An adjustable backstay on a masthead rig is used to change the catenary of the forestay. This affects the depth
, position of draft
of the jib
, and leeward sag of the forestay. Basically a loose forestay provides a deeper draft jib
for more power, but you can't point as high. Tightening the backstay, which is straightening the forestay, provides a jib that is flatter, less power and can point higher. One additional thing that you have to do when straightening the forestay, is to add tension the jib halyard
as the draft will move aft as the sail flattens, so you must move it forward again with halyard
Reaches and off the wind
you let the tension off, which provides a deep powerful jib shape.
If you have forward lowers and slotted spreader attachments, you will also be bending the mast
(mostly the top half) which has the same effect on the main as the jib.
When you have sails
made, tell the sailmaker
you have an adjustable backstay, particularly if the boat normally doesn't come with one. They cut the sails
Downside, more stuff to maintain, my hydraulic backstay needs new seals
. Possible to damage something if you put too much pressure on. When his boat was new to him, a friend was curious as to what was too much pressure, so he slowly increased the pressure. Decided too much was when the mast
made an S shape side to side due to compression
and the middle being locked in place with forward and aft lowers. No damage done, but scary to look at. (Same boat as story below.)
One of the interesting things about boats is that they are flexible. Going upwind we would tighten the forestay, when we turned around to go downwind and wanted to put up the spinnaker
, if we had forgotten to loosen the forestay, we could not get the pole out of its clips on the deck
. The clips had moved close enough together to clamp the pole between them. Loosen the backstay and it came right out.