Michal and Brett, good morning.
all well, then (with the first cup of coffee in hand).

Since you already have the two masts, and seem to have found they (or the tabernacles) will not take up too much space below deck, you should just keep on.

I have never owned a catamaran, myself, so don’t claim to know much about them. I have the Wharram design book and even purchased the building plans for a Tiki 21, but I found it would be too cold and wet to sail about in her, at 59°N. In addition I didn’t like its boomless gaff  mainsail. This was before I had got the idea that gaff mainsails could be replaced with a junksail, as here. My armchair idea of yesterday was based on two reasons:

A catamaran has much wider base for the shrouds, so the compression on a stayed mast will be a lot lower than on a monohull. Therefore it should be possible to just use stays at the mast top., without having it collapse. However, I doubt if the original Bermuda mast could have been re-used, so you need not have any regrets for that reason.

My other reason is that I feel that it is simpler to make and handle one large rig than two smaller ones. However, I can see your point about redundancy. Being an old maintenance engineer, redundancy was a golden word; dual, triple, quadruple r...

I suggest, if you want design help, that you indicate your favoured mast positions (distance from bow), and also let us know the length of the masts you have. I would guess that 2 x 35sqm sails should be handy for a small crew, and make your vessel quick enough.

I hope you get help from some of the catamaran junkmen in JRA, like Bertram Fercot and Pete Hill. However, they are using “Mk3 junks” with quite advanced sails. To begin with, I suggest you go for  simpler “Mk2 sails”; like my ordinary junk sails with cambered panels.

Anyway, good luck!
Cheers, Arne

Arne,

Your input (as given earlier, privately) is always appreciated.

In this case, two masts are sitting in the yard, the central deck living accommodation modifications are already in progress, and the desire for two independent rigs, both balanced as a sloop (for "worst-case" redundancy) having not changed, keep this design on the original track described. Biplane configuration just reaches back to an earlier era aesthetic that is irresistible! 

Your efforts to describe the mounting of a central single sail plan (a more efficient monoplane) will be wasted in this thread, but surely can offer others some good ideas.

We're still looking for a rational, experienced-based measure on how much junk sail area is a "good" replacement relative to original sailplan. In terms of catamarans is best, but input from monohull experimenters is also quite useful and welcomed. Offshore sailing is definitely in the mix, but the Gulf of Mexico (we are in South Texas) Is notable for extremely light conditions on the one hand, hurricanes on the other.

Given a fixed beam and masts in-hand, sail area modifications will be primarily by adjusting aspect-ratio (i.e. chord length) to suit. Our predilection is to increase AR at expense of sail area rather than decrease mast height and keep a lower ARt.

Varying AR affects mast placement, thus tabernacle final design, Those mountings will be on and to the two forward main cross-beams. Initial sail-plan sketch-ups have the mast close on to the aftermost of the two forward beams. Bolting up cleanly now requires a bit of fine tuning based upon finally accepted mast location.

Pax all.

Michael  

A short (sort of) exposition to set the scene:

Brett Baer, my young neighbor and another “newbie” to JRA, has welcomed my free offer to use my engineering ability to give him a tabernacle design.

Design loading has already been determined by using the (two) salvaged mast sections now in hand to design their support to be a bit stronger than the spar breaking strength. For anybody with an engineering bent reading this, I am using the ultimate breaking moment of the mast (flagpole) section as an input for a normal Allowable Stress Design. (Granted, Stress Design is rather old school, but, then again, so am I. I could redo the calcs to Allowable Strength Design or Load Factor Resistance Design, but no recent customer employed grads are looking over my shoulder on this one, so I will stay with my back-of-the-bar-napkin results).

The final design step is exactly where to put the mast, before taking the tedious step of detailing and producing shop drawings. One take , hopefully, would be nice (understanding it never quite works out that way). Got to try anyway.

Brett's boat is a Wharram Pahi 42. Being a third (or later) owner purchase, the original construction prints were not included so the (much more reasonably priced) study-plans are the only documents in hand.

The original is a Cutter Rig. Muscle requirements to handle this rig would tax a young family not desiring extra crew, so decision was made to convert to a side-by-side junk rig, Bertrand F's Pha and Grand Pha (among others) are successful implementations of this concept, so confidence is high to emulate.

The big question is: HOW MUCH JUNK SAIL AREA IS REALLY NEEDED IN COMPARISON TO THE DESIGNER'S?

For pencil work here's what I have for the original design:

Mainsail: 290 sq.ft.

Staysail: 120 sq.ft.

Yankee Jib 230 sq.ft.

Genoa: 315 sq.ft.

Drifter 485 sq.ft.

Pax Vobiscum,

Michael