You have a single fin board. You need a fin. The internet will give you 10,000 words on George Greenough's genius and the shortboard revolution. This page tells you which fin to buy.
The Quick Answer
| Your Board | Best Fin | Size |
|---|---|---|
| Longboard 9'+ | 4A Volan | 9.5–10" |
| Mid-length 7–8' | 4A | 7.5–8.5" |
| Vee-bottom / Egg | 4A | 8–9" |
| Hull | 4A or Liddle flex | 8–9" |
| Edge board | Power Blade | 8–8.5" |
| 2+1 setup | 4A | 6.5–7" + sidebites |
If you're unsure, get a 4A in your board's foot-to-inch size. It works on everything. That's not a cop-out—it's forty years of proof.
The Fins
Greenough 4A
The default single fin. Full base for drive, tapered tip for flex and release. Works on longboards, mid-lengths, eggs, vee-bottoms, and most single fin boards ever shaped.
Two versions matter:
Standard fiberglass — Mass-produced, 3/8" panel. Gets the job done. Good starting point.
Volan — Thicker panel, hand-laid in Santa Barbara. Better flex pattern, more drive and response. Worth the upgrade if you're riding single fins regularly.
The 4A doesn't get in the way. It lets you feel the board. That's why it's been the best-selling single fin for four decades.
Power Blade
The specialist. Completely different animal.
Narrow base. Stiff leg. Wide "paddle" head that twists side-to-side instead of just flexing tip-to-tail. Designed by Greenough for his edge boards, developed with Marc Andreini and Ellis Ericson over five years of testing.
The narrow base means almost zero drag where the fin meets the board. The twisting head gives you "variable tow"—the tail steps out slightly in turns, then snaps back. Feels more like a thruster than a traditional single.
Works on: Edge boards, flat-rocker mid-lengths, boards designed for trim speed.
Does not work on: Hulls, logs, anything you surf from a forward stance. Forum consensus is brutal and consistent: "Very directional. Lacks drive from forward stance. Wants to be turned from the tail."
If you're not on an edge board, start with the 4A. If you are on an edge board, the Power Blade is the move.
Stage 6
The oddball. Roots in Greenough's windsurfing designs from the 80s.
"Stiff leg" cuts through kelp and chop. "Active paddle" head puts more surface area deeper in clean water. The result is loose—almost too loose as a pure single fin.
Works well in 2+1 setups where the sidebites add stability. Also works on edge boards and some hull-bottom singles. Not a beginner fin.
Size up. These ride smaller than the measurement suggests. If you'd normally ride a 9", try the 9.75".
Sizing
The old rule: one inch of fin per foot of board. 9' board = 9" fin.
That's a starting point, not gospel. Here's what the rule misses:
4A runs small in surface area compared to other fins at the same height. If you're between sizes, go up.
Stage 6 rides even smaller. True Ames recommends sizing up a full inch.
Power Blade is sized differently. Follow True Ames' recommendations based on your board length and type.
Fin box position matters as much as size. Start with the fin centered in the box. Move it back for more hold and drive. Move it forward for a looser feel. Quarter-inch adjustments make a real difference.
What Doesn't Work
Nobody else publishes this part. We will.
Power Blade on hulls: The narrow base and directional feel fight against hull surfing. You need drive from the middle of the board. The Power Blade wants to be surfed off the tail. Mismatch.
4A on short, high-performance boards: The wide base needs rail pressure to engage. If your board has tons of rocker and you're surfing it aggressively off the tail, the 4A can feel sluggish. Look at a smaller, more upright template.
Stage 6 as your only fin: Unless you like a very loose board, the Stage 6 works better with sidebites adding control. Great in a 2+1. Squirrelly as a pure single for most surfers.
Undersized fins on longboards: "I'll just run a 7" fin on my 9'6" for a looser feel" sounds good in theory. In practice, you lose drive in small waves and control in bigger surf. Size appropriately, adjust box position for feel.
The Upgrade Path
Start: 4A standard fiberglass. Learn what a proper single fin feels like.
Upgrade: 4A Volan. Better flex, more drive, more responsive. The thickness and construction matter.
Specialize: Power Blade if you're riding edge boards or want to push single fin performance toward thruster territory.
What We Stock
We carry the 4A, the 4A Volan, and the Power Blade in various sizes.
Not sure which one fits your board? Bring it by the shop. We'll match the fin to what you're actually riding.
Sources: Swaylocks archives, Jamboards forum threads, True Ames product documentation, Marc Andreini's Power Blade documentation, and years of fitting fins to boards at 4051 Judah.