Return to Winged Keels

Zonker

Super Anarchist
9,292
5,225
Canada
Ah ha. A double tandem keel.

If you have a big CFD budget yeah you can make it work. For the rest of us, you'll have to recognize that it won't be as efficient as a regular keel but it's a good solution. Thanks I didn't understand what you meant at first.

 

pschwenn

New member
That's a weird way of thinking. Vortexes don't help with lift; they're because you have 2 different pressures mixing.

If the glider and airplane builders could make 100m long wings that wouldn't fall off and fit in airports, they would. They would have small tips.

The only big vortex is at the tip. If the tip is tiny, tiny vortex. Tiny drag.

Short stubby keels have big vortices.  And the 8m should have stayed away from the all you can eat buffett.
I agree that tip-vortexes don't help with lift, they can only detract from it.

But the rotational momentum inherent in the greater speed on the lifting side of the keel than the other side cannot exist without the equal and opposite momentum of the "well behaved" vortex sheet flowing off the aft edge of the keel or sail.  Conservation of momentum may sound theoretical but it underlies the real magic of lift. 

You can see Circulation initiate and fully develop in a flow chamber.  What you won't see is flow "bouncing off" the bottom of the wing, or a symmetrical foil like a keel or acrobatic wing forcing flow to go further, faster over the bulge on top of the foil than the flatter underside. 

Among the valid alternative ways of looking at lift you might find quicker or simpler ways to get the answers, even more accurate answers [the turbulence in circulation vortexes is not understood well enough to compute correctly], just as Ptolemy was used in space flight calculations before computers could do any more complex calculation on time.

regards,

 

pschwenn

New member
Ah ha. A double tandem keel.

If you have a big CFD budget yeah you can make it work. For the rest of us, you'll have to recognize that it won't be as efficient as a regular keel but it's a good solution. Thanks I didn't understand what you meant at first.
You could double down on the CFD versus "regular keel ... good solution".  Many of the puzzles at the technical edge of design seem beyond the experience+insight+tried'n'true approach, but for many of those puzzles, you get an answer with CFD, and it looks precise and formal and authoritative.

But, for example, turbulence cannot be yet treated correctly in CFD (if ever; like the weather it's not even known whether it is predictable period).  For another, CFD'rs change the manner and level of reducing things toward micro-scale elements that they find better at the reduction of our-scale stuff to, and in the meshing used to represent the sails, water, hull, ... .  The principal reason for this is to correct faults in existing CFD results, which also implies that some of the "successful" use in the past was incorrect. Third, CFD sounds good, like "AI", and the renderings of CFD look good.

regards,

 

Panoramix

Super Anarchist
Third, CFD sounds good, like "AI", and the renderings of CFD look good.
As one of my colleague who is a fluid engineer (for buildings so not as fancy!) puts it, CFD stands for "Colours for Directors". Nevertheless he still uses it to simulate natural ventilation systems and his designs seem to work!

I wish I had the skills to play with his toys as I would try to optimise all kind of keels!

 
Last edited by a moderator:

Zonker

Super Anarchist
9,292
5,225
Canada
I think it's a pretty valuable tool in my world. I'm designing some shallow draft tugs with conventional props in fixed nozzles.

The foil struts supporting the nozzle and aft shaft bearing were aligned fore/aft. The CFD showed how there was a lot of turbulence (ha) to this approach as the local flow was not fore/aft, but rather was influenced by the flow into the prop which exceeds the nozzle diameter. Basically the flow was coming in at an angle enough from the sides that the struts were very draggy. Conventional wisdom is to align the struts fore/aft but it's wrong in this case. It's very draggy. You don't see that in a towing tank because you're probably not even looking for it.

We've also used it to simulate waterjet efficiency at low (12 knots) speed. Typical waterjets go fast, and the transom is dry so the waterjets are thrusting into open air. Easy thrust = mass flow type of calculation by the waterjet maker. But at slow speeds the transom was partly immersed and the jets were pushing against the water so were less efficient (there is also considerable drag due to water hitting the curves inside the tunnel). The waterjet maker couldn't easily predict the first effect.

You'll drag our CFD cluster from my cold dead hands. Talk to other NAs. They won't give up these tools once they have experienced or used them.
 

They are not just pretty pictures. Compare this with the CFD (skip ahead to 4:00)






 

Panoramix

Super Anarchist
Yes, it is a pretty powerful tool. My colleague has managed to design ventilation for large schools that only use hot air bouyancy and wind. No mechanical ventilation, super cheap to run and eash to maintain. He learnt his trade by simulating fires in the London tube. "Same thing but not quite as hot" he would say. Cool guy to work with.

