20 interesting statemente re quad v tri

[waka]

well said
Steeno.

[waka]

headwax
who is the unamed guy?
tris , quads- so what
we had a guy rideing a twinnie today!
and everybody who showed up today
are all winners

[willli]

but headwax i do get it, the assertion
I just don't get the method of proof
and because so many simultaneous events are occuring
assigning causality is problematic
and I've heard "boost" arguments here regarding hi aspect flex fins as well
only then it was spring
now sails and wings
and only one thing is certain
a good board is a good board


once the chief ordered me to dewater a basement
I tried to explain to the firemen setting up the eductor how adding water would enable us to remove water from the basement,
but an argument ensued about Bernouli being an astronomer and nothing else
so in principle he never got credit for the falling water level
I think by acclamation they assigned this wondorous event to DaVinci

[red]

Good work on the calculations. So is it better to have a fin within the turbulent flow of another? This aspect could do more for developing a real hypothesis on board design that could support/disprove a whole heap of the empirical (i.e. people trying different things) efforts out there.

[john -]

or as Ron Romanosky relates on his site

quote as near as i can remember

" I make a board for my self and i feel its a dog ...it does nothing for me...a friend has a go and loves it"

more people are perhaps tuned into the process of a tri/thruster but a number of extreamly good surfers ride quads

and then there steeno whith an interesting spin that distabilisers most arguments

and theres the photo of Bud, Farrer and Parkes chating at Bud's place at the recent gathering - ( a powerful photo) - a quad man, a tri man and a man who rode both a tri and a quad at the north shore 05


even

dorje

[MTBarrels]

...<snipped>...So is it better to have a fin within the turbulent flow of another? This aspect could do more for developing a real hypothesis on board design that could support/disprove a whole heap of the empirical (i.e. people trying different things) efforts out there.

I don't think there is any easy answer.

As you have noted, the flow aft of a fin will generally be more turbulent than the flow incident on the forward fin (especially as more lift is being generated). Hence if the aft fin lies within the wake of the forward fin, it is probably true that the aft fin may be capable of achieving of a higher lift coefficient without stalling--if the Reynolds number for the aft fin is less than 1 million (and especially if substantially less). At Reynolds numbers in excess of 3 million, the effects of increased turbulence in the water incident on the aft foil probably has virtually no effect at all.

However, generating lift means that the fins are deflecting the flow somewhat (i.e. adding transverse momentum) in the direction away from the direction of lift. Hence the direction of the flow incident on the aft fin (i.e. the bulk flow) will be from a different direction from that on the forward fin (i.e. the angle-of-attack of the flow for the aft foil will be reduced). Hence, everything else being equal, the lift generated by the aft foil (per unit area) will be less than by the forward foil. Whether this is desirable, or not, probably depends on what the rider likes, and to what extent this occurs (i.e. a function of the angle-of-attack, fin separation, fin planforms, etc.).

Equally important (or perhaps even more importantly) as a fin generates lift (especially at high angles of attack--e.g. approaching, and or entering a stalled condtion), it is also likely that the wake from the forward foil is a mixture of turbulence combined with a less chaotic, less isotropic mix of eddies that do not have the random nature of turbulence. These larger-scale flows are also likely to have a major impact on the lift generated by a second foil lying in the wake of the first. For example, over short time scales (but longer than those characterizing the true turbulence), the eddies present in the wake may alternate between increasing and decreasing the angle of attack of the flow incident on the aft foil. Hence it is also possible that the the angle of attack at the second foil may be momentarily sufficiently increased so that the aft foil prematurely enters a stalled condition. Because of the hysteresis involved in reestablishing a non-stalled condition, the foil may not revert back to an unstalled condition when the eddy flow subsequently results in a momentarily reduced angle of attack for the aft foil.

Perhaps one means of exploring this interaction between the two fins would be by replacing one of the fins with one of the swiveling fin/box systems being developed by Blakestah. Another possible approach might be altering the aspect ratios of the two fins (so that the change in lift coefficient per unit change in the angle-of-attack compensates to some degree for this interaction). It's also likely that there are many other possibilities. However, it's beyond my ability to envision whether the end result(s) would be beneficial, or detrimental from the standpoint of the perceptions of the rider. Hence I guess one would be back to empirical studies again.

mtb

[Jerry]

This is just an irrelevant opinion of an old geezer, but here goes: I ride tri's, quads and twins. For late drops, bottom turns on big waves that I can throw all my weight into and projecting down the line, and for tube riding I like quad's. For going straight from the bottom of a wave to the top of a wave and quickly back to the bottom again and for quick cutbacks in the pocket I like tri's. For going out on the flat and making long sweeping cutbacks both quad's and tri's do fine, but do it differently. As for twins, some of the best boards I ever rode were twin fin swallow tails that the late Bill Caster shaped for me back in the late 70's, early 80's. Reproductions of those boards still work like magic today. But most of all, I just think it's fun to ride different kinds of boards. And ain't fun what it's all about. :D

[red]

MTB

The most intelligent post on this thread.
Thanks. It'll take a while to absorb, but it's opened up a range of ideas about water flow under (and over) boards in general.

If a good board is a good board, why do 90% of kneeboarders feel the need to ride a custom board rather than a clone of a "good board"?

[john -]

red

because excepting that a good board works for the person who rides it wheather it is custom or not is the most diplomatic and sensible way to avoid over analysing and suggesting that some style or some shaper/brand has the market cornered ....it keeps us open


further my board, a quad 5'10 rounded square is a good board for the conditions i get here and i seem to be surfing well upon it.... however, as i learn more about design i will enjoy chating about possibilities with the shaper and trying some new things...i might even return to a tri and employ different elements in terms of length a tail type...i mihgt not


but if im happy with it then its a...." good board " and some one then telling me if only i had one more fin or less fin or this or that is just shooting the breeze and if they go further and suggest that there style is the only style then they are not really broad enough of mind to relate to


truly ....i have not seen that many kneelos who get to surf in such prime conditions consistantly and who are that good that that they need to be that locked in to a design

my board is a good board my next board will be different and a custom and with luck and care it will also be good

fair ?

[ScottMac]

..

[lowrider]

an interesting discussion ( for those that can understand it....ahem) over at Swaylocks on this:
Surfboard and Fin Hydrodynamics at Swansea University
http://www.swaylocks.com/forum/gforum.cgi?post=199085;

MTB,

love your flex-spoons; do you still ride 'em?
Big single fins appear the way to go for spoons, due to the flex characteristics you want to encourage in the outer corners of the tail...
but what's your take on singles for use in more 'conventional' craft?
I ask, as I want to experiment with a narrower plan shape, say 20-21"x5'6", say (and thicker than the norm, maybe more of a 'hull').
I guess I'm thinking retro too, heh..just want to try something different.
any comments welcome