When you try to compare two boats made of similar materials -- a flexible boat with a stiffer boat -- the issue of rocker becomes rather variable.

In your question the answer here can REALLY be variable. When you compare two s&g designs made with 4mm plywood, you have boats made from similar materials which will flex similarly. That is not going to be the case when you compare a plywood S&G with a skin covered frame. They have so little in common when it comes to flexibility.

While you can easily draw a boat design where the ends are higher than the middle -- which gives you your rocker -- the real boat is going to flex under the weight of the paddler and the effect of the displacement of the water by the shape of the hull. When you DRAW rocker for you plans you look at side view of the boat. But when you go to considering the amount of flex you will get when the boat is on the water, and you step into it, You'll have a better reference ( though not a perfect one)if you look at things from a top view. a boat that is shaped like an arrow -- long and thin -- will rise considerably at the bow and stern, giving the effect of greater rocker. A long diamond shaped boat will flex a little in the middle and more towards the ends. You still get rocker, but it is more pronounced at the ends and the center of the boat is (or might be) straighter along the keel. The cigar shaped boat should have the least upturn at the ends and the straightest keel -- unless it is built of materials that flex a lot -- in which case it works like the arrow shaped boat.

Put a heavier paddler into a flexible boat and that paddler effectively has more rocker, as the center of the boat sinks a bit deeper to displace water equal to the padler's weight.

The ultimate example, I think, is to look at what happens when a person steps into an inflatable boat. It sags in the center. It is SO flexible that the center can be many inches below the bow, stern or sides.

I think you have a bigger issue here, though, and that issue is not rocker, but maneuverability. If I guess right, you want to keep, or improve the maneuverablity of your boat, and it is your belief that changes in rocker will do this for you. Ain't necessarily so.

The basic factors you have to consider are how much water you have to move aside to turn the boat, how much leverage do you have to overcome to move that water, (or from the boat's perspective, how much is the water acting against the length of the boat).

Consider a boat that looks like an ice cream cone. (a mathematician would call this an inverted cone with a sphere on top) This looks a bit like a kid's toy top, and no surprise, it turns like a top. While it goes deep, it's shape allows it to turn easily. Now consider stomping on that icecream cone, and flattening it a bit. You have something that looks like the keel on a sailboat. by stomping on it you have made it thinner, so it will go through the water in a forward direction rather easily, but it is also now wider, so it resists any action of the boat slipping to the left or right. but, becasue it is located in the center of the sail boat, the boat can still spin almost as well so it can turn to change directions. Now imagin you've put that poor icecream cone down on the street and run a steamroller over it. You've just stretched this keel to the full length of your boat, and made it difficult to turn.

Here is another analogy. Think of two canoeists in a canoe. One at the bow, the other at the stern. If they stick their paddles into the water just one inch and push away from the boat the boat will move slowly. If they stick their paddles in 6 inches the boat will move more briskly. If they move closer to the center of the boat with their paddles dipped jsut one inch they will not get anywhere near the turning effect of sitting closer to the ends and dipping the paddles 6 inches.

With a boat with rocker the ends of the boat are "dipped" (to borrow from the paddle analogy) less that the center of the boat. with a rigid boat that has extreme rocker, sometimes the ends of the boat are out of the water completely. With a flexible boat the amount of the boat that is immersed at the ends is dependent on how much the boat flexes, and how much the ends float.

Not sure I've made this totally clear but I hope I could give you some things to think about.

PGJ