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 Post subject: RETRACTABLE CENTREBOARD!
PostPosted: Thu Sep 23, 2010 12:41 pm 
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Joined: Sun May 23, 2004 4:32 pm
Posts: 101
thanks, Rabidric, that's the keyword i was waiting for.

the next idea that struck me was the twist cut fins. i don't want to go deeper into this topic cause it led to a too conventional design which worked upwind totally grand. downwind was, well, let's say, no fun. see this for more info.
http://www.kiteforum.com/viewtopic.php?f=159&t=2364350

then i said to myself, fuck it, follow the very early idea of a retractable centreboad. can't be that tough to build. well and here it is. i used two backfins, because i think that it delivers more stability. later i want to try a single back fin. there was one testrun so far. the cernterboard is 36 cm in length. when set vertically (90°) the board is not stable at all. the problem i know too well by now. if i angle the board to one side the board will turn to the other side. at around 60° the board will not react to leaning to eighter side. it starts to work around 50°. at this angle the centerboards tip is around 30 cm away from the board. seams to be the magic number. anyways, for this angle the straps were mounted to far to the front. also i think i can use bigger back fins. i have not found the right setup yet, but i hope this concept works. why? because downwind is just awesome! it totally feels like a waveboard. i installed a mechanism to retract the centerboard with rubberbandrope that extracts the centerboard and a rope that pulls the centerboard back into the board. the rope runs through a clamcleat so the centerboard can be set to any position. plus when running aground it will give way.

more to come...
i call it raveboard.
h.


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 Post subject: Re: New Board Race 1 fin
PostPosted: Fri Sep 24, 2010 8:06 pm 
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Joined: Wed Aug 10, 2005 9:49 am
Posts: 224
wow great work!

I'm sure you'll figure out the right setup soon.
multiple front footstraps might help testing stance and centreboard combos i guess, and you'll want to try out a big single rear fin no doubt, to take full advantage of drag reduction when the centreboard is retracted for downwind.


there is a chance that you are at the start of a new paradigm of raceboard here....interesting times. :thumb:


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 Post subject: Re: RETRACTABLE CENTREBOARD!
PostPosted: Fri Sep 24, 2010 10:57 pm 
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Joined: Thu Jun 05, 2008 1:52 am
Posts: 1187
Location: Thailand
holden wrote:
thanks, Rabidric, that's the keyword i was waiting for.
....
then i said to myself, fuck it, follow the very early idea of a retractable centreboad. can't be that tough to build. well and here it is....

more to come...
i call it raveboard.


:thumb: :thumb: :thumb:

The Raveboard, with it's retractable fin is clearly paving the way for Raceboards, allowing them to perform to the max upwind as well as downwind, with the added bonus of easier handling.
Although I understand the efficiency argument, I personally do not believe going with less fins will improve overall performance. A modern raceboard is difficult to ride and keep flat and much harder on the legs because the torque imposed on the long fins at a point deep below the board's bottom. The purpose of the multiplication of fins is precisely to reduce the torque.
I imagine a Raveboard with three smaller high aspect ratio coupled fins would provide a much easier ride, with hardly any loss of efficiency.
BTW, if a shaper with the right skills can do such a 5 fin (or more) board for me, I'll be happy to order it right away. I have a suggestion for the outline as well. I'll take the blame if it turns out a failure!

I think the surfers out there should remember the season when Kelly Slater experimented single fin boards, he did absolutely no result, and resumed podiums when he returned to 3 & 4 fins. And earlier Mark Richards ended the era of single fin surfboards with his twin fins. There is a further technical advantage for surfboards to have multiple fins, but it's outside the scope of this thread.

:bye:
Alex

@Rabidric
"I'm sure you'll figure out the right setup soon. "
That can be achieved strapless initially for the correct foot placement.


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 Post subject: Re: New Board Race 1 fin
PostPosted: Fri Sep 24, 2010 11:57 pm 
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Joined: Fri Jan 15, 2010 4:52 pm
Posts: 12
Thats Cool!!!!

Now you can figure out where to put a windsurfing mast!!!

:baby:


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 Post subject: Re: New Board Race 1 fin
PostPosted: Sat Sep 25, 2010 6:14 pm 
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Posts: 224
@alexrider

i hear what you are saying about multiple fins and torque etc, but i feel that in racing every degree of efficiency will ultimately count sooner or later, and single rear fin might not have much of an edge over twin, but sooner or later that will matter to someone. furthermore you don't have to make it ridiculously high aspect- there is definitely an exponentially diminishing return to excessive aspect ratio and it may hinder speed on the downwind. (on the upwind, it is the centreboard you are relying on for efficiency).

oh and strapless is fine in smooth, but to figure out the balance when driving it at max competitive speeds over banging chop, multiple strap options will make it easier to experiment on the ragged edge of control.


