Lots of great answers above. It is not at all simple--there are many different hull types. It is way beyond the scope of this forum to discuss all types of hull form--that is something that JimmyG needs to do on his own. I sound like a broken record--but at least do some reading--the best book is "the nature of boats" by David Gerr. But there are internet articles about the different hull forms.
From the Hull design forum: "Displacement hulls are designed to travel in the water at a typical hull speed of 1.34 *(square-root LWL) in knots. This formula applies to the distance between bow and stern wave crests. At faster speeds, wave making resistance increases exponentially because the vessel is trying to climb on top of the bow wave - meanwhile the stern is being sucked down by the dynamic forces from the "hole" created in the water as the vessel moves forward. Displacement hulls tend to have pointed bows and sterns because this form poses the least wave making resistance at "displacement" speeds. It takes a relatively small amount of power to push a displacement hull at its "hull speed."
Semi-displacement hulls tend to have wide, flat aft sections - like a New England lobster boat. These hulls are designed to partially climb on top of the bow wave and separate the transom from the stern wave. Semi-displacement speeds are usually in the area of 1.5 to 2.5 *(square-root LWL) in knots. It takes a *lot* of power to drive a hull in the semi-displacement speed range. The flat wide stern sections help to provide additional lift in the stern to partially overcome this problem.
Planing hulls are designed with straight sections aft. A typical deep-V bottom hull has the same angle to the 'V' (the same "deadrise" angle) from midship to transom. They are designed to climb completely out of the water at high speed and "hydroplane" on top of the water. At planing speeds, water is breaking cleanly from the transom and the hull is riding on its straight aft sections. The greatest resistance at planing speeds is frictional resistance. It takes more power to climb out of the water over the bow wave than it does to maintain planing speed once this is achieved. At very high planing speeds (>25kts) any change in deadrise angle in the aft sections of the hull can adversely affect performance. Hulls with a "variable deadrise" angle in the aft sections (where the angle of the 'V' decreases and flattens toward the transom) are a further modification of the semi-displacement hull form. They are more easily driven at speeds < 25kts, but at higher speeds tend to push the bow down due to the higher dynamic lift in the aft sections. Variable deadrise hulls can actually become unsafe at very high speeds >30kts because of this tendency - it is possible for the bow to dig in and cause the boat to broach at high speed."
The Tom Cat is a planing catamaran (Most cats are semi displacement hulls), and is faster than most cats with the same HP (also more economical at higher speeds, but not some lower speeds), but is more likely to slam as you get into higher seas. A 3 foot wave is considerable--and the Tom Cat will handle a 3 foot wave better than any of the other boats you have mentioned. Something like a Glacier bay will do even better, at least going into the waves--but not as well going beam to the seas or down wave. Realistically how often will you be boating in more than 3 foot seas?
The Rosborough is a semidisplacement hull (like a lobster boat--rounded chines, small keel, fairly flat run aft, and deep forefoot). It is going to take more power to push at the same speed as the C Dory--and will not come on a plane at a lower speed, but can be pushed to a plane with enough HP. If you want to run at 8 to 15 knots in moderate seas--this may be your best choice--if the layout and other features of the boat are satisfactory.
The C Dory will plane at a very low speed--it is a semi dory--very seaworthy--but can pound if driven into a steep sea. The way around this is to put the bow down with trim tabs and permitrims--this presents the V of the bow to the seas--it will throw some water, but you are in a pilot house boat--so no problem. On the other hand--the C Dory will ride better down waves than the semidisplacement boat. The C Dory and other boats you name are quite different--and behave differently. The C Dory probably has one of the slowest planing speeds of any boat built,, because of the hard chine (reverse chine)--flat run, with a little rocker and the use of tabs and Permatrims.
JimmyC--you cannot read some forums and then go out and buy a boat. That is a major mistake. You need to ride in every boat you are considering--and some others in all different conditions. Some folks push a certain boat, because they own one--or because they are convinced it is the best boat...for them. There is no "perfect" boat.
You don't want to rush into buying a boat. Take your time, do lots of sea trials and enjoy the process--much less likely to make a mistake.
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