Thread: Recycling a Jib

AJ,
A jib is generally cut with more chamber in the forward third of the sail and fairly flat in the back half. The object is to provide lift up front and at the same time the back half speeds the air flow over the lee of the mainsail. From memory the theory for mainsail is maximum fullness is about a third back from the luff.

As for using your find as a main, a cat sail I would think it is cut fairly flat for higher boat speeds and higher angle of attack so the amount of difference in cut probably will not matter.

Now I will wait for someone the correct me

Mike

Ta Mike

Once I work out an extension for my mast, I'll be able to test the theory.

This will happen soon after I install the rain water tank (waste of time
really - it's forgotten how to rain), poison the soursobs, sand & repaint
the hull, build a bigger rudder, do some CFS'y things, etc, etc.

cheers
Alan

Howdy AJ,

As you probably know the luff (front edge) of a mainsail is cut with a convex curve.

The reason for this is when it is put up against a more or less straight mast the excess curve is pushed back into the sail and that is responsible for the camber (depth or 3D shape) of the sail.

Now most masts have some bend too - so you cannot assume that they are straight.

MAINSAIL
So for mainsails there are two convex curves that are added to each other.

1/ one to produce the depth of the sail and is proportional to the width of the sail at each point (good enough explanation for these purposes)

2/ to that is added the curvature of the mast when you are sailing normally because if you do only 1. but the mast is bent to a similar curve the sail will go back to being completely flat.

So you add the two convex curves together to make a bigger curve - the sail with have the right depth when the mast is bent the normal amount then flatten as the mast bends past that amount.

JIBS
For jibs it is similar for number 1
1/ a convex curve to produce the depth of the sail and is proportional to the width of the sail at each point.

2/ but to get the starting point for a jib ... the wire is never straight it sags toward the back of the boat when the jib is working. It sags most in the middle.

So what happens here is that you have a concave curve being taken away from a convex one. This gives most jibs an S shaped leading edge with the bottom end a bit convex and the top part a bit concave.

If you put a jib on a mainmast usually the sail will be way too flat and set even more weirdly toward the top.

It is a free sail - so you can give it a shot and see. Or you could recut the luff to suit the mast and the amount of camber you want and put a simple hem on it.

I'd be surprised if the sail had much more area than what you have on the Teal now. I am happy to be surprised!

Michael

AH-HA !!!
THAT explains why the top bit of a jib always seems to lie all saggy & floppy when not up the mast and under sheet tension.
Well.... the price was right. It is about the same size or a tad smaller than my existing polytarp bag. Just thought (hoped) with a better cut it might have more power.
Another option is to convert the boat to a rear-masted jib-only beastie like one of the designs in "Boats with an Open Mind"...

cheers
Alan

Howdy AJ - you don't have a better shaped sail - you have one that was cut wrongly on purpose!!!!

Mast aft is a fun concept to play with but it is quite technically difficult to get even average performance without a lot of compexity and tension in the various bits. The big argument that proponents suggest is "the clean entry for the breeze to the front of the sail.

I haven't seen any aft mast rigs win an event of any kind ever - not once. I think it fits into the "surprising turn of speed" marketing spin cycle - which means OK if you don't sail too close to anything else.. But I have seen plenty of fast conventional mast sail rigs on very high performance boats.

A good big polytarp sail would probably only set you back about $50 - if that. Ducks have the good cloth stitched to the widths that you need. Bit of luff round and you could double the speed of the Teal - then the rudder and centreboard and stub fins would be going so much faster that the whole thing will work heaps better. Remember you only need to change speed from 3 to 4 knots to get nearly double the lift out of your foils or steering power out of your rudder.

MIK