Seems like 9/10 questions that i ask on here have something to do with fins.

This is a question that has been bugging me for a while now, and thus far haven't found an answer (a great surf craft designer said he'd tell me all about it over a beer one day, but i haven't had the time to go catch up for said beer).

Theoretically, much consideration goes into designing a fin, right? At least some times (i've got some fins that would perhaps suggest a lack of consideration).
Rake, flex and foil etc are all designed with hydrodynamic properties and characteristics in mind.
Then we have beautifully designed boards, again with bottom shapes/contours made with the flow of water as a key (only?) consideration.

But then we have a big arsed fin box. Unless the fin is positioned right at the leading edge of the fin, there's a lovely big hole in the board, right in front of the fin. The gap in the fin box prior to the fin has to produce a bubble trail. The bigger the gap, the bigger the bubble trail?
This has to effect the flow of water past the fin to some extent. Perhaps only minutely. Perhaps considerably?

Similarly, empty fin plugs are going to throw bubbles right down through where we're wanting water to exit most efficiently.
(on that note, one can buy 'infills' for unused plugs, but they don't seem too common - maybe only for over-thinking, under-educated fools like me?)

Then there's those 'adjustable' fin screws, with a lovely tab on them so that you can adjust by hand.

So, think of a board with empty side plugs, running a single fin set right back in the box, with an adjustable screw so that the fin can be adjusted in the water.... That's got to be somewhat of an hydrodynamic nightmare.

Somewhat different ballgame, but i can't help but think of props on boats and the loss of drive if they cavitate.

So my question really comes down to this - why all the carefully foiled fins and bottom channels if we're going to start throwing air where water should be?

And as a complete secondary Q, why not 'spacers' to fill in the gaps in our fin boxes, or at least reduce them?

(Again, this might be the simplest question ever asked on this site, but it's been bugging me for ages!)

Cheers!

Duct tape mate. Fixes everything

You can get inserts for empty fin boxes & you can easily tape over gaps in front or behind the LB centre fin...if you are that concerned.

Personally I'm not at all, I have 5 fin option shorties that will always have empty fin boxes. Shorties are bound to suffer more drag on glide performance than a LB but I don't feel it & I have spent a sh1tload more time surfing boards with glassed in fins than surfboards with fin boxes.

I really don't think its an issue unless your fully pro with a stick up your butt or surfing waves with so little push it becomes an issue, like your barely able to ride them & then your probably better off jumping on a 12 to 14 foot LB with no rocker. Something I would like for such occasions

To be clear, i'm not particularly concerned about it - my skill level is far, far away from any point where i'd need to be concerned about the impact of some air by the fins.

Reality is that i'm a scientist (but not an engineer), so i really tend to over think things like this on a daily basis.

More asking from a curiosity perspective more than anything else. Why be overly concerned about the way water flows under the board, and past the fins, if we're going to dump a bunch of air into the equation?
On the flip side, some sections of waves are quite aerated due to backwash etc from a previously broken wave etc, so bubbles from a gap in a fin box etc would be completely insignificant to that.

Really just wanted to hear thoughts from the experienced and learned masters of the surf.

(And Chrispy, it's funny that you mention the duct tape. it's really quite sad how often i've pondered the above question, and literally thought 'duct tape would make a difference'. Granted, duct tape is unlikely to fix my keyboards inability to produce an uppercase i).

Don't underestimate the power of the duct tape bro, just tape that shift key down and it will do the job

Your over thinking it was the exact first thought I had in replying to your thread opener

Not sure how exactly your dumping a bunch of air into your fin boxes, do you have a compressor attached to them or something lol

Maybe it is that pocket of air that negates the need for cashing out on fin inserts or wasting duct tape

I did have the same consideration when I first thought about the 5 fin box issue of empty fin boxes & considered taping them but never did in the end coz I was content. But then for me the switch to 5 fin boards coincided with wider, less rockered boards with shapes & dims that were way better than the low volumed, narrow, highly rockered boards I'd surfed in the past.

Maybe just whack that duct tape on their & pull it off in the same session & test it for science. Then let us know how you go

Kelly Slater would prob be the only person that could tell you if the "gaps" are a negative situation..

I believe your all right as much as your all wrong.
####Finboxequalityforall

I think you're right D67, what's the point of doing all those great things to ensure a perfect laminar flow over the fin when you're just going to bugger it up with holes and bolts and screws. That's why I've always said that fancy fins catch more surfer than waves.

