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IOR bulge


DrWatson

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Can someone explain to me the IOR 'bulge' that I hear a lot about?

 

I've done a bit of reading around and never really figured out what exactly they're all talking about.

 

Is this the really fat midsection that I think I can see?

 

How did a rule create this bulge and how and why do various people go on about the 'unseaworthy characteristics' which these rules created in the boats of the era?

 

Or am I barking up the wrong end of this stick?

 

R

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watching and looking back at the 70's Ton class developments and changes with regard to girth beam and length is a bit like reading of the antics of a fat chick full of gin, odd thing though is that some of the designs were and have been proven time and again to be very very seaworthy vessels, yeah they "selftacked" to windward and could resemble a pianists metronome running flat but they were a progression and the eventual trickle down was the " family frightener" when pushed, could sail over the water as opposed to through it (stewart34 + pied piper were exceptions here), for the full version wait for Mr "fossil" to kick in, he was there and pretty good at it

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My understanding t=is that there was a measurement point somewhere just in front of the rudder. So the boats took a sudden turn on themselves about that point, trying to trick the rule into thinking the boat was smaller than it was, This led to disturbed flow over the rudder at speed thence major wipeouts downwind.

Let's see what the experts have to say.

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If you're talking about the tumblehome seen on mid-70s IOR designs, it comes from the fact that IOR measured waterline beam and max beam at the sheerline as factors in the formula. To achieve greater stiffness when heeled, designers introduced tumble home which was largely unmeasured but gave greater stiffness when heeled beyond about 5 degrees, which would not be picked up by the inclining test. The tumblehome also increased apparent girth which gave an impression of greater displacement than was in fact the case. Unfortunately tumblehome also made the designs of the period even more diamond shaped when heeled than they were already and thus made them very prone to wipe-outs when the rudders were lifted almost our of the water.

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Not an expert, but I think you are refering to what was known as a "bustle' which (as L4's analogy suggests) was an inordinately fat arse at the waterline.

 

This had the effect of increasing a sailing waterline length without incurring a rating penalty. Somewhat similar in effect as a bulbous bow which tricks the ship into thinking she has a longer waterline length than she actually has.

 

In some cases (some 1970's S&S designs, for example) this protuberance was clearly marked and, IMHO, very ugly.

 

On the other hand, when this bustle was smoothed out -as in the case of my current boat, Bintang (a Bruce Clark adaptation of a 1970s S&S theme) it does improve load carrying aft (i.e. you can put a lot of bodies in the cockpit) and it does reduce pitching - both aids to seaworthiness.

 

Off the wind Crew Ogre is probably right, though the IOR type (even without the bustle) were pigs.

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If talking about the aft bustle, very simply put it arose from the way the rule applied an after girth station and an after inner girth station - shortening the distance between the two, and reducing rated length, could be achieved by squeezing the hull at AGS (usually where you see a crease in the aft buttock line) and plumping out AIGS which is just forward of this point, usually incorporating a cleverly designed skeg. An overhang beyond AGS allowed the boat to pick up waterline length while sailing, and Waverider was a great example of this approach. If you'll pardon the plug, it's all covered in my book 'A Lighter Ton' (www.lightertonyachts.co.nz), which also details how Bruce Farr managed to straighten out these buttock lines in his 1977 generation boats but still get them to rate, until of course the rule makers penalised light displacement.

 

The bulge in the topsides was a result of measuring rated beam at a point below the sheer (1/6 of max beam) - Lidgard's half tonner Demijohn (moored in Westhaven) is an extreme example of this, but you'll see a hard turn in the topsides at this point in most IOR boats.

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I waited for Richard to answer this one - having read his book it covers all of this brilliantly. Great book Richard, as a former owner of Black Fun it was really interesting for me. Well worth getting hold of a copy.

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I waited for Richard to answer this one - having read his book it covers all of this brilliantly. Great book Richard, as a former owner of Black Fun it was really interesting for me. Well worth getting hold of a copy.

 

+1 :thumbup: :thumbup: :thumbup:

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If you're talking about the tumblehome seen on mid-70s IOR designs, it comes from the fact that IOR measured waterline beam and max beam at the sheerline as factors in the formula. To achieve greater stiffness when heeled, designers introduced tumble home which was largely unmeasured but gave greater stiffness when heeled beyond about 5 degrees, which would not be picked up by the inclining test. The tumblehome also increased apparent girth which gave an impression of greater displacement than was in fact the case. Unfortunately tumblehome also made the designs of the period even more diamond shaped when heeled than they were already and thus made them very prone to wipe-outs when the rudders were lifted almost our of the water.

 

Not correct.

 

The primary beam measurement (B) was measured at one sixth of maximum beam (BMax) below the sheerline. Tumblehome maximised the important B measurement, by making the greatest beam fall at the same place as the measurement. A bit of weight and windage could be saved by reducing the beam at deck level, which was not measured.

 

Girths were only measured at bow and stern. They were based on half B and threequarters B. Having a larger B brought the FGS and AGS closer together, essentially meaning a shorter length (L) measurement.

