Put a stiff, upright performance single fin in a displacement hull and the board feels braced. You lean on a rail, the hull starts drawing the arc it was shaped to draw, and the fin holds a straighter line than the rail is asking for. The turn begins and stalls halfway.

Nothing is wrong with that fin. It holds the tail steady while the rider turns from the tail, which is what it was drawn to do. A hull does not turn from the tail. It is answering a question your board never asked.

If your board is a mid-length with no hull in it, stop here — that post is already written, and it covers size and box position on a flat-to-vee bottom. Stay if yours is convex-bottomed, a vee bottom, a transition-era shape, or a modern mid-length drawn off a hull.

The rail does the turning

True Ames describes the bottom on Greg Liddle's boards better than most manage: it "has a convex bottom contour which enables riders to trim more forward and find plane in waves with less power." Rounded across its width, the board sits into the water rather than on top of it and rolls rail to rail with almost no effort. That rolling is the steering: bury a rail and its curve, working against a bottom already displacing water, sets the radius.

Which leaves the fin one job and one prohibition. The job is directional stability: stop the board sliding sideways off the rail line, and give it something to project against on the way out. The prohibition is that it must not resist the roll — a fin stiff enough to hold a tail through a tail-driven turn locks the board onto the line it is on.

Flex is the spec that matters most, and the one with no number on it

Flex is how a fin resists sideways slip without resisting the rail, which is why it is the primary spec here and a footnote everywhere else.

A flexing fin loads under the pressure of a turn and bends away from the flow, so while loaded it fights the rail less than a stiff fin would. Returning to centre it gives that energy back as drive where a hull is slowest — the bottom of the turn. True Ames puts it flatly: "Flex fins have slightly delayed responsiveness that ends up providing unequaled projection and acceleration."

Delayed is the word to keep. Quick response is a tail-driven fin's virtue and a hull's problem.

Now try to buy on it. Two quantities would settle any argument — how much force it takes to bend the fin, and how quickly and reliably it returns.

We went looking for either one. Across all 466 True Ames product pages, the number of fins publishing a numeric flex figure is zero. Eighty-one of them use the word "flex" in prose. None of them puts a number on it.

So everything this post says about flex is our judgement, arrived at from outline, foil thickness and the maker's own wording — not a measurement. Anyone quoting you a flex rating made it up.

Tip flex and body flex are different purchases

What you can read is where the area sits and how thick the panel is, because those produce the flex. And the received wisdom that a hull fin must be low and wide is wrong — there are two ways to build a fin that bends.

Wide base, thin section: area low and broad, foiled thin so the tip gives first.

Narrow base, thicker section: narrow from base to tip with plenty of rake, and the foil has to be left thicker, which moves the bend into the body. True Ames spells that out on the Fantastic Acid PMH — "Due to the narrowness from base to tip, a slightly thicker overall foil is required to enable flex into the mid-body of the fin."

Knowing which one you are buying is most of the decision. The L-Flex in Volan is both at once, and worth working through: the Volan panel carries some of the bend into the body, while the thinner precision foil puts the give in the tip. That combination is why it suits a hull.

Your tail decides how much hold you want left over

Flex decides whether the fin fights the rail. What the tail is doing decides how much hold you want left over — and it is the difference between two opposite recommendations on the same length of board, so establish it before you read anything else.

A flat tail has released the back of the board on purpose. Nothing back there generates hold, so the fin supplies it, and a stiffer full-based fin has a job to do.

A vee tail rolls rail to rail and asks for less.

A double concave tail is already gripping — water is channelled and the tail holds on its own. Put a fin chosen for maximum hold on top of that and you stack hold on hold: the board locks through the first half of a turn and then lets go all at once instead of releasing progressively. That is the specific complaint that sends people back to the shop.

This is our experience advising hull riders, not a published specification. No maker prints a hold figure, and the bottom contour of your own board is something you establish by looking at it in the light.

Move it before you replace it

Every single fin here has a US Universal Bahne base, so it slides. That is a property of the box routed into your board, and it is the cheapest adjustment in surfing: forward is looser with less hold, back is more hold and longer turns, and half an inch changes it.

The board that feels stiff off the top and the board that slides out on the drop are often the same board with the fin in the wrong place. Hull riders move their fin far more often than they replace it, for good reason.

What position cannot tell you is what a different fin would do in the same board, and that is the larger lever by a distance. It is also what makes the free adjustment worth more afterwards: once you have felt two templates in the same board on the same day, depth and box position stop being guesses. One fin is an anecdote. Two is a measurement.

The myth: that a displacement hull wants a floppy fin

Ask around and the advice is to buy the flexiest thing you can find. The maker of the fin most explicitly designed for displacement hulls says the opposite, on its own page.

The PHD (Volan) was drawn by Kirk Putnam and Brian Hilbers "to accompany boards of the displacement hull variety." True Ames' explanation of why it is built in Volan: "It would be far too flexy made of any other material, but with Volan it retains a stiff and powerful flex."

A fin designed from the ground up for hulls uses the stiffer panel specifically so it does not bend too much. The target was never maximum bend but a controlled, quick-returning one — the second half of True Ames' definition, and the half a floppy fin fails. Too much flex feels vague, washes out on a steep entry, and gives nothing back because it never loaded properly.

The shortlist, and the fins that keep getting recommended wrongly

Prices are ours, taken 2 August 2026.

Drawn for a displacement hull. The L-Flex, $89.75–$107.50 from 7.5" to 10", is the answer if you own one. Greg Liddle designed it in 1968 for the modified displacement hulls he pioneered, and True Ames has made it for "more than 40 years."

