: When designing kayaks, I use a pre-determined weight for the kayak based on the build methods, weight of cloth, factoring in first time builder experience and then add the paddler's weight and use those combined weights to determine the waterline and performance ratings of the kayak like stability and hull speed.

I've always found this argument somewhat flawed, because you almost never have a paddler that weighs exactly X weight, nor do you know exactly how much gear he's going to carry, how heavy his paddle is, lifevest is, tow rope, VHF radio, nor do you know what kind and how much camping gear/food, etc. he's going to bring with him, nor how big of a first aid kit, what fittings he's going to use, etc... Or if he's going to use it unloaded one day and loaded the next. When I design a kayak, I design it for range of weights and expect it to perform somewhat similarly at different weights. Knowing that it will be less responsive at heavier weights. Ideally, the paddler wants a kayak that fits right with the height and size of the cockpit area reflecting the paddler's size. That fit is far more important than if the build comes under the designed weight. Although, it's a given that you want to match the paddler with the right-sized boat. It's always easier to add weight than subtract.

On a kayak build, we talking about a difference of 15 lbs. either way, which is 15 lbs less that the paddler has to yard around if it comes in lighter. I'd be very suspect of a design that couldn't handle that range either way.

:so that a novice paddler may not judge the boat to be as stable

On Stability: Typically, stability curves are higher for lighter weights. An example included below. Stability curves don't always transfer to reality, but a reduce of 25 lbs. in weight on a sea kayak typically translates to a 1/4-inch difference in waterline. It really takes about 50 to 100 lbs, to really change the feeling significantly. For kayaks builds, we're discussing the difference between a 50 lb. boat and a 35 lb. boat. Those difference make a difference to responsiveness and efficiency, but not so much on stability.

:If a customer wants a lighter, more responsive kayak, I prefer to design it to meet those requirements by changing the parameters like increasing waterline breadth, adding rocker, changing hull shape, etc. Thsi way it should perform as designed.

Yes, you can design for responsiveness and the chosen design should reflect the intended usage, but that's not relevant to my point. When comparing the same hull to the same hull at different weights, a kayak can be more responsive. For example, my NDK Explorer is far more responsive when unloaded than loaded. It works well in both states though. When coming through surf landing a heavily loaded boat, I'll take any extra responsiveness that I can get. Responsiveness isn't just displacement, it's also the total weight working against the force you place on the paddle. If you yard around more weight, it takes more force to perform the same maneuver than with a lighter boat. This also means that if you use the same force it would take to maneuver a heavier kayak and apply it to the lighter kayak, it's going to react quicker and go further.