# How do you calculate spinnaker size needed



## Stillraining (Jan 11, 2008)

OK this is a two for one post

1) how do do calculate the size of spinnaker you need?

2) same question for an inter-stay storm sail?

I = 50.50
j = 19.00

displacement 29000

hull form medium heavy IOR concept..Ketch configured


----------



## artbyjody (Jan 4, 2008)

This may help: *The New Book of Sail Trim - Google Book Search*


----------



## Ilenart (Jul 23, 2007)

Stillraining,

found these formulas on a website.

Racing Symmetrical Spinnaker = 1.8 x J x I

Racing Asymmetrical Spinnaker = 1.8 x J x I

Cruising Asymmetrical Spinnaker = 1.65 x J x I

Have to admit, 1,584 ft sounds like an awful big spinnaker.

This site here has the formulas.

Ilenart


----------



## Quickstep192 (Jan 6, 2001)

I would let the sailmaker determine the size based on my criteria.


----------



## Jeff_H (Feb 26, 2000)

One very important note, there is no relationship between your hull form and an IOR hull form. The IOR rating rule produced hull forms which were tender and which could point very high up wind and very deep down wind but which were not well suited for reaching. Consequently, IOR main and jibes tend to be cut very flat to reduce heeling and take advantage of their ability to point higher than other designs and IOR spinackers were cut full with huge head girths for deep downwind angles. 

The Irwin cruising models were far more moderate, and had rigs and hullforms that were not all that great upwind or dead downwind, but which were optimized for reaching so their jibs and mainsails need to be cut fuller, and their spinackers cut flatter than an IOR influenced design. 

A quality sailmaker would be aware of the different needs of an IOR boat vs a more moderate cruising boat. If you come into a quality sailmaker and tell them that you have an IOR design, the sailmaker will make you sails that would not work well with your boat.

Respectfully,
Jeff


----------



## jgeissinger (Feb 25, 2002)

*Irwin ketch spinnaker.*

I could only find data for an Ayso, but here it is: Luff=50.81, Foot=31.35, Leech=46.75, %lp=165, area 1195sq. ft.


----------



## Stillraining (Jan 11, 2008)

OK..thanks everyone...It was hopefully a simpler formality then obviously it is..just another case in point of ignorance being teachable.

There seems to be a plethora of true symmetrical spinnikars out there used for sale..For learning purposes I thought running/ruining.. a couple 300 to 500 dollar sails would be prudent before having a custom made.

So Jeff... what do you think of the various calculations presented and knowing my boat what would you go with? Yes I would rather have an Asymmetrical but am not afraid of learning with the limitations of a cheap symmetrical. Or is this folly as well?

PS: jgeissinger..where did you find that information?


----------



## Stillraining (Jan 11, 2008)

Bump........


----------



## jgeissinger (Feb 25, 2002)

*Spin measurement*

Stillraining, the source with rig and sail dimensions for just about every boat out there is Sailrite.com. Look under the Sail Quoting tab, then type in Irwin. They will list all the boats under that name. They have yours listed as a 42, but it's a ketch and the rig measurements are the same, if I'm not mistaken. They are usually very accurate, but sometimes the may get models listed wrong. I had an old Ericson that had a mark 1 & 2 version. You had to know the I,P,E, and J to figure out which model you were looking at. Take a look, again, I'm pretty sure this is your boat. Sailrite has a wealth of information for anyone trying to compare sail areas for different boats. Hope this is some help.


----------



## Stillraining (Jan 11, 2008)

jgeissinger said:


> Stillraining, the source with rig and sail dimensions for just about every boat out there is Sailrite.com. Look under the Sail Quoting tab, then type in Irwin. They will list all the boats under that name. They have yours listed as a 42, but it's a ketch and the rig measurements are the same, if I'm not mistaken. They are usually very accurate, but sometimes the may get models listed wrong. I had an old Ericson that had a mark 1 & 2 version. You had to know the I,P,E, and J to figure out which model you were looking at. Take a look, again, I'm pretty sure this is your boat. Sailrite has a wealth of information for anyone trying to compare sail areas for different boats. Hope this is some help.


Will do... thats one of the few I didnt check.. and yes mine is referred to as as the 42 sometimes as its 41' 8" LOD but 45 overall and thats what the J mesurment would be calculated out to include the sprit.

Thanks


----------



## kraken495 (Jul 9, 2021)

2841 sq. ft that sounds expensive


----------



## jtsailjt (Aug 1, 2013)

One mistake I made when I got an asymmetrical spinnaker is that I got one that’s so big and bulky it’s really tough to store it onboard so if I had it to do over I’d get one made of lighter material so it wouldn’t be so bulky. Combined with the ATN sock, it is hard to stow it below so when it’s onboard it’s usually in its bag clipped to the lifelines. 

Maybe you were looking primarily for performance considerations but for a cruiser I think it’s also important to try to envision how you will use it and how it will fit on your boat.


----------

