Thanks Dave, good answers. I might be “barking up the wrong tree” on this question of “balance” and terminology does, as you say, create its own problems. Its hard to express a simple idea without using lots of words. I’ll try again, please forgive me if this is becoming tiresome, but I would like to continue the discussion a little further. I have taken your hint about forum topics, and revived an earlier forum entitled “A Question of Balance”, which also alludes to a relationship between yard angle and permissible sail balance.

Hi Graeme, 

Glad you liked the write up!

Ill respond, briefly, here, as much of this probably belongs to other Forum topics than this one.

Also, please keep in mind that our only previous junk sails were flat cut, so have no experience with contiguously cambered. In other words, my opinions are ungrounded, where others would have better informed input.

Dave Z

Graeme wrote (in red)... full text in his earlier post:

I understand the use of camber (scows need power) – but, where maximum performance to windward is not a priority, what were your main reasons for choosing the split rig?

Our scows did quite reasonably well with flat cut. We chose to try SJR hoping to improve windward performance, not to maximize it.

We chose SJR over contiguous camber since it is, by design, unnecessary to reposition on the mast, simplifying rigging and handling. Given a double sailplan, however, I believe neither style requires repositioning.

I am wondering if a “plain vanilla” cambered sail might be a whole lot easier than SJR and maybe just as good for a scow. I would be interested in your thoughts on that.

From the racing experience of others, SJR appears to yield a head-to-head performance advantage over simply cambered, all things being equal (which they very seldom are!). Whether there is enough to justify the added construction and setting difficulties is a judgement call.

While we're likely to stick with SJR for our purposes, it's nearly a coin toss.

It seems to be an often-repeated wisdom that a major advantage of the SJR is the ability to place the mast further aft – but nobody has explained why this should be so, and I have some doubts.

You'll likely want the sail plan to best balance when close hauled. In simple cambered rigs, the sail is usually hauled aft on the mast, while SJR stays put. If you drew the sail in close hauled position (hauled aft for cambered; fixed for SJR), the cambered mast would have to be ahead of the SJR mast relative to the hull.

Sailing off the wind, the cambered sail usually hauls forward of that drawn position, when it doesn't hurt to put the horse before the cart.  8)

Of course, if the sail is split then the mast has to go where the split is. 

Alternatively, you might say the split goes where the mast is... both have to work together in the sail plan as well as the deck plan. For POPPY (successful design), Slieve went mast @ 25% of batten length, and thought up to 35% might be an improvement. That gives a range of working options.

But effectively we are talking about the chord balance and I wonder if this has anything at all to do with the sail being split, or whether in fact it is simply due to the geometry of the sail with its low yard angle. What is your opinion on this?

I don't think that yard angle plays a role in this beyond halyard/slingpoint mechanical considerations.

Rather than chord balance, the split is a gap between two aerofoils; that of the jibs and that of the main each with their own chord and points of maximum camber (chord balance, also often expressed as a percentage of chord). In this sense the balance percentage (jib : after_panels), has everything to do with being split into two distinct aerofoils. Its location determines that balance.

Lug sails refer both to the area ahead of the mast OR the proportion of that area to the rest as balance. To my mind this overlap of meaning is confusing. It would be good to coin a more distinct term, probably for the area ahead of the mast.

Of all these advantages - the ease in which the sail can be draped, without the need for running parrels of any kind – and the ability to carry a jib to after_panel balance of up to 33% - how much of this is due to the sail being split, or is it mainly due to the geometric shape of the sail with its low yard angle?

I think that those advantages are independent of the sail planform, shaping or yard angle for basically parallelogram sails. It's their parallelogramicity (is that even a word??) that makes it possible.

Rather, advantages stem from positioning the uppermost batten where the lower sail can hang vertically, and from forgoing the ability to position the sail on the mast via running rigging (allows standing batten parrels to be rigged short).

A lower yard angle improves the transfer of loads, and multiple sails improve balance control without individual sail repositioning. A larger balance eases some mechanical loading, and reduces weather helm when sailing off the wind.

And a couple of more specific questions: What camber and sheeting angle did you use for the jibs? (I used 10% and 12 degrees – maybe a bit close to the limit as they tend to collapse rather easily).

Hmm... seem to have mislaid that info. We went with Slieve's then current specs for POPPY... maybe 9 degrees and 12%? But I could be way off. Ours don't collapse easily (compared to the after panels)... maybe 8%?

What is the chord balance of your mizzen?

Our mizzen was lashed flat cut... what camber we developed was in fabric stretch and twist control.

If we were to camber it, we'd likely match or halve the main's 8%.

And any comment on aspect ratio of the jibs? (Slieve has preferred taller jibs, slightly under-square, but I note that you have used five lower panels instead of four, resulting in shorter wider jibs.)

I'm not aware of any aspect ratio (AR) discussion of the jib or any other JR panels... didn't even occur to me! However, judging from the wide range of successful panel heights, I doubt AR of individual panels plays too much of a role.

We worked our panel heights according to fabric limitations (stayed under 54in). By increasing the jib:after.panel balance, we lowered the jibs' AR as an unexamined side-effect.

Mark,

that could well be. 
However, in 1991 I tested the same flat sail with hinged battens (see NL24). That camber was symmetric, but still worked splendidly, except for increasing the weather helm a little bit ( no show-stopper). Therefore, I had little fear of putting the tucks in both the luff and leech. The actual horizontal camber curve became a smoother arc than one should think.
If one only shortens the sail at the luff, the sail’s planform will be altered as well. One should check what happens on paper before doing it in the sail.

Arne

Dave,
Like Graeme, I think your write-up was very good.
As for camber in the mizzen, I don’t think you need nearly as much as in the main.

Here is a simple, non-destructive way of introducing a bit camber in a flat sail, which I tried on my Malena, back in ’93 (look up JRA NL26). What I did to Malena’s sail was to introduce two 10cm tucks at luff and leech of each panel. Short lengths of webbing held the tucks together, and these could be removed any time.

In your mizzen, I suggest you only fit one 10cm tuck (or two at 5cm each) at the luff and leech of each panel, to only make a modest camber. You will need to take the sail off the boat to do the mod, but on a fine day, you could bring the sewing machine out-doors and sew on the short lengths of webbing. No need to remove the battens.
The mod will only shorten the mizzen with a total of 50cm, and should give you some answers. If you find you were better off with the flat sail, then just bring out the ripping needle and remove the tucks with the sail on the boat.
Done in less than an hour.

Good luck!

Arne

I've submitted the write-up of WAYWARD's prototype rig to the JRA. 

It's currently readable here.

Dave Z