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what is making my board .. hum on bigger waves

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Created by Tardy 2 months ago, 20 Mar 2020
Tardy
3506 posts
20 Mar 2020 4:09PM
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I have a 8.7 placid ,I'm using it as a quad ,it was quite big waves today and fast down the line stuff
on the big drop i get this humming stuff happening ..wave size is probably 7 foot .
is it
1. too big a fins ...size 5 front 2 rears
.2. the quad set up
3. the type of fin .
4 .bad alinement ,of fin factory placement .

note this board is very good in medium size waves ....its just the big waves at a ripper speed it starts humming ...and getting a little bit airborne

any advise ?

wazza66
QLD, 497 posts
20 Mar 2020 6:42PM
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Are your fins smooth on the edges and are your grub screws all the way in?
You might need to sand the leading edges of your fins.

Nozza
VIC, 2363 posts
20 Mar 2020 8:01PM
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wazza66 said..
Are your fins smooth on the edges and are your grub screws all the way in?
You might need to sand the leading edges of your fins.


Trailing edges I think.
Square them off.
Or was that smooth them?
Helmy?

Tardy
3506 posts
20 Mar 2020 6:07PM
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wazza66 said..
Are your fins smooth on the edges and are your grub screws all the way in?
You might need to sand the leading edges of your fins.


ok ..so need sharper fins ...they are new fins ...

i have two other rears i have sharpened or sharpen to make them like rears ,i will try that ,as the rears are thickish and kinda flat on one side ..

yeah...that could be it ..wazza ,,,.they are not working as rears as well ...

i will try tomorrow ,and report back ...come to think of it they do feel a little draggy ...and do stiffen up as speed increases ..

colas
3836 posts
20 Mar 2020 7:05PM
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You need to round the trailing edges,. but ONLY ON ONE SIDE.

DO NOT SHARPEN them, it is too dangerous.

See my post in www.seabreeze.com.au/forums/Stand-Up-Paddle/Review/Fin-Whistle

MangoDingo
NT, 392 posts
20 Mar 2020 9:23PM
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Hang on a minute. Dude - did you say you're getting "a little bit airborne"? On the face?
Where are you? Jaws!?!
Jesus H Chrast. I'm struggling to get waves with enough guts to get the 14 Search goin' at the mo'...
I'm livin' In the wrong joint.
Whistling fins - 7 foot waves - it's a nice problem to have me old china plate!
Keep shreddin' brus!
Sick.

micksmith
VIC, 1397 posts
21 Mar 2020 9:09AM
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Hey mate because they're new they could have an imperfection, cast your discerning eye over them to see if there are any. We're talking slight rises or dips in material, the trailing edge as has already been stated, just some 100-150 grit and a couple of runs should do. Surf and repeat sand paper runs if necessary

Tardy
3506 posts
21 Mar 2020 7:22AM
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colas said..
You need to round the trailing edges,. but ONLY ON ONE SIDE.

DO NOT SHARPEN them, it is too dangerous.

See my post in www.seabreeze.com.au/forums/Stand-Up-Paddle/Review/Fin-Whistle


ha ..ha yeah i know what ya talking about .......thanks colas ...

i have put my rears on that i have sanded to imitate a rear shape ....both the same edge ,,shape ,like a rear fin ..should be ..

but yes being a windsurfer i know the danger of sharp fins ...

Tardy
3506 posts
21 Mar 2020 7:32AM
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micksmith said..
Hey mate because they're new they could have an imperfection, cast your discerning eye over them to see if there are any. We're talking slight rises or dips in material, the trailing edge as has already been stated, just some 100-150 grit and a couple of runs should do. Surf and repeat sand paper runs if necessary


thanks mick ..i will take a good look at them all ...and a light sand .....

mango ...its autumn pumping swell here dude ...gnarly ...sorry to say ..you can't visit ..borders are closes ...lol..bloody virus ..

lift off is like floating ..31.5 wide and quad ...you do get a little lift ...lucky the placid has lots of rocker ..its under control roger ...

Tardy
3506 posts
21 Mar 2020 9:35AM
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just looked and yes a very sharp trailing edge ...gave it a good sand ...the fronts where like razors ..

SunnyBouy
436 posts
21 Mar 2020 4:13PM
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It's a similar problem for windsurfing fins, the quicker you go the fins cavatate (sp) and draw tiny molecules of air off the back of the fin and those molecules rub together and create the noise.
Same with dinghy centreboards/rudders.

