Winglets have been used on props, and still are on some low speed aircraft, but the advantage they give from reducing tip vortices, and hence wasted energy, are offset to a large degree by the greater drag they produce. They are at the very worst place in terms of drag, right where the prop blade is travelling through the water (or air) at its fastest speed. As drag is proportional to the square of speed and power is proportional to the cube of speed it doesn't take much to massively increase prop drag loss.
Ducts can increase prop efficiency more effectively than winglets, at least at low hull speed, because they aren't rotating to don't suffer the tip-speed related drag problem. Generally a duct, with proper prop design (the prop blade needs to be designed to match the increased tip loading) can give a 10 to 20% gain, making a 60% efficient prop of a given diameter better than 80% efficient. The downside is drag again. Whilst a duct makes the prop more efficient it makes the hull less efficient, as it adds drag. This isn't really an issue at low speeds (say, sub-10kts) but gets to be a real problem at speeds over around 15kts (that square law drag with speed and cube law power with speed relationship again).
There is a mass of research available on this topic, both in the aircraft world and the boat/ship world. Ducts were tried on both aircraft and ships around 80 odd years ago, and advantages were reported. As speeds increased though ducts were gradually dropped because of the drag they produce.
Nevertheless they can be a good solution for low speed craft that need high thrust from a small diameter prop, maybe for shallow water operation, provided you can overcome some of the practical issues, like their propensity for sucking in weed and plastic bags.
Adding fixed (or adjustable) flow straightener vanes in a duct aft of a prop or fan can slightly increase efficiency, by reducing swirl losses, but generally the increased drag they give (as they are operating in a stream that is at higher velocity than the free stream) cancels out most of the benefit, at least for boats and ships. Flow straighteners are used on gas turbine engines and hovercraft fans, but this is primarily to get good pressure recovery and aid even air distribution to combustion chambers or a lift plenum.
Jeremy
--- In electricboats@yahoogroups.com, "John Green" <v_2jgree@...> wrote:
>
>
> Hi there again Eric,
> I dunno about building a few winglet prototypes.
> Like my past idea for making lead acid batteries, I was throwing the
> idea out so that others can work on it, not me!
> I am a talker rather than doer, but eventually get things done if I live
> long enough, and the wife can back off on loading me up with useless
> jobs like mowing the lawn.
> Seems like any gains with the winglets might be minimal, if at all.
> Not to throw away the idea, but there are probably things that can be
> done that reap greater effect. There might be a good reason why winglets
> are not used on airplane props, and sadly, it might be as simple as
> nobody thinking of it, or if thought of, being rejected out of hand by a
> supervisor with no vision or risk tolerance. After all, from what you
> say, winglets were known about for years before they became common.
> Your Kort Nozzle comment caught my eye.
> My understanding, based on reading it somewhere, was that propulsion via
> a ducted waterflow was far less efficient that a prop. This was relative
> to jet-ski's, I believe, so it might very well be different due to the
> pump system.
> Now, I can get well enthused about the ducted prop idea. I noticed long
> ago that jet engines have static turbine blades that prevent the column
> of gases from rotating. Now, apply this to a prop in water, and it is
> obvious that part of the rotating energy, due to prop pitch, is spent on
> turning the water, rather than thrusting it straight backwards.
> Ideas lead to ideas, I guess.
> I have also had the passing thought that seeing as we are trying to
> create thrust, and by nature are pressurising the water behind the prop,
> that the water, being incompressible, might well be spraying out in all
> directions, thus wasting power. The prop slippage energy is going
> somewhere. So, it is possibly the case that the flow behind a prop
> creates a funnel shape of moving water, with the prop at the narrow end.
> So, if that splayed out water was 'crushed' back in line with the prop,
> it would add to the flow already there, I think. One wonders ('One'
> being in this case 'I') if the blades of a prop could be curved inwards
> at the tips, to direct the flow inwards towards the propshaft
> centreline, but behind the shaft. There would still be water moving
> outwards though.
> So, has anyone tried ducting a prop using thin sheet stainless for low
> wetted drag, and together with the duct, having static blades (that
> support the duct tube?) to lessen the column of water from rotating?
> Maybe Kort has! I should have looked that up before posting.
> This has probably never been looked at by ICE designers, as it is easier
> and cheaper to just increase the engine power.
> I have just taken a quick look at the Kort idea, and the drag of the
> duct might well be a factor, due to the airfoil shape of the duct
> itself. As most electric applications are probably for sailboats, the
> penalty of drag increase is greater, unless it can be lifted clear when
> not in use.
> There is one thing that I can and will test though, unless someone beats
> me to it, and that is a straight, thin parallel metal cylinder duct to
> contain the 'funnel' of water. The drag would be virtually zero, and it
> somehow strikes me that a gain in thrust would be almost certain to
> occur.
> I haven't checked, but I suspect, guessing, that if you look at the
> aftermarket outboard ducts, you will find that they extend in front of
> the prop as well as behind it. This would cancel out the 'good' effect,
> by restricting the inflow.
