Right-Size Your Fin

Right size your #SUP #fin, a #finsciences post about #standup #paddleboard fin size, #paddling ease, speed, and drag. @finsciences
by Doug | Jul 19, 2014 | Blog, Surfboard fin science | 0 comments

How Much Fin Are You Dragging Around?
“How much fin do I need?” is a question I am often asked by standup paddlers looking for more performance.
The answer requires looking at a number of factors that go into SUP and surfboard fin design—especially fin drag. Drag is the hold-you-back force of a fin that you have to overcome with every stroke, the force that slows you down and tires you out.
But fin drag isn’t the only consideration. If it were, you’d simply remove the fin, and voilà, no fin drag! But if you took off your fin, your tail would kick around with every stroke–it’d be hard to go straight, and hard to turn. So you need at least some fin. But how much?
This post addresses that and other questions, such as why my fins look the way they do. Basically I chop off about 30 percent of a fin’s least-efficient area, then add back about 5 percent—repositioned either as winglets or otherwise to maximize efficiency—the same idea as modern airplane wings, boat keels, and race-car spoilers.

WG2 fin in a blue SUP race board, Tybee Island, GA
A Fin Is Part of a System
Some paddlers are happy with whatever fin came with their board, or with whatever their paddling buddies might have.
But some paddlers are looking for more speed, or easier paddling, or longer paddling sessions–or just want to know more about fins and how they work with standup paddleboards. This post is to help understand about optimizing your SUP setup, to ease paddling effort, to get more speed, and to have longer sessions.
Ultimately, this is a question about drag. But more about that in a moment.
How much fin you need depends on a number of factors. When I’m asked “How much fin do I need?” my answer is, “Only just enough fin to track straight when you want to go straight, and to turn when you’d like–any more than that minimum amount, and you’re probably dragging around excess fin area, which saps your energy, your speed, and shortens your sessions.”
A fin is part of a system of board, rider, and fin. And the fin that’s right for one board+rider+system in one set of conditions might be different in another set of conditions. For example, a windy day might require more fin, or paddling in kelp might suggest a fin with more rake.
Not all boards or paddlers are the same. Stronger paddlers pull harder, and need more fin to resist the tail kick.

FSI fins in a quad SUP
Not all fins are the same either. Some fins are more “powerful” shapes than others. More about this below. For now, what I mean by “power” here is a fin’s ability to create hold (sideways force that promotes tracking and resists tail kick) and drive(forward force) as compared with drag (that slows a board down). See also post about hold, drive, lift and drag.
As a general proposition, heavier paddlers probably need more power in their fin than less. Likewise, stronger paddlers probably will want a more powerful fin than less. Lighter paddlers, or those who stroke with less power, will need less-powerful fins.
Paddleboards with harder chines, harder rails, a V-shape bottom, flatter rocker, and displacement boards in general will track better in a straight line simply by virtue of their “hull” shape, and so they’ll need less fin power to go straight. Keeping a board going straight is yaw stability–a paddleboard with greater yaw stability will tend to go straight.
But those boards with straight-line or yaw stability will need more fin power to make them turn. So a distance race with few turns might suggest one fin over another.

Small FSI fins as sidebites in a 2 + 1 setup with a WG2
On the other hand, boards with more rocker (the fore-and-aft banana shape—down in the middle and raised somewhat at the ends) have less straight-line-tracking yaw stability will need more fin power to keep them going straight. But a board with a fair amount of rocker and hard rails when set on a rail, as when surfing or rounding buoys, will turn fairly easily, so less fin assistance, and less fin power, is required.
If your board is wide, it has inherent roll stability–it will resist tipping side-to-side. Narrower boards will be tippier. If your paddleboard’s bottom is flat across, or has chines or harder rails, these factors will promote roll stability. Deeper fins help roll stability–if they have enough area at depth. Think of fin depth as the length of a wrench, and of the area as the amount of force that can turn the wrench. Like a longer wrench, a longer fin will have more power than a shorter fin to inhibit rolling. Actually there’s a subtlety here: fins that have a deeper center of area will make a board less tippy. Deep fins also can increase your speed and decrease your paddling effort by decreasing drag–if properly designed.
A fin that works best for one board-and-rider system might not be the best for another. But here are some thoughts on how you can approach the question of how to choose a SUP fin.
Fin Power
Fin power as I’m using it here is not a technical term–it’s just a handy way of looking at how SUP fins work, and how they can be compared.
Think of it this way. Drag is the hold-you-back force. It can be measured in pounds. If your fin has 1 pound of drag force at six knots of speed, pretty clearly this is better than a fin with 2 pounds of drag force.
Drag is a bad thing in paddling–it makes you paddle harder, and tires you out more quickly. Less drag means more speed. And less drag means easier paddling, faster acceleration, and longer sessions because you won’t tire so quickly.
Fins, like airplane wings, boat keels, and race-car spoilers really create only two forces: lift and drag. In general, you want more of one force and less of the other–more lift with less drag. It’s kind of like the beer commercial from the old days: more taste, less filling.

