My thruster is "humming" from the fins. How to diagnose the problem?


I’ve a 6’1" x 2 3/8" x 19" round pin thruster, and I took it out for the first time this week.

The fins were “humming”/“whistling” when I got up to speed, and it felt as if I was trying to drive my car with the emergency brake applied.

I thought the problem might be the fins themselves (which I bought through ebay for a modest price). The side fins were transparent material with the K 2.1 outline, and the center fin was an FCS black G3000.

So next day I changed the side fins to FCS Yu model, thinking the problem was the side fins. I left the same center fin (FCS G3000).

Waves were very nice.

Although the board didn’t “hum” as intensely as it had the day before, it still felt like the parking brake was on. Especially when the board was planing flat (flat over the water surface = not on a rail).

So I am thinking that I should start looking at the fin placement.

I checked the cant angles on all three fins. The center fin is slightly tilted to one side (maybe 1 degree). The side fins both have the same amount of cant (about 96degrees or 6 degrees off the vertical). And yes, the cant angle on the side fins is pointing away from the stringer and towards the rail.

Which brings me to my question: Having checked that the typical measurements for fin placement were present (3 1/2"~ 4" from tip of tail for rear fin, 10 1/2" ~11 1/4" from tip of tail to trailing edge of side fins, 1" ~ 1 1/8" from edge of rail to trailing edge of side fins), I’d like to know what is meant by “2 inches off the nose” when measuring toe with a straight line.

Is **“2 inches off the nose” **referring to 2 inches to the sides of the nose, respectively?

Or is it referring to 2 inches beyond the tip of the nose going forward from tail to nose?

Any thoughts will be appreciated.







Hi Castrillo,

I've been working with a fin company to find the reason a lot of recent fin designs are humming (or shrieking) at

higher board speeds.  It mostly boils down to the trailing edge-  it's too sharp.  Quick fix is to sand it dull or better

yet put a slight squared-off trailing edge (like 1/64")  That said, there is another component, it appears that a lot

of the recent molded fins (translucent/off-white plastic) are too thin in foil in the last third of the chord (running

from the front edge to trailing edge)  The last cause seems to be a high-pitch shriek from the rail edge in the tail


In some shapes the rail transitions in such a way so as to make acoustic vibrations.  All tolled, it takes energy to

make these vibrations/oscillations which can cause drag.

Look up "Karmann Vortex Street" and you will see info on shedding vortices.  A good visual is a flag waving in the

wind, you can see the ripples shedding off the flag pole onto the flag-  Vortices can act like dragging parachutes of

turbulence (although when wisely placed, can be beneficial, like with vortex generators on aircraft which maintain

attachment of air flow.)


As for the bogging board- have you ever gotten the board to go well?  Not sure it is all the fins, as there are larger

stronger factors like rockers (rail and stringer) and bottom contours.  Once you know the hull is a good one, a fast

one, then you can start testing fins and finding the truth...


As for " two inches off the nose "  yes, that means Laterally outward from the nose tip.  A common fin distance for a shortboard

is about 2" off the nose* so this means a straight line will come within 2" of the nose tip, off to the side.


Hope this helps,



*this standard is becoming obsolete, or will be pretty soon...

I had a similar issue with my fins so thanks for the suggestions to dull the trailing edge.

I’m wondering where the standard 2" inches off the nose came from.  I’ve only shaped 3 board and so far I’ve only copied what I own or saw from others.  I’m hoping to build a base of knowledge so I can undersatnd why the numbers are as they appear on the boards I want to shape.  I have a basic understanding of how toe in functions, but if you could fill me in on why the 2" works, and why you think it might become obsolete, I’s sure appreciate it. Thanks!

Thanks for your reply George.

No, I have not gotten the board to go well yet. I have only surfed it 3 times since I bought it, but all 3 have been in the past 4 days, so the feeling is still fresh.

I strongly suspect that the hull is a good one, mainly because the shaper is one of Puerto Rico’s oldest and most legendary. He has been shaping basically non-stop since 1976, and is pretty much the most well-known surfboard builder in the place.

That being said, I am very aware that even someone of his high qualifications could make a crooked board every once in a while. But on inspection the board has clean flowing lines and is pretty much aesthetically pleasing to the eye.

The board does have a pretty deep single concave going through the whole board, deeper in the back third (maybe 1/4" deep).

