| Re: Foil surface
[Re: arbo06]
#141205 04/25/08 07:00 PM 04/25/08 07:00 PM |
Joined: Jan 2005 Posts: 169 Santa Barbara CA sbflyer
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Posts: 169 Santa Barbara CA | Like polished glass! In the Bethwaite book they measure the turning force in pounds of rudder foils with 1200 grit finish, mirror polished, and they even chromed one (would never work in the real world as it flakes off too soon) and it was mid 80 pounds for 1200, mid 90's for mirror, and mid 120's for the chrome! On our Magic 25 which has a slender high aspect keel and rudder it was SO noticeable if there was the slightest film on the foils. The boat had gradually been broaching more and more, and the rudder didn't LOOK bad, but after spending a few hours mirror buffing it, it was an unbelievable difference! You could feel when the flow was about to shake off, and it would actually make this cracking sound as it stalled (way later into a broach) and you could quickly straighten the rudder and then steer again and the flow would reattach instantly. With the dagger boards any vertical scratching from going up and down makes tiny channels that wick air down from the surface, killing the extra bit of pointing you need to be competitive upwind...
Last edited by sbflyer; 04/25/08 07:02 PM.
| | | Re: Foil surface
[Re: sbflyer]
#141206 04/26/08 03:48 AM 04/26/08 03:48 AM |
Joined: Sep 2007 Posts: 757 japan erice
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Posts: 757 japan | and yet sharks that are more fully evolved to aquatic life than dolphins have a skin like sandpaper and currently someone has developed lumpy blades for wind turbines that seem to allow them to work better across a wider wind range. the idea being based on the knobs along the edge of humpback whale fins http://boatdesign.net/forums/showthread.php?t=17382&highlight=whaleso i wouldn't sweat a lumpy, rough surface if i were you
eric e 1982 nacra 5.2 - 2158 2009 weta tri - 294
| | | Re: Foil surface
[Re: erice]
#141207 04/26/08 07:57 AM 04/26/08 07:57 AM |
Joined: Jan 2005 Posts: 169 Santa Barbara CA sbflyer
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Posts: 169 Santa Barbara CA | I wouldn't either, as long as you don't mind not pointing high and missing tacks because the boat won't come around, or hitting a transom or two because you couldn't quite duck somebody. Bird are more adapted to air than us, but I never saw a aircraft with feathers. After watching Flipper beat up on sharks (which haven't changed since the dinosaurs) I know which one I think is more evolved. Another example from the Bethwaite book, they spend a bunch of time towing hulls two at a time on this balance beam arrangement, comparing known performance hulls against experimental ones. After a road trip to a different location they couldn't reproduce some results until they realized that the test boat had gotten a layer of road dust on it that wouldn't come off in water (think spraying your car down without washing it, know how it leaves that film). After soaping and wiping the hull down, it was right back on their old numbers...with the 1000x greater density of water than air little turbulence adds up.
Last edited by sbflyer; 04/26/08 07:59 AM.
| | | Re: Foil surface
[Re: sbflyer]
#141208 04/26/08 08:17 AM 04/26/08 08:17 AM |
Joined: Jun 2001 Posts: 9,582 North-West Europe Wouter
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Posts: 9,582 North-West Europe | I seem to remember that the rough skins of sharks is mainly benefical at low speeds where they tend to swim at 99.5% of the time. Their rare bouts of high speed attacks are limited only to catching food and make up only an extremely small portion of their total energy expenditure. Meaning evolutionary it was more beneficial to them to optimize low speed drag then high speed drag. Of course sport fish like marlin and tuna and dolphins tend to spend much more time at high speeds and have highly polished skins.
Evolutionary result must always be seen in such balancing of things. Birds never suffer from icing on their non-metal wings and therefor don't need special arrangements for de-icing. I have yet to see a winged airplane take off in an almost vertical direction from a complete stand still, a critical ability for a bird to survive. Although rockets (single flight wonders) could be regarded as exceptions to that rule.
Wouter
Last edited by Wouter; 04/26/08 08:59 AM.
