Main sail luff curve.

Rappey

Rappey

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I have a nice condition mainsail but it appears to have a lot of luff curve. If you take a straight line from top to bottom along the luff the curve protrudes up to 290mm beyond the straight line.
I assume this sail was cut for a boat with a fair amount of mast bend?
My mast is straight so the amount of curve needs reducing.
From what I've read it should never be straight but have some curve ?
How much Curve should I have , ie how many mm should the sail extend beyond the straight line ?
Open to ideas about how to tackle this. What else is there to do with all this time off ? ?
I hope my explanation makes sense.
 
Some of that curve is there to put fullness into the sail.
If it's a nice sail, it's worth getting this done properly by a sailmaker.
 
Outwards curve is normal in a mainsail, more if made for a bendy or prebent mast. Genoas typically have a slightly S-shaped luff curve, outwards in the bottom 2/3rd or so, going slightly inwards near the head. A headsail made for a saggy stay may have completely negative (inwards) curve.

If you want a good result get a sailmaker to do it, or at least mark up how much to remove for you.
 
What matters is how the sail sets under way. A sailmaker may be able to look at it and advise, but it would be better to set it in F3-4 and photograph it from various angles, better still if it has black stripes. You could include shots taken with varying degrees of luff tension.
 
jwilson said:
If you want a good result get a sailmaker to do it, or at least mark up how much to remove for you.
Click to expand...

Two votes to get a sailmaker to do it... (y) That was my initial thought. All that is round here now are larger sailmakers with big offices and probably big bills.. the smaller and easily approachable one/two man operations have gone. The cost of having it done may exceed the used cost of another more suited mainsail.
I do have a very large room and an industrial sewing machine plus have eylets and press.
It can all be unpicked, cut back then all the bits that were unpicked put back on in the right place.
The sail has long battons, well over 3/4 length. When under sail the top and bottom of the sail sit perfectly, how one would expect with no sags but the sail draught in the middle is at the mast rather than a distance away from it.
I have tried to photo but its like taking pictures of large seas, it never looks like it really does.
I was hoping that for a 12m luff length and a straight non bendy mast there would be some magical average figure of how much luff curve there would be..
The boat is a one off , mast from a nicolson 42 with shorter boom , genoa from a rival 42 and had a few not so great mainsails.
"Try fitting a Cunningham control " - just had a read about that, not sure it will help with a sail that does not quite fit properly to begin with but thanks for the suggestion.
 
Rappey said:
Two votes to get a sailmaker to do it... (y) That was my initial thought. All that is round here now are larger sailmakers with big offices and probably big bills......
Click to expand...
Where is 'around here'?
There are lots of good indy sailmakers around, the trick is finding the right one.

There are some DIY sail design programs, might be on the web, see what that spits out, but the amount of luff round will vary according to how much fullness is done by broadseam and the cloth properties?
There are a few books worth a look, a few blogs about DIY sailmaking ?

A sailmaker might tell you a good average figure for luff round, 'wilkinsonsails' (?) posts on here now and then?
 
Around here is gosport.
I sometimes find that you can't get past the front office with a large business and if you do get to talk to someone you can be made to feel your taking up their valuable time. (There are brilliant ones too)
A small business seems more than happy to help and share their knowledge.
I have read a fair amount online about sail design and why but it's not a subject that I find that stimulating.
I carry the rig of a 42ft on a very heavy long keel 33ft so no matter how badly I trim (or dont) I have plenty of sail area to whisk me along. The genoa is more like the size of a code zero.
I rebuilt my boat from essentially bare hull onwards buying parts that were same dimensions as original . Had to learn a lot including all about masts and rigging as I replaced and rigged it myself. After 28 years I'm looking for the easy route, chop a bit off, sew it all back up and be ready for the mad rush when we can go sailing again :)
 
We have recently had a new mainsail computer designed for our 42ft cat, the mast is dead straight. The luff round on this sail is 60mm at the mid point. Some of the sail shape is achieved by broadseaming and some by the luff round.
Sail sets very well and has very nice shape so the 60mm luff round seems perfect. Not sure if mono's require more or less fullness in the sail, and it probably depends on the type of sailing you do.
Hope that gives you a starting point anyway.
 
That is interesting. Thank you.
Unfortunately the cost of new sails is a long way down my list of necessities.
I was thinking of removing somewhere between half and 2/3 as I think that would be about right ?
Posting here allows people to suggest things I've not thought about ,or tell me it's what they do for a living and it's a bad idea !
It will either work or it won't. If it fails I will have learnt that I've no idea at all cutting down sails and £30 lighter in materials.
I think its more likely to work. ✂?
 
I would suggest that DIY reshaping the luff will be very difficult. You would need to detach the bolt rope cut back the actual sail cloth in a suitable line of taper then reattach the bolt rope. Probably by hand sewing.
I would much more suggest you cope with the sail shape. As said try cunningham eye tension. Outhaul tension and try to get the mast to bend a bit. You should be able to get some improvement. Presumably the sail was made for a bendy mast fractional rig and presumably yours is a straight mast stiff mast head rig. Perhaps a professional can recut it but I think you should persevere actual sailing longer first. ol'will
 
The Sailmaker's Apprentice is a good read and will cover luff curve. I might dig mine out later and see what he says. Managed to tweek my ankle in the garden/wilderness yesterday and Nurse Ratchet tells me to rest it today, so a bit of reading is in order.
 