> I would try having small static vanes in front, and the duct with static
> vanes behind. Another test would be to have the rear static vanes very
> slightly curved, as, if the water column is rotating, straight vanes may
> sap away energy by trying to straighten the flow out too much. The trick
> would be to have them curved less than the natural curve of the water.
> It might be too small to consider, especially as the flow increases.
> Also, in the final design, for least drag, the duct would have to be in
> line with the boat motion, rather than the (angled) prop shaft.
>
> John
>
>
>
>
> 3b. Re: Propeller Idea
> Posted by: "Eric" ewdysar@... ewdysar
> Date: Mon Aug 22, 2011 9:38 am ((PDT))
>
> Hi John,
>
> Those "little vertical fins" are called winglets. Here's a quick
> history on the concept.
>
> The initial concept dates back to 1897, when English engineer Frederick
> W. Lanchester patented wing end-plates as a method for controlling
> wingtip vortices. In 1905, the Wright brothers used a concept they
> called "blinkers" on the canard of their Flyer III and later, on their
> Wright Model A aircraft. In 1910 they installed "side curtains" and
> modified blinkers on their production Wright Model B aircraft, to
> improve its stability. In the United States Scottish born engineer
> William E. Somerville patented the first functional winglets in 1910.
> Somerville installed the devices on his early biplane and monoplane
> designs.
> Dr. Sighard Hoerner was a pioneer in the field, having written a
> technical paper published in 1952 that called for drooped wingtips whose
> pointed rear tips focused the resulting wingtip vortex away from the
> upper wing surface. Drooped wingtips are often called "Hoerner tips" in
> his honor. Gliders and light aircraft have made use of Hoerner tips for
> many years.
> Hoerner's concept was further developed by Richard T. Whitcomb, an
> engineer at NASA's Langley Research Center, in response to the sharp
> increase in the cost of fuel after the 1973 oil crisis. Whitcomb's
> designs were flight-tested in 1979–80 by a joint NASA/Air Force team,
> using a KC-135 Stratotanker based at the Dryden Flight Research Center.
> Even before NASA did flight testing on winglets, Burt Rutan incorporated
> them in his innovative Rutan VariEze homebuilt aircraft design, which
> made its first flight with winglets on May 21, 1975. The VariEze
> pioneered glass-reinforced plastic composite construction in homebuilt
> aircraft, which simplified fabrication of the winglets. He reduced the
> resulting drag penalty by assigning double duty to the winglets; they
> also serve as vertical stabilizers and rudders in his canard, pusher
> configuration aircraft....
>
> I wanted to mention Rutan's contributions. During the 1970's, my father
> and I spent a reasonable amount of time in Burt's shop in Mojave as he
> was developing many of his "revolutionary" ideas. So, as a teenager, I
> was privy to numerous engineering discussions while he vetted various
> concepts with aeronautical engineers and rocket scientists like my
> father and others. So I've known why and how winglets work for 35
> years, and I was pretty jazzed when Boeing added them to their airliners
> almost ten years later.
>
> Here's a brief description of what winglets actually do.
>
> The upward angle (or cant) of the winglet, its inward or outward angle
> (or toe), as well as its size and shape are critical for correct
> performance and are unique in each application. The wingtip vortex,
> which rotates around from below the wing, strikes the cambered surface
> of the winglet, generating a force that angles inward and slightly
> forward, analogous to a sailboat sailing close hauled. The winglet
> converts some of the otherwise-wasted energy in the wingtip vortex to an
> apparent thrust.
>
> So, from what I know about props, they don't generate a significant tip
> vortex. Notice that we don't see winglets on aircraft propellers, wind
> turbines or similar devices that have had billions of dollars of
> research thrown at them to make them more efficient.
>
> What we do know is that props can be more efficient if they are put
> inside a duct (like ducted fans). With very tight tolerances, the duct
> kind of acts like a stationary endplate for the propeller blades. So
> one would believe that ducting a boat propeller might increase the
> efficiency of the drive. That brings us to Kort nozzles.
>
> The Kort nozzle is a shrouded, ducted propeller assembly for marine
> propulsion. The hydrodynamic design of the shroud, which is shaped like
> a foil, offers advantages for certain conditions over bare propellers.
> Kort nozzles or ducted propellers can be significantly more efficient
> than unducted propellers at low speeds, producing greater thrust in a
> smaller package. Tugboats are the most common application for Kort
> nozzles as highly loaded propellers on slow moving vessels benefit the
> most. The additional shrouding adds drag, however, and Kort nozzles lose
> their advantage over propellers at about ten knots (18.5 km/h)
>
> There are number of after-market ducts that can be added to the drive
> legs of outboard motors, but they haven't really delivered on the
> theoretical improvments on boats like ours.
>
> Now that you have a little more background on your general concept, you
> can start making a few prototypes and collecting some data on whether or
> not the concept actually helps in the scale that we're dealing with.
> Even though I don't think that this will produce a siginficant
> breakthrough, I'm personally interested to see how well your prototypes
> will work.
>
> Keep us posted.
>
> Eric
> Marina del Rey, CA
>
>
>
>
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