But as mentioned, I would need to learn about the hull construction back then. Also researching IOR race boats, I am guessing I need to understand more about the stability as well?
Below is a very long article that I wrote for another purpose but it includes a lot of discussion on IOR hull forms and rigs. Its also a little out of date since the IMS rule dies about a decade ago due to complexities in scoring a race.
But first, I will note that Bruce Farr was a kid in his mid- 20's when he designed this boat. He had been a wonder, taking the racing world by storm with what were truly innovative race boats that were fast and easier to handle than the standard IOR racers of the day. But this was still a period when race boats needed huge crews and counted heavily on that crew weight out on the rail for stability. The standard IOR 2 tonner was a physically demanding boat to sail even with a big crew. Farr's designs were typically a little less demanding. Typical IOR boats tended to be short on stability and squirrely to steer. Farr had come out of racing dinghies and so his designs tended to be have a little more form stability and more predictable steering. They still were typically sailed by big crews.
"IOR vs IMS type forms
If you ever needed to cite an example that demonstrated how unintended consequences can have a disastrous impact, at first glance the IOR rating rule would seemingly be a good case study. In modern times there is a tendency to revile the IOR rule and its adverse impact on yacht design during the period that it existed, but like so many things in sailing, that hindsight view tends to be an over-simplification. To understand the IOR rating rule it is important to first understand what rating rules are, and to look back at the events that resulted in the IOR rule.
Rating Rules- Their purpose and Origin:
Almost from the beginning of yacht racing, there has been an effort to develop a way of comparing in a fair and equitable manner the relative performance of vessels of differing designs. Since the factors that control the speed of a vessel are extremely difficult to quantify, over time a wide range of simplified rating and handicapping rules have been crafted and used. It is important to understand that there are essentially three ways that boats are rated relative each other.
The first simply looks at the past performance of individual vessels and based in their observed speeds and tries to assign a rating that corrects for their relative performance. This is often referred to as a ‘handicap’ rating system. In the U.S. a good example of a handicap rating system would be PHRF.
The second method of rating boats would be a measurement type rating system. Under a measurement type rating system a discrete number of measurements are taken and these measurements are fed into a formula that attempts to predict the relative performance of the vessel.
Measurements can include (both on deck or at the waterline or at some point in between) lengths , beam, draft, sail area, displacement, girths and a wide range of other increasingly esoteric measurement points. The rules typically stipulate where on the vessel and its gear that these measurements are taken and will often stipulate the state of loading, etc. as well. The Universal, International, MORC, CCA, and IOR rules are prime examples of measurement type rating rules.
The short comings of a simplified measurement rule typically result from the fact that there will always be a limited number of measurements taken and these measurements are taken at known points. Therefore, it becomes possible to distort the boat so that these measurements produce a boat that will appear to be slower under the rule in question than it is in real life. Designers have consistently quickly learned precisely how to distort their design to beat the rule. Generally, speaking these distortions mean that boats actually are slower in real life sailing, but they still are not as slow as the rule thinks they are. And because the hull and rig have been distorted to cheat the rule, often ease of handling, seaworthiness, motion comfort and other desirable sailing characteristics are compromised as well. This becomes even more the case where the rating formulas have been skewed in an effort to respond to some particular goal of the rule writers.
The third rating method is a comparatively modern extension of the basic principles behind the measurement rule. Starting in the 1970’s, with the advent of the then available and somewhat primitive computers of the era, yacht design theorists began to quantify the speed generating factors of rig, weight, and hull data at a comparative fine level so that they could relatively accurately predict the performance of a given vessel, in a given loading, wind speed, with a given sail inventory. These computer programs were generically referred to as a VPP (velocity prediction programs). With better computers, these early VPP’s became easier to share, and operate, and it wasn’t long before the idea occurred to a group at MIT that a high enough quality VPP could be adapted to rate vessels fairly. And since individual designs do better or worse depending on the specific conditions on race day that they are sailing in, these early VPP based rules also included mechanisms to adjust the boat’s rating to the wind speed on the course and the sailing points of the race. This early VPP based rating approach eventually became the MHS, which later evolved into the IMS rule.
In theory this should have been the major break through that allowed boats of any type to race against each other. But of course, like any other rating method, designers eventually learned to ‘game the system’ and ended up producing IMS type form boats. The good news about VPP derived boats is that Designers quickly began to develop boats which were well rounded designs and which were designed to be fast relative to their lengths on almost any point of sail. The theory is that a fast boat can sail a little further in order to reach portions of the race course that then allows them to take tactical advantage of better wind or current.
Type Forms are hull shapes and rigs that are optimized to a specific racing rule. The IOR produced very specific hull and rig characteristics. It was hoped that IMS would fairly rate boats without a specific Hull Type Form being developed. That didn't happen. While there are notable differences between IMS designs, for the most part, IMS boats are just as Hull Type Formed as MORC and IOR boats. On the other hand, just about any boat can be rated under the IMS rule and can race to one degree of success or another.
Initially, the IMS actually produced really good sailing boats. Designers began paying attention to motion, since a boat with an uncomfortable motion for humans, also had a motion that disrupted the flow of air over the sails, and water over the keel. The IMS originally included minimum accommodation standards so these were comparatively comfortable boats down below (for a race boat). Sail plans and deck layouts were optimized for efficiency and ease of handling. Without the artificial incentives of a measurement rule, masthead rigs and large overlapping head sails became somewhat obsolete.
But like any other rule, there were short comings to the IMS as well. Some were intentional. The IMS almost from its inception was intended to allow and encourage dual purpose boats, racer-cruisers, or cruiser-racers to race against each other. But with the demise of the IOR rule, racers began complaining that the IMS had also killed grand prix level racing. Eventually the IMS developed a parallel rule that encouraged the development of grand prix level race boats, and with that the rule seemed to go into decline as racer-cruisers were no longer able to compete with these purpose built race boats.
The other issues that lead to the decline of the IMS rule was that it was wildly expensive to digitize a boat at the fine level that was required to produce a meaningful VPP and managing races where the Race Committee had to choose the rating based on wind speed and direction, made race management a nightmare.
This led to an effort to produce simplified rating rules such as the ORC, and IRC, which is where we are today. Unfortunately these simplified rules represent a giant step backward in terms of the designers learning how the rule works in great detail, and producing rule beating designs which are not necessarily better sailing boats and certainly are not dual purpose boats.
IOR vs IMS hull type forms
This brings us back to the original topic of this article, IOR vs IMS type forms. There are substantive differences between the IOR and IMS type forms. The IOR hull form tended to have a deeper canoe body, more of a raked stem and reversed transom, a wider entry angle, more flare, a narrower run, longer shallower keel forms for the same draft, and a very distinct section that resulted from the measurement points used in IOR.
The IOR rule was actually a series of rules, each intended to correct the excesses of the one before it, so this distinct hull form was less pronounced in early and last IOR boats. The earliest of the IOR rules was the IOR-1 rule, which produced the best all around sea-boats of the IOR era. But these boats tended to have massive tumblehome (a product of the measurement points and methods). This tumble home resulted in boats with a lack of reserve stability. Almost any design has a point at which the stability of the boat drops off suddenly. This is a product of the geometry of the topsides and deck and usually occurs near the point at which the deck enters the water. In the case of these early IOR-1 boats, this point occurred earlier as there was no flare to continue to build stability. While it can be argued that the loss of stability occurred progressively rather than suddenly, it was easier to get lulled into trouble since there was no sense that you were on the edge as there was with later designs.
To correct this, the IOR adopted a different set of measurement points and formulas and almost instantly the IOR-1 boats were obsolete as race boats, and the heavy distortions of IOR-1 also meant they were also pretty mediocre cruising designs.
From IOR-2 onward, IOR boars had a very distinctive hull form, marked by a unique three-plane sectional shape. This section had a relative flat area on the bottom that radiused into relatively flat sections on either side that sloped up toward the waterline. These are then radiused into a hard turn of the bilge at or above the water line that proceed to fair into fairly straight flared topside. This profile tends to have mediocre initial form stability but quickly develops more stability as the hull turns in the bilge at the water line become immersed.
This shape was solely IOR rule driven because of the IOR location for measurement points and the big penalty for initial stability. It was not fast and as a type did a lot to foster the common impression that that all light boats automatically have unbearably quick motions regarding motion in a seaway, particularly as described by C.A. Marchaj in his book "Seaworthiness". I just had the chance to review this book again and the critical research was clearly performed on IOR type forms.
IOR Boat knockdown
, on Flickr
An IOR-III hull form in extremis
Mojo wipe out
, on Flickr
An IMS hull form. In comparing the two note the differences with the IMS boat having the elliptical sections, more form stability and more powerful stern sections, and its center of buoyancy located further aft to improve the boat’s motion characteristics and tracking.
In many ways the IMS Hull form addresses the criticism of the IOR hull form and the concerns raised by the Fastnet Tragedy. The IMS type form, by contrast to the IOR, has a shallower canoe body and a much rounder underbody. The shallower canoe body gives greater form stability while the rounded sections give a bit less. The combination results in a similar, but perhaps a bit greater initial form stability. But because the hull form does not have the hard bilges, the hull does not have the acceleration as it flops toward the turn of the bilge or the snap de-acceleration as the reserve buoyancy of the second turn of the bilge kicks in. (The IOR’s hard bilge was also a contributor to the IOR boat's ability to produce spectacular wipe outs and death rolls.)
