More Propeller Thoughts

Some time back I did a mind dump of some thinking about boat propellers.  And one of the comments on that post hit a nerve - one that I have been thinking about for years.

Tip vortices.  What are these?  They are the spiraling water that slips off the ends of the prop blades when it is turning.  They come from the fact that water on one side of the prop is at a higher pressure than water on the other side.  This arrangement holds just fine until you get to the end of the blade, and then the high pressure water just spills off the blade and joins the low pressure on the other side, making a vortex.  For visualization, the same thing happens at the ends of an airplane wing, causing sometimes beautiful effects.  And drag.

Wing Tip Vortices

Making vortices uses energy - energy that could have been used to propel water astern giving thrust.  So, how to stop this waste?  On an airplane wing (or a keel...), one way is to put up a fence to stop the spill-over, thus the development of winglets and winged keels.

So what would a fence on a propeller blade look like?

• Start with a conventional propeller.  
• Add a ring that goes all the way around the ends of the blades.  
• Extend the blades profile to meet the ring.  

This is an interesting example - the ring here is being touted as a guard, which of course it is.  But it almost meets the purpose of a fence.  It falls short only in that the ring is not wide enough to fully cover the ends of the prop blades.

Stationary ring bolted to engine

Why doesn't this prop guard achieve the purpose?  In fact, this is probably worse than no ring at all.  The tips will still be forming vortices, which will then immediately impact the (stationary) ring, creating additional turbulence and drag.    It is important that the tips extend to and attach to the ring, and that the ring rotates with the propeller.

Ducted fans have been using (stationary, however) rings forever.  And the cross section of the rings is designed to minimize flow turbulence as the fluid enters the duct (look at the leading edge of a jet engine cowling for an example).  If the rotating ring had such a cross section, drag could be reduced even further.

Now, if only I had a bronze foundry and some propeller tooling to play with...

If someone out there wants  to do the experiment, I need a RH 20x14 prop to fit a 1.25" shaft...

More Propeller Thoughts

Keel? Why Is There a Keel?

(For those of you reading because you are attracted to the romance of sailing into the sunset, or the peace of being anchored in a quiet cove at dawn... There is no romance in what follows - it is the technology which enables the romance. But I promise: no equations or vectors, just common sense and analogies)

Why would you take a perfectly good boat, and then handicap her by hanging 6 tons of lead (in Eolian's case) from her bottom?

Two reasons. The keel actually performs two separate functions that, in a wonderful bit of convergence, work out to require the same structure.

First: Keep the boat from falling over.

A sailboat has a tall mast, and carries a lot of sail up there, The weight of the mast and rigging alone, plus the sails would make her unstable and likely to fall over without the weight down below the waterline. Then there is the tremendous force that the wind adds! The hull actually acts as a fulcrum on which the mast/keel arm pivots. As the mast descends due to weight and force of wind, the hull rolls and forces the keel to rise, like a teeter-totter (<-- I think that may be the first time I have ever typed that word... it looks strange to me). As the boat rolls farther, the keel is pulled out farther away from directly below the hull, increasing the roll resistance. At the same time, as the mast goes more and more toward the horizontal, it becomes less and less effective at catching the wind. therefore the system is stable and self-correcting. As the boat heels more and more, it becomes more and more difficult to increase the angle of heel. The greater the weight, and the lower in the water below the hull it is suspended, the greater the stability which results.

Second: Keep the boat from sliding sideways in the water.

When the boat is sailing in any direction except dead downwind, the force of the wind on the sails is not along the boat's line of motion. To make this possible, something is needed to create a force which will resist the hull's desire to drift downwind. Once again, the keel comes to the rescue. In this case, the shape and surface area are the controlling factors; weight is irrelevant.

In the early days of yacht design, it was the "keel-as-a-barn-door" theory that prevailed. The idea being that a barn door would be really hard to push sideways thru the water. And in fact this is true. It leads to the full and modified-full keel designs which have been with us from antiquity. These boats (including Eolian with a modified full keel) are additionally very stable and easy to hold on course.

Tho the full keel design works well at optimizing one parameter: lateral resistance, it fails badly at optimizing another: wetted surface. Dragging anything thru the water takes effort - an effort that is proportional to (among many other things), the amount of surface area submerged in the water. Given a constant propulsive force, decreasing wetted surface will result in an increase in speed.

In the middle part of the last century (that would be the 20th century...), the search for a keel design that would provide the needed lateral resistance with decreased wetted surface was on. This led to the modern fin keel, when it was recognized that the keel moving thru the water could be viewed as a wing (much as the sail can be viewed as a wing operating in the air above), and thus could be designed as a hydrodynamic lifting body, - long and narrow with an airfoil cross section, instead of a barn door. Fin keel boats are faster, but are a little less stable - it takes more steering to keep them on course. However, they are far more maneuverable in close quarters, like a marina, since the boat pivots easily on the narrow fin.

