Smarter and stronger.

One of the sailors skills on a sailboat is the ability to use mechanical advantage to assist  in being able to handle the large loads encountered when handling the sails. The most traditional device is the block and tackle. A block is what is called a pully by most non-sailors. A traditional block consists of the parts as shown below:

 

A traditional block would have been contructed of a wood shell, metallic sheaves(bronze or steel) and straps and fittings of bronze or steel. This is the traditional block as seen on old sailing ships or "classic" sailboats. The principle is that a line is run thru the sheave and the direction of the line is changed or with multiple blocks, a machanical advantage can be realized. The sheave turns on a simple pin and bushing to help minimize friction, but a traditional line is also stiff enough to impart frictional losses.

Here are examples of basic block and tackle theory from "Sail Power" by Wallace Ross:

On modern sailboats, the blocks are made of more sophisticated materials. The shell and sheaves are often made of high strength plastic and the fittings are stainless steel. Also, the sheaves often turn on ball bearings for minimal friction. Furthermore, line is often a small diameter, high strength synthetic line that runs through the sheaves with less friction.

Racing sailors are especially savvy at designing block and tackle systems with very high mechanical advantage to control the very high loads seen on racing sailboats. Most cruising sailors do not encounter the loads that racing sailors see on a regular basis and therefore utilize less exotic blocks and line and their systems are not as powerful.

One of the racing sailor's tricks to increase power in a block and tackle is to have one block and tackle pull another block and tackle. This multiplies the mechanical advantage of the system. This is known as a cascading system. It is often used for high loads that do not require much movement and on "two speed" systems with a coarse and fine adjustment.

Here are three examples of cascading systems with high purchase ratios on my boat, a J35:

This is the mainsheet system. This is an example of a cascading 4:1/16:1 system with a coarse adjustment and a double ended fine adjustment. The blue lines are the mainsheet. The solid blue lines are the 16:1 double ended fine trim and the grey/blue line in the fiddle block is the 4:1 coarse adjustment. The cam cleat on the fiddle block is a special trigger cam cleat that can be released under high load. A normal cam cleat cannot be released under the high loads that we see. Also the big sheave in the fiddle block is not round, but is a hexagon. The groove is a v-shape instead of an arc. This sheave ratchets one way(when pulled in) and helps hold the line from running out. All the blocks are ball bearing for minimum friction losses and the line is spectra for minimum stretch.

The red line is the traveller cross haul system to adjust the position of the mainsheet for trimming the boom for differant conditions. The system is a 4:1 continuous line that is cleated on either cockpit wall and because it is contiuous, the far side can be uncleated without crossing the cockpit.

This is the boom vang. This is an example of a double cascading, double ended system. The tube has an internal spring to support the boom and a 3:1 wire purchase to compress the tube(and hold the boom down). Attached to the wire is a 4:1 tackle made with high strength, small diameter spectra(dyneema) line. Attached to the end of the 4:1 tackle is a block that is part of a 2:1 double ended line that ends at cam cleats near the cockpit. This allows: 3:1 X 4:1 X 2:1= 24:1 purchase. Of course, there are some friction loses. The wire, small diameter spectra line and ball bearing blocks minimize friction. The length of the spectra line is critical to allow the maximum movement of the boom vang. If the spectra is too long or short, the single block of the final 2:1 cascade can be "two blocked" against the upper or lower blocks. The strong purchase allows us to vang sheet the main and reduce main sheet loads in high winds. This may be a bit of overkill to have such a high mechanical advantage. My main reason was to be able to lead the line to the cockpit to make it easier to control. But in a recent race, we needed to adjust this system often to keep the mainsail and boat under control. The crew person was able to rapidly ease and take in the line without undo effort. This helped our speed and the ability to keep the boat under control.

This is a 4:1 system for adjusting the genoa car position. The black line runs from the cam cleat, turns 90 degrees through the rachet block and goes forward through a double becket at the front end of the track, and through the double block on the genoa car for a 4:1 purchase. By pulling on the black line, the genoa car goes forward changing the angle of the genoa sheet to the car. By releasing the line, the car is allowed to move aft and is assisted by the solid black shock cord. The blocks are all ball bearing as is the genoa car on the track.

In addition, there is a light gray spectra line that is attached to the back of the genoa car. It turns 180 degrees through a cheek block and then runs forward to the short track for the #3 jib. It goes around a single becket block at the front of the track, through a block on the jib car and ends on the becket block. This gives the forward car a 8:1 purchase(4:1 genoa purchase X 2:1 cascading jib purchase = 8:1 total jib purchase). By pulling the genoa car forward, the gray line also pulls the #3 jib car forward. In addition, the distance that the forward car moves is half the distance that the genoa car move. So, the genoa car can move the full distance of the longer track for any sail combination on that track without the #3 jib car hitting the end of it's track. Again, all blocks are ball bearing, altough there is a lot of direction changes that increases friction.