The following is my version of Yak Design 101. Believe me, boat design is one part science and one part art but there are certain basic principles that very much apply and that you need to know. I'm learning everyday and just wanted to share. Your comments, experience and ideas are very much welcomed. We're in this together!
Factor #1: Speed
You will often hear that a boat is "fast" or "slow". The number one factor, by far, is something called LWL or waterline length. This is NOT the overall length of the boat (or LOA) but the length of the boat that is IN the water. An excellent example is the Necky Dolphin with it's swoopy bow. The overall length is about 14 ft. But the part that's actually in the water is only about 12-1/2 ft. (waterline length).
The longer the waterline, the faster the boat (potentially). Period. Just that simple. All other things being roughly equal, don't let anyone tell you their 12 ft. yak is as fast as a 15 footer. Just ain't so.
However, there is another related factor called "wetted surface". This is simply how much yak is in the water. Let's take two 14 ft. yaks. One is wide (wide "beam" or BOA), the other narrow. The wider yak has more surface area (BWL) and there is more yak in the water. More area means more friction and the yak with more wetted surface will have more drag to overcome.
Again, all other things being roughly equal, a longer yak also has more wetted surface.
Here's the hard part. Yes a longer yak is POTENTIALLY faster, but you (the paddler) have to be able to overcome the friction of the water so it CAN go fast. A rough rule for hi-tech touring yaks is that although those longer than 17 ft. can go faster, few paddlers have the strength and power to overcome the tremendous friction.
And this factor - wetted surface friction - affects all yaks. Go try a Tarpon 16 and you'll see what I mean. Yes, it can go faster, but it's a little like a train getting started. Slower and takes a lot of power. And to go at top speed you have to work more.
So here's the rule:
Longer mean potentially faster but also means more friction and more work by you. How does this relate to yakfishin? Well, most yakfishin does NOT require great speed. Most yakfishers paddle at moderate speeds where yak length isn't nearly as important. Where a 12 ft. yak can keep up with the 16 footer, and with a lot less effort.
Let me quote a leading designer "...at normal touring speeds a longer yak may actually end up moving slower than...a shorter boat with less wetted surface area".
There's another problem with potentially fast long yaks. They are hard to turn, and in yakfishin that is a BIG problem. Which brings us to...
Factor #2: Rocker
Rock and roll baby. How sweet it is. Rocker is the front to back curve of the bottom of a yak. Some yaks are literally flat, won't "rock" (rocker) at all. Others are like a teeter-totter. And it's easy to check. Just put the yak on a flat surface and take a look. See how high the bottom is off the ground at the bow and at the stern. Go to either end and push down, "rock" the yak end-to-end.
So what good is rocker? Simply a yak with rocker will be a lot easier to turn; let me try to explain. Let's say a yak is totally flat bottomed. Go to the front and push it to the side, to try to spin the yak around it's middle (which is what happens when the yak turns). Because the yak is flat the entire side of the yak must move through the water.
Regarding a yak WITH rocker, the ends of the yak are out of the water, in the air. When you try to spin or turn the rockered yak, only the center part of the yak must move through the water (with the ends sticking out). This takes much less force so the rockered yak "turns easier".
Rocker has another advantage. When a wave comes at you, the rockered bow, which is already in the air, tends to go over the wave (rather than through it). The wave tends to roll smoothly under the rockered yak, not smashing the bow, or throwing the yak around.
So in rough water, good rocker helps you go over the waves, maintain your speed or go faster. The flat bottomed yak gets belted, pushed around, stopped in its track.
Factor #3: Beam or width (stability)
A lot of new yakkers think wider is better. Roomier and "stable". If they only knew. All yaks are stable, but there are really two kinds of stability: primary and secondary. When you first sit in yak you experience primary or intial stability, or lack of "tippiness".
A wide flat bottomed yak would have extreme primary stability. Almost no tip at all. But at some point, and especially in rough conditions, flat yaks WILL capsize and will do so without warning. This is why most good yaks avoid flat bottomed designs.
Secondary stability is what keeps you from capsizing. A great example of this is the Necky Dolphin or Spike. When you first get in, these yaks feel tippy (low primary stability). But as you lean you find that the sides of the yak come into play and resist further lean and capsizing. At that point the yak "hardens up".
Remember this....a barge is designed with tremendous primary stability, is designed to stay level to carry things, not to tip. Good yaks are SUPPOSED to tip and to allow a certain amount of leaning (just like a bicycle). Most yaks are equipped with eyelets placed so you can use thigh straps to better control the lean.
Thus, a "tippy" yak is actually much safer, more seaworthy, turns better and easier. And you WILL get used to it. Just like learning to ride a two wheeler. Remember a tricyle doesn't lean and is a lousy performer.
Oops almost forgot. Width creates a couple more problems. First, it increases wetted surface and makes paddling harder. The yak has got to move more water sideways around it's "belly". Width forces you to buy a longer paddle and to paddle wider (more horizontally), forcing you off track.
