How Wings, Sails, Keels, and Rudders Really Work.

[Roger Long]

I think you've been editing! I'm ok with the last two pp of your site now (I bet you'll sleep better knowing that )-- though I still am not quite sure what it brings to the party.

I looked at it the next morning and decided you were right so the last two paragraphs are now gone. I was anticipating the most annoying of the responses that came back when the original article appeared but I think it's best to save those points if someone raises the question.

 

[RichH]

So after reading all the above I'm left wondering; is this "lift" the keel producing, and presumably the rudder also, going to make my boat go faster or just keep me from heeling a lot?

Indeed yes.
Back in the 60s & 70s a 'compendium' of such interaction was available at most engineering schools under the title "Ancient Interface". This was a collective of many papers from the pre-NASA days that investigated foil shape and the resultants .... also included hydro, ccomplex foils, composites, etc. The 'standard' accepted hydro shape for most 'all-purpose' boats remains the NACA64-010-.(do websearch for same to get into the total listing of NACA shapes, etc.) .. . foils for keel, rudder, and even staysail, etc. shape, etc. The amazing aspect of the NACA series shape profiles is that hydrodynamicists and fluid scientists find that these aero shapes work equally well in 'liquid' applications. Just about all modern 'production' boats use the NACA64-010 as their shape profile for keel and rudder as its probably the best optimized shape for 'recreational' sailing. (hint - if you're racing, consider to change/alter at least the leading edge keel shape to the 010 for 'best' advantage).
Faster being a relative term, VMG is a better term to compare this sail/keel lift ratio efficiency. Its still done today in the 'utra-racers' but since its all 'commercialized'/greedy/shysterized, unfortunately not much is shared.
Back in the late 70s there were many (unsuccessful) attempts to apply canards and 'leading edge' trim tabs to the keels of the 'ultra race boats' with Tom Blackalers AC entry being the most visible attempt. I think I remember that C. J. Marchaj included a discussion of variable AOA of keels in his earlier work. (BTW The same C J Marchaj now cites Arvel Gentry as one of his primary cite-sources for 'how sails work')

With the current regime of non rotating keels (now a preference to canting keels) it still comes down to the AOA of the sails *matching* (by some value) the AOA of the keel .... with respect to a keel that (horizontally) *lifts* to windward .... chasing that magic Velocity Made Good (VMG) number. The use of a GPS set to VMG function will help one realize WHEN (of IF) that keel is 'flying' and producing lift to 'weather' .... neat!

Most interestingly, All of this for either aero or hydro all comes down to 'viscosity of the fluid' as the main controlling factor.

The Term "Ancient Interface" ... referred to the foil above the water optimized and interacting with the foil below the water. Im sure that this 'compendium' still exists in some engineering schools.

;-)

 

[tsmwebb]

Indeed yes.
Back in the 60s & 70s a 'compendium' of such interaction was available at most engineering schools under the title "Ancient Interface".

Oh yes. I wasted a lot of hours in the library reading those. AIAA symposium on aero/hydronautics as I recall. Kind of eclectic but lots of fun...

Just for the record rudders are often 63A or four digit foils. If you make them 63A or 64A then some blunting on the leading edge gives them better stall characteristics. The "A" designation just means that the hollows in the back of the foils are straightened out so they are easier to build. One of the nice features of the four digit, and 6 series is that they are scalable. There's nothing all that magic about 10% thickness and the sections are generally scaled to take into account volume needed for structure and or ballast and differences in flow as you move along the span. These sections are old and there are better sections available but they published in lots of places and well understood. You could certainly do worse.

--Tom.

 

[Hermit Scott]

tsmwebb-so you have a site for your boat? I think it's cool that you are hanging out in Mexico on your boat. Do you live on your boat all the time or are you just cruising?

 

[tsmwebb]

tsmwebb-so you have a site for your boat? I think it's cool that you are hanging out in Mexico on your boat. Do you live on your boat all the time or are you just cruising?

Thanks for the kind words. No, I don't have a web site for the boat. We have lived aboard full time since January of 2001 and done a bit of cruising and a bit of working in that time. Our home port is Honolulu, Hawaii and we've done three large slow loops through the Pacific Islands and Australia and New Zealand. We spent last summer in the Bay Area and then cruised down the west coast to La Paz where we're awaiting boat bits (ain't it always so).

--Tom.