SUP fin hold + drive = lift
When I use the term lift, I am not talking about up-and-down force. I am talking about horizontal force. See related post about hold, drive, lift and drag. My fins’ winglets are not intended to lift a paddler and board out of the water vertically like a hydrofoil any more than an airplane’s winglets are meant to make airplanes fly sideways. My fins’ winglets range from the size of a dime to about the size of a quarter–way too small to lift a board and paddler out of the water. By comparison, the Laird Hamilton hydrofoil fin was enormous–maybe 18 inches or more across–and had enough drag that he had to be towed in to get onto the wave.
When I am talking about fin power, really what I’m referring to is a fin’s ability to create the most lift with the least drag. In terms of science, what we’re talking about is a ratio between the two, the lift coefficient (Cl) compared to the drag coefficient (Cd), which can be graphed.

Cl and Cd for four foil sections
For example, the graph compares the lift and drag of four foil sections. First, look at the green arrows and look left to right. At a Cl of 0.7 on the left axis, the red foil has less drag than the green foil–the red curve is further left on the graph. So the green foil would take more energy to pull through the water than the red foil. Every stroke would take more effort to pull a board with the green-foil fin through the water than the red-foil fin. The red foiled fin is more powerful–it has less drag per unit of lift, at least when measured at a Cl of 0.7.
Likewise, if you look at the yellow arrows and read the graph up and down, you can see that for a Cd of 0.03 on the bottom axis, the red foil has more lift at that same level of drag than the green foil has. What this means is that the red-foil fin area, more “powerful” per unit of drag, can be made smaller in size than the green-foiled fin, yet the red foiled fin can produce just as much side force in that smaller package.
If I say a whole lot more about that, most folks’ eyes will glaze over–if they haven’t already.
The general concept is to aim for a fin that is efficient—a fin that has a lot of lift per square inch, and no more square inches than you need—and your paddling will be easier, you can last longer, and go faster. If your fin doesn’t have a lot of lift per square inch or it is just too large, then you’re dragging around unnecessary fin area, and that’s slow, tiresome, or both–it’s a drag in both senses of the word!
Fin Area and Position
In general, less area means less drag. This is because of friction (skin-friction, specifically). Every square inch of fin is a square inch of friction. So more fin area means more skin friction, more drag.
More and more board designers are recognizing this, especially those with a background in naval architecture, and especially in racing standup paddleboards. You might hear about race-board designers’ efforts to reduce boards’ wetted surface–because each square inch of wetted surface is a square inch of drag, and drag is slow.

Bulb on bow reduces drag
But fin area also affects lift (again–horizontal side force called hold and the forward-force called drive). Lift is a good thing, at least up to a point–you only need as much side force as you need–and no more.
Lift–the sideways force called hold, helps the board track in a straight line, and helps you turn. And the forward force called drive can help you along the way, or make you faster when in surfing mode.
Different portions of a fin produce various amounts of lift. In other words, not every square inch of any fin produces the same lift.
For example, the intersection of a fin and the board causes interference drag–turbulence at the base of a fin. So the area at the base of a fin produces less lift per square inch than others. Lots of designs devote a lot of area to the base–which makes up for the inability to create lift. But that extra area causes more drag–more square inches means more skin-friction drag.

Bulb on tail reduces drag
Probably you’ve seen ships with bulbous bows. This is an effort to reduce interference drag at the intersection of hull and water. Modern airplane rudders have a similar drag-reduction method, decreasing interference drag at the intersection between tail and airframe, although you might never have noticed it.
I use a bulbous forefoot and a small cutaway to reduce interference drag. This helps make the area of the fin that we use more efficient. In other words, I put area where it’ll do the most good in terms of lift while doing the least harm in terms of drag–I reposition area toward the middle and the fin tip, devoting little area where it does less good–at the base. I also reposition area into winglets at the tip, where efficiency is less compromised by the board’s interference drag.
Many designers put the greatest area of their fins at the base–where interference drag is at its greatest, and where the area’s efficiency is most compromised. Some folks say that a fin with a longer base has more drive. I am not sure whether this longer-base-gives-more-drive belief is folklore that has justified dolphin dorsal fin design since the 1950s, or what. I don’t see hydrodynamics evidence that a longer base gives more drive. But I think a well-chosen foil section and other features give more drive, and can explain why I think that.
Anyway, the takeaway here is that efficient area is good, and you only need the minimum amount to produce the side force necessary to go straight or to turn as you’d like–any more area than necessary is a drag that tuckers you out–if your fin is inefficient, then your paddle, arms, legs, and back have to work harder to overcome the inefficiency.
Judging from all of the shapes out there, there are as many approaches to SUP fin design as there are designers. I don’t know enough about their specific design principles or the fins’ performance to weigh on on other designs. Some fins have a lot of area. By way of illustration, my SUP-longboard fin has about 30.6 square inches. On the other hand, some SUP fins have as much as 58 or more square inches. This would be like adding the area of a tennis ball–28 square inches–to my fin, and dragging that through the water too. Some boards or riders might need the extra area, but unless you need the area, overcoming the extra skin friction drag will take more energy from you, all else being equal.
With my designs, I try to maximize lift while minimizing drag, and to get enough lift for the average person on the average board. We have more designs on the drawing board including weed fins and speed fins. One size does not fit all in all circumstances or for every day.