Factor #4: Weight
Some yakkers equate weight with strength or solidity. It's just not so. Yaks with large flatter areas tend to "oilcan" and bend, while those with complex curves are rigid and stable. The Scupper Pro and Prowler are excellent examples of bulletproof rigid designs that are also relatively light (55-58 lbs.). The heavier Tarpon 160 (67 lb) or Malibu Extreme (80 lb). have more flat surfaces
and are more "bendy".
The point: don't be swayed by weight, and remember that long yaks much over 55 pounds or so are significantly harder to transport and load.
Factor #5: Prismatic Coefficientt
Don't panic at this term. This refers to the "fullness" of a design when looking down on it. Say one yak is of medium width in the middle and remains fairly "fat" at the ends (a "full" yak). Another is the same in the middle but very sharp and narrow at the ends (a "narrow" ended yak).
A "full" yak is said to have a high prismatic coefficient. A lot of fishin yaks are designed in this "full" manner so they can offer a wide space both up front and near the stern for storage, large hatches and wide tankwells. But what does this fullness mean in terms of performance?
Oddly enough a "full" yak can potentially go faster (has to do with the distance between bow and stern waves, don't ask). But as before, only if you have exceptional strength to overcome the extra friction of that big bow and stern. Most yakkers don't.
The "narrow" ended yak is potentially slower, but at moderate speeds may actually be faster simply because the narrow yak takes less force to paddle. At low and medium speeds the narrow ended yak has much less friction, doesn't have to push so much water out of the way, and is a joy to paddle.
The big "full" yak gets to feel "heavy" and gets progressively harder to paddle at all speeds.
Caveat: An ultrafine and narrow ended yak has a tendency to "dig into" waves, especially in surf or big swells. The Tarpon 160 has this problem. Better designs can still have fine ends but add "flare" (get wider) toward the deck, and "rake" (long extended "dolphin" bows) to provide extra buoyancy and prevent "digging in". Example: Dolphin, Prowler.
Factor #6: Deck Design
Most yakkers think of the deck as just a place to mount or bungee things. Thus they look for flat decks. However the deck is very important in providing hull rigidity and strength. A flat deck may permit the yak to actually bend ("oilcan") and twist.
Notwithstanding the need for "places" to mount things, look for complex curves. Great example: the Scupper Pro. Look at all the modest curves, ridges and edges over the entire deck and cockpit. Add to that the curved sides and hull and you have a light but extremely strong and rigid yak that will resist deformation.
Compare especially to the Mars which exhibits broad flat areas. These yaks may twist or deform, require care in storage and transport. Extreme twist can even cause hatch leakage.
Factor #7: Seat Height (aka "wetness")
So many yakfishers seek "dry" yaks. In northern climes that may be an issue, but it's really a false one even there. I won't bore you with the numbers but when you raise your 150-220 lbs. an inch or two you have dramatically increased the chance of capsize. To quote a leading designer:
"Lowering a kayak's seat an inch or two can result in a dramatic increase in stability. All the weight in a kayak should be placed as low as possible, including the kayak's and your seat."
According to one leading designer, most yakkers will notice even a half inch difference. This graph was for a 200 lb. yakker sitting at the waterline who'd normally capsize at a 30 degree tip. Raise the seat just one inch and he'd capsize at 24 degrees, two inches and you're in the drink at just 17 degrees!
The area under the curve represents general stability. Look how fast it disappers. Raise the seat two inches and you have almost none!
I'm not kidding. The difference between a wet yak and a dry one is just an inch or two. The difference in stability is noticeable. My Scupper is very wet and very stable. Sue Sea's Scrambler is dry, high and noticeably less so. The Scrambler XT addressed this issue and is wet. And much more stable.
One day Sue Sea added another inch to pad her butt and we got caught in a squall. She went down without warning in conditions that were wet and windy, but not extreme. Said "it just went out from under me".
Enough for now. Please know two things:
1.   All these factors are related and affect one another.
2.   Any factor (like speed or primary stability) carried to an extreme will cause degradation in other related factors. Example: a long "fast" yak will be very hard to turn. A very short and maneuvreable yak will be much slower. A very flat yak will be very unstable, even dangerous in rough water, hard to turn and noisy.
3.   Do not be swayed by any one factor. These are general principles and all yaks are compromises. The key
is matching your needs with key factors.
My bias is toward an all around design for our Southeast Florida waters. These range from ocean surf, swells, bays and chop, 10-20 knot winds, fast running inlets, narrow mangrove channels and quiet flats. I exclude special purpose yaks, e.g. for fly fishing, standing or motoring.
Due to the wide ranging conditions and waters in South Florida, our fishing yaks require compromise among these many factors. I have come to call this the Magic Compromise:
1.   long enough to have reasonable speed
2.   light enough for easy handling out of water
3.   rocker enough to turn well
4.   storage enough, and accessible enough
5.   minimum important features (like tankwell, console, etc.)
6.   deckspace enough to add things
7.   good primary and better secondary stability
8.   complex curvature and rigid design
9.   rugged and solid
10.   minimal hull slap (quiet)
And made by a quality manufacturer with plenty of experience, that will hold it's value and that is widely available both new and used. Buy a bad design and you will pay - over and over. Get lucky with a good one and you'll pay homage to the yakfishing gods.)