Max speed with displacement hulls, all kayaks are displacement hulls, is roughly the square root of the waterline length times 1.34.

For narrow boats the "hull speed" formula is not particularly useful. All it tells you is the speed at which the wavelength of your wake will be the same as the length of your boat. For wide boats, this indicates when the form drag of the hull starts to rise dramatically with the result that you can't go much faster than "hull speed".

However, slender hulls like kayaks routinely exceed the hull speed. 17' ICF sprint kayaks (hullspeed 5.5 knots) go 10 knots. Not all kayaks are "slender" and some are more slender than others, but the upshot is the sailboat "hull speed" formula doesn't provide a very useful judgement on the speed of a kayak.

That said, the Mystery has much higher speed potential than the Petrel. The Mystery creates very little wake and as such it can more easily exceed the theoretical hull speed. It creates very little wake because it is quite narrow at the waterline. A result of this is it is quite unstable.

Now, time for my discussion about fast kayaks. There is no such thing, only fast paddlers.

Imagine two kayaks: a "slow" one with 1 unit of drag at 4 knots and a "fast" one with 1 unit of drag at 6 knots. If the paddler powers both kayaks with 1 unit of thrust it would be logical to believe that it is just as easy to make the fast one go 6 knots as it is for the slow one to go 4 knots, after all it just needs the same amount of thrust.

This obvious, logical conclusion is wrong. Because the faster boat is moving faster, the paddler has to work harder. After paddling one minute the faster kayak will require 50% more work because it has gone 50% farther. Going farther requires more effort.

The result is that faster kayaks require more power from the paddler to paddle for a given time span. So, to make a fast kayak go faster than a slow kayak you need to have the physical fitness to sustain the speed. While you may be able to sprint the fast kayak substantially faster, chances are that when you revert to your cruising pace you will end up going nearly the same speed. It is even possible that at your cruising speed, the slower kayak is more efficient due to lower wetted surface area, so you could end up being faster in the slower boat.

A common experience to help understand the faster-is-harder concept is surfing down wind. As you are paddling downwind there may be times when it feels like you can just take off with the help from the waves, so you sprint briefly and you just fly downwind. You are thrilled by how easy it is to go really fast in the right conditions. But, you will also notice that those sprints are very tiring. After a period of "effortlessly" surfing downwind you will be completely exhausted.

So, back to Robert's original question. The Mystery can be much faster than the Petrel, but you may end up swimming because it is unstable, and when you learn to stay upright, be prepared to be exhausted after what should feel like an easy paddle.