Hi Don, I know that in order to point high you should reduce headstay sag. However, there are times when you need to power up the headsail. This can be done by easing backstay tension which will cause the headstay to sag to leeward. What would you consider the maximum amount of headstay sag on a masthead rig? Is there a practical limit to maximum sag? Thanks. Ted
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Match the luff 'hollow' already cut into the sail
There are TWO answers to this question ..... First, the maximum amount of sag depends of what you are trying to do! Down or off-wind you can put as much sag into the rig as you want; but, just less than that which would make the sail unstable (too much ‘roundness’ at the luff). Upwind, the amount of sag depends on how high you want or need to point .... The less sag the higher the boat will point .... up to a point at which the shape of the sail becomes too flat in the middle portion of the ‘entry’ sections ( the rounded area right behind the luff ). Second, and perhaps more important is: how much luff ‘hollow’ DID the sailmaker originally put into the sail to compensate for 'normal' forestay sag? huh? Yup, every jib or genoa is lofted (cut) so that the typical normal amount of luff/stay sag for that particular type and model of boat is 'compensated' for in the design of the sail’s luff. Such calculations consider or assume that the rigging is tensioned at 12-15% of its normal breaking (yield) strength and the sail is or becomes windloaded at or near its design maximum wind range at ~17-18 kts. So, if the tension in the forestay approaches 12-15%, and the boat is being sailed in 17-18 kts., the sag that you will encounter will be compensated by the design of the sail. If you want to know what the designed ‘luff hollow’ in your sail is – lay the sail on a flat floor, make ONE accordian pleat/fold about 1-1/2 to 2 feet back from the luff and such that the sail is entirely FLAT on the floor; and then, sight along the luff or measure the difference between a string, etc. pulled tight between the tack and head .... and measure, .... use this dimension as your ‘target’ luff sag. Then, if you need to point higher - add tension to the backstay (which reacts with the forestay) and the luff will straighten out a bit. How much is too much, when you get up to above 30-50%% rig tension ... somethings gonna 'go' soon! Note: If you overtension the jib sheets (pulling super-tight) you will ADD more sag to the forestay ... and will need to add in more backstay tension to compensate. So, how much sag? .... match the amount of 'hollow' that the sailmaker originally cut into the luff is the answer.
There are TWO answers to this question ..... First, the maximum amount of sag depends of what you are trying to do! Down or off-wind you can put as much sag into the rig as you want; but, just less than that which would make the sail unstable (too much ‘roundness’ at the luff). Upwind, the amount of sag depends on how high you want or need to point .... The less sag the higher the boat will point .... up to a point at which the shape of the sail becomes too flat in the middle portion of the ‘entry’ sections ( the rounded area right behind the luff ). Second, and perhaps more important is: how much luff ‘hollow’ DID the sailmaker originally put into the sail to compensate for 'normal' forestay sag? huh? Yup, every jib or genoa is lofted (cut) so that the typical normal amount of luff/stay sag for that particular type and model of boat is 'compensated' for in the design of the sail’s luff. Such calculations consider or assume that the rigging is tensioned at 12-15% of its normal breaking (yield) strength and the sail is or becomes windloaded at or near its design maximum wind range at ~17-18 kts. So, if the tension in the forestay approaches 12-15%, and the boat is being sailed in 17-18 kts., the sag that you will encounter will be compensated by the design of the sail. If you want to know what the designed ‘luff hollow’ in your sail is – lay the sail on a flat floor, make ONE accordian pleat/fold about 1-1/2 to 2 feet back from the luff and such that the sail is entirely FLAT on the floor; and then, sight along the luff or measure the difference between a string, etc. pulled tight between the tack and head .... and measure, .... use this dimension as your ‘target’ luff sag. Then, if you need to point higher - add tension to the backstay (which reacts with the forestay) and the luff will straighten out a bit. How much is too much, when you get up to above 30-50%% rig tension ... somethings gonna 'go' soon! Note: If you overtension the jib sheets (pulling super-tight) you will ADD more sag to the forestay ... and will need to add in more backstay tension to compensate. So, how much sag? .... match the amount of 'hollow' that the sailmaker originally cut into the luff is the answer.
Finding the groove
Rich, thanks for your explanation which brings up another related topic. Assume I am using kevlar or some other low stretch fabric for my headsail which does not stretch like dacron. If I am having trouble steering the boat upwind (staying in the groove) could the cause of this problem be too little headstay sag? In other words, with a very tight headstay, the area just behind the luff would be too flat and therefore the groove would be narrow. I know that with a dacron headsail you can increase halyard tension to move the draft forward which helps steering upwind in lumpy conditions by providing a wider groove. With a kevlar sail the shape is pretty much fixed by the sailmaker and will only be slightly affected by halyard tension. Any thoughts about this?