 

pschwenn

New member
Yes, it is a pretty powerful tool. My colleague has managed to design ventilation for large schools that only use hot air bouyancy and wind. No mechanical ventilation, super cheap to run and eash to maintain. He learnt his trade by simulating fires in the London tube. "Same thing but not quite as hot" he would say. Cool guy to work with.


12 hours ago, Zonker said:

I think it's a pretty valuable tool in my world. I'm designing some shallow draft tugs with conventional props in fixed nozzles.

The foil struts supporting the nozzle and aft shaft bearing were aligned fore/aft. The CFD showed how there was a lot of turbulence (ha) to this approach as the local flow was not fore/aft, but rather was influenced by the flow into the prop which exceeds the nozzle diameter. Basically the flow was coming in at an angle enough from the sides that the struts were very draggy. Conventional wisdom is to align the struts fore/aft but it's wrong in this case. It's very draggy. You don't see that in a towing tank because you're probably not even looking for it.

We've also used it to simulate waterjet efficiency at low (12 knots) speed. Typical waterjets go fast, and the transom is dry so the waterjets are thrusting into open air. Easy thrust = mass flow type of calculation by the waterjet maker. But at slow speeds the transom was partly immersed and the jets were pushing against the water so were less efficient (there is also considerable drag due to water hitting the curves inside the tunnel). The waterjet maker couldn't easily predict the first effect.

You'll drag our CFD cluster from my cold dead hands. Talk to other NAs. They won't give up these tools once they have experienced or used them.
 

They are not just pretty pictures. Compare this with the CFD (skip ahead to 4:00)


I did not mean that it cannot be very useful and have broad scope.  It can and does, as you point out.

It can even be very useful when its not quite right - for example when its better than alternatives, or no better but much faster.  If it weren't valuable in use, few would find the inadequacies and look into them - its difficulties push fluid dynamics and computers ahead.

The fact the visuals can be beautiful and photo-realistic doesn't mean that they aren't among the most informative graphics.  My favorite Mystery video is of an imposing ship in open ocean developing roll momentum, hardly visible, in resonance with a not very heavy sea, then roll over and sink.  CFD permits studying such visually in whatever range of wind and sea conditions.

Now that many PCs have graphics processors with the power of super-computers of not so long ago, CFD expands rapidly be taken up by designers and engineers in areas "denied" it previously (perhaps like Bitcoin creation working down to home desktops.) 

-----------------------------------------------------------------------------

The guilt by association with AI was unfair to CFD.

regards

 

Zonker

Super Anarchist
9,292
5,225
Canada
Well we still invested something like $0.5M in our CFD cluster. Not supercomputer territory but not exactly cheap either. It uses 15 kW of electricity just to cool the room.

A few years back it would have been more profitable to mine bitcoin with it than do NA CFD, but then bitcoin mining starting using very specific processors to be more efficient.

 
Last edited by a moderator:

SemiSalt

Super Anarchist
7,752
269
WLIS
Just by the way, there are a lot interesting CFD pictures on the internet. This one is from The Physics of Sailing, a CFD Analysis.

2022-01-11_20-38-15.png

 

pschwenn

New member
That's a weird way of thinking. Vortexes don't help with lift; they're because you have 2 different pressures mixing.

If the glider and airplane builders could make 100m long wings that wouldn't fall off and fit in airports, they would. They would have small tips.

The only big vortex is at the tip. If the tip is tiny, tiny vortex. Tiny drag.

Short stubby keels have big vortices.  And the 8m should have stayed away from the all you can eat buffett.


I don't get it either, but I keep in mind that something I've heard, more often from the airplane guys than the boat guys, is "no vortex, no lift". Possibly the wings reduce the flow under the forward 2/3 of the keel which tends to reduce the pressure difference between the two sides of the keel.

Most of the boats with fancy keels are in classes with draft limitations.
The underlying key is "No drag, no Lift" !                [Yes, "No tip-vortex, no lift" is simply false, but the "forward" component of the fin's downstream vortex sheet is Lift.}

To repeat an earlier post "Lift" is simply a drag component in a direction you prefer.  But confusion results from considering only the positive connotation of "Lift".  One is thinking: more Lift, good deal.  But the real deal is the tradeoff between the Lift and the Drag that comes along with it.

To illustrate: when the trim tab on a keel is in use, toward good effect, the keel is no longer a symmetric foil and the Lift increases.  But, as with "Flaps" (there is no essential difference in effect between a keel trim tab and a Flap}, the Lift increases but so does the Drag, and that combination helps an aircraft land by lowering the stall speed and by slowing the airplane down.

