Surfboard Fins: Thruster Vs. Quad: When and Why
If your board has five fin boxes, you've got the option of running a thruster or a quad fin set-up (or even a twin + 1, but that's another article). Knowing when and why to use each results in more fun for you!
In broad terms, the differences between thrusters and quads are thus:
Quads: more speed and less forgiving. Great for small and/or slower waves, or bigger, clean waves where outracing sections and holding an edge are key. Placing the trailing fins farther back and closer to the stringer makes a board feel more stable and predictable, which is why you'll see this on big wave boards.
Thrusters: Slower, but more forgiving for vertical surfing and snappy turns. Great for top to bottom surfing in steeper, small-to-medium waves, and big waves with bump and/or wind complicating things.
A big difference is that center fin. If it's there, you've got more drag, limiting your top speed. However, you've also got a pivot point, and that fin will keep your board engaged through tight turns, especially in the most critical part of the wave. Without the center fin, and with added drive from the extra fin area of the two trailer fins, you'll go faster, in just about every situation, but your board won't feel as planted and stable through turns in steep sections. Quads are great for maintaining speed in long, drawn-out, on-rail turns.
Big Waves
It's common to see quad set-ups on big wave boards, almost more so than thrusters depending on the wave and the day. Here's a paraphrase of what Peter Mel has to say about quads and thrusters for big wave surfing, but the principles hold true for the small stuff too—Quads will build speed quickly so you can get in and get going while the wave sets up, and then have the faster top speed once you've set your line. Thrusters will help point your board where you want to go when knifing in, on late drops, or in bouncy conditions. When it's bouncy, choppy, or you are otherwise thrown off your line, thrusters have more of an "autocorrect" effect that's helpful.
Small/Performance Waves
Quad groveler and small wave performance boards often place the trailing fins closer to the rail and the front fins for a looser and snappier feel. Think about riding a skateboard, pretending to go down the line on a wave. Those flicky, tail-slide off-the-lip "snaps" you're doing feel a lot like the quick turns you can do on a quad with a tighter fin cluster for small wave surfing. In this case, a quad will break free easier and feel livelier than a thruster because there's no "anchor" at the tail.
As the waves get a little steeper, however, a thruster will enable tighter, harder changes of direction. Since there is less fin base, the single leading fin of a thruster is a tighter pivot point compared to the two fins of a quad. Also, the thruster's center fin keeps the board from sliding in a tight-arc turn, where a quad is more apt to spin out. However, for edge hold in the barrel and flat out speed to make long sections, quads have the edge. Quads also love drawn-out turns with a wider arc, maintaining and even gaining speed where a thruster is apt to bog down a bit.
Fine Tuning
Switching up between quad and thruster sets depending on conditions gets the most out of a board. Keep experimenting with fins until you have a favorite quad and thruster set that works for most of your daily surfs. Some boards you'll ride exclusively as a thruster, some as a quad, and some as both according to the waves. Switching fins is an economical way to stretch your quiver. With a different set of fins, you'll find some boards are more versatile than you thought.