Hull Fins: What Flex Actually Does on a Displacement Board and How to Buy One Without a Number

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. The fin gets out of the way.

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.

The variables

Flex, the primary spec that nobody publishes

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 center, 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. True Ames' glossary names the two quantities that would settle any argument — "how much force it takes to flex the fin (i.e. how stiff the flex is), and how quickly and reliably the fin returns to center" — and then publishes neither, for anything.

We checked. Across 466 True Ames products there is not one numeric flex figure — no index, no scale, no deflection measurement. Forty-five of the 105 fins in their single-fin collection describe flex in words; none in numbers. And it does not stop at flex: not one of the 105 carries a spec block. True Ames prints HEIGHT: / BASE: / AREA: on 161 of the 228 FCS- and Futures-compatible fins it lists outside that collection — and on none of the 105 single fins. No single fin publishes an area at all, and five state a base in a sentence of prose — the D-2, the Derrick Disney Farm, the Troy Elmore Flex, the Fantastic Acid Wide Base and the Fantastic Acid Vee Bottom. That is the category's published geometry, entire. Otherwise you get one number, and the option is literally labeled Size.

So a fin with "the right amount of flex for a hull" is someone's account of how it felt. That can be excellent information. It is not a specification, and anyone quoting you a number made it up.

Base and section: flex is not one thing

What you can read is where the area sits and how thick the panel is, because those produce the flex. Hull fins are supposed to be low and wide; our own shelf falsifies that. Two ways to build a fin that bends, and we stock both.

Wide base, thin section: area low and broad, panel 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 the second one 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."

Tip flex and body flex are not the same purchase, and knowing which you are buying is most of the decision. Which is where the copy stops helping. Take the L-Flex (Volan) page. Two sentences above its feature list it says the Volan panel gives "flex coming more from the body rather than just the tip" — a line True Ames prints on fifteen of its Volan listings, so it describes the panel and not this fin. Then the feature list says "Thinner style precision foil with optimal flex," which appears on exactly one product in their catalog of 466: this one. The specific claim and the generic claim point opposite ways, on one page, about one fin, and the specific one is the tip.

So the fin is foiled thin, and the panel is what moves some of the bend into the body. That is the only thing about flex on that page you can rely on, and you had to read two lines against each other to get it.

What the tail is doing, which sets how much hold you want

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 first thing to establish because it is the difference between two opposite recommendations on the same length of board.

A hull entry that runs out to a flat tail has released the back of the board on purpose. Nothing back there is generating hold, so the fin supplies it, and a stiff 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 channeled and the tail holds on its own — and a fin chosen for maximum hold on top of that stacks hold on hold. The board feels locked through the first half of a turn and then lets go all at once rather than releasing progressively, which is the specific complaint that sends people back to the shop.

This is our experience advising hull riders rather than 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, not by reading a product page. Establish it before you read the roster. The decision section has a branch for both, and they do not recommend the same thing.

The box, which is your board's and costs nothing

Every single fin here has a US Universal Bahne base, so it slides. That is a property of the box routed into your board, not of any fin below, and the cheapest adjustment in surfing. Our mid-length post sets the direction out: 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 one 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, and this is why: one adjustment costs nothing and the other costs ninety dollars. Use the free one first.

Material, and what the Volan actually costs

Volan is a glass layup that flexes from the body rather than only the tip, which is why so much of this roster is Volan-only; on the L-Flex it costs $10.00 to $12.75 more than the clear or smoke at the same depth. Against the seafoam grey the gap is wider and it widens with size, because the seafoam is flat-priced and the Volan is not: $12.75 at 7.5", $18.75 at 8.5", $27.75 at 10". The Volan argument has its own post — its title quotes $30, which is not today's price on any template we stock in both.

The myth: that a hull fin is 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 roster

Prices are ours: the roster was taken 2 August 2026, the Skip Frye and the Volan comparison 7 August. One dimension appears below — one of the five base figures in True Ames' single-fin collection.

Drawn for a displacement hull

L-Flex — $89.75–$107.50, 7.5" to 10". Greg Liddle designed it in 1968 for the "modified displacement hull" boards he pioneered, and True Ames has made it for "more than 40 years." If you own a hull, this is the answer. One thing worth knowing 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 $89.75 in seafoam against $107.50 in clear — $17.75, or 16.5%, for a color. It comes in six straight sizes and two wider W sizes. The L-Flex (Volan) is the same template on the Volan panel, $102.50–$117.50.

PHD (Volan) — 9", 9.5" and 10", $111.50 to $117.50. Built for hulls with "thin rails and vee to flat bottoms," and the only fin in this roster True Ames calls stiff: full base, thick leading edge, narrow tip. Buy it for down-the-line waves with push. Wrong fin for a gutless summer point — more fin than a knee-high day needs, felt as work rather than glide.

Fantastic Acid PMH (Volan) — $117.50, 9.9", one size. Tristan Mausse's own fin, narrow-based and 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

Fantastic Acid Vee Bottom (Volan) — $117.50, 10.15", one size, and a 9.25" base, "the maximum width compatible with a standard fin box." A homage to the 1960s Wave Set fins, drawn for vee bottoms and transition-era boards. Not for a Liddle-style hull; the base alone will overwhelm it.

Channel Islands Spicy — $92.75 at 8", $95.75 at 8.5", $99.50 at 9". Made with Devon Howard and Britt Merrick for CI's Tri Plane Hull, a mid-length blending "a Greenough-inspired planing hull together with Al Merrick's egg outline." Upright, slender, conservative rake, and drawn for a bottom that is part hull rather than all of one. There is also a Volan version at the same three depths, $105.50, $108.50 and $111.50.

Hobie DP Flex (Volan) — $102.50 to $111.50 at 7.75", 8.25", 8.75" and 9.25". Dana Point, medium body, "somewhat upright with a gentle amount of rake," drawn for mid-lengths and point surf. True Ames tags it hull; its own copy never mentions a hull. Read the copy, not the tag — the same list takes in the Greenough 4-A and leaves out the L-Flex (Volan).

Skip Frye — $80.75–$103.50, 6" to 9.25", and 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 arrived at from a different direction. Also in Volan, $96.75–$111.50, at four depths. Note the ladder: it goes 6.5" then 7.5" — there is no 7", in either construction, and it is the size people ask for.

Flex fins that get recommended for hulls and shouldn't be

Wilderness Flex — $89.75, 7.6", six colors. Bob Duncan drew it for Wilderness Surfboards, the Santa Barbara label founded in 1966, as "the perfect complement to stubby-style designs and fun shapes." True Ames calls it "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.

Josh Farberow Pro Model (Volan) — $111.50 to $117.50, 9" to 10". A longboard fin, tagged longboard, developed from the Farberow Flex by "increasing both the base width and rake" for noseriding. Buy it for a log that wants spring. It has nothing to do with hulls.

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. Our Greenough guide puts a 4-A at 8"–9" beside the L-Flex, and in the shop the 4-A 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 the 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. Move it in the box before you move on from it. Take the seafoam grey: same template, 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 is generating its own hold, so the fin's job shrinks to directional stability and the roster's stiffest options 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, and 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.