Jan 28, 2008
Author: Derek Rijff
Fin Buyers’ Guide
by Derek Rijff
In a properly balanced windsurfing combination, the fin is just as important as the board or sail, yet is often overlooked by screwing in the stock fin and forgetting about it. A switch in fin size, shape or quality can be an inexpensive way to increase range, carve tighter turns, or travel straighter. With a goal of increasing fin awareness, here is an entry-level guide to what makes a fin work and the main elements of fin shaping that designers aspire to work in harmony: size, flex and foil.
How a Fin Provides Lift
Lift is a complex topic. With help from Chuck Ames of True Ames Fins and Stan Pleskunas of Fumunda Marine Products, we have put together a basic explanation.
Despite being symmetrical, fins work in a similar means to all other foils, like airplane wings or sails. To achieve lift, the air or water molecules flowing around the foil must have farther to travel around one side than the other before meeting up at the trailing edge. The shorter side represents high pressure; the longer side is low pressure. As the low pressure (partial vacuum) seeks to become full, it pulls or sucks the foil in that direction, causing lift. Lift always pulls to the low-pressure side. Picture this as an asymmetrical foil, like an airplane wing, before moving on to understanding fins.
A fin may be symmetrical, but the water flow around it is not due to the concept of angle of attack. As your board moves across the water, it travels forward but also downwind (leeway angle) in a crab-like fashion. The forward motion provides the water flow on the fin, while the leeway direction results in the water hitting the fin slightly off-centre (from the downwind side), creating the angle of attack. Lift is achieved because the water flow is asymmetrical. The fin’s leeward side becomes high pressure because the water has a shorter distance to travel, while the windward side is low pressure, forcing lift in this direction. The amount of lift is a function of the fin’s angle of attack, area, shape and speed.
The direction of the lift is always towards the windward rail and perpendicular to the centreline plane of the fin. Simply put, pushing on the fin (rolling the board to leeward) angles the lift in an upward direction, while riding with heel weight (rolling the board to weather) holds the board’s rail down as the lift is angled more downward. An “over-finned” board will want to roll too far to leeward and tail-walk, becoming difficult to control due to excessive upward lift. This is why having more than one size fin per board will allow control over a greater range of conditions and sail sizes.
When it comes to fin size, length is important but not everything. Just as you wouldn’t choose sail size by mast length, there is more to choosing your fin size. A short, wide fin with a swept tip will have as much surface area as a straight, narrow fin a couple inches longer. Gauging the exact size of fin needed is often a matter of personal preference. Sailors who push hard against the fin need something of substance to hold, whereas others who transfer power to the board using the rail can get away with less. Length does become an issue for anyone riding wider boards with footstraps on the rails, as they require a longer fin to provide necessary support.
Just as a sail’s top twists off, a fin flexes as you move across chop, change direction or weight it differently. This flexing helps keep water moving across it and can dampen the lift generated. Generally, softness offers better control while stiffness generates more lift and speed potential. Three things influence the flex of a fin.
Outline: A common guideline is that a long, narrow fin will be designed for speed and early planing, while a short, curvy outline is for manoeuvrability and control.
Material: Designers use different material and production techniques when building fins to influence the flex. Milled fins are cut from a block of material, with poly/vinyl-ester being the softest and G-10 being stiffer. Adding carbon layers makes it even stiffer. Molded fins use different materials that are added or removed to control flex. Assume a more expensive molded fin has the flex more refined than a cheaper one.
Thickness: I learned the hard way as a kid climbing trees that thicker branches are stiffer than thinner ones. Fin designers must have done the same, as thickness affects flex. With a keen eye and sense of touch, or calipers, differences that will be noticeable on the water are usually discernible.
Fin designers try to maintain water flow across a fin while maximizing lift versus drag. Unless you have a degree in hydrodynamics, choosing a fin based on the foil is not recommended, but since this is an article on how fins differ, we’ll point out the main concepts.
