TECHNICAL PURCHASING POWER

Sam Fortescue explains how to check you have the power needed to handle your running rigging safely - and how to fix it if you don't

- SAM FORTESCUE

What does lifting a sodden MOB out of the drink and tensioning the backstay have in common? Well, unless you're sailing a very small dinghy, they both make use of mechanical advantage to multiply the force of the human arm. Also known as purchase, there are several ways to turn a small force into a big one without having to resort to electrical power.

Our boats are full of such systems. Even dinghies have a system of multiple blocks to help trim the mainsheet, for instance. If you sail a larger boat, you're likely to have the same thing to move the sheet travellers. Winches are nothing if not an age-old mechanism to gear down for extra power, as is the big wheel on the backstay tensioner. Hydraulic power achieves the same thing using fluid dynamics.

So, we’re surrounded by purchase systems on board. But do they handle their job well, and can they generate sufficient power to make your task lighter? New-built yachts should come with winches and blocks correctly sized to handle the sails, but it’s not always the case with older boats. Sometimes builders will have miscalculated the forces acting on the rig, and sometimes previous owners will have made changes to the original sail plans or rig loads – by adding a bowsprit, for example, or a square-top main. Changing rigging runs with extra blocks to bring mast foot lines back to the cockpit is another classic example.

Common purchase systems

Mainsheet: Even 30-40ft boats will often have a winch-free mainsheet that relies instead on an arrangement of triple blocks giving 6:1 purchase.

Backstay: Stay terminates well before the deck with a block, through which a line is run, itself terminating in an ever-decreasing cascade of blocks. In this way you can achieve enormous purchase – I found an example of 48:1.