You are almost certainly surfing on fins somebody else chose, in a factory, to hit a price. The set in your shortboard was specified by a person who has never seen you surf and does not know what you weigh.

It is the only part of the board nobody picked on purpose, and the cheapest one to change.

What the third fin is for

Two fins near the rails hold on rail. Put the board over, the outside fin loads, the inside fin unloads, and you go around the arc. That works and it is fast, because nothing in the middle is making drag.

It fails in one specific place. When the tail is unweighted and the board is past vertical — coming off the lip, dropping late into something steep — nothing behind the rail fins stops the tail swinging through. The board rotates further than you asked. That is sliding out.

The third fin is not there to add grip. On rail it is barely loaded. It is there to resist rotation, and because it sits furthest from where the board pivots, it does that with more leverage than its size suggests. It does almost nothing until the exact moment you need it.

You pay going straight, where it sits in clean water making drag. Every thruster set trades trim speed for a tail that stays put. So the question is never whether to run three fins. It is how much fin, and where the area sits.

The thing nearly everyone has backwards

Your rear fin is almost certainly the same size as the front two. Not smaller, not a stabiliser, not a trailer — same template, same size.

True Ames says so outright on three of the sets we stock. The TA Thruster: "These 3 fins are all the same size." The CI Tri in Medium and in Large: "All three fins are the same size."

The confusion comes from twin-plus-trailer, where the third fin genuinely is about half the height of the other two — a different arrangement solving a different problem. A trailer is a safety net bolted onto a setup designed without one. A thruster's rear fin is load-bearing.

Which matters because four sets do run a smaller rear, and each is making an argument. The CI Tri Small, the Skip Frye and the Mackie are buying release at the tail — True Ames on the CI Small: "the smaller back fin loosening up the tail of your board."

The Power Trailer Tri is doing the opposite. Its rear is smaller in area but far more upright, and an upright rear catches the tail later rather than letting it go. Less rear is not one thing.

Two numbers, not four

Every set gets described with the same three words — drive, speed, versatile — and none of them tell you how much fin you are buying. Two numbers do.

How much fin in total. From the Boomerang at the small end to the Mackie Mini Keel at the large, the big one is 58% more fin than the small one. That is not something you adapt to over a few sessions.

More area holds harder, drives harder off the bottom, and takes more force to break loose. Less area releases earlier and accelerates in weak water — and lets go sooner when the wave has power in it. Heavier surfer, heavier waves, more fin.

Where the area sits. Height divided by base. Around 1.0 is the modern default — roughly square, moderate rake. Higher and the fin is upright: tighter pivot, less base to drive off. Lower and it is keel geometry: drive from the base, hold from the length, and a turning radius you do not get to choose.

Almost every set in the line sits in that 1.0 band. Two do not, and those two are the real choices in this collection rather than variations on one: the Boomerang, distinctly upright, and the Mackie Mini Keel, the lowest-slung fin True Ames puts in any three-fin set.

Everything else is close enough that nobody has ridden two of them back to back and correctly named which was which from feel alone. Nine of the eleven configurations we stock live inside a 17% band of total area. So pick on the two numbers, then stop.

What is not a lever. Front-to-rear ratio barely moves across the line, because shrinking the rear much removes the thing you added it for. Foil is a default rather than a choice: asymmetric rails, 50/50 rear.

Construction changes feel, not template — Hexcore is lighter and gives more through the tip, solid fiberglass is stiffer. And cant and toe are cut into your board's boxes by your shaper, so no fin you buy can have them.

Which one is yours

You are riding the plastic fins that came in the board. TA Thruster in Futures, in the size that matches what you pulled out — keep the box, it is the only size reference you have. On FCS, the single-size version at $128.50, where you match the template instead. Medium body, medium rake, no strong opinion — and matching what came out is exactly the job it is best at.

Standard shortboard, decent waves, and you want to keep snapping. Timmy Patterson Tri — moderate rake, so the fin pivots rather than drawing the turn out. Hexcore at $123.50 if you are light or the waves are soft; solid fiberglass at $128.50 if not.

Hollow, powerful, and you want hold. CI Tri Large — the most fin of the same-size sets, high rake, and the one True Ames points at step-ups and guns. In solid fiberglass either way: Futures — $128.50 or FCS — $128.50.

Heavier surfer who keeps sliding out. Same answer — and check size before template. CI Small to CI Large is a bigger change across the set than switching designs is.

Small, weak, gutless waves. CI Tri Small in Futures solid fiberglass at $128.50 on a shortboard — the smaller rear is there to let the tail go. On FCS the Small is Hexcore only — $123.50.

Mid-length or glider, and what you want is trim speed rather than looseness. Boomerang Tri — the least fin in the line and the upright one: Futures — $128.50 or FCS — $128.50.

Mid-length, egg or hybrid you surf off the front foot. Skip Frye Tri in Futures — $128.50. Fronts slightly larger than the rear, which True Ames describes as "a tad more forgiveness without any loss of control." Not sure your board counts? The mid-length post sorts that out — it answers for a single-fin box, so come back here if yours takes three.

Fish, groveler, anything with real width behind your back foot. Mackie Mini Keel Thruster — the low-slung one, and nothing else here is drawn for that board. FCS — $128.50 or Futures — $128.50.

Price is deciding it. KC Tri in FCS solid fiberglass — $105.50. A middle-of-the-ladder set at the bottom of the price list.

The board reached you with no fins in it. You want a measurement rather than a box to match, and which one depends on your boxes. On Futures, the CI Tri Medium in solid fiberglass at $128.50: True Ames publishes the same per-fin area for it as for the TA Thruster, so it is the same amount of fin with a number attached. On FCS, the TA Thruster at $128.50 — same figure, no size to choose. Do not hunt for a CI Medium in FCS: that size is Hexcore in that system.

Whichever you land on, the set matters less than the time on it. A month in one set teaches you more about what you want next than three sets in a season, because only one of those is a comparison.

Bring the board in and we'll look at what came out of it.

Related reading

Sources

All dimensions from True Ames product pages, retrieved 2 August 2026: https://www.trueames.com/collections/tri-fins-thruster. Prices and availability from our own catalog on 7 August 2026. Ratios, totals and aspect figures are computed from those published areas. Simon Anderson's dates from https://en.wikipedia.org/wiki/Simon_Anderson. Full working, and every figure we derived, on file.