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 Post subject: Re: New Board Race 1 fin
PostPosted: Sat Sep 25, 2010 6:35 pm 
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Posts: 735
tautologies wrote:
Why does not anyone try to create a sandpaper-ish type surface under the board? If we add covered in tiny ‘V’ shaped ridges, known as dermal denticles that should reduce those small whirlpools right?

Look at the shape of a golfball...it is not smooth, yet the surface reduces drag by creating turbulence....wouldn't this be feasible on a race board too?

You pretty much want the board to cavitate...and then use the fins to completely control the board...I guess intuitively you might be able to create the same surface on fins too...

anyways...I really don't know what I am talking about...but it has been stuck in my head for a while....

./A


Around 1990 I had a windsurf slalomboard with a bottomfinish similar to what you mention made by a local shaper. The board was very fast but I honestly think that had more to do with the overall shape than the finish but I unfortunately never polished the bottom and quantified any possible effect.


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 Post subject: Re: New Board Race 1 fin
PostPosted: Sun Sep 26, 2010 10:31 pm 
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Joined: Mon Feb 16, 2004 5:36 am
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Location: Oahu
SBBeachbum wrote:
Around 1990 I had a windsurf slalomboard with a bottomfinish similar to what you mention made by a local shaper. The board was very fast but I honestly think that had more to do with the overall shape than the finish but I unfortunately never polished the bottom and quantified any possible effect.


Well how about sharks..they have a rough type surface...it might have to do with them not being entirely straight, which we can have boards be. I think the idea is that by having that little irregular water movement between the board and the water we avoid getting major cavitation...

For racing tho it must be possible to quantify this with a different means than putting the board on your feet. I guess the margins aren't that small yet...oh well.

:-)


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 Post subject: Re: New Board Race 1 fin
PostPosted: Mon Sep 27, 2010 8:08 am 
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Posts: 224
hey tautolgies

I really hate to be a downer on ideas :oops: , but as far as i know that sharkskin thing has been a bit of a non-starter on boats and boards for years and years. sorry! 8)
as i hear, some success has been made with it in olympic swimmers suits where flow is completely turbulent anyway, but it doesn't work for objects at different scale/speed as it increases surface area a fair chunk(bad). I think it helps sharks at a low speed to be more efficient(they only go fast 0.00001% of the time), which is a bonus for them at their specific size etc, especially since they have to keep swimming to breath...

a different but intuitive take on it, is that dimples work on golfballs, but you don't see them on planes and cars etc.

the super technical explanation would be that at some specific combination of reynolds number and other conditions, a rough surface may reduce the drag you get from an already turbulent flow. however, it you can achieve laminar flow at all, that will usually be better. i.e. it ain't practical in nature to maintain a complete mirror smooth finish and an ultra sleek outline(need eyes,nose, mouth, gills etc), but mirror smooth finishes are better in 99.9% of circumstances.


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 Post subject: Re: New Board Race 1 fin
PostPosted: Mon Sep 27, 2010 8:23 am 
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Joined: Sun May 23, 2004 4:32 pm
Posts: 101
@Rabidric & alexrider: thanks for your mental support. i will find out the perfect strap positions.

finding out strapless is a bit hard i imagine because you kind of ride the board very differently. with straps you force the board into the perfect position and with speed and upwind angle the fins start to work better and better. multiple frontstraps is also not easy, because you can only make big steps. it is more a matter of 10cm. but it is ok. after a max of 2 sessions i will have it figured out. i hope...

i will stick to the 2 back fins for a while. it is an ok compromise. with the twistcut the turbulances are not too bad. plus you got truster fins if you angle you board too much. this saved me form one or the other crash already.

next testrun hopefully this week. back straps are further back, centreboard box will be sealed properly.

h.


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 Post subject: better by far
PostPosted: Wed Sep 29, 2010 9:23 am 
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Joined: Sun May 23, 2004 4:32 pm
Posts: 101
but not there yet. back straps way better than before. the board ist a lot better controlable now with the centreboard at between 50° and 60°. i had the feeling that the front straps need to be moved backwards for five to ten cm.

the centreboard sealing came off at the first 100m. i need to figure out the best way how to mount it. the gps readings where not too well. speed was ok, the upwindangle needs to get better. i need a testrun with a seal that works to draw final conclusions.

i also had the feeling that the grip of the back fins has some room for improvement. maybe i will make bigger ones earlier than planned. it is kind of fun, the back straps are more to the back than the back fins.


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