MacGregorSailors.com

Close enough for govt. work

sailboatmike wrote:Foresails are measured by the distance from the foot of the forestay to the mast, a 100% jib would go back to the mast, 110% goes 10% of the distance past the mast.

The 150% genoa goes from the foot of the forestay back to the mast and then past it by 50% of the distance from the forestay to the mast.

Eg, if the forestay to the mast is 10 feet, a 100% jib would have a foot of 10 feet,a 110% would be 11 feet and 150% genoa would be 15 feet,

I think thats right anyways

Now, for advanced level maths - what does that mean for the percentage of sail area fore and aft the mast?

bobbob wrote:Now, for advanced level maths - what does that mean for the percentage of sail area fore and aft the mast?

That depends on the shape of the sail and the angle of the mast and the luff, but it's probably more relevant to look at the location of the centroid of the triangle in relation to the mast or keel. And even that probably isn't that good, as the sail is a wing, and the location of the centers of lift and drag, I would think, are more meaningful.

To determine the centroid of the triangle, just offset each straight side by 1/3 the perpendicular distance from the opposite corner to that straight side. The offsets intersect at the centroid.

Another way is to draw a line from each corner to the midpoint of the opposite side. They all intersect at the centroid.

Here's a paper you helped write :

bobbob wrote:Now, for advanced level maths - what does that mean for the percentage of sail area fore and aft the mast?

sailboatmike wrote:Foresails are measured by the distance from the foot of the forestay to the mast, a 100% jib would go back to the mast, 110% goes 10% of the distance past the mast.

The 150% genoa goes from the foot of the forestay back to the mast and then past it by 50% of the distance from the forestay to the mast.

Eg, if the forestay to the mast is 10 feet, a 100% jib would have a foot of 10 feet,a 110% would be 11 feet and 150% genoa would be 15 feet,

I think thats right anyways

That's a commonly repeated "definition", but it's wrong.
The nominal size of a sail is measured by the Luff Perpendicular, not the foot.
The % of the sail is determined by the "Luff Perpendicular" measurement compared to the base of the fore-traingle (aka "J").

Here's the correct definition:
A 100% jib has an LP (luff perpendicular) equal to the base of the fore-triangle (aka "J": the horizontal distance from the mast to the forestay)

Here's Restating that using math symbols:
%LP = LP/J
or
A 100% jib has an LP = 1.0 * J
A 150% genoa has an LP = 1.5 x J





How far the clew goes past the mast cannot be used to calculate the % of the sail either.
The measurement of the foot cannot be used to calculate the % of the sail.
If you use the foot measurement, you will be overstating the % of the sail. The foot measurement is always greater than the LP


A 100% jib has an LP (luff perpendicular) equal to the base of the foretraingle (aka "J": the horizontal distance from the mast to the forestay)
A 100% jib with a clew above the deck will overlap the mast, and the dimension of the foot is more than 100% of J.
A 100% jib with a high clew will have a foot that overlaps the mast. The higher the clew is, the more it will overlap.

A 100% jib can have a low clew (almost on the deck) or a high clew (almost to boom height). The 100% with a low clew will overlap the mast a little, and the 100 jib with a high clew will overlap the mast a lot.

Same definitions work for a 150% genoa.

With the notable exception of Highlander's boat, virtually everybody here uses either a 100% jib or a 150% genoa on his 26X or 26M. All the headsail sold by me or BWY are either 100% or 150%. All the headsails have a very specific clew height above the deck, so that they will sheet correctly to the tracks that were installed at the factory.

Judy B

PS. For those of you who recall your basic geometry about triangles: You need more than the LP to determine the area of the sail. Area of a triangle = Base * Height * 0.5 . In the case of a headsail, area = LP * luff. So a 150% genoa with a short luff is smaller than a 150% genoa with a long luff.

The 110, 130, 150 stuff is all about the foot distance to the forestay - it's about the length of the foot for the most part, but not completely - there are other things involved like the distance to the forestay and other stuff I don't really know but not all sails are the same so there is a pretty big fudge factor when it comes to a Genoa. You may have what is listed as a 130 - but if you actually measure the thing the real percentage is 128.56% and so forth. The one on 'boat' is listed as a 150 but it's much closer to 163% than 150 - I assume it was probably a sail listed as a 170 that was really a 160 but someone discounted it to a 150 to get it out of the loft, (no one wants a 160 Genoa - that's an odd one). When I got the sail it was listed by square feet (230) but I think the average for a MAC is about a 198 to 200 square foot head sail genoa - they are not all the same because the head sail comes from the dealer - not the factory. Mike Inmon sole me the one on 'boat'.

If the furler is hanging up and it's not at the drum look at the plastic furler and see if it has bad bends in it (sight up the forestay like you sight a pool cue) - sometimes a bad bend in the plastic can make it go "flop, flop flop" as it unfurls that last three feet and make it hang on the forestay. If the plastic is bend let it sit out in the hot sun for a while with the mast up.

There are two types of stock furlers on the MAC - the ones with the closed drum (CDI) and the ones with the open drum (factory) - the factory one is cheaper but it works better because you can actually SEE the ling wrapping onto the drum and it's easier to clear in a snag. Also when you let the sail out make sure you are holding the furler line and watching it wrap in the drum (if you have an open drum) and if the line is not wrapping well, stop, pull the line out, and start again. Don't let if roll up in a messy blob. You really need the furler in one hand and the jib sheet in the other as you unfurl your headstall. It takes a little skill.

Hey Judy, I guess I was writing my post when you did that - thanks for the info - can you tell me what the square footage is for the sail you sell as a 150 Genoa?

