Arne,

 I was recently following your notebook designing a junk rig conversion for my lateen rigged dinghy (but apparently I was doing it wrong and using 10% mast balance for the 70 degree sail). I found it very useful for a conversion with an existing mast step to find the fixed CE-to-mast dimension (1030mm in my example), then apply the fixed mast balance to find out how long the boom must be (because Johanna70 has 35% of the chord between mast and CE). Then I could pick the aspect ratio to get the sail area that I wanted.

Freeing the angle constraint as you did here now gives two degrees of freedom. I can choose a different mast balance to get a different boom length, then I could get my target area with a different aspect ratio.

Do you have any thoughts on how to choose among the resulting one-parameter family of sails? Or in this case, the three resulting sails with yard angles of 60, 65, and 70 degrees? It seems like you are suggesting that they will all “work” but will be taller vs shorter. In a design sense, do you think that there is any sort of “best” choice among those? Or particular strengths or weaknesses?

Thanks for all you have done in making these sails accessible to beginning sail designers and makers,

-Neil

Thanks for sharing, Arne. Even though my current boat still needs time and attention i find myself thinking about the next, bigger, boat. I have been wondering if it would be feasible to fit a single sail to a Catalina 30 while keeping the mast more or less at the original location. I dont think i have the skills to make a SJR work. Your lower yard angle plans might be just the thing to make this happen.

Anonymous wrote:

Increasing the mast balance flexibility of the Johanna sail.

In the beginning.
The junksail I designed for my Johanna in 2001 was given a yard angle of 70°. This was to allow me to have minimum mast balance in the sail.

In 2014, after having learned to make use of the QCAD program, I made a whole range of ‘master sails’, based on Johanna’s sail (AR=1.87), all with 5.00m chord. The AR of these sails range from 1.80 to 2.25 with steps of 0.05.

The need for more mast balance.
The mast balance of the Johanna 70 sail (‘70’ stands for 70° yard) can only be adjusted between about 12 and 17%. This sometimes give little flexibility with respect to positioning the mast. My original idea with this was to avoid that the mast should distort the camber on the port tack of sailing. However, other JRA members, like Paul Thompson, had shown that 22% mast balance works just fine.

I have therefore liberated myself from the 70° yard angle, lately. As the diagram below shows, four rig versions will now cover any need for balance between 12 and 30%. For instance, a Johanna 60 sail is now propelling JRA-member Ketil Greve’s Kelt 8.50 in Stavanger. This is being set with 21-22% balance.

I haven’t got around to make a full range of Johanna 65 and Johanna 60 master sails yet. My procedure these days when designing a rig for a boat, is to start with finding a place for the mast(s). Then I draw the boom. I know that the CE of the sail will sit within 1% of the boom length from its middle, so just use the middle for CE.
Since the sail area, SA= B² x AR x 0.78, I can quickly calculate how long a boom I need to get a desired sail area. The resulting test boom on the plan will tell me how big the sail’s mast balance need to be. If the number is 19%, for instance, I see that I need a Johanna 65 sail.
In case I already have a master sail with the right AR and yard angle, I just pick it and scale it up or down to the correct size.
If I don’t have that particular master sail in my library, I make one. I do so by scalping the three top panel off the Johanna 70 master sail and build up three new panels (each about same area as the lower panels).This way, I am now free to vary the mast balance over a wider range.

Johanna 55?
I have still to make practical use of the shown Johanna 55 sails. I hesitate with adding more than 25% mast balance until it has been tried on a small boat first. However, a Johanna 55 sail with, say, 28% balance would be very desirable (if it works) on really big sails. The increased balance will offload the battens a good deal, and the sheet loads will be much lighter as well. That would be fun  -  to be able to control a 100sqm sail...

Cheers,
Arne

From  Arne's sketches, section 6