I ran across this bit of info while researching why my fins hummed, could prove insightful??? Why do my fins hum – it’s a Thruster? You’ve just banked into a hard bottom turn and suddenly your board begins to sound as though you are being chased by a 500-pound bumblebee. Your board feels as if it is slowing down – it certainly won’t go any faster. This is classic fin hum, and you’ve just run into a hydrodynamic wall, similar to that of a subsonic aircraft trying to exceed the speed of sound. Fin hum has probably been around since Tom Blake put the first rudder-like appendage on a surfboard back in the mid-'30s. Though today’s modern multi-fin boards hum for different reasons than the crude, blunt fins of yesteryear, there is still a common underlying cause: the various forces of waterflow rushing over both sides of a fin. Without going into dense detail, a fin works properly when the water flowing around your fin(s) follows and adheres to the fin surface with the greatest “efficiency” and meet again at the trailing edge (rear of the fin), with the least possible amount of separation and turbulence. Now, this simplistic explanation has just leapfrogged over volumes of physics that deal with aero/hydrodynamics, so I must beg the indulgence of any readers from Cal Tech and JPL. Different types of fins and foils present various waterflow problems; fins that are foiled on both sides allow the flow of water to wrap more or less evenly around the fin (how evenly is sorely tested in hard turns), while the side fins on a Thruster create an unbalanced flow. Being flat on one side and foiled on the other, the waterflow travels over the rounded side of the fin at a higher velocity than it does across the flat, inside surface. This creates a lower pressure and, thus, a bit of lift on the foiled side of the fin. So, Thruster fins are fixed onto the board in such a way to exploit this; the front fins are toed-in a little and the foiled side of the fins face the rails. This causes drag, of course, but also (combined with the outward-pulling pressure of the asymmetrical foil) a certain degree of torque or leverage that the gives the Thruster its characteristic bolt-out-of-the-blocks speed and snappy maneuverability. These turns and maneuvers further complicate all of this and throw in the spoilers of water flow separation and turbulence that designers must contend with. There are a number of fairly common reasons why your fins may hum on your Thruster. Usually it will boil down to two things. First, improper “foiling” (how the thickness tapers, from both base to tip and through the cross section of the fin) may disturb the waterflow to such an extent that it breaks away from following the surface of the fin the way it should. When the water flow is disrupted or forced to separate from the fin, turbulence will rear its head. This turbulence can often result in vibration in the thinner tip of the fin, and voila, you have your hum. Second, a blunt or squared-off trailing edge causes turbulence when the two separated waterflows don’t dovetail together again behind the fin efficiently enough. This second example seems to be, in my experience, the most common cause of garden-variety fin hum. There…we’ve made some ground without having to resort to algebra or brain-numbing terms like “vortex,” “laminar flow” or “cavitation.” You can usually prevent the humming by re-sanding the fin a bit. I haven’t come across too many fins that hum because of the way they are placed (or rather, misplaced) on the board, so you’ll mainly be faced with re-foiling the fin and/or eliminating the bluntness along the trailing edge. With glass-on fins, you’ll have to check the foil: look at how the fin is shaped from both leading edge to trailing edge, and from base to tip. The transition from thick to thin shouldn’t be abrupt, especially toward the tip area. Overly flexible fins can intensify your problems, too. Before you do anything drastic, though, check the trailing edges of all three fins and see if you can detect any bluntness anywhere. If you find any, take a piece of fine-grit sandpaper, wrap it around a piece of dowel (a cut-off tip of an old broomstick or the plastic core of a roll of fax paper both work great) and try and feather the edge so that the rounded, foiled side of the fin meets the flat side as flush as possible (don’t saw away at the fin with the dowel, but rather use it like a knife sharpener). The trick is to have the two sides meet seamlessly without having to make the edge dangerously sharp. It’s better to leave it a little dull than to contend with three razor-sharp fins pronging back at you from the end of your leash. Occasionally, I will dull such a set of re-foiled edges by pushing them back and forth through the wet sand (this depends on the type of sand at your homebreak, of course) before paddling out. Sometimes you may find a resin drop or a beaded line on the edges of glass-on fins; this is usually a left over build-up from the glossing or sand-coat phase of construction. These will often contribute to humming and should be sanded off. If none of these remedies seems to work, then your fins may need to be replaced – it’s probably a problem with the foil and thickness taper, and not so easy to just reshape. Removable fins systems are another matter. These fins usually have fairly proven foils and shapes by the time they are put on the market and are manufactured in such a way that there is little variance in quality. Also, they aren’t put through the glassing process and caked with resin and fiberglass and then sanded, so there is less chance of human error or faulty re-shaping. If you have removable fins that hum, check the trailing edges for bluntness. If you find any, use a new straight-edge razor blade (for non-fiberglass fins) to scrape the trailing edge into a proper foil. If the fins still hum, try other models of fins; sometimes the shape, foil and flex of one model may not work with a certain type of board or surf condition. It is probably certain that every surfboard in the world would eventually reach the fin-hum threshold. It is just a matter of getting them to a high enough speed; most of our boards aren’t designed to handle the kinds of speeds other water or aircraft have to contend with. Our fin systems cater more to maneuverability and control than dealing with high speeds – sooner or later they would all begin to sound like that 500-pound bumblebee. An interesting idea relating to fin hum concerns the possibility that sharks may be attracted to just the sort of vibratory hum given off by an off-kilter fin. I’d always wondered about this, and a number of years ago, I was able to ask one of the resident shark experts at the Natal Anti-Shark Measures Board in Umhlanga, South Africa. Of course, it wouldn’t be proper science to speculate on this amateur hypothesis, but he agreed that it was entirely within the realm of possibility. Cause enough to sand your fins, eh? – Dave Parmenter
Dave, Nicely stated.