BOAT wrote:Hey Judy, I guess I was writing my post when you did that - thanks for the info - can you tell me what the square footage is for the sail you sell as a 150 Genoa?

The sails I sell are very close to the factory dimensions.

Here's the factory drawing.


Keep in mind that the edge dimensions are the shortest distance between to points on a spherical shape (AKA geodesic). When you lay the sail out on the ground, it's not flat. The LP is measured along the surface of the sphere.

If you draw out the triangle in 2D, the 2D LP appears shorter than the 3D LP.

Judy

Okay, that's is VERY informative - based on that drawing my headsail is WAY bigger than a 150 Genoa. I would estimate the headsail on my boat as closer to 210 square feet (at least).

I'm not sure what you would call that. Mike just called it "California Cut". The loft is Kettenhoffer Marine in Costa Mesa - those guys run pretty fast an loose - (half the time they are never there because they are out racing every morning). I see a lot a sails coming out of that place with odd sizes.

(P.S. okay, I just realized what this means: "When you lay the sail out on the ground, it's not flat. The LP is measured along the surface of the sphere." - that means I should be off by about 2 feet???

I don't even have the math skills to figure out the rounded part with the PIE equation and all that crap I found of the web to deal with the "sphere" but I can estimate that I am off by at least a foot so that would make my sail about 208 square feet.

What is that? 208 Square feet? 160? 170? 160 and a half? (GEESE, I guess that's why those Europeans went to metric sizes on sails???)

BOAT wrote:Okay, that's is VERY informative - based on that drawing my headsail is WAY bigger than a 150 Genoa. I would estimate the headsail on my boat as closer to 210 square feet (at least).

I'm not sure what you would call that. Mike just called it "California Cut". The loft is Kettenhoffer Marine in Costa Mesa - those guys run pretty fast an loose - (half the time they are never there because they are out racing every morning). I see a lot a sails coming out of that place with odd sizes.

(P.S. okay, I just realized what this means: "When you lay the sail out on the ground, it's not flat. The LP is measured along the surface of the sphere." - that means I should be off by about 2 feet???

I don't even have the math skills to figure out the rounded part with the PIE equation and all that crap I found of the web to deal with the "sphere" but I can estimate that I am off by at least a foot so that would make my sail about 208 square feet.

What is that? 208 Square feet? 160? 170? 160 and a half? (GEESE, I guess that's why those Europeans went to metric sizes on sails???)

What are the dimensions of your sail, straight line from pointy corner to pointy corner?
Luff?
Leech?
Foot?
(You can directly measure LP using a tape measure, with the sail on the floor, but I don't have time to write up the instructions on how to do that accurately)

Using those I can calculate the approximate measurement for LP.
With the LP, I can get an approximate % (150%, 160%, etc)
Using Luff and LP, I can calculate the approximate area.

Judy

BOAT wrote:
I don't even have the math skills to figure out the rounded part with the PIE equation and all that crap I found of the web to deal with the "sphere" but I can estimate that I am off by at least a foot so that would make my sail about 208 square feet.

What is that? 208 Square feet? 160? 170? 160 and a half? (GEESE, I guess that's why those Europeans went to metric sizes on sails???)

Using straight line measurements, (without hollows and rounds on edges) the OEM genoa calculates to be 185 square feet, not 171.

I'm not sure the 171 sf number for total area on the OEM drawing is accurate, but I don't have time to check it with my fancy software right now.

Judy B wrote:

BOAT wrote:
I don't even have the math skills to figure out the rounded part with the PIE equation and all that crap I found of the web to deal with the "sphere" but I can estimate that I am off by at least a foot so that would make my sail about 208 square feet.

What is that? 208 Square feet? 160? 170? 160 and a half? (GEESE, I guess that's why those Europeans went to metric sizes on sails???)

Using straight line measurements, (without hollows and rounds on edges) the OEM genoa calculates to be 185 square feet, not 171.

I'm not sure the 171 sf number for total area on the OEM drawing is accurate, but I don't have time to check it with my fancy software right now.

Okay, I actually went to my records and looked up the old Bill of Material on the boat from Captain Mike when I purchased it. That cleared up a lot of thing - the Genoa Mike sold me is 208 square feet - that's what it says on the document - there is no mention on the document about the % of the Genoa, just the figure: 208 Square feet. If I use your calculations I get a little more than that but I am not so good at the math so I may be off by 2 square feet.

One mistake I made was using a measurement from the fore-stay to the tack - your notes say I need to use the luff to the tack. My sail does drag down well inside the bow pulpit when fully deployed, so I think it might be a foot taller too. I would need to get out there and re-measure everything - but according to Mike Inmon's documents the sail is 208 Square feet. I have no idea what size Genoa that would make it. Perhaps I should use a metric on this one!

I'm guessing around a 165, BOAT, based on Judy's 171sf drawing being a 150, and yours reported to be 208sf.

Tomfoolery wrote:I'm guessing around a 165, BOAT, based on Judy's 171sf drawing being a 150, and yours reported to be 208sf.

Yeah, that's a really odd size - obviously they had the sail made to the largest possible dimension they could fit on the fore-stay without it actually dragging on the deck. I guess that's what Mike meant by "California Cut".

It's absolutely a 208 square foot sail - that is not a standard size. I really love the sail - I can tow a car behind the boat when the thing is all the way out - When I do someday replace it I want the sail maker to use the same dimensions as the one I have now because I like the sail so much.

Just looked at the data sheet for the X, and it shows the 150 genoa as 206sf. So now I'm a bit confused. I guess the dimensions need to be checked against the fore-triangle dimensions.