blocks, Genoa blocks)

Headsail clew sheets run through jib or Genoa blocks and back to sheet winches. These blocks mount on slides attached to strips of track along the edge of the boat or coach roof. Plunger or screw pins keep the slide in place, but also allow you to reposition the block anywhere along the track.

Snatch Blocks

These versatile blocks have a snap shackle on one side for quick attachment to a deck fitting. One cheek can be opened and shut using a simple bail and latch mechanism. That way, you don't need to reeve (thread) the line though the block. Use a snatch block to redirect a line or remove jammed turns from a headsail winch.

Size Your Line Based on Block Length

Match the line diameter to the diameter of the sheave. Choose a line too big, and it will wear from rubbing on the cheeks of the block. Select an oversized block if necessary to prevent this problem. Follow these rules of thumb for a perfect match: