I just discovered this old but very interesting post by Jeff. I have some personal experiences that differ from what would be predicted by some of Jeff's comments though, so I'll add my comments to the selected passages below.
Unlike earlier rules where a slower boat with a big rating advantage could win, in the early days of the IMS, there was no advantage to designing a slow boat because you could not cheat the rule.
To my mind this was the core failing of the IOR. I loved the look and windward ability of IOR boats but could never understand why, when it was revised so often, they never really moderated the most extreme effects of it. Why, for example, were keels mandated to be no wider at the bottom than the top? Bulb keels existed before in many shapes - my own Columbia 43 has one. They are obviously more stable than an IOR delta shape so why outlaw them? I can fully understand penalizing speed producing factors when handicapping - adding weight to a race horse is a well known example - but when the penalty actively encourages slow design aspects in new boats it is overdone.
Which brings us to fine bows, plumb stems and the afterward re-location of the center of buoyancy popularized by the IMS typeform. If you have ever beat or motored to windward in a short chop, try to remember the motion that you felt as the boat encountered each wave. Initially, there was a moment as the boat collided with the wave that to one degree or another you could feel your body thrown forward as the boat de-accelerated. And while you were feeling that force, you could feel the bow being be jerked upward, the whole boat rotating about the pitch axis of rotation. Then you could feel the boat heave (moving vertically) as the wave passed under the center of buoyancy. There is a brief moment during which the boat seems to hang in the air, before the bow begins to rotate downward. And lastly, you felt the bow jerk to a halt or even upward as it hits the back of the wave below.
What can be designed around is the amount of impact force imparted into the hull by the collision with each wave, and the amount of change in speed with which the boat pitches. To begin with, visualize two boats with equal displacement, equal longitudinal centers of gravity, the same deck shape when viewed from above and in profile, same depth of canoe body, same mid-ship cross section, and the same reserve buoyancy (meaning volume of the hull above the waterline forward of the center of buoyancy), but one has a plumb stem and the other has four feet of overhang at the bow. In this example, by its very nature, the boat with the overhang will be more blunt, less knife like, than its plumb stemmed sister. Instead of cleanly slicing through the wave, the boat with the overhang, actually collides with the wave with a greater impact force, and that impact force, both slows the boat down, and also is wearing on the crew.
Because both boats have equal reserve buoyancy, the deck will stay equally dry, but because the waves act more suddenly upward on the blunter ends of the boat with the longer overhangs, there is more concentrated rotational force imparted and applied more rapidly (i.e. more of a collision than a gradual application of force) into the forward end of the boat with the longer overhangs. All other factors being equal, greater concentrated force applied further outboard forward means a greater rotation angle; more rapid application of force means a more rapid change in direction. Greater rotation angle and greater speed of motion means a larger flow interruption over the sails and foils and less comfort for the crew.
On the boat with the modern hull form, as the bow moves upward, the fuller stern sections build reserve quicker than the overhanging sterns of more older hull forms and that earlier progressive building of buoyancy serves to further dampen the speed and amount of rotation.
At this point the boat with the longer ends is moving upward at a greater speed and will have greater kinetic energy causing it to rise higher than the shorter overhang boat. Because it has farther to fall, and the acceleration of gravity is a constant, it will hit the water later on the wave, and with a greater impact force.
Another factor that further improves the motion of the IMS type form is that the center of buoyancy is located further aft. If you visualize two boats having their bows lifted an equal distance by a wave, but one boat rotates about a point that is further forward than the other boat’s axis of rotation, the angle of rotation on the boat with the longer distance to the axis of rotation will rotate through a smaller angle than the boat with the axis of rotation located further forward. And since the boat with the center of buoyancy located further aft ends up with not only small rotation angle, but with a shorter distance from the axis of rotation to the cockpit, there is less vertical distance experienced by the crew in the cockpit.
This is the commentary my experience differs with. I owned a fairly extreme, mid 70's, custom IOR quarter tonner for many years. This was a 26' boat with a 20.6' WL, 9.5' beam and 4300 Lbs Disp. It had the typical IOR diamond plan shape with an extreme "stinger" stern. Internally that boat had nothing but a head in the bow back to the mast and nothing in the stern aft of the cabin top - ALL the weight was located over the keel.
That boat had the least tendency to pitch of ANY boat I have ever sailed. As the fine bow (with typical overhang for the type) met a wave, it would slice into the wave without rising. As the crest of the wave approached the beamy midship section the boat would start to rise, levelly since there was very little polar moment due to the centralized weight. As the crest rolled aft the radically tapered stern wouldn't rise, or rather, wouldn't depress the bow, for the same reason the bow hadn't risen originally - the relatively low percentage of total buoyancy in the ends. The boat never slammed when descending either although waves sometimes "slapped" under the highly flared topsides
The net effect was the boat basically only heaved over waves, it didn't pitch much at all - it would rise up, over and down, staying very level the whole time.
Now, on the other hand, I have a fair bit of time on a modern cruising boat with a near plumb stem and a very wide stern with a bit of a sugar scoop - not a race boat though.
That boat pitches relatively more although not badly but it slams mercilessly. Generally, when motoring into head seas, the slamming is so bad you have to tack - under power
- to make the ride bearable. It has been so bad I worried about the rig, it was banging and shaking so badly - to be fair, that was motoring into 40 knots and fairly short seas.
The boat has a shallow wing keel so generally its motions are quite soft and comfortable but the contrast in pitch & slam between the two boats would seem to conflict greatly with Jeff's commentary.