How to size an asymmetrical spinnaker?

[Privilege]

I'm looking to purchase an asymmetrical spinnaker for my cruising catamaran. The sail will be flown from the bow sprit with my top-down furler. I can't seem to find guidance online on how to size it. The luff is never taught on this sail so I'm guessing that you don't measure the luff the same as you'd measure a jib or mainsail. The 'I' measurement doesn't really offer guidance as it will be flown from a bow sprit.

My spinnaker shackle to the furler shackle and the length of my anti-torsion rope is 60'. Any advice is appreciated.

[StuM]

This may help: Asymmetrical Spinnaker - MPS; Spinnakers and Accessories

"The luff length is the deciding factor when choosing the right size Asymmetrical Spinnaker. As a general rule, if you are going to fly the spinnaker just above the front pulpit, the luff length should be about the same length as the mast height is above the deck.For a more accurate measurement, raise a tape measure up on the spinnaker halyard to it is at the full hoist position and measure down to where you would like the tack of the spinnaker to be. (lets call this measurement "A"). If you have a bow sprite, this would be to the end of the bow sprite. If you dont, it will most lickely be to the top of the forward railing (pulpit)
The luff thength of the spinnaker should be Measurement "A" x 1.03 (give or take 300mm either way)"

Click on the small image of a table on that pagefor actual sizes

[Privilege]

Thanks Stu. I'll aim for something around 62'.

[Roy M]

Plus, you can always purchase smaller used ones or different weights.

[GrahamHO]

Originally Posted by Privilege

I'm looking to purchase an asymmetrical spinnaker for my cruising catamaran. The sail will be flown from the bow sprit with my top-down furler. I can't seem to find guidance online on how to size it. The luff is never taught on this sail so I'm guessing that you don't measure the luff the same as you'd measure a jib or mainsail. The 'I' measurement doesn't really offer guidance as it will be flown from a bow sprit.

My spinnaker shackle to the furler shackle and the length of my anti-torsion rope is 60'. Any advice is appreciated.

I'm using a Selden top down. Even if you have another make there is a lot of helpful information on their web site.


I made my AT rope a little shorter than the bowsprit eye to the mast turning block as it's important to have the AT very tight. So tightend right up there should still be halyard visible between the top swivel and the mast. I am using a spectra halyard which works OK for my boat which has a mast 40' above the deck with a fractional rig.


Even though the gennaker luff is not tight in use you can get a measurement by pulling it tight. Pulled tight it should be shorter than the AT rope. That's so it can wind up OK. On your 60' AT rope you probably want it to be tied so it is around 1' below the attachment eye at the top. You need a short length of attachment line at the top to start the winding up process. Likewise at the bottom you probably need around 2' minimum clearance with the gennaker pulled tight. That also depends on how much clearance you want at the bottom for visibility or clearing lifelines etc.


As you point out the luff won't be tight in use. Nor do you want it to be tied tightly on the AT rope as that might prevent the AT rope being correctly tensioned.


If you can get a sailmaker to measure up on your boat so much the better. If you are going mail order, some photos showing each end with a tape measure showing at the bottom is a good idea. I used a Tele lens for the top photos and with a long tape measure I could measure the top clearances.


It's a little risky specifying all the actual sail measurements to a sailmaker as you have no comeback if you get it wrong. They allow for stretch and luff round etc etc. But do give them the AT measurements etc. I once had a radial cut genoa made by a sailmaker who made it too long by mistake even though he measured on the boat beforehand. They then had to modify the top so I could use it while they made a completely new one the correct length at their expense.


I did my initial gennaker roll up in a park tying the complete AT along the ground between some posts and used a purchase to tension it while I rolled up the sail. A yachtsman driving past who I didn't know came and helped.


Check out the Selden web site. To go from one gybe to another I wind up, gybe then wind out again. Happy sailing.