And now a simple question - what rupture stress are you using for tthe wood mast calculations? I have seen tablulated figures of 1,400 psi to 10,400 psi.

I want to use the Practical Jung Rig equations (with Arnes corrections) but want convert to steel masts.

Having three masts makes calculating mast size from the righting moment a little hit and miss.

Arne and David,

Most helpful, thanks again.

Stavanger, Friday

A quick way of sketching up a sloop rig.

Ash, could I suggest my quick sketching procedure?

·         I start with deciding where the CE should sit compared to the boat’s CLR, centre of lateral position. Normally I reduce the lead, compared to that of the Bm rig, a bit (5% of the wll).

·         Then I draw the boom with 10 deg rise. The CE of my Johanna-style sails is just about on the middle of the boom, or just an inch or two forward of it. The position of the mast will decide how long the boom has to be to get the CE right. On my Johanna, the deck layout forced the mast to sit far forward, so the boom had to be long, and thus the AR low.

·         Then I draw a vertical luff line, a bit longer than I need.

·         From this luff line I now draw a yard with 70 deg peaking and same length as the boom.

Now I am almost there: I just measure the vertical length, h, from the peak of the yard, down to the boom. I have found, by checking several sail plans, that the

                             sail area = 0.79 x boom x h.

This correction factor, 0.79, has proven to be constant within 1 - 2% with various aspect ratios, as long as the boom rise is 10 deg and the yard is 70 deg.

This lets me quickly try with several yard positions until I get the sail area I want. Now the AR can easily be found as

                             AR=h : chord

This sketching method lets one easily assess several rig sizes and mast positions without needing to draw the full sails.

From there on it is the task of drawing the sail with as equal sail area as possible in each panel. To save me from work, I now use the CAD to scale up or down a standard sail plan that I have stored. It can of course also be done manually. Now that the CE and SA are known, there should be less need for drawing several sail plans.

Good luck!

Arne

PS: I still use HK parrels, but they see less load, now that the throat hauling parrels have been re-discovered.

                                                   Stavanger, Thursday

Oscar

I would not alter the SA of the Johanna-style sail by just stretching or crimping it along the y-axis. That would stretch/crimp the yard and two top battens  compared to the lower ones and I design my sails with equal batten length.

This is why I design a series of 7-panel standard sails with AR between 1.8 and 2.2. These have been carefully designed to give the top panels the same area as the lower panels, or just a bit smaller.

From these standard sails it would of course be possible to feed the data into spreadsheets.

Personally I still tend to work in semi-manual mode:

• I first print out the boat, with the original rig removed, to a useful scale.
• Then I sketch on a preliminary rig
• Then I draw an accurate sail in QCAD, based on the sketch and a standard sail of same AR. This is very quick.
• Finally I print out the sail on a transparent in the same scale as the boat.

This transparent sail can then be given the final position on the boat drawing and I can then scan the final result (sheet 1). When constructing the sail I use the CAD-based sheet 2, 3 and 4, where the numbers are written on.

Actually, all this focusing on computer techniques may make members think that computers are a must  to get the JR right. They are not. One may well design a good junk sail with just pencil and paper, a scale ruler, a pair of compasses and a calculator. A piece of chain for the chain calculator may also be an idea to get the round right for your intended camber. I mainly have taken CAD into use to save me time. The gain in accuracy is nice to have as well, but it's not rocket science to make a JR, after all.

My method may sound primitive and slow, but a JR project is not just a keyboard exercise. I generally end up sewing the sail, and that takes five times as much time as making the finished drawing for it. (I don't reckon the time used on "pre-design speculations"). And then comes the construction of the mast step and partners, and the mast , and, and...

Arne

PS: I admit that I am not without scepticism to spreadsheets: Their hidden algorithms may be right  -  or wrong. Expanding the sails with CAD makes it easier to see the result.

PPS: The sailplan of Frøken Sørensen  shows the four sheets of CAD-generated sail plans. They turned out to be very easy to work from. You find the plan under here:

http://www.junkrigassociation.org/arne