But if someone held a gun to my head and said, "stop be a such a fat ugly troll and give us a frigg'n answer", this would be it:

at a rough guess, a 10 in high fin with a base of 5 inches probably has a total surface area or around 24 squ inches, (I hope) the total surface area of the remaining openings, and assorted iron mongery, may only add up to 1 or 2 square inches.

so that's 24 squ inches that you can definitely get working for you in terms of a good shape and a good foil, and 2 squ inches that may, or may not, be working against you.

If that's the case, it makes good business sense to get the maximum working at its maximum.

Overthinking and fins - now we're talking.
I'm with Mick - the "hole" or "gap" may introduce a low pressure area, but can't generate air.
I once put a video camera underneath a 666 with a big single concave, camera behind the rudder / fin. (yes, my life is that sad)
Paddled about 5km with it, trying to ignore the massive drag.
The amount of air sucked under the board in flat water was astounding, would be huge in surf or disturbed water.
I was watching the air bubbles being directed at the rudder / fin from the concave, and thought "is that really a good thing?"
Have been fitting a forward fin box to a 666 to aid light wind windsurfing, and had the same thought - "will this empty thing be bad when paddling / surfing it?"
On line search found some blanking plates for boxes from the UK.

Being British, they are too long for an 8" US box, and too short for a 10" box.


But I guess the "hole" is shorter.
Lacey, my new Alkali Twin Fin Equality set.

Longboard fin blankers are just too easy to make.....plastic chopping board, shaped to suit, standard fin pin, screw & plate

I'I say it doesn't matter, you don't see pros filling the blank plugs.

Wave surfaces aren't flat, smooth and undisturbed and surfboards are usually turning, not going in a straight line with the bottom entirely in contact with the wave like say a boat on flat water.

Beer thinking now...
Isn't a concave supposed to cause lift, so would an air pocket really be a bad thing?

There were even boards with dimpled / jet bottoms to disrupt water flow to make them looser.. Some Body boards have dimpled bottoms too that are supposed to trap air an make them quicker.

Could it be similar to a golf balls dimples?

Sadly, every response makes sense in it's own way. Good discussion though.

Going down the golf ball path to dimpled body boards etc. Perhaps the trapped air is effectively reducing the surface tension and friction? But if that was the goal, would the bottom shape be different too?

Again- not up with my hydrodynamics, but wont the pressure difference of the 'hole' produce bubbles ('vapour')? A boat prop turning through the water will produce bubbles - granted bigger surface area, a bunch of torque etc, so different ball game.

And i like the idea of the amount of surface area and subsequent little/no impact of bubbles based on a reduction of effective surface area.

it also seems like i'm not the only one that has considered this to some extent, so i may not be too lonely in the looney bin!

Further, there is no way i could ever tell the difference. Just thought it was interesting that there's all this consideration in regards to water flow on/around fins, through bottom channels, yet the boards have all these essentially squared-edged holes in the board. To me it just seems a little counter-intuitive. i guess in my mind, it either matters or it doesn't.
if it doesn't matter, why pay a bunch for a hand-foiled fin?
if it does matter, why have square-edged holes in the board?

Cheers!

Don't feel sad bro, your threads one of the most thought generating & interesting this week

Non square edged fin boxes...better be quick to beat me to patenting that idea lol...hey why not. Your thoughts are far from being alone as demonstrated by the fin inserts already available. Non square fin box holes is certainly thinking outside the box & I'm sure someone would probably buy them. Plenty of punters these days opting for the options available...please never feel restrained from sharing your thoughts...cheers!

Regarding paying a bunch for hand foiled fins, I never have...I did pay $145 for Futures Solus fins about 5 or 6 years ago, but hey were not hand foiled. Moulded ka'ching - ka'ching, second most expensive set was a 5 fin set for $79. Otherwise all my fin sets have been $40 to $55 & I'm more concerned about their size, shape, construction, materials than exactly how they have been finished. Last point doesn't really concern me.

Just interested if anyone has used rear quad fins as sidebites for a thruster set up on longboard

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Ricardo1709 said..
Just interested if anyone has used rear quad fins as sidebites for a thruster set up on longboard

Yeah I use Shapers Asher Pacey AP1 sides & Ruben Roxburg 5.5" centre on the Whalebone & Tolhurst....love'em