Rudder fences, mentioned earlier, were tried and discarded in IOR days.
I sailed IOR boats offshore from '79 to the mid '80s, including four Sydney-Hobarts and on a variety of boats from almost metre-style Peter Cole designs through to Kaufmans, Farrs and Dubois designs. MHO one significant issue is the rolling (itself partly caused by heavy displacement and pinched ends, plus large spinnakers) which caused the C of E to sometimes end up well off the centreline. Part of the reason this is less of an issue now may be better kite cloth, perhaps?
My own boat, a pre-IOR half tonner by John Spencer of Ragtime fame, has a narrow stern but with no distortion in the buttock lines. She displaces about 2000kg on a 28' LOA (so much lighter than her contemporaries) and while she rolls heavily at times despite having chines, directional stability was excellent with the old shallow skeg-hung rudder and remains so with the current deep spade. There has been no significant in downwind stability with the big spade, which tends to indicate that a big rudder may not be a major issue in this case. In boats that are on the limit downwind in a breeze, things may be different.
Some time ago I talked to a couple of legendary Sydney-Hobart winners who had owned top boats from before IOR all the way through the IOR period and on to IMS and IRC boats. They both emphasised that minor tweaks to an IOR design could have enormous benefits to handling. For example one of them (Lou Abrahams) noted that when his 1970 S&S design had her mainsail foot shortened for rating purposes, the rig imbalance downwind turned it into a bit of a pig. The same theme of minor tweaks greatly affecting boats was followed in Lou's later boats and in Fisher's "Ragamuffins".
Directional stability may not come without issues. We've recently started racing my boat again, this time doing beer cans right up the end of Sydney Harbour, where the channels can get down to a couple of hundred metres wide and the wind bounces down in very fluky gusts. I'm just sailing with the wife and kids and rushing a half-rigged boat to the start so things are far from perfect, but I am still struck by the way that because the boat is so directionally stable, we are struggling a little bit in handling fluky gusts in big breezes, compared to boats that can turn faster when you get a 20+ knot increase in breeze and a 45 degree angle shift in a couple of seconds.
Modern wide sterns can certainly be an improvement, but back in the IOR days when sailcloths and rigs were less well developed, the extra drag of the wide stern did cause a significant speed loss in the light winds that most boats normally sail in. For many people, speed downwind in a breeze was less important than light wind speed. And while modern boats use bigger and better rigs to get around the issue and therefore can drag wide sterns, those rigs cost a lot more than the smaller older ones and dollar for dollar, we may not have gained too much in some ways.
PS - as noted, the J/24 was not an IOR boat and not noted for nosediving. On the other hand, my experience with old RORC designs (including owning my own for decades and doing two Hobarts on another) indicated that plenty of them handled better than later IOR boats, although that was not universal.