 

[All U Get]

Hi Guys,
Sorry I'm a little late with this site https://www.kqed.org/quest/television/the-physics-of-sailing
but try the NASA simulator. I changed the angle, camber, and thickness to make a variety of results. Them NASA fellas might be way out there, but I think they got the wing style figured out.
All U Get

 

[Hermit Scott]

All you get, that is a cool video. I guess even the NASA engineers partake in controversies concerning lift.

tsmwebb, what kind of boat do you have?

 

[alangbaker]

Airplane wings don't create thrust f=m*v^2, they create lift by a pressure differential.
There is a localized low pressure above the wing. If you could move the air above the wing upward to create the low pressure volume of air above the wing it would lift all the same. That the air goes slightly downward behind the moving wind is completely incidental to the pressure difference. The air can move sideways, up, down it doesn't matter as long as it gets out of the way of the top of the wing.

You are entirely wrong.

If there is no air moving downward, then there is no lift. PERIOD.

I'll say it again: if, after the aircraft has passed, there is no net movement of air downward, then the aircraft was creating no lift.

 

[Hermit Scott]

Alan- https://www.kqed.org/quest/televisio...ics-of-sailing
Me and this engineer from NASA disagree with you. Don't get your underwear in a twist over it. Apparently there are engineers that disagree with him. This argument is really kind of silly. So there is a low pressure above the wing that causes it to rise and air being thrown down behind the wing that casues the wing to rise. Everyone wins yeah!

 

[Roger Long]

This argument is really kind of silly. So there is a low pressure above the wing that causes it to rise and air being thrown down behind the wing that casues the wing to rise. Everyone wins yeah!

So, after all the arm waving, we come full circle to what I was saying in the first place. Engineers don't really disagree on this. If you are designing an airplane, all you really need to is the flow velocities, and thus the pressures, right at the aircraft surface. It's easy, both in the calculations and your mind, to just forget about the rest. Whether you have to look at the entire picture to define and describe "lift" is more of a philosophical and semantic question than a technical one.

However, if you are an aerodynamacist studying the behavior of a fighter aircraft coming up behind a tanker to refuel, or aircraft flying in close formation, you will be very interested in the full explanation.

Why should we care? Well, if you have a boat with more than one sail, you might be interested in how the movement of air from one sail that is inextricably linked to the the flow/pressure differentials effects the others. If you are a racer, it might help to understand how the movement of air from the complex airfoil that is yours or your compeditor's boat effects the other.

BTW that Quest piece is full of errors. The lift vector of the keel is not forward as shown. Lift vectors are pretty much perpendicular to the surface so, with the AOA of leeway, your keel is actually pulling backwards. It's only that little forward component due to your sails being off centerline and twisted that pulls the boat forward.

The fact that the NASA scientists don't mention circulation doesn't mean they don't understand it. I'm sure they do. They were talking for a half hour show directed at the general public. There's quite a bit from their years of education and research I'm sure they didn't think there was time to get into even if it wouldn't go over the heads of the audience. When they say there is stuff they disagree on, they weren't talking about the stuff we've been discussing here. There are lots of mysteries but circulation and the movement of air dictated by conservation of energy is not one of them.

 

[kloudie1]

Yup, if there was no downward motion of air, there would be no downwash from helicopters.. Those high aspect ratio rotating lifting surfaces are very well developed airfoils!

 

[Roger Long]

Your prop is also a rotating foil although of much lower aspect ratio. The same principle is why you can use wash against your rudder when the boat isn't moving and why high power outboards have a rooster tail.

I could have pointed this out earlier but then, think of all the fun we would have missed

 

[kloudie1]

Yup ! I have enjoyed the discussion! Again, thanks for the articles, Roger !

 

[Bill Roosa]

Keel practicality

So the premise is keels act like lifting devices turned on their side and keep my boat from heeling as much once I get the boat moving. Otherwise or at low speeds the keel is just a convenient way to stick a weight further from my COB (center of buoyancy) to give it a longer moment arm.
It would seem that if the racing community could find a way to make the keel to produce lateral resistive force (LRF)(I'm going to start calling it what it is and abandon "lift") more effectively then we would not see canting keels.
Canting keels operate on the moment arm principal and not on increasing the LRF. They even have to supplement the keel's contribution to LRF by adding a dagger board to leeward!!!

What I glean from this is that the keel LRF resists the motion to leeward while the big weight at the end keeps the boat upright. It is a combination, of course, but those are their primary functions. Also means that a rudder turned down wind (tiller to weather or weather helm) is going to be a better pointing arrangement because it is "helping" the keel resist motion to leeward.

