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I have a jib which came with my boat that "should" fit, but doesn't. It's too long in the luff. I think I must be measuring things wrong.

Is a headsail's luff measured from head grommet to tack grommet (center to center?), or is it a more theoretical measurement, from the imaginary points of the triangle at the head and the tack?

The standard 150 genny for a C22 has a luff of 26.25'. When I lay this sail out on the lawn and put a stake through each grommet, from the head stake to the tack stake it measures 26' 4" (with just a little snugness in the sail). Seems like it should fit, right? But it doesn't.

To help explain the mystery, I'm also measuring my forestay. What is the proper way to get this measurement? The mast is down, boat is on a trailer, if that matters.

Thanks for helping me clear this up!!
 

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Once known as Hartley18
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I have a jib which came with my boat that "should" fit, but doesn't. It's too long in the luff. I think I must be measuring things wrong.

Is a headsail's luff measured from head grommet to tack grommet (center to center?), or is it a more theoretical measurement, from the imaginary points of the triangle at the head and the tack?
It's head grommet to tack grommet (center to center), so you're doing it right.. :)

How long is "too long"?

The standard 150 genny for a C22 has a luff of 26.25'. When I lay this sail out on the lawn and put a stake through each grommet, from the head stake to the tack stake it measures 26' 4" (with just a little snugness in the sail). Seems like it should fit, right? But it doesn't.
If your research is correct, 26.25' would be 26' 3" on the luff - maximum. If your sail is 26' 4", then, no, it won't fit and might need to be cut down.

To help explain the mystery, I'm also measuring my forestay. What is the proper way to get this measurement? The mast is down, boat is on a trailer, if that matters.
I've only measured the headsail luff with the mast up (which is probably easiest since everything is then tight and straight). Basically connect a tape measure to your usual jib halyard, haul it up as far as it will go and measure off tight at the foot. Certainly you'd expect the luff measurement this way to be quite a bit shorter than the forestay itself.

I hope this helps :)
 

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Laying out the sail between two stakes isn't enough. When raised the sail's luff will be under tension, and you need to duplicate this at home too. Fixing one end to an inmovable object and using a 4:1 tackle (like your main sheet or vang) on the other end will let you simulate that tension. I bet that when you do this you'll find that it's more than a few inches too long. Doing it with a tape measure on the boat is a little easier.

You might be able to fit a slightly too long (like 1") sail by changing the tack or halyard hardware. I was helping a friend with a borderline too large genoa on his boat this weekend and we had to change the tack shackle on his furler to get a little longer effective luff length.
 

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Barquito
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Do you have roller furling gear? If you do, maybe this sail was cut for the full stay, rather than the reduced length of the stay with the furling drum.
 

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If your forestay has turnbuckles, you might also check to see if they can be eased off. This can end up lengthening the forestay by several inches. This may be enough to make your jib fit. If the mast was removed for winter storage, the crew may have slapped the forestay in and tightened it up without thinking how it might affect the sails.
 
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