 

Girths did not affect the nominal displacement (D), which was largely a factor of midships depth immersed (MDI) and forward depth immersed (FDI).

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I think the bulge just refers to the style of boat back then (narrow in the bow big beam and pinched in the stern) Now days we would call modern boats "the wedged" because of their shape . This bulge shape came from the IOR rule itself as designers tried to trick the rule and this was the shape that was the end result.

 

The missconception is that all ior boats are pigs is simply bullshit. Most boats in NZ that are dubbed IOR boats actually arent. They weren't designed to the rule itself but based on that style of boat. They are a IOR styled boat.... it's quite different. It would be like calling a new Elan a Volvo boat because of how wide they are in the ass when they totally arent a volvo boat. Big asses are just the style of at this time.

 

Its true IOR boats did use to roll any race boat has its thrills and spills thats what happens when your pushing the edge in boat racing. A lot of ior styled boats sail upwind like a dream and with new kites and a sails thy are pretty easy downhill too just keep them strapped down!

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:thumbup: As said by Sow1ld, with today's sails the downwind issue some of the IOR style boats had, are no longer an issue and were in fact only a result of boats being pushed to the limit. Back then, everyone sailed dead down wind,now-a-days the fastest way to get downwind is to reach and an IOR style boat is just as stable reaching as any other.
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Bow down trim.

 

IOR.JPG

 

 

From an article in Yachting by Rod Johnstone (May 1984)

 

Flotation measurement trim - stand the boat on its nose! One can only regard the modern IOR yacht much the same as a novice rider regards a horse - too much behind and not enough in front. This imbalance is accentuated by the rating advantages gained by trimming a yacht down by the bow for measurement. One of the greatest deceptions of the IOR rule is that the values of two of the most important ingredients of the rating formula, (L) and (D), are drastically affected by a yacht's fore-and-aft rating trim.

 

Bow-down trim tends to reduce (L) and to increase (D). It also tends to increase DSPL slightly, important on lightweight yachts which are assessed a high displacement/length penalty (DLF). In addition, bow-down trim reduces a yacht's static stability by submerging the narrow, deep end and by lifting the wide, shallow end clear of the water. The effect is a slight reduction in (RMC).

 

Taken separately, the rating factors affected by bow-down trim may seem to change little, but take together significant rating reduction can be obtained.

 

(lots of other really intriguing nuggets in the article, including how Johnstone was able to effectively get 5 feet of unmeasured sailing length on the J-41 by manipulating variables to move the AGS as far forward as possible, because it puts length aft of AGS "where it is not measured".

 

From a book (not sure which one, I only have a few pages of it)

 

Generally speaking most boats will rate better with bow-down trim. In many hull configurations the rating is highly sensitive to stern trim and for every fraction of an inch that the afterbody is lifted out of the water there is usually quite an appreciable reduction in rating.

 

[the author opines in a different section that this is because the IOR's approach of using the AOCP line, a measure which seeks to define the steepness of the stern profile, is intended to help approximate the horizontal distance from AGS to a point where a line drawn at (0.018*LBGC) below the AOCP profile intersects the waterplane. In practice, this slope-derived waterplane intersection is heavily affected by measurement trim.

 

From "A yachtsman's guide to the rating rule", chapter 8 (Peter Johnson, published in UK)

 

[...now we have] trimmed the boat 0.2 ft up by the stern, so that the forward freeboards reduce, the after freeboards increase, the MDI remains the same and FDI increases a bit. There is no girth difference, and so the after overhang component is markedly reduced, with the result that L is reduced and the rating drops from 21.6 to 21.1

 

Note the very powerful effect of simply lifting the stern of the boat. If we want to rate at 21.6, we can gain something like 40 square feet of sail area for the same rating by simply altering trim.

 

The net of all of it is based on the IOR's belief (?) that the AOCC and FOC lines help derive an "effective sailing length" that is self-adjusting each time new value for fore-and-aft trim are obtained. Because those lines typically have very different slopes, though, a change in trim affects FOC's intersection with the waterplane very little, but has a great effect on AOCC's intersection with the waterplane. That can cause a significant change in (L).

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So I guess most of its already covered.

 

The IOR rule went about measuring boats and attempted to rate them just as the IMS rule later did.

After the rule was designed, designers started deliberately designing to the rule and found that by exploiting the rule they could make the rule think a boat was slower than it actually was.

 

One thing not yet mentioned is that the IOR rule puts a big penalty on stability.

So a lot of genuine IOR boats have substantial weights in Internal Ballast and really quite small and light keels.

Due to the distorted shapes, narrow waterline beam, small fins, and lack of stability, they can be very difficult to manage downwind.

Some of them most definitely are Pigs off the wind and it is not just the sails that make them so.

 

When the first few IMS boats came out they were nice and fair but it later came about that there were even bumps built into the later IMS boats as well. But because the IOR rule focuses so heavily on only 3 main measurement points, FGS, BMax and AGS the distortion and bumping is much more pronounced. The IMS rule used a lot more measurement points and so distortions aren't as severe.

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Guest Saturday Night Special

Ganbare had Pigs of Lead screwed around the deck to decrease the moment of inclination

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