FSI fin-tip flow at winglets
Winglets’ Effect on Fin Area–Why We Use Them
One way to look at my fin designs is that basically I take a typical fin, chop off about 30 percent of its area, then add back about 5 percent, repositioning area in winglets and other places where most efficient–just as is done in modern airplane wings and boat keels. This is the basic design principle behind my fins, and it’s in my patent too.
The idea is to reduce area overall, and reduce skin-friction drag, and then to maximize the efficiency of the area I have left. I also do a couple of other things, like add the bulbous forefoot and cutaway, for reasons discussed a bit below.
I want to create a very powerful fin, a fin that creates a lot of lift per unit of drag. This is why you’ll find winglets on airplanes and boat keels–winglets help efficiency, and they help a lot. Next time you’re on a Southwest 737, notice how short the wings are–if the wings have winglets. Wings that have winglets are short compared to 737 wings without winglets because winglets help maximize the lift per square inch. See Southwest’s 2014 Superbowl commercial about winglets.
And fin efficiency is why, when customers ask me about the right-sizing of our fins for them, I recommend that they downsize by about 20 to 30 percent in total fin area compared to their current favorite setup’s total area.

Paddle vortex–water flowing from the high-pressure to the low-pressure side
For those wanting a bit more detail, the idea behind winglets is that they decrease the flow of water from one side of the fin to the other–from the high-pressure side to the low-pressure side of the fin.
In the canoe-paddle picture, you can see the swirl, or vortex. This vortex forms because when you pull on your paddle, you pressurize the water on one side, and the pressurized water wants to leak to the other side of the paddle–to the low-pressure or back side of the paddle. And it happens with every stroke.
And with each stroke, you’re pressurizing your fin too, because a paddle stroke creates a rotational force that pressurizes one side of the fin. So with each stroke, water is trying to migrate from the high-pressure side of your fin to the low-pressure side, and it creates a vortex. Likewise, as your board tips side to side, you pressurize one side of your fin, then the other, especially at the fin’s tip, which travels the greatest distance with each rolling movement.

Vortex beginning at a typical fin’s tip
A vortex is slow. In scientific terms, a vortex is a kind of drag–induced drag. Winglets help reduce induced drag, which is why they are on airplanes, boat keels, race car spoilers, and elsewhere. Here’s an Article about Richard Whitcomb, NASA, and Boeing Research on Winglets. On airplanes, drag costs gas. In paddling your SUP, drag costs energy–your energy–and taxes your muscle power or endurance.
So overall, I recommend looking for a fin that has a lot of power–a lot of lift for its drag–not just a big fin or a small fin.
Fin Foil Section
Fin foil section is very important in creating high-lift-low-drag fins. Foil section is the slice of the fin if you chopped it in half horizontally. Different foil sections have different lift-versus drag ratios. Check out the Cl and Cd graph up above, which compared several foil sections. Some foil sections are better at producing lift, and some are better at decreasing drag–some are better at creating a powerful fin–lift per unit of drag. Some designers shape foil sections by hand. And some use different foil sections in the same fin.