On the practical side of the fence, this thing is an absolute failure. It has been noted several times here that this thing is a flotsam trap and makes "going on the hard" require a frame to support the boat. Presumably going aground is not a problem but I suspect being left high and dry would have its share of tense moments.
I for one would never buy a boat with one.
It is not materially better at positioning the ballast
It relies on hydrodynamics to produce LRF to resist motion to leeward which probably only works when the boat is moving fast. This would make light wind beating a real no-go proposition.

 

[Roger Long]

It relies on hydrodynamics to produce LRF to resist motion to leeward which probably only works when the boat is moving fast. This would make light wind beating a real no-go proposition.

Your keel always works this way; even in light winds. As long as it is moving, hydrodynamics is the most important component of LRF. Traditional hulls with very long shallow keels work this way as well. They are simply very low aspect and inefficient foils which is why boats and ships of that configuration go to windward so poorly.

BTW the lift or LRF produced by your keel makes the boat heel which is why you need ballast and hull form to resist the forces.

The loop keel has a lot of foil that isn't lifting in the optimum direction. OTOH, it cleverly doesn't have any ends which are the source of a lot of the drag in an air or hydrofoil. I'd have to see some rigerous and objective test results. These thing are often surprising.

My boat came in both keel / centerboard models and a shoal keel in which the area of the centerboard was added by making the keel about a foot deeper. You would think that the high aspect effecient foil of the centerboard, sticking far down into the water, would make the earlier boats faster to windward. They weren't. It turns out that the drag of the centerboard slot, due to the leeway, more than offset the greater effeciency of the centerboard.

 

[tsmwebb]

You are entirely wrong.
http://www.plantassolares.com.mx/
If there is no air moving downward, then there is no lift. PERIOD.

I'll say it again: if, after the aircraft has passed, there is no net movement of air downward, then the aircraft was creating no lift.

This is really semantics but http://www.arvelgentry.com/origins_of_lift.htm does a reasonable job of showing why in a 2D world there is no net downward movement of air in the far field. It isn't as easy to read as Roger's stuff and what the heck all 3D foils do have a net downward movement... And, in the near field Roger is right even in 2d and that's good enough!

--Tom.

 

[tsmwebb]

So the premise is keels act like lifting devices turned on their side and keep my boat from heeling as much once I get the boat moving.

No, no, no... Well, mostly no. The lift from your keel or centerboard or leeboard and from the hull itself reduces the leeway caused by the sails and creates a couple between the center of effort of the rig and the submerged part of the boat that causes heeling. Ballast in whatever form it may take (eg. keels, canting keels, crew, hull weight &c) is used to counteract the heeling moment. IMO, canting keels are a stupid way of moving ballast around but they work within some racing rules. If you cant your keel significantly the strut between the hull and the ballast will be nearly horizontal and will no longer help to reduce leeway. So, boats with keels that can be canted to large angles also need leeboards or daggerboards to prevent leeway. Without some kind of leeway prevention device you can not sail in any direction save directly down wind. While it is possible to build a boat that can sail upwind without any heeling moment or even with a negative heeling moment all conventional sailing craft need to create and then balance out some heeling moment (and some pitching moment) in order to sail.

--Tom.

 

[tsmwebb]

tsmwebb, what kind of boat do you have?

I have an Atlantic 42 catamaran. It is a Chris White design only slightly modified from the stock design on his site:http://www.chriswhitedesigns.com/atlantic42/index.php.

--Tom.

 

[Todd Smith]

keel-centerboard

My boat came in both keel / centerboard models and a shoal keel in which the area of the centerboard was added by making the keel about a foot deeper. You would think that the high aspect effecient foil of the centerboard, sticking far down into the water, would make the earlier boats faster to windward. They weren't. It turns out that the drag of the centerboard slot, due to the leeway, more than offset the greater effeciency of the centerboard.

A boat designer wrote (in another forum) that the full-keel centerboard combination has another problem. The 2 parts have very different lift characteristics and want to operate at very different angles of attack for efficiency. They end up with neither the keel nor the centerboard working at best efficiency. I remember (possibly faulty) that this was more of a problem when the centerboard was not deep enough the create all the leeward resistance.

Todd

 

[Todd Smith]

This is really semantics but http://www.arvelgentry.com/origins_of_lift.htm does a reasonable job of showing why in a 2D world there is no net downward movement of air in the far field. It isn't as easy to read as Roger's stuff and what the heck all 3D foils do have a net downward movement... And, in the near field Roger is right even in 2d and that's good enough!

--Tom.

Good stuff. I urge any new student of fluid flow and lift to understand the 2D world, before going on the the 3D world. The 3D flow is so much more complicated and requires understanding the 2D principles before beginning.

Todd