- Oct 22, 2014
- 22,076
Is bigger always better?
Intuitively we know without a block, sheave or low friction ring the line will rub on the boat parts, be harder to pull, or chafe and break.
The answer in many situations is to use a block.
Blocks come in many sizes, shapes and made from different materials.
The basic design is simple. There is a housing holding the block together, a center pin and a wheel or sheave that rotates around the center pin.
The burning question: How big of a block do I need. Let’s focus on the forces involved. I recently discovered this Harken tech site tool that provides their engineering knowledge to help us address this issue.
https://www.harken.com/en/support/tech-articles/block-loading-vs-angle-of-deflection/
The deck organizer is found on many of our boats. It is used to direct the line from the mast base to the cockpit. It is an example of a block. The line is turned by this block. Most often the angle turned is shallow (for example 30 degrees). The block used in this example will experience a load equally to 52% of the load on the line. It is no wonder that the low cost white plastic sheaves wear down and break after a few years hauling the sails up the mast.
There is the issue that you need to work out for your own boat.
What is the force involved on the line?
Armed with the load force on the line and the angle of deflection (the angle you are turning the line) you can select a block the will provide years of safe service on your boat.
Intuitively we know without a block, sheave or low friction ring the line will rub on the boat parts, be harder to pull, or chafe and break.
The answer in many situations is to use a block.
Blocks come in many sizes, shapes and made from different materials.
The basic design is simple. There is a housing holding the block together, a center pin and a wheel or sheave that rotates around the center pin.
The burning question: How big of a block do I need. Let’s focus on the forces involved. I recently discovered this Harken tech site tool that provides their engineering knowledge to help us address this issue.
https://www.harken.com/en/support/tech-articles/block-loading-vs-angle-of-deflection/
The deck organizer is found on many of our boats. It is used to direct the line from the mast base to the cockpit. It is an example of a block. The line is turned by this block. Most often the angle turned is shallow (for example 30 degrees). The block used in this example will experience a load equally to 52% of the load on the line. It is no wonder that the low cost white plastic sheaves wear down and break after a few years hauling the sails up the mast.
There is the issue that you need to work out for your own boat.
What is the force involved on the line?
Armed with the load force on the line and the angle of deflection (the angle you are turning the line) you can select a block the will provide years of safe service on your boat.
