Topic : vortex tip drag from rudder

hydrologists, a question

Will a small tube protruding aft (12mm aft 6mm dia) off the tip of the rudder blade [505 shape blade] carry the vortex off the blade?

Posted: 19/07/2005 15:50:56
By: Ponderer
No more than the general water flow past the blade does already......

Posted: 20/07/2005 10:13:45
By: BmaxRog
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.

As edge wings are out, was wondering if the drag we're stuck with could be utilized.

Posted: 20/07/2005 11:32:16
By: ponderer
Is the short answer. Long one would fill several volumes. Best you can do is make the rudder tip the best shape you can to generate the smallest possible vortex. An approximation of the classic "spitfire wing" is as good as any, but just be aware that you need to make the thickness of the foil less as the chord gets less towards the tip, so the thickness:chord ratio stays the same-ish.

Posted: 20/07/2005 13:31:33
By: BmaxRog
Would B Max rog advocate a sharp but square or razor sharp TRAILING EDGE? Emotion says the latter but science?

Posted: 20/07/2005 13:49:51
By: Geek
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. 

What are your thoughts as to best practical shape for a rudder blade that doesn't stall?

Posted: 20/07/2005 14:10:21
By: ponderer
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.

Stalling is a whole topic in itself. You need to differentiate between stalls caused by ventilation (air being dragged down from the surface) and just plain old mega turbulent flow (bolloxed tack in a drifter). (BTW cavitation does not and will not occur on dinghy foils, and never will 'til someone gets a dinghy to do 50 knots.) Tip vortices may well actually help prevent stalling, despite their drag, as, if you think about it, they're lowering the negative pressure on the back of the blade, hence decreasing the tendency to ventilate. This is possibly why spitfire wing shapes are low drag (good) but stall prone(bad). A more conservative square tip is probably more forgiving in extreme conditions-save the sculpted carbon scimitar blade for the light stuff.
Sorry to be anoraky, hope this makes some sense.

Posted: 20/07/2005 14:57:33
By: BmaxRog
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.

Posted: 20/07/2005 15:10:04
By: Geek
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.

Posted: 21/07/2005 15:58:07
By: NeilAncientG
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.

Posted: 21/07/2005 17:41:12
By: Ponderer
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.

Posted: 21/07/2005 18:44:54
By: Stuart
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?

Posted: 22/07/2005 11:47:45
By: Andrew M
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.
If the Turner rudder is like mine, I doubt that the trailing edge angle would have been sufficient to to induce a vortex. Actually I think it was a bit of flash design. The bottom tip of my old turner rudder certainly was sharp enough to induce a vortex... until it got broken off at Midland Sailing club!

Posted: 22/07/2005 12:31:13
By: NeilAncientG
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.
Vorticies are generated when there is a pressure difference between two points in a fluid which aren't seperated by a physical barrier. I can't see how you can get a vortex in the middle of a rudder blade, at half depth, as the blade itself seperates high from low pressure?

Posted: 22/07/2005 13:22:52
By: BmaxRog
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.
You can make a tight vortex wherever the 'induced' (side) flow over the rudder is moving towards and you introduce a 'slip-off' point.
It's a very complex issue, agreed. It's a lot more than just high and low pressure! Actually it doesn't just apply to rudders, but all foils including the centre-board, and even hull surfaces. You might even say that having a top batten sticking out of your sail is benficial in the same way. Induced drag is, without doubt, the cinderella of hydrodynamics.

Posted: 23/07/2005 00:42:50
By: NeilAncientG
I remember that in the 70's Stars had a fashion of ptting knobs on the side of their masts sames things?

Posted: 23/07/2005 10:06:12
By: Ancient Geek


To Reply, please join/renew membership.

Owners Association

Developed & Supported by YorkSoft Ltd


Merlin Rocket Owners Association