Another distinguishing feature of the IMS type form is the keel and it relation to the shallow canoe body. IMS boats have very high aspect ratio foils. This can occur because the span between the shallow canoe body and the slightly greater draft is longer than can be achieved in other hull types. Normally this would result in two problems, lack of stability from the minimal amount of volume in the keel foil and lack of directional stability from the short foil chord length.
The stability problem has been addressed with creatively shaped bulbs that are also intended to reduce drag due to tip vortex generation. The added weight of the bulb keel also increased the limit of positive stability and with it the chance of quicker capsize recovery. These bulbs result in tremendous stability but as a side effect they also greatly improve seaworthiness and the comfort of the boats in a seaway. The long foil span has a dramatic dynamic dampening effect on the roll rate and angle. The bulb also increases the boat's roll inertia and thereby helps decrease the roll rate as well.
Directional stability is handled in a different manner that would be traditional. In the case of the IMS boat, the long water line and exceedingly fair hull result in relatively straight waterlines, (even when heeled a bit). The directional stability of this hull type form comes almost exclusively from the canoe body. This actually works reasonably well. One issue with the IMS Hull form is the heavy dependence of dynamic loadings. At very slow speed the keel and rudder foils are next to useless, but as soon as there is a little way on they become very powerful and efficient. At even low speed they make very little leeway and are very maneuverable with very light helm loads. This makes the IMS hull type very suitable to vane type self steering or electronic autopilots with low battery usage.
To some extent the lessons of the IMS hull form has filtered into production boats. Recent hull designs by major manufacturers such as Beneteau, Catalina, Hunter, Dehler and Sabre to mention a few clearly reflect some aspects of the IMS and Open Class Hull type form. I consider this trend to be generally good, but I believe that to really work a boat with the IMS hull type needs to take advantage of modern materials engineering and be light. As the bow becomes blunter, the foils shortened, and the canoe body deeper, the hull type form looses its advantage and can quickly become a far less successful vessel. Also, without intending to start a verbal food fight, some of these manufacturers are not doing the careful engineering that is necessary to safely achieve the light weights necessary. The grand prix versions of IMS boats feel amazingly rigid. These wannabe’s do not.
IOR vs IMS typeform rigs:
One of the most distinctive features of the IOR rig is the proportion between the size of its headsails and mainsail. By the early 20th century it was understood that the majority of the drive came from the leading edge of the sail. Other areas of the sail (leech), added some drive, but that these areas also added disproportionately to the drag of the sail. It was also understood that as a headsail overlapped with a mainsail, at some point, the impact of the overlap decreased the efficiency of both sails.
In an effort develop a rating rule which compensated for the performance implications of luff length relative to sail area, and the impact of overlapping headsails, in rating rules in the early 20th century began rating headsails based on the boat’s 100% foretriangle, ignoring the amount of overlap in the equation. The IOR over compensated for this problem more extremely than most prior rules. The modern spinnaker further added to this perfect storm, by further incentivizing huge foretriangles since the size of the spinnaker was proportionate of the size of the foretriangle and a bigger foretriangle mean a much bigger spinnaker.
Adding to this problem was the fact that the IOR rule over-penalized standing sail area 100%foretriangle plus mainsail. Since mainsails were penalized strictly based on their sail area, mainsails became tiny, high aspect ratio sails. This tendency towards small mainsails is in stark contrast with the most efficient rigs in terms of sail area to drive, and in terms of ease of handling.
And since there as such a penalty on standing sail area, IOR typeform rigs start out under-canvassed but came to depend on huge headsails to compensate for the lack of sail area. Rather quickly this led to the development of modern multi-speed geared winches and crewmen who had been former football linebackers. But it also led to the need for very large sail inventories.
This need for large sail inventories came to be primarily as a result of the relationship of sail carrying capacity of IOR boats to their small standing sail plans. Because IOR boats tended to lack stability, which of course translated to a lack of sail carrying capacity, they were not able to stand up to a lot of sail area in a breeze, but because their rig proportions were inherently inefficient they needed a lot of sail area to sail in light air. As a result, it was critical to have a large enough sail for the windspeed, yet not so large a sail that the boat was overpowered. Even small IOR boats carried numerous genoas and spinnakers and a whole arsenal of specialized sails like bloopers, spinnaker staysails, reachers, and drifters. These were not easy boats to sail at speed, especially in the light to moderate end of the wind speed range.
By contrast, the IMS rig tends to be a fractional rig with a much greater standing sail area. Improvements in stability coupled with the greater efficiency of the low stretch sail materials, and advanced sail shaping controls, permit these boats to sail with less dependence on large over-lapping headsails and consequently smaller sail inventories. Since the low drag and high stability that is typical with IMS hull forms allows big gains when sailing hot angles vs dead downwind, IMS boats tend to have rigs optimized for deep reaching rather than running dead downwind. As a result, IMS rigs tend to employ asymmetrical spinnakers in all but the heaviest winds. "