Rich, thanks for your explanation which brings up another related topic. Assume I am using kevlar or some other low stretch fabric for my headsail which does not stretch like dacron. If I am having trouble steering the boat upwind (staying in the groove) could the cause of this problem be too little headstay sag? In other words, with a very tight headstay, the area just behind the luff would be too flat and therefore the groove would be narrow. I know that with a dacron headsail you can increase halyard tension to move the draft forward which helps steering upwind in lumpy conditions by providing a wider groove. With a kevlar sail the shape is pretty much fixed by the sailmaker and will only be slightly affected by halyard tension. Any thoughts about this?
Ted
It doesn't matter what your sails are made from, sail controls affect the shape of the sail the same. When a control is changed, it merely reshapes the cloth, it does not stretch by distrorting the cloth. Naturally, the grove will be narrow on a beat and increase as you head off. You cannot beat very well with a loose headstay. A large headstay sag is of no affect to the draft location on the sail. This is controled by halyard tension and car location.
It doesn't matter what your sails are made from, sail controls affect the shape of the sail the same. When a control is changed, it merely reshapes the cloth, it does not stretch by distrorting the cloth. Naturally, the grove will be narrow on a beat and increase as you head off. You cannot beat very well with a loose headstay. A large headstay sag is of no affect to the draft location on the sail. This is controled by halyard tension and car location.
Strecth vs. shape ... plus boltrope shrinkage.
Alan's states it correctly. Just like bending the mast, sagging the forestay will change the shape. Even sheet steel is elastic, and will take a curve .... when you pull on one edge of a triange the opposite edge loses tension - its just that less stretchy materials require more stress to do so. Its interesting that most of the modern sail panel cutting programs seem to depend more on luff edge shape and mast/luff shape than tapering the 'boadseams' (olden days) to get the proper curve into a sail ... this seems especially true with the high tech fabrics including the newer dacron cruising laminates. I still make my own sails, just have someone else do the cutting, and I am amazed by how critical the dependency on mast/luff shape (-hollow) and forestay shape (- sag) has become so important. Boltrope shrinkage. These shaping/materials discussion brings up an interesting phonomenon on dacron sails (even laminates) still made today that have 'bolt-ropes' sewn into the luff (and or foot). Take ANY rope and stretch and relax it many times ... and the result is a fatter and shorter rope !!!!! Doesnt matter if its spectra, dacron or nylon, etc. they ALL do this. Three strand ropes are the worst and still sailmakers usuallly put in a three strand rope as the luff boltrope. The object lesson here is when you have a new sail with a boltrope, you should carefully measure this dimension and watch for changes over the life of the sail. The luff especially will shrink continually due to strech while sailing and of course hoisting/tensionioning to shape it. The usual result is the sail requires more and more halyard tension over time to get the same original shape. If you have the precise original dimension its an easy task either by yourself or by a sailmaker to 'reset' the proper 'preload' (approximately 1 inch shortening for every 9-10 foot of luff length) and restore the luff back to the original length (and shape). I sail all my boats quite hard and find that I usually have to 'reset' the luff bolt-rope at the end of nearly EVERY season of hard sailing or after a very long passage. With restting the boltrope, I can make a sail keep good shape for many many years. Hope this helps (and saves you $$$$$)
Alan's states it correctly. Just like bending the mast, sagging the forestay will change the shape. Even sheet steel is elastic, and will take a curve .... when you pull on one edge of a triange the opposite edge loses tension - its just that less stretchy materials require more stress to do so. Its interesting that most of the modern sail panel cutting programs seem to depend more on luff edge shape and mast/luff shape than tapering the 'boadseams' (olden days) to get the proper curve into a sail ... this seems especially true with the high tech fabrics including the newer dacron cruising laminates. I still make my own sails, just have someone else do the cutting, and I am amazed by how critical the dependency on mast/luff shape (-hollow) and forestay shape (- sag) has become so important. Boltrope shrinkage. These shaping/materials discussion brings up an interesting phonomenon on dacron sails (even laminates) still made today that have 'bolt-ropes' sewn into the luff (and or foot). Take ANY rope and stretch and relax it many times ... and the result is a fatter and shorter rope !!!!! Doesnt matter if its spectra, dacron or nylon, etc. they ALL do this. Three strand ropes are the worst and still sailmakers usuallly put in a three strand rope as the luff boltrope. The object lesson here is when you have a new sail with a boltrope, you should carefully measure this dimension and watch for changes over the life of the sail. The luff especially will shrink continually due to strech while sailing and of course hoisting/tensionioning to shape it. The usual result is the sail requires more and more halyard tension over time to get the same original shape. If you have the precise original dimension its an easy task either by yourself or by a sailmaker to 'reset' the proper 'preload' (approximately 1 inch shortening for every 9-10 foot of luff length) and restore the luff back to the original length (and shape). I sail all my boats quite hard and find that I usually have to 'reset' the luff bolt-rope at the end of nearly EVERY season of hard sailing or after a very long passage. With restting the boltrope, I can make a sail keep good shape for many many years. Hope this helps (and saves you $$$$$)
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