hydrologists, a question |
No more than the general water flow past the blade does already...... |
yes but there is drag caused by the vortex moving from leeward to windward, if this 'drag' which we get anyway can be redirected, it can be used to advantage down wind. |
Er-no. |
Would B Max rog advocate a sharp but square or razor sharp TRAILING EDGE? Emotion says the latter but science? |
The spitfire wing shape i agree has less vortices produced, it does however stall almost evenly along the trailing edge, = completely hair raising sailing on a fast run. |
Geek; Ooo, a tricky one that. Some books say that a squared off trailing edge acts like a kind of anti stall device, by generating turbulent flow off the back edge of the blade. By squared off, they mean shaping a perfectly razor-edged foil, then cutting off the last 5% of the chord, PERFECTLY SQUARE. I suspect this to be the practical downfall-how the hell do you keep your beautifully square edges undamaged as you bang the rocks in Salcombe? Razor edged is probably better in the real world. |
It makes very good sense, on the profile front years ago I'll admit Alan Warren & Robin Judah had the same profiles a paralell sided blade with a rounded (semi circular) bottom - rake to personal choice and need for "feel". Bob Hoare cut his square but that meant it had to be perpendicular. |
There was a discussion similar to this a while back. It's something I studied a few years ago. Vortex control certainly is important, but the most effective method depends very much on the blade shape and orientation. If the trailing edge is completely vertical then one small 'concentrator', meaning something sticking out behind or a point or a spot a sharp cut off, might be sufficient and it would make sense to to put it at or near the bottom. If the trailing edge rakes forwards at the bottom, then another concentrator or one higher up would probably work better. If the rudder rakes aft at the bottom, (watch out for capsizes,)then the concentrator must be at the bottom. The concentrator need not be big if well postioned. Look at modern commercial aircraft wings like the Airbus and you'll see quite a few concentrators and flow controllers... mind you they are just a little bigger. |
Thank you NeilAncientG, Tis a long time since I studied fluid mechanics at university, thought I wasn't pondering over nonsense. Refreshed my knowledge on nasa stuff, whitcomb winglets etc.. but couldn't quite find anything about what I thought should work. |
Don't know if this helps, but I've just bought Merlin 3428 and it's got a funky rudder tip apparently for just this purpose. Does seem to handle differently but not sailed it enough to be convinced if it's better or not! Need to sort out the free play in the pintles first. http://www.smoothinternet.com/Merlin3428/RudderSideView.jpg |
OK, here's one for NeilAncientG - my Turner rudder (actually both of them; I have a fixed and a lifting) has the then fashionable shape of vertical leading edge going to a rounded tip with a trailing edge in 2 straight bits, meeting at a slight angle at mid-depth, which is therefore the widest bit of the rudder. The present Winder rudders pretty much copy the shape but with a tip that rakes forward, presumably to hook up the weed? So is there art or science behind this shape? I suspect the reason is about control of vortices and that it will encourage a small vortex at mid-depth as well as a controlled tip vortex. But does anyone actually know? |
I had one of those rudders too! To be truthful I couldn't tell it apart in terms of sailing behaviour to a parallel sided vertical rudder with not quite flat but aerodynmically shaped and angled tip. I wish I could draw it coz it's shape is very difficult to describe - actually it was designed by prof Markaj (Spelling?)of 60s boat designing and research fame. I don't think anyone has done serious research into lifting rudders for a long time. |
I defy anyone to really tell the majority of rudder shapes apart, unless there are really gross differences in planform (profile shape). Far more important are factors such as section shape and nose radius/sharpness. A tight radius on the leading edge, combined with a section shape with a high negative pressure peak will make the rudder stall at the slightest provocation-it's planform could look like fried egg for all the difference that would make comparatively. Many rudders that stall easily may not even have a vertical leading edge 'cos they aren't securely fixed, and anyone who's sailed a Laser in a blow knows how stall prone that makes a rudder. |
The whole point of concentrating a vortex is to make it as tight and as small as possible, thus shifting as little water around as possible and wasting less energy. |
I remember that in the 70's Stars had a fashion of ptting knobs on the side of their masts sames things? |