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DrB 09-12-2008 09:48 AM

Sail Area Defined
I can't find an answer to this so I solicit the input from the esteemed and knowledgeable group here.

When they advertise or state a boats sail area, what sail dimensions are they using to calculate this. I can figure this out for a boat with one sail, but say for a sloop, is it the area of the main + plus the area of 100% jib or something else? Similarily for a yawl or ketch, how is the area calculated?


chucklesR 09-12-2008 10:21 AM

100% main and Jib is supposed to be the standard.

A lot of manufactures fudge that and true displacement for better numbers on the SA/D ratio so beware the manufactures published numbers.

My Gemini might have weighed the advertised 9600 pounds dead empty, it doesn't now :)

My boat is listed at 340 on the main and 350 on the jib, which is a 150% roller, and then 490 on the screacher, which is a light air sail that PCI says is the equiv of 200% if it was a jib. Doing that math doesn't work out so good. because 280 is the I x J / 2 number and 280 x 2 is not 490 :)
In fact, none of their numbers really works out.

The mast top is 40ft from the deck, and 14 ft from mast to furler. Basic formula Is I x J /2 , or 14x40 / 2 or 280 sq ft.
A 150% jib should then be 420, and a 200% should be 560.

It's all marketing hype

In fact Practical Sailor magazine (known for cutting through the hype) lists the Sail area of the Gemini as 540 sq ft - I have no idea how they came up with that as 340 + 280 = 620, unless the roached area of the main doesn't count either and it should because it's always in play.

Anyway here's a link for the basics: Wildwood Sailing Club - Sail Calculator

it includes yawl and ketch designs.

knuterikt 09-12-2008 10:38 AM

AFAIK there are no standard or regulation for this and it will probably depend on context.

I think that in normal condition it would be the max area when beating.

So for a standard boat from a manufacturer it could be one of two
1) Designed max sail Area
2) Max area with standard wardrobe

For a used boat it could be max area with curent wardrobe

flyingwelshman 09-12-2008 10:51 AM

A quick question on this topic:
When you see 150% Genoa (or 100% Jib) for example, what does this mean?

My understanding is that a 100% sail goes as far back as the mast, a sail of greater than 100% goes from the forestay to past the mast. So a 150% Genoa would go past the mast half the distance than from the forestay to the mast.

I asked a sailor at the marina this question a couple of weeks ago and he told me that the numbers were the sail area (a 150 Genoa was 150 sq ft etc.) I didn't think this was the case, but I thought I would find the answer here.


chucklesR 09-12-2008 10:52 AM

Read the link I posted, it defines it the same as the many many books I have on it. Can't post a link to a book on my shelf :)

Ilenart 09-12-2008 10:57 AM


what it should be is the following:

Mainsail = (P x E) / 2
100% Jib = ( I x J ) / 2
Mizzan = (PY x EY) /2
"P" is the luff length of the mainsail, measured along the aft face of the mast from the top of the boom to the highest point that the mainsail can be hoisted or black band.
"E" is the foot length of the mainsail, measured along the boom from the aft face of the mast to the outermost point on the boom to which the main can be pulled or to the black band.
"I" is measured along the front of mast from the genoa halyard to the main deck. The main deck is where the deck would be if there were no deckhouse.
"J" is the base of the foretriangle measured along the deck from the headstay pin to the front of the mast.
PY" and "EY" are, respectively the luff length and foot length of the mizzen of a yawl or ketch measured in the same way as for the mainsail.

So for my boat the numbers work out as:

Mainsail area = 40.672 x 14.76 / 2 = 300.15936

Foretriangle 100% = 1 x 22.57 x 47.232 / 2 = 532.9281024

Mizzan = 32.8 x 10.824 / 2 = 177.5136

Total 1,011sqft

However, what I have found is a lot sales brochures use 135% or even 155% genoa, which really overstates the sail area.

This site here lists the formula and measurements for thousands of boats.


camaraderie 09-12-2008 12:05 PM

As a practical matter the formula for a 100% jib may be recognized as the old tirangle area formula of 1/2 the base x height. Since the mast height does not vary...the measurement of the foot of the sail in relation to the 100% measurement will yield the actual % just as easily.
Example...I have a boat with a 50 ft. mast and 10 foot j measurement which yields a 250sqft. sail area.
Adding 50% to the J would create a 150% genny and this can be seen by multiplying 15x50/2 which yields 375 sq. ft. or 150% of 250sq. ft.

So...both methods are correct.

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