On a sailboat it can help with leeway, rig/keel balance, aiming the centerline to permit getting the best sail trim, and as Yves-Marie Tanton indicates at the top of the posts below: in case the rudder falls off, or to reduce the autopilot load.  It can also remedy small keel/rig imbalance in the design. (I don't see how it would facilitate shallow draft.)

Sailboats don't often want to brake but trimming the sails to turn one way and the trim tab the other has the same effect as a speedbrake flap on a glider: for landing, Lift is decreased and Drag is increased, no worry on stalling - a glider's stall-speed is inherently so low.

Many Lift advantages.  But the vertical circulation vortex (not the tip-vortex) which in a sense is the Lift, also has the Drag component which aircraft use to land, and which slows the sailboat as well.

Also detracting from the keel Lift are the trim tab's reduction in the effective height of that vortex sheet by height of the gudgeon&pintle at the base, any kick-up of the aft tip to protect the trim tab from grounding, and any gap opened at the top of the trim tab when it swings off the CL.  Not much effect on a deep keel, more significant loss the shallower the effective keel height.

Below you can see the reduction in height for the shallow draft of the boat (sliver of drawing first below), same boat as built shown last below (fuzzy, look close), and on the taller keel in the middle image.

regards,

[everyone hates the tip vortex; the shark fin rudder in the middle (sometimes repeated on keels), may originate in the rocket fin fixation of the late 50s.  That tip helps generate, with the large sweep angle, a tip vortex energetic enough, even at modest yaw, to initiate stall at modest yaw.  A notable Soviet contribution to yachting.]

68ForumNewTop.gif

68ForumEndTrimTab.gif

68ForumSharkTips.gif

68HauledKeel.png

 

The Q

Super Anarchist
Trim tabs are another thing I spent hours trying to get information on. They seem to work well only in a narrow band of conditions.

For my own purposes, racing along narrow tidal rivers. The extra lift they gain at the expense of more drag, could keep you off the bank but close to it in the low current, where others may have to tack away. If the tacking away is across a strong current against you, it's a huge loss of forward distance.

I've not fitted a trim tab at the moment, preferring to sort out the rest of the design first. Though there is provision to fit one.

As for the boats requirements for shallow draft, the broad we use for 1 race day in 3 , is closed over the winter. For the first few races of a season the boats leave a brown trail behind them as the old years rotting leaves and silt gets stirred up by passing keels.. the bottom below that is soft silt.  The average keel depth? 3ft, the majority of the boats are in the 20ft to 30ft LOA range though there are up to 45ft sailing cruisers..

As I previously said I decided wings were not the thing for the boat, dragging wings through the sludge just wouldn't be a good idea..

 

pschwenn

New member
The underlying key is "No drag, no Lift" !                [Yes, "No tip-vortex, no lift" is simply false, but the "forward" component of the fin's downstream vortex sheet is Lift.}

To repeat an earlier post "Lift" is simply a drag component in a direction you prefer.  But confusion results from considering only the positive connotation of "Lift".  One is thinking: more Lift, good deal.  But the real deal is the tradeoff between the Lift and the Drag that comes along with it.

To illustrate: when the trim tab on a keel is in use, toward good effect, the keel is no longer a symmetric foil and the Lift increases.  But, as with "Flaps" (there is no essential difference in effect between a keel trim tab and a Flap}, the Lift increases but so does the Drag, and that combination helps an aircraft land by lowering the stall speed and by slowing the airplane down.

On a sailboat it can help with leeway, rig/keel balance, aiming the centerline to permit getting the best sail trim, and as Yves-Marie Tanton indicates at the top of the posts below: in case the rudder falls off, or to reduce the autopilot load.  It can also remedy small keel/rig imbalance in the design. (I don't see how it would facilitate shallow draft.)

Sailboats don't often want to brake but trimming the sails to turn one way and the trim tab the other has the same effect as a speedbrake flap on a glider: for landing, Lift is decreased and Drag is increased, no worry on stalling - a glider's stall-speed is inherently so low.

Many Lift advantages.  But the vertical circulation vortex (not the tip-vortex) which in a sense is the Lift, also has the Drag component which aircraft use to land, and which slows the sailboat as well.

Also detracting from the keel Lift are the trim tab's reduction in the effective height of that vortex sheet by height of the gudgeon&pintle at the base, any kick-up of the aft tip to protect the trim tab from grounding, and any gap opened at the top of the trim tab when it swings off the CL.  Not much effect on a deep keel, more significant loss the shallower the effective keel height.

