When someone says "roughwater boat" I think of a degree of maneuverability in conflicted water where lean/turn characteristics of the hull are responsive to paddler skill with less priority put upon theoretical top speed. YOu can make any 4mm s&g design durable enough. The use of rudder will eliminate what I feel is an attribute of the hull which is a lack of turning response beyond about 20degree lean for ease of turning. Most kayaks are easier to turn the more you lean them. The Chesapeake has that characteristic to a certain degree but the ease in turning with increasing lean kind of stops long before you hit the capsize angle. Nothing catastrophic about that but it's frustrating to try and turn the boat at a particular point on a wave when there's one moment to do it and it doesn't move,,having the choice to move it by leaning the kayak hard is nice,,so you lean it hard with a sweep and it doesn't turn. But as you get into choppier water the ends remain stuck so you have to pay more attention to turning on tops of waves,,again with a rudder the point is moot. There are some production kayaks with rudders that have bad weathercocking characteristics that require a rudder and they're profitable. In this case the weathercocking is moderate but somewhat unresponsive to paddler skill. Another characteristic of a boat that is good in "rough water" is the ability to "unhook" the bow before a broach begins,,again with a rudder this is less of a priority but the consequences are that the rudder is fighting the bow with waves from the stern more than a design that is easier to turn. So a maneuverable boat with a rudder won't have the rudder fighing the broach as much as a kayak with a rudder where the bow is stuck .
If this is your first experience paddling in rough water with a ruddered boat then it's good enough. If you are familiar with handling in rough water there are better designs where going in a straight line requires maneuvering over a changing surface with every paddle stroke in order to go in a straight line. It's kind of like driving with a big engine over a bumpy road compared to driving over a bumpy road with a smaller engine but a better suspension. Some folks like big engines,,some folks like good suspension.

: Hi.I am considering building the Chesapeake lt17 from a kit.

: I don’t really have an alternative to Chesapeake as I live in Europe and
: don’t know of any other kit boat available here (and I don’t have the
: skills required to build from plans).

: I need a reasonably stable boat – if I were buying a glass boat I would go
: for the skerray.
: I am buc 3* and hope to graduate of 4* in the next couple of years
: but,realistically my skills are never likely to get above that level.

: I am 6’ and 176# .
: I would be using the boat on the atlantic.
: How good a rough water boat is it?
: Much of the information online seems to be from flat water paddlers.
: .
: I would probably fit a C trim rudder.

: I would really appreciate your opinions on the seaworthiness of this boat.
: Many thanks in advance.