I will sand a little off of the trailing edge of the side fins as you suggested, to see what happens.

Thanks a lot for clearing up the “2 inches off the nose” situation for me. I had been dragging that question around for at least a decade, since I saw John Carper’s shaping video initially.

Also, why is that “standard” being replaced?

As for the turbulence/vortices, funny that the visual you mentioned (the flag in the wind) is exactly what I thought of when I initially attributed the humming to the new side fins. These side fins are significantly more flexible (less stiff) than regular FCS fins, which is something I failed to mention before.

But, the situation didn’t correct when I switched those fins for the Yu FCS side fins. Which is what prompted me to suspect there might have been something wrong with the placement of the FCS plugs (like for example a dysfunctional toe angle).

What I ultimately want to accomplish is to have the board ride well. And that could mean re-placing the fins.

I will let you know how the board goes next session.


Thanks again George,



99% of the time fins humming are not too sharp, but on the trailing edge there is a resin bead, or in the case of an fcs fin the fin was not trimmed properly after it was taken out of the mold. Take the fin and feel how it contours from front to back and you will feel mold flashing on the back edge of the fin; now where you feel it remove it either by sanding, or scraping with the edge of a razor blade. The water goes around the fin, and where it should release it is impeded and expands, so at higher speeds, it starts to whaaaaaaaaaaaa,whaaaaaaaaaaa because of a bad trailing edge. When people made fins by hand, and glossed them it used to be a more common error, but it is easily remedied. Really has nothing to do with fins being too sharp, but that can be an even worse problem.

And castrillo your “crooked board” theory has nothing to do with the fins humming.

True, fin hum can come from a bead, like after a gloss coat.  That kind of hum is lower in pitch,

more like a true hum. The higher pitch shriek is from the camber thickness of the foil becoming

too thin when used on raked fins (trailing edge template curvature.)  It is more prevalent than

only 1% as stated, in fact the high pitch hum/shriek is being found on many of the production

molded fins, but only when they reach higher speeds, and that is a lot of fins (and a large

problem that is being addressed.)  Maybe the 99% is just a figure of speech, I'm okay with that...ha.


In the 70's I had a board with a very vertical trailing-edged fin, and as Ghettorat has described

the fin had a pretty fat bead of resin drip down the entire trailing edge.  During a bottom turn

the board would HUMMMMMMMM so much that the vibration would actually massage your back

foot!  We called that board "the Hummingboard"   It had a pretty low pitch hum, like that you might

experience on some sailboats, like small catamarans.


I've tested a lot of fin angles (using the South African 4-way Fin System that allows big articulation

in toe angles) and I was very surprised that drastic changes in toe (like -2" off the nose) did not

affect speed too much and turns still had speed.  BUT, the boards paddled noticibly slower!

Also, the boards went pretty well with the fins toed OUT!  I didn't think it would work at all, but

they had a pretty good feel, except when going off the top and getting stuck in the lip.  Amazing

how Simon got it dialed so right so quickly...


As far as not questioning the hull.  If it was hand-shaped even the best of them can falter, and

the issues can be very subtle, even a laminator or sander can turn a decent shape into a

stinker.  Try the same fins on a buddy's board, and do some cross-testing.  See if you can

go fast on his board with YOUR fins, and report back.  That is a good simple test to seperate

fin problems with shape problems.  (Fingers crossed it is just a fin problem...)



Hi Wbrame,


I don't mean to hijack Castrillo's thread but this is related, so...      I meant that the shortboard

is starting to become obsolete with newer designs that are much shorter but surf just as well

or better.  Since they are so short, we do not use the +2" off the nose on those boards.


The reason 2" off is seen so much is because that's what seems to work best all-around on

conventional shortboards.  This was found through trial-and-error, and most shapers

including myself (35+ years building boards) check out what the "other guy" is doing, ha ha. 

(monkey see-  monkey do...) 

If it doesn't work, we're off to the next thing, and keep a lookout who's ripping and on what.


Thus, 2" off the nose has become a paradigm.  I've had looong conversations with a well-known

Hawaiian shaper about how to mark fins on the blank and he was still using 1/8" in (front, front fin

dots) and sometimes 3/16".   His boards work killer, he's well-known for it, so hey, why change?  

