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  #21  
Old 10-11-2011
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Quote:
Originally Posted by bobperry View Post
Smacker:
You were thinking of "Tubercules".

Very soon my blog will be attached to a site where I will be paid for hits. I can't give out the details yet. but it is happening.

I appreciate your kind words as ever. I'm having some fun with it and the traffic is quite good.

Still got the light on for you. You must have some reason to come to Seattle.
Yeah that's him!

I'm glad you're figuring out a way to monetize your efforts. They deserve it.

And don't worry, I'll get up there. And I'll bring the scotch. Promise.
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  #22  
Old 10-11-2011
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Smacker:
I'm not drinking. I'll watch you drink.
I have now gone 23 days without drinking and I think I like it. I had a big Sunday dinner with company, roast beef and baked spuds with Caesar salad and while the others drank nice wines I drank tea. Tea is really not a good drink to go with dinner. But I'm a better cook when I am not drinking. I just, from time to time, find myself reaching out for that glass of wine that isn't there. Damn!

Good news is that I have lost 14 lbs. and I am looking my old kumite weight of 197 lbs. right in the eye. Beware of angry old guys.
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  #23  
Old 10-11-2011
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If you ain't drinking I ain't drinking. We'll just sip tea on the porch and be angry and old to all the passers by. I already have the black knee-high support hose with suspenders and bermuda shorts to complete the motif. This could be fun.
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  #24  
Old 10-11-2011
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That's actually the standard uniform of Australian angry old farts. With the obligatory short sheeved white shirt.

"Passers by"? There are no passers by up here unless you count seagulls, seals, sealions and bald eagles. I haven't seen anyone all day. I saw the mail truck go by but all I saw was an arm out the window. One of my clients signed me up for the NRA so I get a lot of mail.

I have no problem being around drinkers. I am not judgemental although I'm going to work on that.

It's blowing about 20 with gusts to 25 or so. Not the right weather to sit on the porch.
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  #25  
Old 10-11-2011
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I think we still have a lot to learn with whales and fishes in what regards hydrodynamics:

The skin of a shark is like sand paper. Why is not smooth like silk?

Well, natural selection does not make mistakes in what concerns improvements and the shark is one of the oldest spices alive, quite primitive in some ways, but in what regards fundamentals is just a very efficient animal and one of the most important things for a sharp is speed.

It skin is not soft like silk because its rough skin works better in what regards hydrodynamics and makes it faster.


"The structure of the shark’s skin is made up of tooth-like elements and of tiny grooves running parallel with its body. The grooves somehow mute the development of turbulence in the water that’s being dragged by the moving shark.

The water that a boat or a fish or a swimmer drags with it is one of the major reasons it takes so much energy to move through water. A dolphin also has a special skin. It's soft and compliant. It yields to your touch.

People who can do the mathematics of fluid flow, struggle to describe the action of either a shark's or a dolphin's skin. If they ever manage to do so accurately, they might be able to create artificial skins for ships -- skins that would save an enormous amount of energy and greatly reduce fuel consumption."


If you want to know more download "Passive and Active Flow Control by Swimming Fishes and Mammals" from F.E. Fish and G.V. Lauder. Just google it and you can download the full paper.

I know that experiences have been made in navy submarines (and those are top secret) and even on Americas cup boats (these ones without any conclusive result) but I dream the day I would put some anti-vegetative with a rough touch, to help to improve boat speed

Regards

Paulo

Last edited by PCP; 10-11-2011 at 06:23 PM.
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  #26  
Old 10-11-2011
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On the AC boats those were called "Riblets" or something like that and I'm pretty sure the rule banned them later.
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  #27  
Old 10-11-2011
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Bob,

I looked at van Nierop et al. and there seems to be something of a tradeoff here. They compared foils with smooth leading edges to foils with varying sizes of tubercles, but otherwise similar shapes. They found that the smooth foils had the highest coefficient of lift (Cl), but that the relationship between Cl and angle of attack had an abrupt inflection point at about 12 degrees (i.e., the foil stalled very suddenly at 12 degrees). In contrast, if tubercles were added to the leading edge the foil, the peak lift coefficient was reduced somewhat, but higher angles of attack could be used without a resultant stall. They show that a smooth foil had a peak Cl of about 1.2 at about 12 degrees, with the aforementioned precipitous drop-off in lift above 12 degrees. Their "tubercled" foils had a peak Cl as low as about 0.9, but a much flatter Cl/angle of attack curve at stall. (Note: The foils used in van Nierop et al. have tubercles along the entire leading edge, rather than just near the root of the foil, as in the pic on Bob's blog. So I suspect that the relationship between angle of attack and Cl would be a bit different for the actual boat rudder in the pic.)

Correct me if I'm misinterpreting things here, but it seems to me that a smooth foil should in theory give one the best performance, but the tubercles give the helmsman some feedback before the stall actually occurs. This would allow the helmsman better control, at the sacrifice of a bit of rudder performance.

EDIT: Changed "Cf" to "Cl" (my bad; I just reviewed a paper earlier today that was full of references to coefficient of friction, Cf; in other words, Duh)
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Last edited by SlowButSteady; 10-11-2011 at 09:14 PM. Reason: Duh....typing faster than thinking
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  #28  
Old 10-11-2011
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Yes, I agree but in the field with the new rudder the stall angle is increased. The rudder is effective longer. With the top part of the rudder being the most prone to "ventilating" maybe having the bumps only at the root end is sufficient.

I'm sure if tested with the ends closed the smooth foil would win. But in this case the ends aren't closed and one end pierces the surface of the water.

I did not design the bumps. I designed a smooth rudder. The owner likes the bumps. He thinks they work. Maybe you see what you want to see. I thought the boat was fine as it was originally but I like seeing a happy owner and one who is winning races, i.e. Swiftsure this year overall.

I'm not going to defend the bumps. I'm not going to defend the paper. I'm just showing you what can be done beyond the armchair world of theory.

I was watching the soccer and thinking about your post. I think this is a case of a clever designer reading what the theorists have to say then doing his own thing with the information. Call it being "creative". That's how you stay ahead of the competition instead of just following exactly what the book says.
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Last edited by bobperry; 10-11-2011 at 07:56 PM.
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  #29  
Old 10-11-2011
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Bob,

If you like iced tea. go to macy's or equal, they have/had a 2 gal glass jar with a lid and spigot, much like a mini keg. I've been putting 10 bags of Lipton and 5 bags of Constant comment and about 1.5 cups of sugar. I like it on the sweet side, ie 2 cups, spouse a bit less ie 1 cup, so we've settled on 1.5! Lasts me about a week. put the jar on the lower shelf of the fridge. I usually put HOT tap water in initially, and let it steep for 2-3 hrs.

Then proceed to drink out of my slooper cups! If you do not have one of those, Im sure some one local from the sloop would get you a customized one! I've worked the hard way placing in a few of the races. Hopefully this saturdays fall regatta will be better than the FWB result a couple of weeks ago.

Marty
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  #30  
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Bob,

Yes, the stall angle will be increased, but at a lower peak Cl. (Note that van Nierop et al. used foils with the distal edge unattached, as in a rudder or a whale flipper, for their experiments) I'm not knocking the utility of the method. Without some indication that rudder stall is imminent it would be very difficult to sail very "close to the edge". In other words, the benefit of having a smooth foil might never be realized because the helmsman wouldn't dare get that close to the stall angle, for fear of stalling and loosing control. With tubercles, one could put the rudder further over (i.e., use the rudder at a higher angle of attack), but the lift generated would be lower (albeit, marginally so) than for a smooth rudder.
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