"Out of curiosity, what would be the max CoA shift aft that you'd be comfortable with? The more space I put between the mast and the hatch, the stronger my mast partner and coach roof options."

I don't know, Scott. My inclination would be to work from the other end. Place the mast with a strong deck support across just behind the hatch, and draw up the rig from there. Then see what C of Area shift you get and decide if you are comfortable with it. You would think that a couple of % of the total water length length should not be too critical, but obviously it would be desirable to be as close as possible. Once you start making hard decisions you start to make problems for yourself. Rough it out, and if it looks good it will probably be good.

Not a clear answer, I know, but it is an inexact science.

Cheers, Slieve

Unfortunately your latest diagram won't open up for me, but I gather the numbers are coming together well. I'm no expert on the subject, but I'm wondering about the high aspect ratio. The ballast ratio seems to be only 38%, so would that suggesting getting the centre of area a little lower if practical?

How would it work out if you increased the chord to 5.5, and probably decrease the height to about 11.4m, then use 35% balance and see what you get? Where would the C of Area be then? I do realise that it's easy to sit here and suggest that you do lots of drawings, but it would be good to get the best setup from the word go.

Please remember that you are swimming in muddy waters as you try to define your rig parameters, as there are no real experts on the subject. It's very satisfying when you settle on your final figures

Cheers, Slieve.

Hi Scott,

Thanks for the diagrams. I'm not sure if I'm seeing them correctly, but they look as if you are only using 30% balance. Is this the case? If so then things would change if you increased the balance towards 35% which we have got away with in practice. Then where would the C of Area lie?

By the way, what is the chord of the rig in the diagrams?

And another thought, if the centre of area is 11 cm further aft than the Bermudan centre of area, then that represents only 11 cm on a 7.86 metre waterline length (?) which is 1.4%. Can we be that accurate in our guestimates? Emmmm?

The trouble is that every answer usually raises a few more questions.

Cheers, Slieve.

Scott, If you're not prepared to go for a 100 metre mast then why not do a quick and easy drawing of what is easiest.

Set the mast height as the same above deck as the Bermudan rig, and no-one will laugh at you for that. Then draw in the mast as close as you dare to the back edge of the existing hatch, assuming you could reinforce the deck adequately and draw a mast centreline vertically up from there. Reckon that that line is the 35% chord line, mark in the Centre of Area line from the rig diagram and call it the 50% chord line, so from those two numbers you can draw in the 0% line (luff of jib) and the 100% line (leech of main) and see what the length of chord works out at. Draw in the rig height to get the area of 600sq.ft and see if it fits the mast.

I get a feeling that if you do a lot of calculations and drawing with a variety of A/Rs you will find that the mast position will move very little, something like a couple of inches, so the question is, could you get away with doing the minimum of work?

How critical is the Centre of Area position anyway?? Everything is a compromise. With a well cambered rig, and wider sheeted jibs the Centre of Effort seems to be well forward in the Centre of Area.

Cheers, Slieve.

Joking aside, I think it's the difficulty of sheeting a tall sail that finally limits the AR. The higher you go, the steeper the angle of the upper part of the sheeting system, and the less effective it is at pulling the sail in, the worse it is at distorting the sail vertically. Also, the harder it is to control twist.

If you really want to go very high, I think that separate upper and lower sheets are a must, with the upper sheet able to be shifted up to the weather side.

How high? I don't see any difference between split rig and other forms of junk rig. I'm using a sail of AR = 2.3 with upper and lower sheets, and that's fine. PJR deals with AR up to 2.78, but I think I'd have difficulty sheeting a sail like that. I'd guess that AR = 2.5 is a sensible upper limit.

I don't suppose that there's a practical lower limit for AR. Oryx has had very low AR sails of various types. However, I can't see any reasons for going lower than Fantail, at AR = 1.5