Here goes..
Taking a simple example straight off the page, for a draft of 10% (quite generous) one adds 2.7% of the cord measured paralell to the foot.
e.g. 1/3 up the sail with a cord of 6.1mt, one adds 165mm to the length beyond the straight line from tack to head.
Hope this helps
DW
 
Cantata said:
I can't think why you are even considering altering the sail without trying it out first!
Click to expand...
I totally agree that would be crazy. I have used it for the past year hence having some idea of what is wrong with it.

William_H said:
would suggest that DIY reshaping the luff will be very difficult. You would need to detach the bolt rope cut back the actual sail cloth in a suitable line of taper then reattach the bolt rope. Probably by hand sewing.
I would much more suggest you cope with the sail shape.
Click to expand...
This sail does not have a bolt rope. It's a tasker sail.
The batons are nearly the length of the sail. Instead of a nice curve in sail from end of baton to mast it comes off batton at sharp angle , too much draught?

I have a straight line from head to foot so can measure how much Curve at many places along luff. If I took let's say 66% off each measurement then I would end up with a similar curve but 2/3 less?

My previous mainsail was very badly stretched. This one came from a storage unit clear out, is a little shorter and will certainly do for a while until I find a better one.



DownWest said:
one adds 2.7% of the cord measured paralell to the foot.
e.g. 1/3 up the sail with a cord of 6.1mt, one adds 165mm to the length beyond the straight line from tack to head.
Click to expand...
I'm not fully understanding this. The cord measurement in your example 1/3 up is from leech to luff?
165mm is not 2.7% of 6.1m ?

I really appreciate all the input on this thread ?
 
The roach on the leech is basically there to gain extra area and is supported by battens. From what you say about your rig the limiting factor for the roach will be the line of the backstay(s) to allow tacking without chafing the sail. The aft section of a main is generally fairly flat so reducing the roach will have little effect on the fullness of the sail and the crease at the forward batten line.

The sharp crease at the front of the battens indicates a full sail that may have been built for a curved/bendy mast, as you said. This shape will be created by the curve of the luff and broad seams so this is the area to be altered to flatten the sail. Possibly, going beyond a DIY project.

However, as you have space and a suitable machine fitting full length battens may be the simplest way to provide a serviceable, but rather full, sail on your straight mast.
 
Rappey said:
I'm not fully understanding this. The cord measurement in your example 1/3 up is from leech to luff?
165mm is not 2.7% of 6.1m ?

I really appreciate all the input on this thread ?
Click to expand...
Yes, the cord is the distance from the leading edge to the trailing edge i.e. luff to leach, but following the airflow, so sort of paralell to the boom. 1% of 6.1 mt is 61mm, 2.7% is 165 rounded up. Like I said, 10% draft (curve of the sail between luff and leach) is quite generous. 8.3% draft would give you 2% round. So the example would give 122mm past the straight line.
This applies to any cord measurment up the sail, apart from very near the tack.
As SRM says the shape is a combo of round and broadseaming (sort of 'darts', to use a dressmaking term :) ) It does sound like a reduction would help with the kink between the battons and the luff.
The book I referenced above is very good value and cheap on Amazon. I made my sails from it and they set very well.

Amazon.co.uk: The Sailmaker's Apprentice: Books
 
srm said:
fitting full length battens may be the simplest way to provide a serviceable sail
Click to expand...
That would surely incur high costs for new batton and special mast sliders ?
I'm after a low cost improvement where labour is free (me) and will give me something constructive to do and and degree of enjoyment.

Thank you "DownWest". Very useful as using your information ties in with the amount I thought right for the amount to remove.
 
You should watch the recent 2 part YouTube videos on sail design on RAN Sailing.

I don't think it's possible to work out how much curve to take out without analysing the sail on the boat. You need to sail in a strong breeze with the wind ahead of the beam. You should to be able to move the draft forward with full control of the of the outhaul, halyard tension, mast bend and cunningham tension. When set properly it should have vertical creases up the luff. If you have horizontal creases there's too much sail for the scope of control you have. I bet a sail maker could tell you how much curve to take out from a few pictures of the sail under load.
 
Rappey said:
I have a nice condition mainsail but it appears to have a lot of luff curve. If you take a straight line from top to bottom along the luff the curve protrudes up to 290mm beyond the straight line.
I assume this sail was cut for a boat with a fair amount of mast bend?
My mast is straight so the amount of curve needs reducing.
From what I've read it should never be straight but have some curve ?
How much Curve should I have , ie how many mm should the sail extend beyond the straight line ?
Open to ideas about how to tackle this. What else is there to do with all this time off ? ?
I hope my explanation makes sense.
Click to expand...
The normal rule of thumb is that your mast should have curve of half the chord. It should not be straight. The sail is cut to suit this curve plus a little bit more as tension on the luff stretches it
 
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