G’day guys, I hope you’ll let me throw a spanner in the works! Most of what is stated above is true, foil being the most important, real FOIL that is. It starts where the water first makes contact with the fin and ends at the point where the water releases. Anywhere between those two points not blending into the next can cause vibration-----hummmmmmmmmmmmmmmm. The fact that most fins these days have a curved leading edge profile, that’s side profile, automatically lends itself to bending and pushing water in the wrong direction, I should say, moving water ineficiently, to the point where the flex of the fin vibrates. The cross section profile throws another co-efficient into the equation. Too much water one way or the other can cause problems with cavitation and differences in pressure on either side of the fin. I realise this is needed under turning force, but it can cause those strange sensations that you don’t like while coming out of turns or during transitions. I have to flat out state again (see - progression of surfboard) that flat surfaces are not dynamic, so this, coupled with a foiled surface, can cause fin hum as well. In my experience the old dribble of resin on the trailing edge is probably the leading exponent of the fin hum phenomenon. Get rid of it, but DON’T round off the trailing edge in any way. You are better off with both fin surfaces having a hard trailing edge, leaving the fraction of an inch or millimeter squared off between them. This allows each of the working surfaces to release cleanly without any water distortion. Scientifically there is a percentage of the size of the foil which should be left this way, and having worked with highly qualified engineers, they emphatically state this case. The fact that most modern fins have a curved trailing edge side profile as well, just throws another negative factor into the equation. I’ve tested all this shit in lots of waves on lots of boards, from tiny junk to perfect barrells, and the sensation of taking the brakes off is astounding. I have not ridden waves bigger than just maybe 10 foot, and I have never done an arial. But I have the deepest respect for the people who do this and admire their ability to push boundaries the way they do. I just think that they can go further, faster, if some of the restrictions were taken away. I really look forward to watching what guys will be doing in fifty years time, and what sort of equipment they will be riding.
Cavitation? Cavitation doesn’t exist in surfboards or surfboard fins. It only exists in props, high speed hydrofoils, turbines and pumps. And yet this “cavitation” has been repeated to me as the cause of spin out for over 35 years. Fins spin out because they either stall or ventilate. Stall happens because the angle of incidence (angle of the water as it meets the leading edge of the fin) becomes to high for the attached water flow on the low pressure side of the fin to remain attached. Ventilation happens when surface air is sucked down the low pressure side of the fin interrupting attached flow. The low pressure side of the fin is 2/3 its power. In other words the suction side is twice as powerful as the pressure on the high side of the fin. Fins rarely stall. They ventilate 99% of the time. Cavitation doesn’t happen until pressure on the high pressure side, or on the low pressure side near the leading or trailing edge, raises to near vapor pressure of the liquid which vaporizes the liquid. The air pockets or cavities formed by this vaporization are cavitation. This can also happen around tip vortices on props. At the speeds we are capable of going it is doubtful that this could ever happen. So we need to amend our vocabulary to properly identify spin out as ventilation. The football, S, Fence and Boomerang fins discussed recently on this site were all designed to eliminate ventilation by using either forward rake or “fences” built into the fins to reduce traveling bubble ventilation. These designs were all eventually discarded from sailboards when it was found that proper foil could better eliminate ventilation without the disadvantage of excessive drag caused by these early fins.
Point taken Greg, although the technicalities of the use of a word, or misuse of a word commonly used to describe this phenomenon, seems a bit pedantic and doesn’t worry me too much. I’m sure most people know what I mean.
Yes, I agree. Time to change that one though. Ventilation is a better term.
As I read your post I was wondering just who “Dave” was… When I reached the bottom it made sense. Good post, thanks for the information. Magoo p.s. I think that you wrote The Anatomy of a Surfboard? That was excellent. M
Magoo, and Gregs et al I, the “Dave” that posted am not Dave Parmenter. My name is Dave Sherrier, I live and shape in Manhattan Beach CA. ! With that said, I don’t want to cause any misconceptions as to who this info came from, nor am I looking to take any credit from one of the “guru’s”
Dave, I appreciate your integrity and character. It is something that is sometimes missing these days. I will keep an eye out for your work. What shop do you work for or is it your own label? Regards, Magoo
I am a “backyard” shaper under my Babylon label.