Nomad revisited
So will the sail have a “good” side and a “bad” side because of the jackstays—like a lug of any sort does with the mast?
And that brings us to the ama. Did you test that with models? I’m fascinated to see how it works.
The stay at the leading edge should clear the sail and rear stay should be fairly slack with some tension in the sail.
No models. A couple of real down and dirty 14’ boats that sometimes got backwinded led me to suspect having more lateral resistance in the float would work better. P52 had that setup and worked pretty well. Initial configuration had the float on a swing arm type geometry with the idea that the boat could be trimmed by swinging the float fore and aft. Initial trials were inauspicious at best, terrifying at times. By the time P52 was sold the hydro side of the boat was pretty well sorted out, sail much less so.
A lot of this comes from my background, have spent a lot of time designing racing canoes over the years and had zero experience with the aero side of proas. I’m learning.
Skip
After twiddling my thumbs for a couple of weeks visiting friends and family while shoulder heals I figured I should try to do a little calculation on what I hope to do when both hands can be put to the task(s).
Cambered Panel Sail
A lot of this is based on Robert B"s insight regarding how loads are distributed on rigid batten sails and Chris L’s work on Snelson’s Canoe. There’s some corroborating evidence from the experience of members of the Junk Rig group that started breaking battens when going to cambered panels.
The following math is probably not 100% correct but it’s a good place to start to get in the ballpark. Calculations are based on !an Hanny’s “RIG THEORY” AYRS publication 113 page 52.
Nomads’ 75 square foot sails are divided into cambered panels separated by rigid straight battens. A large portion of the stress in each sail panel is directed into compression in the battens. The vertical stress in each panel is added to the adjacent panels so that the halyard stress is the sum of all the vertical stresses. Vertical stresses are proportional to the square of the vertical length of panels and the sag of the panel.
Assuming a wind speed of 20 knots being the worst case scenario (I’d reef long before that) the total halyard load is only 282#‘s. For comparison an equivalent unbattened staysail would be looking a 1,390# !!!
No wonder I broke P52’s mast.
A bonus with this configuration is the boat can easily be tacked in lighter winds, I’ve done that before both accidentally and deliberately.
For reference the Bionic Broomstick’s future 68 s.f. sail is shown. Stresses are 333# battened, 999# unbattened. Should be interesting theoretical allowable compressive stress on mast is 318#.
Very interesting calcs on the cambered rig. Its kind of funny to think of a batten in compression but without deflection.
Hope you feel better soon. I’m excited to see broomstick testing these ideas out!
best,
chris
Skip,
Are the battens attached to the mast in some way like a standard junk rig? If not, the sag you use to calculate the halyard load should be the total sag (not the sag for each panel) and the halyard load should be the same regardless of whether you have battens or not. If you have high halyard loads, you could consider using spreaders or lower shrouds to reduce the chance of buckling the mast.
Cheers,
Mal.
Are the battens attached to the mast in some way like a standard junk rig? If not, the sag you use to calculate the halyard load should be the total sag (not the sag for each panel) and the halyard load should be the same regardless of whether you have battens or not. If you have high halyard loads, you could consider using spreaders or lower shrouds to reduce the chance of buckling the mast.
.
Hi Mal,
Thanks for the input. Short answer is no, the battens aren’t currently envisioned as being connected to mast.
Long answer….I don’t know, but want to find out. Original thought was to have a bidirectional sail with the battens “tied off” at the quarter points as shown in the earlier iterations on this thread. Further cogitation and doodling led me to think that maybe Robert B was right that the stresses were additive and came up with a rationalization even.
Common knowledge and experience shows that stay sails, jibs and the like won’t function for windward work unless there in significant tension in the system. My line of reasoning is that the sail(s) suffer from the luff and leach being pulled together, bagging the sail with more camber when you really want less. Having rigid battens should keep this from happening. Only way to find out is to try both setups.
Another week and hopefully my arm will be out of sling and I’ll be able to slowly get back to finishing boat to find out some of this stuff. Assuming there’s still enough water around to float the boat, been terribly dry around here for a while.
Cheers,
Skip