"Hey PO which of the three is the fastest? Love my load monster 15.5 but might try a demo 17 to compare it to."
This is a great question, but unfortunately a very subjective one. Having spent a fair amount of time in all three of these Deltas I have a pretty good feel for them, so which one is "the fastest"? To determine that we need to understand what "fast" means in a kayak. A big part of how fast something is, is determined by the motor, and in this case, the paddler is the motor.
I know from paddling my Seventeen with a GPS that I can cruise at 3.5 or 4 knots, and sprint to 5.5. A couple of times I have hit 6, but I can't keep it up for long, and everything needs to be perfect for me to hit that speed. I am actually not a very fast paddler. So that is the first part of the equation, The speed of the boats will of course vary with the ability of the paddler, and I wouldn't be surprised if Gecko paddles faster than me.
The boat of course plays a role, The longer a boat is, the faster it will travel. The reason for this is pressure. Pressure on the hull. You have to propel the boat through the water, and to do that you have to physically push the water aside, as it moves down the hull. The front half of the boat is pushing the water ever further apart until it reaches the midpoint. So at the midpoint of the boat - where it is the widest - is the point where pressure starts to decrease. So the longer the boat is, the more hull you have from the narrowest point - the bow - to the widest point, at the midpoint. So if that distance is longer, the pressure on the hull is less because it is distributed over a larger area. And of course, the wider the boat is, the more you have to press the water apart. Which is why a long, narrow boat is faster than a short wide boat.
So the Delta 15.5 is of course fifteen feet, six inches long, and 24.5 inches wide. The Sixteen is longer and narrower, sixteen feet long, and 22 inches wide. When you are talking about kayaks width, 2.5 inches is a lot. Then you have the seventeen. Longer at 17 feet, but 22.5 inches wide. So if we are talking about absolute speed, my guess is that the Sixteen and the seventeen are pretty close to identical, with the 15 being slower. But we are probably talking about a total difference between the 15 and the 17 of half a knot.
But if you have ever driven a high performance car, you learn that top speed isn't really that important. It is getting to that speed that is the fun part, and if we are only talking about half a knot speed difference, then that just proves that top speed isn't that important. What is important is acceleration. And without a real way to measure acceleration in a kayak, it is all about feel.
When someone gets into a boat, and does a handful of strokes, and says "wow, this feels fast" what they are really saying is, wow, this accelerates fast. And I will say right now, the Delta Sixteen accelerates fast. It is a nimble, responsive boat. I think the fifteen is pretty fast, but definitely accelerates slower than the other two. And the Seventeen is right in the middle.
The Fifteen has been pretty consistently referred to as "the minivan" of the Delta line. It hauls a lot of gear, and for a boat of its width is a lot of fun to paddle. The Seventeen is the classic touring kayak. Seventeen feet is a great length for touring, it holds a lot of gear, accelerates fast, and cruises all day long. the Sixteen is like getting in a corvette. Fast off the line, responsive, with a slightly smaller payload.
So, Gecko, I would definitely demo a seventeen.