Wouter Hijink Formula 16 NED 243 (one-off; homebuild) The Netherlands
| | | Re: Foil surface
[Re: sbflyer]
#141209 04/26/08 08:25 AM 04/26/08 08:25 AM |
Joined: Sep 2007 Posts: 571 Hamburg Smiths_Cat
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Posts: 571 Hamburg | The question of surface quality is not easy to understand. I will try to make a short explaination: Between fluid and surface of the foil is always friction. This friction causes the fluid close to the surface (the boundary layer) to be slowed down. If the slowing down is to strong the boundary layer changes from a laminar state to a turbulent state. In general laminar flow has less friction drag than turbulent flow. The surface roughness is a major factor of this slowing down, but if the shape of the foil is not designed to have laminar flow, you will never achieve laminarity even with super smooth surfaces. Conclusion one: If you have laminar foils, surface roughness is very important.
However laminar flow is very sensitive for early seperation (stall). That means that a non laminar foil produces in general more lift than a laminar foil of the same size. If stalling is a problem (manoeuvring) , rough surfaces may be helpful, especially if it fixes the transition from laminar to turbulent boundray layer close to the foils nose. Conclusion two: Laminar foils are not always useful.
Shark skin or riblets are not only rough surfaces, but rough and ordered surfaces, which may reduce the friction drag of a turbulent boubndray layer by about 5%, without having the problem of early stall. This is a not pure theory, but also tested in real life on aircraft and boats. However this technology is not for free, I think 3M once produced riblet films, but I don't know about costs and stability of the glue. Conclusion three: Shark skin is more than just rough surface
Hope that clarifies a bit the situation.
Cheers,
Klaus | | | Re: Foil surface
[Re: Wouter]
#141212 04/26/08 01:22 PM 04/26/08 01:22 PM |
Joined: Aug 2001 Posts: 1,307 Asuncion, Paraguay Luiz
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Posts: 1,307 Asuncion, Paraguay | Summarizing the previous posts:
- Rudder's turning forces (LIFT) occur and are measured for optimization at HIGH angles of attack. - Board's lateral forces (also LIFT) occur and are measured for optimization at LOW angles of attack. - Hull's DRAG forces occur at LOW angles of attack and are usually measured for optimization at NO angle of attack.
Optimizing a foil's surface at HIGH or LOW angles of attack are two different things.
Optimizing for minimum absolute DRAG is not the same as optimizing for MAXIMUM LIFT or for BEST LIFT/DRAG RATIO.
Conclusion: the optimal surface for each of the three situations must not necessarily be the same.
Luiz
| | | Re: Foil surface
[Re: Luiz]
#141213 04/26/08 05:14 PM 04/26/08 05:14 PM |
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Posts: 571 Hamburg | Hi Luiz, - Rudder's turning forces (LIFT) occur and are measured for optimization at HIGH angles of attack. Yes, for the sectional coefficients, it would be clmax/cd0. And the planform would be close to rectangular. The rational is a combination between elliptical (low induced drag) and low cl close at the surface to avoid cavitation. - Board's lateral forces (also LIFT) occur and are measured for optimization at LOW angles of attack. - Hull's DRAG forces occur at LOW angles of attack and are usually measured for optimization at NO angle of attack. I think that the angles are around 6° or so? Do you have any numbers? Optimizing a foil's surface at HIGH or LOW angles of attack are two different things.
Optimizing for minimum absolute DRAG is not the same as optimizing for MAXIMUM LIFT or for BEST LIFT/DRAG RATIO.
Yes, and foil optimisation is only one part, planform, area and span are even more important. As you wrote already in the asymmetric foil posting, hull friction dominates over foil friction effects. Hence the optimisation should not be a pure daggerboard optimisation, but a complete drag optimisation. In general this leads to a cLmax optimisation for both rudder and daggerboard and a minimisation of lateral area. However you will be able to optimse either for light wind or strong wind but not for all the range of possible wind speeds and courses. Or you make a fair compromise between the two, so you have neither a light wind design nor a strong wind design. I would go for the compromise. Cheers, Klaus | | | Re: Foil surface
[Re: Smiths_Cat]
#141214 04/26/08 06:30 PM 04/26/08 06:30 PM |
Joined: Aug 2001 Posts: 1,307 Asuncion, Paraguay Luiz
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Posts: 1,307 Asuncion, Paraguay | - Board's lateral forces (also LIFT) occur and are measured for optimization at LOW angles of attack. - Hull's DRAG forces occur at LOW angles of attack and are usually measured for optimization at NO angle of attack.
