GRF's rig school

Originally posted by G.R.F.

   So - here's the how to bit.

Given a mast how do dinghy sailmakers decide how to cut their sails?

Do they set up the rig, with the boom in place a length of none stretch line 

between the end of the boom and the mast tip and another between the 

end of the boom and the foot, then tension the rig?

That's how a windsurfer sail maker pre bends the mast, describes the 

curve of the mast under load kind of draws it back like a bow.

That curve is then traced onto the bench and forms the basis for the luff 

curve of the sail. The way it works with windsurfing sails, by matching the 

curve of the mast to the curve of the luff in a positive way (introducing 

enough seam shaping between the lower panels) you get the belly you 

need for drive, then by reversing the curve to slightly negative from the 

point at which you want the sail to twist off in ever increasing increments 

(millimetres) by varying the curve of the mast with increasing downhaul or 

tightening it up with less downhaul.

Downhaul and outhaul being the only controls available to windsurf rigs.

Now with a modern dinghy rig with say a GNAV, the only system I can 

speak of with any experience of having studied, there are more controls.

It doesn't appear to me that there is much going on with the application of 

luff curve seam shape. It all seems to be from broad seam shaping (that's 

the panels of the sail being cut slightly curved or high pointed to generate 

a curve. Some of the sails I have don't seem to be cut with a tight enough 

leech to start with, some can be made to tighten up by virtue of the fact 

the boom is sheeted from the rear and by really sheeting in hard, almost 

block to block in fact the leech can be made to tighten, but when released 

the leech tends to sag from the middle rather than twist off at the top.

Twisting off at the top rather like a propellor blade has been found to be 

the more efficient means of exhausting rigs.

There doesn't appear either to be much attention paid to the flattening 

effect a GNAV has over the leech tightening effect a kicker has for 

example.

Go back to the opening description with the rig set up with bare none 

stretch line, pull down on a kicker the tip of the mast will bend, apply a 

GNAV and the middle of the mast will bend forward almost before the tip 

of the mast bends, at least that's the way it appears on the three masts 

I've experienced, so did they research that? And how did they research it, I 

can't find any specs on t'internet.

There are sailmakers here, possibly reading this, how do you do it?

I'd be keen to hear.