Spinnaker sizing

[Tony Wilcox]

I am looking for a cruising spinnaker for my H376. I know the I & J measurement is how to order from sailmaker. However I have seen Assymetrical spinnakers being sold using Luff, Leach and foot measurements. Does anyone know how to size an assymetrical sail knowing the I & J. What are the Luff.Leach and Foot measurements that are needed to fit my boat? Perhaps there is a formular that can be used for any boat using the I & J .Tony

 

[Miles]

Algebra

Well, if you take the square root of (I squared + J squared) you should get pretty close to the forestay length which is what you'd want for the luff more or less. Then take 1.5 to 2 times the J for the foot. You don't have to be precise, I got a used spinnaker from another boat that works just great, a few feet won't make much of a difference since the whole thing flies free. If you want to do it perfectly right there are some nice 'chutes for sale in the chandlery here. Have fun!

 

[Bob Howie]

Save the headache

Most sailmakers have complete sheets of sail plans for most all boats. If they don't have one for yours, part of the deal is they come measure it. My recommendation to you is to let the loft do the math; their math, their screw-up, it's their pocket out of which the money will come. Otherwise, you might find yourself paying for something you can't use. FYI, I just had a cruising spinnaker built for my 78h30 and my LOCAL loft handled all the math work.

 

[Lee Hadjiosif]

Spinnaker Measurements

Tony,Don't just go by the published I and J numbers but rather confirm them with a measuring tape. The manual for my H310 lists the I as 37.08' but I measured it as 41'-6". The reason as best I can explain is that they measured it to the foot of the mast instead of the top of the deck.-not the cabin-. Also the J is for the jib and if you have a fractional rig with the spinnaker halyard exiting the mast above the headstay, you'll need to add that dimension. The best way is to tie a soft 100' tape to the spinnaker halyard and raise it up. Then mesure the distance down to the deck(not the cabin top) and also measure to the point where you will tie the tack for the luff dimension. -or you can calculate it with the Pythagorium theorem! The sailmakers usually add 5% to the correct I dimension to arrive at the luff. The spinnaker should fly about 2-3' above the deck for visibility and clear the rail. I know all these because I'm going through it right now myself. I decided to buy the off the shelf spinnaker from West Marine which is the same as the cruisingdirect.com spinnakers. You can get them right away and the price is right. Good luck.S/V Calaloo

 

[Bill Sedgwick]

Spinnaker Dimensions

FWIW if you want to race or sell to someone who may, here's what PHRF SoCal says:7.STANDARD PHRF SPECIFICATIONSSAILSHeadsail LP shall not exceed 1.55 x JSymmetrical Spinnaker SL shall not exceed .95 times the square root of (I² + J²)Symmetrical Spinnaker SMW shall not exceed 1.8 x JAsymmetrical Spinnaker SLU shall not exceed 1.15 times the square root of (Ic² + Jc²)Asymmetrical Spinnaker SF shall not exceed 1.8 x JcAsymmetrical Spinnaker ASMW shall not exceed 1.8 x JcAsymmetrical Spinnaker ASMW/SF ratio shall be equal to or greater than 0.75POLESSpinnaker pole shall not exceed 100% of JExtended “bow pole” (BPL) shall not exceed original manufacturer’s designed lengthWhisker pole maximum length shall not exceed 1.4 x J