Last update, December 3, 2009
This is for your information in a tool to make repeated settings. It is more than you need to know…buy or make one…you will love it. Rick West, 2009
This device is used mainly for setting the backstay tension (BST). While this adjustment is at the aft end of the boat, the purpose of the tension is transmitted to the mast and onto the jibstay. The luff of the jib will sag against wind pressure on this wire that only has two points of attachment. A sailmaker will cut the sail with a curve to match a predetermined amount of sag. This is called the Jib Luff Allowance (JLA). This cut is measured in 1/32" increments.
Should the sag of the jibstay exceed that cut for the sail, the shape will change and that is not good. So, a chart was developed that would predict the needed BST at various wind velocities for the various JLA used that would keep the sag within limits the sail design could handle. This chart was presented in the EC12 manuals published by Ragged Symmetry and is available here in Excel format for printing with their permission. This chart is now representative of what you may want to develop in your tuning trials with sails from various makers. If you record them it would give you a repetitive setting when you need to have it. Regardless, the device will give you an idea of where you are at the lake.
It should be noted that the JLA increments are quite small and not cut with the precision of a computer run laser. The tension meter will give you a base starting point that is usually very close. However, should you note any knuckling of the jib on the water, it means the wind pressure on the sail plan exceeds the design sag of the jib by the sailmaker and more BST is needed. The knuckle creates a break in the jib luff that extends into the camber. This is not good. You can read more of this in On The Water.
The example use shown here is for a set of PX75 sails with a #6 jib. This means the jib was cut for a JLA of 6/32". It would be expected that this jib will work well at around 3 mph winds and the BST at 2.4 pounds.
The EC12 manuals include detailed drawings for the construction of the tension meter. Mike Zellanack provides this meter as part of his products if you would like one ready to use.
Hint: The scale on the device will weather in time as shown. You can copy and replace the scale easily. Be sure you make the marks at the same place and position the scale in the same location. You can also make your own scale by hanging calculated weights from the hook to the right of the pointer and mark.
The best function of the tension meter is through note making when sailing. Your set of sails may not react to the chart listed here. The chart is to show you a base to understand your set of sails so you can preset them at the lake. Likewise, sail design and making changes over time, as does materials. The need to know the correct BST remains and it is a learned process that not difficult.
Rigs have been changing also and in some cases shroud tension is important in the management mast shape and wind pressures.
Those that use a tension meter a lot do not like to guess. It is a very good instrument tool.