#### George Hackett

##### Super Anarchist

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- #21

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- Thread starter George Hackett
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- Thread starter
- #21

Remember? Daily reminder for me

Following a winning trend I suspect.Having never had to subject myself to the IOR rule, why did type formed IOR racers tend to tall skinny mains and huge overlapping genoas (in the earlier days of IOR)?

Or the designers felt the efficiency of such a rig more than compensated for the high rated sail area it incurred. Or that MH rigs worked better for heavier boats

Recalling what I can about how IOR rated sail area:

100% of the geometrical area of the genoa was used in the calculation.

Only 70% of the geometrical area of the main was used in the calculation.

An aspect ratio penalty was applied to both the genoa and main (may have been greater for the main - my mind is a bit hazy on this, but would explain lower AR mains)

There was an RSAF/RSAM calculation (can't remember what it was called) applied to the combined areas - where the larger the proportion of headsail area to mainsail area, the larger the multiplier.

So basically, headsail area got hit with a triple whammy in the S calculation.

I sailed on a local Ganbare 35 in the early 80's and struggled to keep up with the Mk II & III Peterson 35s in the light stuff. Extended the boom and E by some 2.5 feet so actual main sail area increased by about 55 sq ft - but comparing the new IOR cert with the old one, RSAM went up by only 11 sq ft.

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http://www.acmenovelties.net/sailing/ior_rule/

I think the first formula SAM = 0.35 E P + 0.2 E (P - 2E) does underestimate real mainsail area considerably.

For example I have a E = 10 and P = 30.

A plain triangle of E x P / 2 =

(I am ignoring the Reduced Girth Factor etc correction assuming it wasn't usually taken. Also ignoring Corrected E and P (EC PC) because they probably have to do with headboards and tack/clew offsets etc.

I = 30, J = 10, LP = 15

I

30

J

10

LP

15

A genoa of those proportions would have a real area pretty close to the rated area.

I think where the big difference lies is a genoa is more efficient without a mast in front of it

In later years (Mk III version something or other or Mk IV) we started to see many more 7/8 rigs with bigger mains. That might have been because they finally realized they could make a more flexible main for trimming or that main area was under-rated.

(This probably also led to the skinny IOR masts where you were trying to make the mainsail as little shadowed by the mast as possible.)

A fascinating history lesson.

Batten lengths were also restricted to % of E until mid 80's when they could go longer.

The first terms in calculating RSAM and RSAF were to determine the areas of the main and foresail. But RSAM had a .35 multiplier while RSAF had the .5 multiplier. Since you use .5bh to find the area of a triangle, they simplified it by using .35 rather than .7 x .5 for the main and .5 rather than 1 x .5 for the foresail. The second part of the first term for RSAF (1.0+1.1*(LP-JC)/LP) was to account for the area of the overlap and typically came out as 1.33 IIRCSome kind soul scanned each page of Mk III, up to 1985. Read it at your peril. This is really simplified so bear with me if you have designed to the IOR Rule! I've only been exposed to it slightly. I may be cutting important corners

http://www.acmenovelties.net/sailing/ior_rule/

View attachment 483413

I think the first formula SAM = 0.35 E P + 0.2 E (P - 2E) does underestimate real mainsail area considerably.

For example I have a E = 10 and P = 30.

SAM= 0.35 (10 x 30) + 0.2 x 10 (30 - 2 (10)) =125 ft2.

A plain triangle of E x P / 2 =150 ft2

A real main, with roach will be maybe 12% more or 168 ft2(~74%). The 12% might be a more modern figure where we are using longer battens. So probably 12m is correct = about 70% of actual area is rated.

(I am ignoring the Reduced Girth Factor etc correction assuming it wasn't usually taken. Also ignoring Corrected E and P (EC PC) because they probably have to do with headboards and tack/clew offsets etc.

View attachment 483414

I = 30, J = 10, LP = 15

I

30

J

10

LP

15

RSAF

217.5

A genoa of those proportions would have a real area pretty close to the rated area.

I think where the big difference lies is a genoa is more efficient without a mast in front of itfor equal amounts of sail area. So the rule type forms you toward big genoa + small main. So 12 metre is very correct - the main is less penalized.

In later years (Mk III version something or other or Mk IV) we started to see many more 7/8 rigs with bigger mains. That might have been because they finally realized they could make a more flexible main for trimming or that main area was under-rated.

(This probably also led to the skinny IOR masts where you were trying to make the mainsail as little shadowed by the mast as possible.)

A fascinating history lesson.

The second terms were the Aspect Ratio penalties (or adders) I mentioned in my previous post. A bit primitive, but it at least partly took into account the efficiencies of a high AR sail plan. For the main this was 0.2*EC*(PC - 2*E). For the foresail it was 0.125*JC*(IC-2JC). As a result a high AR main was penalized more heavily than a high AR foresail (.2 vs .125multiplier). Hence IOR favoured a low AR main more than a low AR foresail. At least as far as maximizing actual sail area vs rated sail area. In your example, if you decrease P to 25 and increase E to 12, you get the same triangular area of 150 but SAM decreases from 125 to 107.4

Milgram and the French guy just took it to the logical extreme and eliminated the foresail altogether. The rule said you must be rated with a mainsail - but where there is no provision for setting sails in the foretriangle, RSAF shall be zero.

On top of all this, elsewhere in the rule I believe you will find the part about a sliding scale penalty based on RSAF/RSAM

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I vaguely remember his name coming up in I think some Davidson Laboratory (Stevens Institute) publication in my Grandpa's library.

It could have been this one: https://books.google.com.fj/books?id=fVLuI07rKw4C&pg=PA538&dq=jerry+milgram+davidson+laboratory&hl=en&sa=X&ved=2ahUKEwijldyKnZr1AhWQwzgGHdZXCPAQ6AF6BAgJEAE#v=onepage&q=jerry milgram davidson laboratory&f=false

He must have been 32 or so, at the time. Way above my paygrade anyway. Perhaps more appropriate for SA is this obituary, one of many. A great man indeed: https://news.mit.edu/2022/professor-emeritus-jerome-milgram-dies-0104