NACA double-zero foil section
I chose a specific foil section for my fins similar to the red shape shown here, and continue researching and developing high-lift and low-drag foil sections. The foil sections I use have been widely tested and proven to have excellent lift over a wide range of angles of attack. In other words, I chose a foil section that does not stall easily. Stalling is bad, because stalling creates drag, and drag is slow.
Foil Thickness
If a foil is too narrow, it will stall easily as angle of attack widens, as when turning. Of course angle of attack changes a bit with rolling too. If a foil is too thick, then its straight-line drag will increase, and the fin will be slow. So there is a range of thickness that is pretty good or even just right, and within that range can be thicker or thinner depending on the use intended–speed versus turning.
Foil Reynolds Number
Certain foil sections work better at certain speeds than at other speeds–certain foil sections, thicknesses, and chord lengths–produce more lift per unit of drag better that others, within certain ranges. So Reynolds number is a way of analyzing a fin–a Reynolds number is a function of both the chord length (fore-and-aft length) of a fin, and the speed that the fin is traveling. One takeway here is that your fin’s foil section must take into account the designed speed range and chord length of your fin.
Another takeaway is that at higher Reynolds numbers—greater speeds in effect—well-designed fins will increase drive—forward force, helping your paddling effort. This speed-effect of lift is what makes airplanes fly when they reach takeoff speed, and what makes sailboats sail toward the wind. With fins, lift likewise increases as speed increases too. So when starting from a dead stop, and before a fin had much flow of water moving past it, a small fin might tail-kick a bit with the initial paddling. But once it gets water flowing past it, a well-designed small fin’s greater speed potential will increase the lift–both hold and drive–quickly reducing or eliminating tail kick, and helping along your way. Read more about hold, drive, lift and drag.
Foil Stall Angle
Some foil sections are better at stall prevention than others. Stalling is bad—it creates a lot of turbulence that slows you down and forces you to paddle harder. So choosing a foil that avoids stalling is a good idea. This is especially true for paddlers who plan to race in races with a lot of turning. Stalling during a turn is slow. A fin should be designed to inhibit stalling over a wide range of angles of attack, especially if designed for turning. But also, each significant roll of a board wags the fin this way and that, changing the angle of attack, and potentially stalling the fin if it is not designed correctly.

The WG2 is about 30 percent smaller than a typical dolphin fin
Fin Rake
Fin rake is one of the most important aspects of fin design, and is an issue intertwined both with aspect ratio and with weed-and-kelp shedding. Generally speaking, fins are most efficient with a rake of about 15 degrees, and the lift-curve slope, a measure of fin efficiency and ability to generate lift per unit of drag, drops as fins are more heavily raked.
So with less rake—a more upright fin—your fin will perform better—and will help you turn and maneuver around kelp in your path. On the other hand, with more rake, you can expect less performance overall, i.e., the fin will be less efficient, so you’ll have to stroke a bit harder, but obviously the fin will shed kelp or weeds better if you hit some.
Fin Taper Ratio
A low taper ratio looks more rectangular, and a high taper ratio looks more triangular. The taper ratio compares the fore-and-aft length of the tip as compared to the base. More rectangular shapes put relatively more area at depth, away from board interference drag, and help inhibit board rolling. Triangular and dolphin fins devote more area to the base, and less at the tip–they have a high taper ratio.
Fin Aspect Ratio
If you look at airplane wings, we know that they tend to be long and narrow, not short and squat. This is because the high-aspect-ratio shape is a low-drag shape.
In general, tall and skinny is faster, and shorter and squatter is slower. Aspect ratio dramatically affects drag. A tall, thin, high-aspect-ratio fin of, say, 10 to 1 can produce 400 percent less drag in pounds than a short, squat, low-aspect-ratio fin with a ratio of 2 to 1. Thus where a 2:1 low-aspect fin might produce 5 pounds of drag force, a 10:1 high-aspect-ratio fin might produce only 1 pound of drag force.

My WG2 is a high-aspect-ratio design
A possible drawback with high-aspect-ratio fins is that they are more prone to stalling as the angle of attack increases–as when turning. This is less of a problem in paddleboarding when going in a straight line or in a wide curve, but more of a problem when rounding a buoy or making other sharp turns.
So the remedy in high-aspect-ratio fins is to use a foil section that resists stalling, so that the fin not only goes fast, but also turns well. So ask your high-aspect-ratio fin designer about the foil section he or she put into the fin if turning ability matters to you.
Fin Depth
Deeper fins in general help with roll stability. But really what we’re talking about is the depth of the center of area. A deeper center of area and more area at depth will resist rolling–it will help with roll stability. It’s like having a longer wrench and putting more force on the wrench handle to stop the rolling moment. Moments are forces over a distance. With a longer lever, i.e. fin depth, the area to inhibit rolling can be smaller at the end. Or with a shorter fin, the area at the fin tip will need to be greater to produce the same roll-stopping force, because the lever arm is shorter.
Deep fins can decrease your paddling effort by decreasing drag–if properly designed. Those who say that “deep fins are slow” are probably referring to typical fins that devote a lot of area at their base, low-aspect-ratio shapes with a poor foil-section choice, a triangular shape with a narrow tip, or fin features that really increase a fin’s area for a given amount of lift–and thus really increase the drag, making the fin slow. But if designed correctly, deep, high-aspect-ratio, low-taper-ratio fins with a well-chosen foil section not only help roll stability, but can decrease paddling effort and increase speed.
Food for Thought
So if you got through to this point, you’ll have at least one fin-designer’s take on fin design. There are lots of fins to choose from out there. Hopefully this post serves a food for thought. Please contact us with any questions.

Quad SUP with FSI fins