Surfline's resident design-o-phile Nick Carroll responds:
OK -- you've pinned WK down.
Imagine water encountering a surfboard. First -- before it comes into contact with the board -- it's free to move. Then, bang! It runs into, or is run into, by the board. Friction slows the water's passage beneath, around and past the board (hydrodynamic tests show the water moves slowest halfway along the object with which it's come into contact). Then, as it approaches the tail, the water sniffs freedom. Yesss! It accelerates out and away.
Here's what that tells us: water is moving at its quickest at the beginning and end of its journey past our board. And thus, the board's outline is most like to have its greatest effect on waterflow at those beginning and end points.
Thus! The tail shape is a seriously important piece of the surfboard's overall design.
Here's a very crude series of explanations of the effects of the basic tail shapes on waterflow, and how they affect a board's performance.
a) The SQUARETAIL. Water moves straight off the rail line, which is cut off, boom, just like that. It also moves abruptly straight off the square edge behind the back fin. The squaretail gets rid of a lot of water very quickly, which makes it a very fast tail shape; but since the water's all dumping off a straight edge with no modulation of the flow, the squaretail tends to turn awkwardly in "jumps" from rail to rail, with little subtlety in between. The squaretail also permits a wider tail, preserving volume well behind the forward fins, which helps flotation. WK believes this shape suits a beginner surfer or one who's experimenting with short, wide boards for small waves.
b) The SQUASHTAIL. This is an adapted squaretail, with some of the benefits of quick release and width for flotation, but the rounded curves off the rails provide some subtle modulation in turns. A "tradeoff" tail, well suited to a wide range of surfers in most waves between two and eight feet.
c) The PINTAIL, or round pin. Water moves off the tail and rails in a diffuse fashion, in a wide range of angles. Since it has a lot of options in that nice curve toward the final pin, the water takes more time to break free. Thus the pintail doesn't have the raw acceleration of a square or squashtail, but what it lacks in off-the-mark speed it gains immensely in control and subtlety of turning angles, and the generally narrower immediate tail area fits better into barrels than either square or squash. A common tail shape among elite pros, particularly in surf over six feet.
d) The SWALLOWTAIL. Referred to as a "double pin" by at least one of its pioneers (Bob McTavish), the swallow blends a square's instant rail fall-off with some of the pin's tube-fitting and turning subtlety. Swallows release concave bottoms better than most other tail shapes and also allow a wider tail shape, which makes 'em popular among some high performance surfers in the nitro-small-wave hotdogger range.
e) The DIAMOND is a largely superseded attempt to soften the square's hard edges and preserve some of its release speed. Its job is now done more effectively by the SQUASH.