One thing to know before you click: seafoam grey is flat-priced at $89.75 in every size while clear and smoke step up with depth, so a 10" is $17.75 cheaper in seafoam — same template, same layup, different pigment. Also on the Volan panel as the L-Flex (Volan), $102.50–$117.50.

The PHD (Volan), $111.50–$117.50, is the only fin here True Ames calls stiff — full base, thick leading edge, narrow tip, built for hulls with "thin rails and vee to flat bottoms." For down-the-line waves with push. On a gutless summer point it is more fin than the day needs, and it feels like work rather than glide.

The Fantastic Acid PMH (Volan), $117.50, 9.9" and one size only, is Tristan Mausse's own fin: narrow-based, heavily raked, "ideal for convex bottom boards, allowing for deep rail-burying bottom turns." The body-flex school in its purest form, and a lot of fin with nothing smaller to fall back on.

Some hull in the bottom, not all of it. The Channel Islands Spicy, $92.75–$99.50 across three depths, was made with Devon Howard and Britt Merrick for CI's Tri Plane Hull — upright, slender, conservative rake, drawn for a bottom that is part hull rather than all of one. Also in Volan.

The Hobie DP Flex (Volan), $102.50–$111.50, is "somewhat upright with a gentle amount of rake," drawn for mid-lengths and point surf — which is where we have seen it work, and why it sits here rather than in the group above.

The Skip Frye, $80.00–$103.50 from 6" to 9.25", is the one most often left out of this conversation. True Ames drew it "in the mid-1970s to complement Skip's iconic Eggs and Gliders" with "minimal rake, and smooth, controlled flex for effortless trim and flow."

Eggs and gliders are forward-trim boards, which is the same problem a hull sets, and minimal rake with controlled flex is the same answer reached from a different direction. Also in Volan. Note the ladder goes 6.5" then 7.5" — there is no 7", in either construction, and it is the size people ask for.

The Fantastic Acid Vee Bottom (Volan) is a homage to the 1960s Wave Set fins, drawn for vee bottoms and transition-era boards, on a 9.25" base — "the maximum width compatible with a standard fin box." That base alone will overwhelm a Liddle-style hull.

Two that get recommended for hulls and belong elsewhere. The Wilderness Flex, $89.75, is described by True Ames as "slight flex" — on a fin named Flex, that tells you where it sits. A lot of rake on a small area is what drives a stubby down the line; in a hull, that slight flex braces against the roll instead of giving with it, and 7.6" is short of the depth anyway.

The Josh Farberow Pro Model (Volan) is a longboard fin, developed from the Farberow Flex by "increasing both the base width and rake" for noseriding. Buy it for a log that wants spring.

The Greenough 4-A is not in that group, whatever its reputation as a fin for everything. Its copy claims it "works extremely well on just about every type of surfboard" — not a hull claim, and not a reason to dismiss it either.

In the shop it is the fin most people arrive with, and the most useful thing to reason from: ride the one in the board, work out which direction you want, then buy the fin that goes that way. It runs from 6.5" to 10.5", so whichever direction you choose is offered in the same template.

How to choose

A classic Liddle-style displacement hull. L-Flex at 8.5" — $89.75. The fin Greg Liddle drew to go in those boards, in the middle of its own range, and it agrees with what our Greenough guide tells hull riders. Start it central and move it from there — the box is free and it will teach you what this template wants. Take the seafoam grey: same fin, and the one colorway that does not charge you for depth.

A flat tail, and real waves — you want the fin supplying the hold. PHD (Volan) at 9" — $111.50. The 9" is the one to have; the 9.5" is the next step up.

A concave tail, or a board that already holds and turns stiff. Your tail generates its own hold, so the fin's job shrinks to directional stability — and the stiffest options here are the wrong answer.

What you want is a different outline, not simply a shorter one: minimal rake, so the fin stops adding hold behind the base. Start with the Skip Frye at 7.5" — $89.75. Controlled flex on a clean outline, and the shape that lets a tail already gripping do its own work. There is no 7", so 7.5" is the size to take, and 6.5" is the step down if it still feels locked.

A bottom that is part hull — a Tri Plane Hull or similar. Channel Islands Spicy at 8.5" — $95.75, designed for that exact board with the shaper in the room. If there is less hull in it than that, take the Hobie DP Flex at 8.25" — $105.50, drawn for point surf rather than for a hull, which is the point.

You know you want the bend in the body rather than the tip. Volan alone does not get you there — the PHD is Volan precisely so it bends less.

What you want is a narrow base with a thicker foil, and one fin here is drawn that way and says so: Fantastic Acid PMH at 9.9" — $117.50, "narrowness from base to tip" with "a slightly thicker overall foil… to enable flex into the mid-body." One size, so it is a commitment rather than a step — a second fin rather than a first.

If you want to move that way without committing, the L-Flex (Volan) at 9" — $111.50 is the template you already know, on the panel that flexes further down. Buy it for the panel. The page's one claim about this fin's foil is the feature bullet quoted earlier, and it points the other way.

A shorter hull, where you just want less fin. Not the concave-tail branch: that one changes the outline, this one keeps it and drops a size. L-Flex at 7.5" — $89.75. Take this one because you want less fin, not because it costs less than the 8.5" — in seafoam grey the 8.5" costs exactly the same.

Whichever you take, the fin drawn for those boards is still the L-Flex, and 8.5" is the middle of it. Everything else here is a reason to deviate. Know your reason first.

Sources

True Ames pages, 2 August 2026: https://www.trueames.com/pages/how-to-choose-the-right-fins, https://www.trueames.com/pages/single-fins, https://www.trueames.com/collections/single-fins. Prices from our catalog, 2 August, with the Skip Frye and the Volan comparison re-pulled 7 August. Counts from the full products.json feeds; working on file.