Submitted on September 4, 2005 by Robin Leigh

Question: Dear North Sails,

I have a Gemini 3000 cat. I am looking into getting an asymmetrical spinnaker for off-wind and light air use. How do you measure luff length for these sails? I see that the luff can be longer than the headstay length, but I am puzzled about practical sizes and measurement. Please explain the relationship between the boat distance from halyard sheave to tack fitting, the sail luff length and the sail straight line measurement from head to tack.

Any information on this point would be appreciated. While there are several sources for calculating sail area using I, L and J, I have yet to find meaningful information about actual sail dimensions.

Regards,
Robin Leigh

Answer: Hi Robin,

The sail designer and the sail buyer have a fair amount of latitude in determining how big to make a spinnaker for any boat. You have identified the three lengths that come into play for the spinnaker luff.

The distance from the tack fitting to the halyard shackle at full hoist defines the space that the sail will fly in. Lets call that one "Max Hoist"

The sail's luff length is the measurement you will find when you stretch the sail out in a sail loft or on your lawn and measure the distance between the tack and head rings. We call that "SL" for Spinnaker Luff.

The third dimension is more accurately defined as the luff cord length. That is the distance between the tack and head when the sail is inflated.

The difference between the sail's luff cord length and the headstay length is the height of the tack off the deck when you are sailing. If Max Hoist and the Luff Cord are equal, you will not be able to see where you are going because the tack will be right on the deck.

The difference between the cord length and SL will vary with the sail depth and the amount of roach area on the sail. In general terms, a sail with more roach (bigger mid-girth, bigger shoulders) requires more 3-dimensional shape to support the extra area outside the triangle formed by the head, tack and clew. As the sail gets deeper, the difference between the Luff Cord dimension and SL gets greater.

To make a guess at the correct size for your light air cruising spinnaker you can do the following:

1) Determine the MAX HOIST by pulling a tape measure up your rig on a
very calm day, or by measuring off a scale drawing of your boat.

2) Decide how high off the deck you want the tack of the sail. A good
starting place is to put the tack at the same height as your pulpit, or about 2-3' off the deck. Any lower and you will not be able to see. Any higher and the sail will bounce around too much.

3) Subtract the tack height from MAX HOIST. That is your Luff Cord.

4) Multiply the Luff Cord by 1.04. This will get you close to the SL
for a good all purpose cruising asymmetric spinnaker.

The sail leech will be 8-12% shorter than the luff on most boats. The foot length will be somewhere between 1.5 and 1.8* the length of your foredeck (greater multiplier for a boat with a small foretriangle and long boom, smaller multiplier for a boat with a short boom and big foretriangle). The midgirth will usually be 90-98% of the foot length.

Regards,
Dan Neri

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