I have tested some of the elements in Oscar’s Junk Rig calculator against some of my masts and boats. Here is what I found:
Hull Properties.
This appeared to work well, with a plausible righting moment. However, this will vary with the ballast position and percentage, so remember to add a bit if heavily ballasted and subtract a bit if your boat is lightly ballasted (or with only inside ballast). I found the Displacement to LWL calculator handy - I have never used that parameter, but it makes sense.
Spar-mass-strength.
Appears to make sense. The only snag is that the sort of available aluminium alloy varies, and so does the strength. Is this calculation done with yield strength of 250MPa (250Newton/mm2)?
Sail Properties
I have only checked the Johanna type. It appears to be based on Johanna’s own sail, which has a quite low aspect ratio (AR) of only 1.87. Problem is that we often need to vary the aspect ratio quite a bit to get the sail area and centre of effort (CE) right. That is why I have expanded the Johanna-sail into a whole family of “Johanna-style” master sails, with AR ranging from 1.80 to 2.25 (look up chapter 4 in The Cambered Panel Junk Rig - TCPJR).
To make a quick estimation of a sail area (SA) on a Johanna-style sail, if you know the length of boom and battens, B, and the AR, I suggest you make use of this simple formula:
Sail Area = 0.78 x AR x B2
The result is generally well within 2.5% error. This is actually a combination of the formulas for SA and AR on page 6 of Chapter 3 of TCPJR. In real life we often have to start the rig designing process with the boom to get the CE in the wanted place (CE is close to the middle of the boom on the Johanna-style sails). Then it is a question of trying different AR to see if we can get the wanted SA without ending with a super tall or supper low sail.
Cheers,
Arne