why are modern Kneeboards so BIG!!Why even kneeboard?

toofast · Post by **toofast** » Tue May 11, 2004 7:35 pm

Hart...Its kind of funny what you said at the end of your note...Something that we can untilise today..I work with some young surfers and they all talk about swallow tails as being the newest thing around...I told them swallow tails and basic fishes have bee around for yrs,they laugh and say i`m lying......By the way,i`m glad to see that someone else is riding on on...We people see mine,it blows them away.

Bryan Jackson · Post by **Bryan Jackson** » Tue May 11, 2004 11:53 pm

According to MTBarrels surfboards/kneeboards do not experience cavitation. I got into quite an argument (which quickly became overly technical, obtuse, and arcane and thus extremely boring

) with him over this phenomenon on the "which is faster: quad or tri" thread

.

Thus your post, Hart, referring to cavitation on a step tail caught my interest. So I'm curious as to your opinion on this subject? Do you think that surfboards/kneeboards experience cavitation?

Duane Bosch · Post by **Duane Bosch** » Wed May 12, 2004 12:58 am

Hi Hart..

My mate Carl Ebel from CT South Africa had a step tail a few years ago. The board had klinker style channels and a step in the tail.

Carl hated it. I hated it. Basically everone who rode the board hated the way it went.

The board was super fast in a straight line but as soon as you put it on a rail it just plain stopped. We all just assumed it was the stepped bottom and gave the whole idea a miss.

That is the only step tail (Standup board included) I have ever seen and it went bad, real bad.

Speaking of bottoms. I've seen pics of boards that have channels that are not straight. They are curved and intersect a regular intervals. What are these channels called? and what do they do?

In 1992 the Aussie kneelo team came out here and I bought one of your boards off of Gavin Coleman. It was my first 3 fin and I loved it. It was one of those boards that will always stay with me.

Keep uo the good work.

willli · Post by **willli** » Wed May 12, 2004 3:58 am

I'll take a shot at this "cavitation" thing.. and first you have to distinguish between the common tongue usage of the term and the technical engineering usage. In the common tongue 'cavitation' is a catch-all term to indicate all sorts of phenomena including the INTRODUCTION of air into water. For instance.. an improperly pitched outboard motor when the rpm's suddenly rise during acceleration is said to 'cavitate'. Again an outboard whose rpm's suddenly rise in a hard turn is said to have cavitated. Its easy to see that technically none of this is 'cavitation'. But if you've made a prop for a fast attack submarine and in drydock upon examining it find microscopic pits and cracks, how do you explain how they got there? Sand? Cavitation is generally associated with the science of bubbles and their formation, and bubbles can be very destructive..
So for our terms shall we say that the broken wall of foam rushing toward you is the cavitated face of the wave?
Or should we use the rush of air and water exiting a collapsing tube at Pipe as our example? Big time cavitation!
How about the way single fins would "sing" on a really hard fast turn.. was that cavitation?
As surfboard designers introduced marine engineering concepts into their craft wasn't it inevitable that specific phenomena that required deep math to understand were generalized into simple explanations? Some false?
So simply put, for mortal men, I suppose any design element that causes a break in laminar flow and the subsequent introduction of turbulent (bubble) flow is said to cause 'cavitation'.
Take a deep breath V E N T I L A T E. Exhale C A V I T A T E.
Stay low, stay loose, let it go..

hart · Post by **hart** » Wed May 12, 2004 12:22 pm

I agree with willli in that a break in laminar flow will create cavitation.

The step in the bottom of a board..up 1/2" deep will, in my opinion, intentionally cause cavitation and in the case of a Steppie..make the board feel shorter than it actually is.

And Duane..you had me sweatin' for a while there!! But another intentional element I put into Steptails is a fluted deckline..pull the top of the rail off the deck, down really low and pinchy..looks kinda like a flextail..only sculpted out of foam only..no glass.

This seems to allow the tail to bite..whereas without it, the step seems to make the tail want to break..just as you describe.

And curved channels that intersect?..not really sure..but I have shaped channel bottoms that terminate into the step itself..just sent a sixfooter to a guy in Indo who's stoked.

Beeline2.0 · Post by **Beeline2.0** » Wed May 12, 2004 1:02 pm

red · Post by **red** » Wed May 12, 2004 4:17 pm

This piece of a thread may be of interest. Headwax did a fair bit of research into aerated water.
http://www.kneeboardsurfing.co.uk/forum ... &start=210

Seems to me that there are only 2 ways for water to be aerated (call it cavitation in some contexts)
1. Air is extracted from the water
2. Air is introduced from outside

Boiling is a case in point for (1). As you know, water can be made to boil at low temperature if the air pressure above it is low enough. So one could speculate whether the step (or concave - Headwax) creates sufficient vaccuum at certain times to 'suck' air out of the water.

A more likely scenario is introduction of air, possibly sucked in where the step meets the rails. If the step meets the rails at 90 degrees then it may be that the 'sucked in' air does not make it all the way under the board, causing drag. Whereas if the step is sloped towards the tail air may be more inclined to make it all the way under the board and mix with the turbulent water flow there.

But I don't think that aerated water is the reason for steptails' performance.

Steptail function is probably closer related to wings and flyers (water breakaway and speedup effects).