I think that the angles are around 6° or so? Do you have any numbers? When a boat points six degrees higher then its actual path there is probably something wrong with its foils. At this angle the hull itself is generating lots of lift. I believe the drift angle is typically around two or three degrees. Actual figures vary in the same boat depending on point of sail, speed, wind pressure, heel angle, etc. When comparing different boats under the same conditions, each one's drift angle will depend on foil's aspect ratio, section, surface quality, magnitude of the lateral forces compared to foil area, etc.
Luiz
| | | How to measure your boat's slip angle
[Re: Luiz]
#141215 04/26/08 08:29 PM 04/26/08 08:29 PM |
Joined: Jul 2001 Posts: 851 US Western Continental Shelf hobiegary
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Posts: 851 US Western Continental Shelf | To find the amount of "slip" that your boat is exhibiting, use a protractor that is shaped like a "D." These protractors have a flat edge (the left side of the D) and a hemispherical angle gauge.
At the center of the flat edge, a hole is usually found. Tie fishing line into that hole. Drag that line, with any sort of drag weight (split shot sinker) from the protractor. Place the flat edge of the protractor against your rear crossbar and trail the drag weight behind.
The fishing line will align itself with a degree angle on the protactor while you sail the boat. This reading of degrees, where the line crosses the protractor scale, will be the difference in the direction that the boat is aimed, and the direction that the boat is actually moving.
(OR... compare your compass reading with your GPS reading.)
GARY
Santa Monica Bay Mystere 6.0 "Whisk" <--- R.I.P. | | | Re: How to measure your boat's slip angle
[Re: hobiegary]
#141216 04/26/08 10:10 PM 04/26/08 10:10 PM |
Joined: Aug 2001 Posts: 1,307 Asuncion, Paraguay Luiz
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Posts: 1,307 Asuncion, Paraguay | (OR... compare your compass reading with your GPS reading.)
Remember it is aso necessary to compute the differences between magnetic north and true north, like magnetic declination and deviation. The proctrator system sounds easy and potentially more accurate. It requires only one type of measurement that can be easily repeated and averaged. Have you actually tried it?
Luiz
| | | Re: Foil surface
[Re: Luiz]
#141217 04/27/08 05:03 AM 04/27/08 05:03 AM |
Joined: Jun 2001 Posts: 9,582 North-West Europe Wouter
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Posts: 9,582 North-West Europe | believe the drift angle is typically around two or three degrees.
That is my conviction as well. 6 degrees of angle is more associated with boardless cats like the Prindle 16 Wouter
Wouter Hijink Formula 16 NED 243 (one-off; homebuild) The Netherlands
| | | Re: Foil surface
[Re: Smiths_Cat]
#141219 04/27/08 12:08 PM 04/27/08 12:08 PM |
Joined: Aug 2001 Posts: 1,307 Asuncion, Paraguay Luiz
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Posts: 1,307 Asuncion, Paraguay | ok quick sanity check: balancing weight: two guys on the wire =2*60kg 100kg cat =50kg beam: 2.3m mast height: 9m center of sail force: 40% resulting sideforce =((2*60+50)*9.81)*2.3 / (0.4*9) = 1066 N board height in water 0.6m, board width 0.2m, board area 0.12m² aspect ratio AR=3 and hence lift coefficient 2*pi*AR/(AR+3)*1.4 = 4.4 per rad = 0.077 per deg (factor 1.4 for hull effect) Assuming we travel with 16 kts = 8m/s and have one board down: board cL=1066/(0.5 *1000 * 8² *0.12)=0.28 board alpha = 0.28/0.077 = 3.6° slowing dowm to 12kts: cL=0.49 alpha=6.4° cL and alpha would be roughly half, if both boards are down and on a boat with weatherhelm, the rudders take some of the loading.
A 0,60 x 0,20 cm board seems small, but not too far from reality. The calculation considered 16 kts the capsize speed. 18 kts makes more sense to me, perhaps more. Also, I'd expect the speed reduction from 16 to 12 kts to be accompnied by a sifnificant reduction of lateral force. Note that the daggerboard, rudder and hull contribution to lift must be all considered together for this calculation. While the daggerboard is perceived as "the" source of lateral lift, the rudder usually operates athigher angles of attack, generating more lift per unit area. The hull is a low aspect ratio foil, but its surface is big compared to the other foils, making its contribution relevant as well. All considered, the sanity check seems to indicate that 2 to 3 degrees make perfect sense. Thanks! | | |
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