Long post warning…
I guess we?re talking about models here. So in the context of monohull models, the first problem is that we don?t have the benefit of moveable crew weight to use as ballast. Although you are absolutely correct that the beam and hull shape contribute to stability, there are two key factors (and a third perhaps less obvious) that prevent us designing sufficient stability into the hull of a monohull model to compensate adequately for the removal of a ballasted keel. The first is that the improved stability obtained from beam, a flat hull and hard turn at the bilge is ?initial? stability only. It resists the healing moment when the boat is upright, but that resistance decreases as the heel increases. We’d have to be quick at easing the sails, or they’d capsize in the gusts and would not be self-righting.
The stability curve can be plotted, and if you haven?t seen it, you?ll enjoy reading the report that came out of the Fastnet disaster of 1979, which showed some interesting comparisons between the leading boats of the time (which, under the IOR Rule, favored wide beam, light displacement and ballast high up in the keel), and the more traditional (for the time) designs. The light displacement racers showed higher initial stability than the traditional heavier displacement designs, but reached an angle of heel where stability was lost quite suddenly. The center of gravity was quite high in these boats, as it is in a dinghy that uses a centerboard to provide lateral resistance, but crew weight to provide ballast. The greater initial stability was a result of their beam and hull form.
Which brings us to point number two, - they used the wide beam, and the high center of gravity, because the rule they were designed under encouraged it. Beam is slow (according to the IOR rule) and less stability is not good, so the designers were rewarded in other ways, by being allowed (for example) more length or more sail area if they designed in some of these ?slow? features. But if we take a boat like the IOM or the US One Meter, our key performance factors of length and sail area are fixed (or at least the maximums are). With the IOM, no matter what concept of boat we go for, it?s must weigh in at 4kg or more. That?s actually quite heavy for a one meter long boat. It?s not really practical to design a flat-bottomed planning hull within the parameters of the rule. The ?skiff? designs amongst the IOMs are not really planning hulls, but their wider beam allows them a shallower hull for the same displacement. So they are easier to get up on top of the water than a narrow IOM in those situations where they have enough power ? i.e. downwind in a strong breeze. The higher drag of these hulls is evident in the light airs downwind stuff. It?s horses for courses.
When you sailed the laser (the dinghy ? not the RC Laser) you will have worked hard to keep the boat flat when sailing to windward. Lightweight racing dinghies, and light displacement performance keelboats, sail as upright as possible on the wind. Excessive heel gives the boats a very different underwater profile from that they have when upright. I?ll never forget my first experience as a kid onboard a trapeze dingy planning to windward ? I still love sailing trapeze dinghies almost 30 years on. But to get the boats to plane on the wind we need power (hence the trapezes to compensate for the high sail areas) and we need to keep the boats upright so that the planning hull retains the correct underwater shape. To do this without moveable ballast (a careful choice of words), we?d need to go very wide indeed. The boats would end up as barges with a very large wetted surface relative to the power available from the sails. They?d sail like barges too.
As the IOM skiff designs rely on ballasted keels, AND they are sailed at quite an angle of heel on the wind, the designers have put considerable thought into the underwater shape of the hulls at heels angles applicable to windward sailing. Hence the dishy shape of the hulls in the aft sections. If I recall, Lester Gilbert has a great article about this on his website.
Finally, the effects of scale come into play. The smaller the boat, the less inherent stability it will have. Larger boats can achieve a greater length:beam ratio for given inherent stability than smaller boats. Scale comes in with fin area two ? but so does the speed at which a boat is designed to sail. To put it another way, the speed at which a foil is designed to move through the water, or the air. That?s why fast jets have a much smaller (relative) wing area than slow flying aircraft. We use the lift to resist leeway. More speed equals more lift from a given foil, area and angle of attack.
So we can design a fast, flat-bottomed planning hull that will burn the pants off a ballasted keel hull of similar length so long as we have the power to drive it and the ability to keep the correct underwater shape upwind. Assuming we want the boat to sail on all points, then to do that we need moveable ballast. Enter the canting keels. Other ideas seen over the years (and in some cases still very much in evidence) are water ballast, and rigs that ?heel?. Add a channel or two on an RC model and we could have a ballasted model crewmember on a sliding seat (or something similar).
With the surf cats you mentioned, what you really have is two heavy displacement hulls with a high ratio of length to beam. Narrow is fast ? wide is slow. The lack of a centerboard is possible not only because the leeward hull has a sizeable area underwater to provide lateral resistance, but also because these hulls are moving fast. (More speed ? more lift again.) That?s also why fast catamaran sailors have prehensile toes (like the tails on certain species of monkey) ? so they can hang on while trapezing at 25 knots.
By ?heavy? displacement I mean that the hulls are designed to sail deep in the water, not that the hulls themselves are heavy. When a fast cat is flying the windward hull, the entire weight of the boat is carried by the leeward hull in the water ? double it?s upright displacement.
OK ? many simplifications and omissions here, but in a nutshell, this is what we?ve got. Beam gives stability but increases drag, and planning sailboats are sailed upright through control of power in the rig and via moveable ballast. Of course with water ballast, weight can be reduced when the power is not required ? i.e. off the wind.
I haven?t talked about the differences in the drag, and the change in importance of the various factors contributing to drag, that occur when a boat gets up on the plane. The old, hard chine, planning dinghies I used to sail, such as the Cherub (in NZ, Aussie and the UK), where quick on the plane, but I suspect that a good round bilge design of the same size and power would have beaten them at displacement speeds. The sharp edges of the hull and turns at the chine create more drag than a design without chines. But that?s another story?
So its less about what makes a boat fast, and more about what makes a boat slow. Rules make boats slow ? and the need to sail both upwind and down. So the trick is to design a boat that is least slow under a given rule. Ah, there?s the challenge, and the fun, of rules like the IOM.
What the hell - you knew this already, but I’m sure there’s something here to attract other opinions.