We used to either square off the trailing edge or taper one trailing edge (as mentioned) doesn't always work, but does mitigate it a little.

Tardy
3506 posts
21 Mar 2020 6:06PM
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I gave the fronts a really good rounding ,and I 'm happy to say its worked ..humming gone ...IT did seem faster too ...

thanks for the fin hints ...i would never of thought that would of made a difference ...

from now on, all new fins get a sand

colas
3836 posts
22 Mar 2020 3:29PM
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SunnyBouy said..
It's a similar problem for windsurfing fins, the quicker you go the fins cavatate (sp) and draw tiny molecules of air off the back of the fin and those molecules rub together and create the noise.


For windsurfing, it was ventilation (cavitation can only happen above 50knots and destroys even metal propeller blades, so no, no surfing fin ever experienced cavitation. You would see small craters on the fin). Ventilation is when the low pressure on the side of the fin could suck up air from the surface, killing instantly the lift for a spectacular high speed spinout crash. The solution was more in fin placement and board tail & rail shape so that the board could act as a lid to isolate the fin from the surface. It can happen in surfing or SUPing if you do not bank the board enough in turns and the rails stays closer to the surface... happened to me last week, the feeling is unmistakable, just as if your fins were instantly removed.

Humming noise has nothing to do with air. It is the (pure water) trailing vortex that oscillate and creates an oscillating force that make the fin material vibrate (air would dampen this force), reducing its efficiency by messing with the laminar flow. A perfectly sharp trailing edge will lock the vortex in place, but will cut you to the bone (or more). Locking the vortex to one side of a blunt trailing edge is a safe, working solution.

colas
3836 posts
22 Mar 2020 3:30PM
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Tardy said..
from now on, all new fins get a sand


Yes, and you will save a lot of money in leashes too :-)

WaveScience
VIC, 110 posts
24 Mar 2020 4:23PM
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It happens to me on big waves too but I just assumed it was the sound of me ****ting myself.

Tardy
3506 posts
25 Mar 2020 5:22AM
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ha ha ...yeah ..

yeah i also was sheeting myself this day ...so i went in and got the 9,11 insane ...i was hunting for the big ones after that ...
it was epic ...everything was just a bit bigger ..bigger turns ..longer bottom hand turns ..the proners where sheeting themselves when they saw me coming down the face ...one of them said to me in shore ,you proved a point out there today ...

supthecreek
1966 posts
25 Mar 2020 6:13AM
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Showed the crawlers to respect the Dude... sweet!

Nice to be away from the virus troubles.... don't let anyone back on you little paradise Island until this settles down. NO one.

Tardy
3506 posts
25 Mar 2020 6:54AM
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We're in lock down well i should say lock out .

gregjet
QLD, 5 posts
28 Mar 2020 6:50AM
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I have had experience with this...a VERY long time ago.
Made a 5'4" twin fin in the late 60's. We were still figuring out about how and where to put the fins at that stage. The little board would be fine in small and slow waves and in foamy water ( actually VERY good therein), but as soon as you slid down the face of a nice big smooth wall, it would hum directly in relation to the speed. On our board the problem was we had the fins with no cant and toe-in angles to a point at the nose. It was way to much toe-in. The noise came from a mix of cavitation and vibration of the fins themselves from the vorticies produced by the angle.
Check the fin set toe-in angles . I have forgotten what the comtemporary angle is but the faster the board goes, the LESS the toe-in you can have.
Try removing one set then the other and see if it goes away. If it is one set only, try putting bigger fins in the quiet set and smaller ones ( particularly shorter and narrower) in the noisy slots to help maintain fin area.
That's my 2cents worth

LeeD
1454 posts
28 Mar 2020 5:40AM
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Just bad fin causing hum.
Btw, I started shaping twin fins around '67, made over 50 by '68, and brought a 7'6" and 8'6" ers to Oahu for a Nov to March stay at Log Cabins.
I rode mostly Pipe and Sunset, including 2nd reef Pipe, max size Sunset, and 15'+ Pinballs with no problems......except for crowds.
NO toeing, cant from flexing.
Rode 12'+ Tarantulas...Jalama...with my normal 5'6" twin and 8" fins. Only the 3 of us out...from San Francisco. Over 40 guys watching from the cliffs and never went out.



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"what is making my board .. hum on bigger waves" started by Tardy