Is it possible to cut a given sail to fit any mast curve characteristics (excluding stiffness)? Or do some sails need certain bends.

What I'm trying to get at, is whether there is any reason why masts are flex top or stiff top? - other than the cynical reason "we cut it like that so you buy our masts"

Gestalt said...

yep it's posaible to cut a sail to suit any mast curve and stiffness charactistic.

but that's where the grey area starts. sail designers have preferred mast specs that they believe work optimally for a particular sail type and size. They design their sails around this. when you look at a sail range "some not all" go from flexier tops in the smaller sizes to stiffer tops in the larger sizes.

depends on what the designer feels is the best mast/sail combination for a particular use.

But isn't it the case that for a given mast, you can cut more curve into the luff and it means that the sail is held tighter at that point? So for the same mast, you cut a straighter bit at the top on the smaller sails.

I have to admit that I am seriously ignorant here... keen to learn though

There is a topic in the wave sailing forum on sail making by a sailmaker.

I also found this site sometime ago, then lost it and recently found it again, deals with sailmaking and gives a bit of a descripttion about luff curves relating to mast bends and seams. It also shows an example of changing the luff curve to suit another mast. I found it a good read as i am going to try and make one oneday.

Basicly the guy says that luff curve is virtually an upside down shape of the mast bend. I haven't had time to test this on the garage floor yet.

www.sailrepair.co.uk/makingsail.htm

Thanks Gesty,

I'll have a look, but not much of a tech head when it comes to computers so a bit reluctant to download free stuff after a sh t load of free stuff killed the inlaws computer and if I killed this one I'd be shot.
Interesting stuff sailmaking but probably an expensive way to get a dud sail, will eventually get around to having ago.

Just wondering if after this long absence the head has cleared and you have come to the conclusion that a queen is better than a sutton

The shape of a sail is induced and controlled by two methods, luff curve and broadseaming.
Luff curve is the difference between the bend of the mast and the amount of round cut into the luff of the sail, luff curve is the main factor in dictating the overall depth or camber of the sail.
Broadseam is the shape cut into the edges of individual panels of the sail, whilst braodseam does induce a little bit of depth into a sail it has more of a role in where the point of maximum camber lies.
The relationship between the two is very complex and hard to explain for a yachts rig and on a rig as dynamic and flexible as a windsurfer rig near impossible so I am not going to try,
To answer the original question, if you alter the luff curve of a sail to suit a different mast you may have to add or remove cloth to achieve the match, if you do not also modify the broadseaming of the sail when recutting the luff you will find the sail will not be anywhere near the same.
As for the stiff top vs flex top I think different designers have different schools of thought as to how the rig will react with a different mast, so go down the path they like best.

yep i understand,

both myself and bundybear have answered that question though.

it is simply they design with a particular bend curve as they feel it is the best choice. there is no conspiracy that it's done to sell more gear.

yes all sail designers could design to one "standard bend curve" but why should they, forcing them to do this is asking those that don't agree with the chosen bend curve to compromise their design.

if you look at australia, both severne and KA use different bend curves. i can't speak for either ben or andrew but i am certain that both those guys are trying to design the best product they can. they would both have fundamental reasons why one bend curve is preferred over the other.




this is from the KA website

ka.RT…ka Rig Technology

As part of our intensive ka.RT program, we did a huge amount of experimenting with different bend curves and commissioned samples from the leading mast builders around the Globe. Launching off the experience we had from the Mach2 Foiling Moth development program, we were able to select the absolute optimum layups and materials for the KA Mast range.

We have selected a manufacturer in Italy. The build quality and finish is excellent and every mast is machine tested to ensure compliance to the specified IMCS stiffness and bend curve. The masts are numbered and all data is recorded. All masts are constructed using the latest composites technologies to achieve the perfect strength to weight ratios.

After much testing (and we tested some extreme design bend curves) we have settled for a very moderate IMCS bend curve of 64% Base and 77.5 % Tip giving an overall bend curve of 13.5 . This is just very slightly more flex top than true constant curve. This curve proved to be working beautifully across the entire range of sails.

interesting interview with Ben.

But who in their right mind puts a rap backing track to an interview?

How do i work out the luff curve of a sail for a given mast if starting a sail from scratch?
The website above says its basicly an upside down mast shape, I tried to see that in the pics below, but turning the mast upside down doesn't seem to match the luff curve even if taking any shaping into account which may be in the body of the sail. (on another sail it does match, but mast and sail don't go well together)

Also how much mastbend to apply or is it all trial and error.
As mentioned above the relationship between mast bend and luff curve is what gives most of the shape and the broadseams just determine where the draft will be,so does this mean that a sail will work without any broadseams?
thanks