Anthony Cook wrote:

It seems to me that using tucks, rather than broadseams, will create more of a 'box section.  With tucks, the full amount of sail material is available virtually right up to the head and foot of the panel, creating more fullness in those areas. With broadseams, the sail material contained within the sewn seam is lost for ever.

I don't think I'm following your reasoning here, Anthony. Both tucks and broadseam can be applied in the same way, to obtain a 3D shape at the edge of a panel that is reasonably close to the ideal shape obtained with an added, lens-like shelf. Tucks are used where the panel is formed from one continuous cloth, and broadseam is used where there are vertical seams between a number of cloths that make up a panel, but each can do what the other can do, in terms of forming a 3D shape.

My experience has been that it's inadvisable to try to get a completely rectangular cross section to the panel. This seems to put too much cloth into the sail, too close to the batten. Better is to aim to create an angled shelf. Angles of 30 and 45 degrees seem to lend themselves to easy calculation, as one can use a "1:2:√3" or "1:1:√2" triangle to work out the offsets.

Arne Kverneland wrote:

David,

it seems that my head needs to be rewired to fully grasp the 3D idea. What I am rather looking for is a practical rule of thumb, either how much they should slacken the sail along the battens or how much length I should remove by using tucks/broadseams (in percent of B)

Any suggestions?

Sorry, I can't help with this. I have no experience with the first, and with the second, I'm reminded of the old joke about the Irishman who was asked for road directions: "I wouldn't start from here, if I were you".

David,

it seems that my head needs to be rewired to fully grasp the 3D idea. What I am rather looking for is a practical rule of thumb, either how much they should slacken the sail along the battens or how much length I should remove by using tucks/broadseams (in percent of B)

Any suggestions?

It appears to me that I somehow got the tension very close to right on Frøken Sørensen. Here, the sail inflates nicely in light winds and the vertical curve is still very close the trapeze, or even rectangle shape. See below.

Arne

PS: Thanks for the 80/20 rule. I am there for sure, in the good-enough league.

David,

I looked long and hard on your diagram, but I am afraid that instead of clearing things up for me, it left me rather cross-eyed and confused.

To clear my mind, I went out in my Ingeborg to take some measurements. First of all, I found the length from luff to leech along the battens to be about 4.86m (instead of 4.90m). I guess you cannot get it that much longer without really hauling on it.. 

Then I wanted to measure the actual camber. Since there was a tail wind in the berth of some 5-6 kts, I went out in the bay for a little test sail. The rather hastily performed measurements showed that the camber has ended on about 33-34cm. Divided on the slightly shortened  chord (4.86m x cos 10°) the camber should be around 7.0%.

That coincides well with experience on Edmond Dantes’ and Frøken Sørensen’s sails: Unless one slackens the sail a little along the battens, the camber will end up about 1% (of the chord) lower than my chain calculator predicts.

There is one observation more, worth mentioning: The luff and leech of each panel set with a small but noticeable hollow. It must be from here the sail gets most of the extra horizontal length to achieve the camber

In the diagram I made of Ingeborg’s sail in the posting below, the extra length needed to get around 8% camber is 87mm or 1.78% of the B.  My guess is that if I increase the slackness along the battens from the present 4cm to 7cm (1.43% of B), the camber of the sail will approach the planned 8% camber, instead of the present 7%. This way of slackening the sail will inevitably lead to those wrinkles along the battens, as shown on the photo of Johanna.  

If it hurts too much to watch all those wrinkles, one may instead make 2-3 broadseams which eats up the same length, (say 1.5 – 1.7% of B). This too will be an approximation.

Something like that.

Arne

Go to an image stored somewhere on this site (in your photo album?). Right click and select 'copy'. Start a reply and right click in text area .. select 'paste'. Select pasted image and drag corner to resize to no more than 420 pixels wide (if the image is any bigger it causes the text to overflow the right hand edge of the box).

It's easier if you have two browser windows open, one with your post and one with the original Image.

Chris