Thicker foils create more drag at speed even though they generate more lift and hold well at low speeds. Think of an airplane extending its wing flaps on takeoff creating a larger, thicker foil and retracting them once airborne for less drag. Fins with the thickest point forward generate more lift at slow speeds but spin out at higher speeds, as the water is more likely to detach.
The foil’s symmetry and smoothness is important. Flat spots, dimples and asymmetrical foils will spin out more readily. If you see something wrong or a little off, then you can probably feel it on the water. This is why it’s critical to keep the fin in pristine condition.
Slots, Twinsers, Thrusters and More
Over the years there have been numerous alterations to the standard single-fin setup, some working better than others. If a design is born from a fin designer, it usually focuses on eliminating spinning out. Ideas coming from board shapers intend to tweak the board’s performance.
From fin designers we’ve seen fins with split tips, drilled holes, ridges, and even tiny fins set in front of a main fin. The most popular alteration has been the slotted fin. The benefit of these fins is not that they don’t spin out but the ease at which they reattach. Often it happens without the rider even noticing.
Board builders change things up by adding fins. Using two fins of the same size creates more drag but, depending on the fins’ size, can either make it feel looser in transitions or help stabilize a board at speed. A bonus of the twin-fin board is that it can sail in shallower water. The other established multi-fin setup is the “thruster,” with a single main fin and two (or more) side fins. The nitpicker will again point out the slower speeds. Proponents will rave about the extra tracking and hold through carves.
Fin Works produces the highest quality fins available, displaying excellent performance over a range of conditions.
About us: Founded by Larry Tuttle in 1984, Fin Works quickly gained a reputation for high quality and performance. In 2006, Dave and Rene Lassila took over, introducing computerized design and CNC production capabilities. Dave, a mechanical materials engineer by training, designs and manufactures the new fins, while Rene runs the company and is the friendly voice you hear over the phone.
What’s new: Fin Works now uses computer-aided design (CAD) exclusively, resulting in the highest accuracy of foils. Prototypes are created using CNC machining and tested by sailors of all levels. Look for the new G-10 wave/bump-and-jump fin and CNC foiled Formula fin.
A well-planned quality fin quiver is just as important to your windsurfing setup as having finely tuned rig and board quivers.
About us: Gorge Sport takes the world’s finest design shapes and improves them by bringing great fins into precision construction for worldwide distribution. Design flows from what is learned from pro and recreational sailors, allowing developers to improve on all their fins. G Sport’s Bill Kline believes that “using high-quality fins will yield a far greater return on investment than any other purchase in windsurfing.”
What’s new: Refinements in freewave, wave and freestyle designs make manoeuvres and riding smoother than ever. The new SR and Speed fins are faster. The Carve series increases the performance range of bigger freeride boards for easier planing and better control.
Maui Fin Company
Web: mauifin.com; chinooksailing.com
The Maui Fin Company team is on the water 365 days a year testing shapes, profiles and materials to develop new products.
About us: Around since 1986, the MFC believes in the importance of working with the world’s best riders across all disciplines, like Micah Buzianis, Kauli Seadi and Jose “Gollito” Estredo. Veteran designer Pio Marasco invites you to try the company’s fins: “If you come to Maui, you can test all our products by coming to the shop in the Pawela Cannery.” MFC’s lineup includes something to satisfy any type of sailor.
What’s new: The MFC lineup is constantly evolving and improving. For ’07 the brand introduces the 2K wave fin and a Buzianis-inspired slalom line. Next year, look for new shapes, profiles and constructions to keep MFC in the forefront of fin design.
True Ames works directly with consumers, retailers and manufacturers to design the best possible fins for your equipment.
About Us: Founded in 1979 as a fibreglass fin manufacturer for surfboards, True Ames evolved into windsurfing in the mid-’80s and has led the way in design and performance ever since. An enthusiasm for windsurfing is core to the business’s foundation. Working primarily with G-10 and polyester sheet stock, most fins are hand-shaped, making each one a custom product.
What’s New: Joining True Ames’ already complete lineup this spring are the new JD Wave and Surf Grass models. The JD Wave is keeps transition and trick sailing in mind, while the Surf Grass is part of the constantly evolving line of weed fins.