Below you can see the reduction in height for the shallow draft of the boat (sliver of drawing first below), same boat as built shown last below (fuzzy, look close), and on the taller keel in the middle image.

regards,

...


A Simple Example: consider a submarine or an airplane in the shape of a large ring with the same foil shape all around. 

Moving straight and level, the forces on this ring wing are only making drag.  If the ring changes direction, the forces toward the desired direction are called lift, the others are called drag.  If the ring reverses this turn, those same forces, with no essential difference in cause, direction, dynamics or shape -- exchange their names "drag" and "lift".

Why use "drag" to name the underlying fluid dynamic forces?  Might arbitrary, initially even puzzling - which draws attention to the point at hand.  "Lift" like any good thing, is often unreliable or missing altogether, while "drag" is entirely reliable.

regards,

 

weightless

Super Anarchist
5,607
582
^^^ Huh? By convention the part of the vector that is in the direction of flow is called drag and the perpendicular portion is called lift.

 

Santanasailor

Charter Member. Scow Mafia
1,344
701
North Louisiana
A real puzzle is symmetrical airfoils.  Just how can an aircraft fly?  How can a model aircraft fly with balsa wood sheet wings.  Of course the explanation is angle of attack.  Some Hobbies defeated this by having asymmetrical hulls.  If twin keels or better yet, twin lifting keels Not work best.  Minimal drag due to the angle of angle of attack lift (though there will be some, regardless) 

I am not referring to the foils used in the latest America’s Cup craft. This is regarding cruising yachts or small plaining or displacement day sailers.  A good example are the ILYA scows, sailing with bilgeboards for better than a decade.  How much better might a thin asymmetric foil be?  Just asking not complaining.

 
Last edited by a moderator:

DDW

Super Anarchist
6,317
1,012
A Simple Example: consider a submarine or an airplane in the shape of a large ring with the same foil shape all around. 

Moving straight and level, the forces on this ring wing are only making drag. 
Only from a distant point of view. Every inch of that wing is making lift, but the forces are resolved within the structure so the net lift on the whole is zero. Make that ring out of something crushable, and it will crush itself, then you won't say there's no lift. 

Weightless above has the generally accepted definitions of lift and drag. You can make up your own definitions, but then no one will know what you are talking about. "Lift" and "drag" are simply the overall force vector resolved into two equivalent vectors, which are easy to measure and convenient to use in the design of real things. 

 

Steam Flyer

Sophisticated Yet Humble
42,491
8,690
Eastern NC
Only from a distant point of view. Every inch of that wing is making lift, but the forces are resolved within the structure so the net lift on the whole is zero. Make that ring out of something crushable, and it will crush itself, then you won't say there's no lift. 

Weightless above has the generally accepted definitions of lift and drag. You can make up your own definitions, but then no one will know what you are talking about. "Lift" and "drag" are simply the overall force vector resolved into two equivalent vectors, which are easy to measure and convenient to use in the design of real things. 
zackly

I've had a number of students who insist on confusing themselves over "lift" because it sounds like it should be directed "UP" or perhaps away from the nearest surface of some other much much larger body, or perhaps the water. Lift is always always always perpendicular to flow as WL said.

- DSK

 

weightless

Super Anarchist
5,607
582
Some confusion may arise from the way that lift and drag are introduced. Many popular descriptions show an airplane, a thrust vector and drag vector, a lift vector and a gravity vector. That layout implies desirable / undesirable, and up / down parings because of the airplane in the picture. That's a setup that muddles several concepts. Maybe it's good motivation for folks moving on to understanding airplanes? It's doesn't seem at all helpful in the context of winged keels working at various heel angles. I suspect that a simpler picture with flow, an object in the flow, drag and lift would be a better starting place.

 
Last edited by a moderator:

DDW

Super Anarchist
6,317
1,012
In your diagram of an airplane, at least you can have Lift equal and opposite weight, and Drag equal and opposite thrust. That is the "unaccelerated flight" condition. If you want to confuse someone, draw that diagram for a glider. Which is closer to the situation for a sailboat. Which has two such airfoils, in different media, pulling different directions. 

This is why I've often maintained that very few sailors actually know how a boat sails (to windward, at least). The keel is the only thing with any component of force going your desired direction. 

BTW, don't know if any of you are taking in the new PBS adaptation of "Around the World in 80 Days". A bit of fluff really, but I did like the scene with the balloon, which had sails to propel it.....  that must be from the Jon Favreau universe of physics. Not this one. 

 
Top