We took my long straight edge and found his marks were about 2" off the nose in most cases, he

would shift to "a hair less than 3/16" " as the boards got shorter then 3/16" on the short ones.

In that way I see him as pretty much on it without using any "off the nose" stuff....



Hey George you shape a really nice looking board. I know Steve Pendarvis did a really nice study with all kinds of pretty pictures on different foils, maybe when you see him he will share it here or tell people how to get a hold of a copy. I know he uses a fin design by my old friend Rick Vogel’s based on a swordfish. I surfed the bay giant with him and Steve Tiau. King’s reef was macking, and when the sets came in the bay would drain and the water level would drop about ten feet. I told Pendo what a great surfer Vogel was, back then I wasn’t scared. Vogel, Tiau, and Pendo are worthy.

Hi Ghettorat,

Small world! ha!  I see Steve and Cher all the time, straight up good people.  Steve needs to

be more known for all the innovations he's made, including fin making.  I heard some heavy

stories about Kings from JR (worked a lot with Ridds this summer on his hollow balsa gun

for big Bay.)  I have to say that whole zone is a unique testing ground for lots of different

equipment.  That's some huge surf you guys were messing with!!!



That makes sense. Thanks a lot for sharing!

…hello +1!,

that 2" off nose correspond normally to the old 1/4" and when smaller board to a 3/16"



-fiberglass fins:

HUM: protrude or too thick back edge with not too good foil.

WHISTLE: protrude on front edge or too thick bad foiled front edge


-what you say about those new composite fins are very interesting

so, too thin the foil and those fins hum at fast speed?

all the brands?


Report from today’s surf:

Waves were nice again.

Took out the board with the Yu FCS side fins and a G1000 center fin. I sanded the side fins’ trailing edge as suggested.

The board still rides as if it had the emergency brake on. Seriously on.

One detail I failed to mention before is that the board was cracked on the deck and repaired before I got it. The crack is from rail to rail (looks as if it almost snapped in 1/2), and it is located further back from center, about right in the mid-point between my feet.

The bottom is intact.

The board doesn’t buckle or bulge in any way, and the repair looks and feels very solid. I tried to buckle the board by resting it on the nose and applying pressure downward over the deck, but it did not yield at all.

Might the center fin cant be the problem (being a little off the vertical by about 1 degree)?

I will make some pictures and measurements and send them through, to make this more effective.

 George: As for testing the transparent fins on another board to see if they go well, I will report on that when I do it. That will settle the question of wether those fins are faulty or functional.

As for the surfboard in question, I am more inclined today to think that there’s something wrong with the fin placement. I want to be able to put my finger on the spot, so I can solve the situation and have the board surf well.


Thanks for all the input!



A buckled board is a significant detail to leave out of the discussion.  Probably why the board is a dog. I get rid of them when that happens after salvaging fins or anything else on it I might want.  Mike

Report from today’s surf#2:

Took out a different board, “board #2”, a 6’0" x 18 1/2" x 2 1/4" thruster.

Now, this is a board I have owned for at least 6 years. I have ridden it plenty of times before today, and it goes well enough for me.

Today, I put the transparent side fins on it, with a center G3000.

The board whistled and hummed like she meant it, and felt like it had the parking break seriously on, just like “board #1”. That had never happened to me before with this particular board.

Now I think that it’s safe to say that the new fins I bought through ebay are dysfunctional.

Tomorrow, I will again take out “board #2” with FCS fins which have worked well for me, because I want to test a theory:

If “board #2” goes well with the FCS fins tomorrow, then the new transparent fins were definitely a problem that can be overcome by substituting them for fins that work.

If “board #2” doesn’t go well tomorrow, and again feels like it has the parking brake on, then probably the new transparent fins not only don’t work, but  also damage the FCS plugs or something close by in such a way that when a functional fin is placed it will still feel stalled (as is happening with board #1).

Then, back to “board #1”… Is the fin placement crooked, or did it suffer damage from the new transparent fins?

Or is it something else altogether and the defective fins just happend to mix things up a little bit?

My car feels slow on the highway for about 10 days now. Could it be that in this point in my life right now everything that usually goes fast is decelerating forcefully?

Rooster: As for the “buckling” on “board #1”, the repair looks solid and very well done. The board does not buckle when I try to buckle it. Nevertheless, I agree with you that it is a very important point in this discussion which I had left out, for it could very well be the cause of the board’s poor ride.

Nevertheless, my heart tells me it’s the fins where it’s at with board #1.

But, at this point, I cannot be certain.

I will report back tomorrow.


Thanks to everyone.





I like the process you are using to solve the problem.  The detail you left out sounds like a buckled then repaired board.  I don’t think the fiberglass will buckle without snapping the stringer.  I might be wrong.  A snapped stringer is going to change the flex pattern of the board.  Much bigger of a problem, in my opinion, than humming fins.  Might be a dog for you, but someone else’s next magic board.  Mike

I had not made that connection (although it now seems so clear), that if the board buckled the stringer must have snapped.

And yes, I agree that it is a board that once buckled and broke the glass on the deck, and was later repaired.

But I have just carefully inspcted this buckled/repaired board (board #1), and the stringer and glass job are intact on the bottom. The stringer is 1/8" solid shiny wood, and it doesn’t look splintered or altered in the bottom area directly below the buckled deck, or anywhere nearby.

On the deck, in the repaired part, I can see the stringer intermittently, since it has a white resin strip (the filler resin) below the glass. The white filler covers some of the stringer for about 2" (the strip actually goes from rail to rail).

I don’t have a good visual access to it, so I can’t give a precise description of the stringer on the affected part of the deck.

Maybe the stringer snapped on the deck, but the break didn’t go through to the bottom part of the board?

And if matters with this board fail to progress, I agree that it could be someone else’s magic board. Sounds weird, and my first reaction is that I’d be ripping someone off with a proven-POS board. Nevertheless, come to think of it, surfboards are like that. Maybe somebody else’s footing and technique will make the thing go off.

As for now, I wish that somebody who can make the board go off is myself, since I like the shape and see it working, if I can overcome this “parking brake” situation.

Thanks a lot for the thoughts.


Hi Castrillo,


So you sanded the trailing edges and you are still getting a hum?  I'm wondering if you got some knock-off fins,

seen some of those, look like the real thing but the foils are horrible.  Sanding the leading edge could help, but

if there is an issue with humming from the leading edge there is definitely poor flow across the fin making it work

much worse than if the issue was at the trailing edge.


You might want to put in a set of completely different fins in your new board to see if it still bogs.  Also consider

that the particular swell might not be the best, as this can affect performance, for both your old and new boards...


I explained what 2" off the nose is earlier in this thread.



Hey Castrillo, my 1st thruster I made in 81' and took to South Africa went so good. It was abt a month after Simon won Bells and the Coke, so I didn't know much about them, anyway the fins were so sharp they kept cutting my ankles, so I took the board back to my hotel room and sanded the back edges square, as I'd already been making boards for more than 10 years and knew about the hum factor, but always thought it came from the bead left from finish coat not properly removed on the back edge. Anyway the board turned into a dog, over night and hummed the worst of any board I've ever had to now. I thought it was the fins not facing the right way. There was nothing wrong with the board, it was the square edges, round them off a bit and I will bet they don't hum. I sold the board to some guy in Mauritius and that board probably never got to go as it should. Years later I found that out, it's not the cant or the back fin leaning slightly,it's the square edges. Like Ghettorat said the water has to wrap around the trailing edge. cheers  H.

Waves were not good today. Way too much wind.

I will report back with results when I ride both boards with proven functional fins.

Thanks again everyone.


Report from today’s surf:


Took out board #2 (6’0" x 18 1/2" x 2 1/4") with regular FCS fins (side G5s and center G3000).

Board worked fine. Too small for me nowadays, but the board went fast and no stalling from the fins.

After 3 or 4 waves, went in. Switched the fins to Dorsal Fins black model, “fiber-filled”. I also got that set through ebay.

Went back out. Caught a wave. Stood on my board. Board whistled and hummed. Slowed down. Instantly drew the obvious conclusion.

Conclusion: The fins don’t work. Board #2 was not permanently damaged by the dysfunctional fins from the last time I surfed it, because today I rode it with regular FCS fins and it worked fine.


And as for board #1, the board that originated this thread…

I have to ride it carefully with functional proven fins (FCS) to see if it goes well.

If it doesn’t, then it’s probably a case of confounding variables: the dysfunctional fins I put on it when I first rode it only complicated an already existing problem with the board which I don’t know what it is.

I’ll keep you posted on that one.


Thanks to everyone for your input.