Convergence

At first, the fin keels were just (nicely shaped) slabs of lead or cast iron. But then another conceptual breakthru came when it was realized that, if the keel material were strong enough, the bulk of the weight could be concentrated at the bottom in the form of a bulb, maximizing the righting moment that any given amount of keel weight could deliver.

Next, bear with me for a moment as we consider the keel as a lifting body. Consider the more familiar form of a lifting surface: an airplane wing. No matter how you describe the mechanism that makes it generate lift, it is a truism that the pressure on the bottom of the wing exceeds that on the top, if it is generating lift. Now what happens at the end of the wing? Yup... air flows out from under the wing and tries to fill the low pressure area on the top. This leads to the tip vortices that force air traffic controllers to space out flights at an airport. And it leads to a decrease in lift. The small vertical winglets seen on the wingtips of the most modern planes are effective at blocking this bottom-to-top flow.

Now back to the keel... it needs to lift from either side as the boat moves from one tack to the other., making each side of the keel alternately the high pressure and then the low pressure side. And yes, tip vortices rolling off the bottom steal away some of the effective lift of the keel. The same thinking that brings winglets to airplane wings has brought the wing keel - but with winglets on both sides. And in addition, the winglets are usually a modified form of bulb - that is, they contain a substantial fraction of the keel's weight. Because the wings increase the keel's effectiveness as a lateral plane, and because they allow weight concentration at the bottom where it is most effective, wing keel boats can have shorter keels than their fin keel counterparts with equivalent performance. Conversely, with the same depth and weight, a wing keel will increase both the boat's stiffness and its pointing ability.

Divergence

Working in the opposite direction, it is also possible to divorce the functions of roll stability and lateral resistance into two separate structures. The shoal draft/centerboard boat is such a case. In this design, the ballast keel is increased in weight, but held relatively high up, giving a shoal draft configuration. Because this keel generates very little, lateral resistance is provided by a retractable centerboard, which is weighted only enough to keep it in place. With the board extended, excellent lateral resistance is generated; with it retracted the boat can venture into thin water (tho the keel alone will not provide enough lateral resistance to allow thin water sailing without making huge amounts of leeway, unless nearly directly downwind).

The Future

So, what comes next? Venturing into prognostication, I have seen some interesting developments that have been tried on the "any $$ for 1/10 of a second" boats. Some of these are:

• Canting keels - they can be hydraulically tilted from side to side to increase the righting moment
• Keels with tabs - flaps really, keeping the aircraft wing analogy. The flaps increase the lift that the keel generates
• Boats with two steerable keels, fore and aft and no rudder

All of these ideas presume that something movable can be kept working in an environment that features barnacles, mussels, crab pot lines, and the occasional ill-placed rock. To survive in this environment, things have to be simple and robust - no delicate linkages need apply.

We'll see.

Keel? Why Is There a Keel?

Revolutionary Propeller Design

 

I have talked about propellers and propeller design before, here and here.  In fact I proposed a new propeller design with a circular rim, but this revolutionary design  takes it a huge step further.  

Called a 'toroidal propeller', this design claims to eliminate tip vortices, consequently delivering a 20% increase in efficiency, and a huge decrease in noise.  Tho the article seems to be more focused on the application of the design to drone propellers (because drones and quadracopters are new, hip, and cool), it does mention marine applications.

And speaking of marine applications, when you are below decks and a boat passes by, you are certain to hear him - his prop makes a lot of noise in the water, and it is conveyed to your hull where you hear it.  Submarines have spent fortunes trying to eliminate prop noise because this noise carries a long way in the water and frustrates stealthy operation.  Expect these props to be refitted to all current and new submarines.

For now, if you want one for your boat, expect to pay approximately 10X the cost of a conventional prop, but this cost should come down significantly over the next few years, as the design gets adopted widely.

Revolutionary Propeller Design

Winged keels?

If you didn't read that title carefully, you saw "winged heels" and are probably visualizing Mercury's boots.

But that's not what I want to talk about.  Instead I want to discuss winged keels.  I mean what is the big deal, after all?  Sure, they're modern, sure they're cool, and "every boat must have one".  But why, exactly?

It's all about end effects.

Huh?

OK, now I have to build something in your mind here.  Please give me some latitude and play along... Imagine an airplane wing.  Regardless of which theory you consider to describe how it holds the airplane up in the air, all the theories condense to the singular fact that there is lower pressure on top of the wing than on the bottom.  The airplane is held up by suction.  So what keeps the high pressure air under the wing from just flowing up into the low pressure area above the wing?  Well, on the leading and trailing edges of the wing, it is momentum - that wing is moving right along, and the air at the front and rear of the wing just doesn't have a chance to flow "upstream" to the low pressure area.

But what about at the ends of the wing?  Well, at the inboard end, there is the fuselage in the way.  Ah, but at the outboard end, there is... nothing.  And air does indeed flow up from under the wing around the end to the top.  This is the source of the "wingtip vortices" that cause the spacing out of flights at airports.

So, if you were an aeronautical engineer, how might you stop this flow around the end of the wing?  Well, you might put up a fence.  On the end of the wing.  In point of fact, this is exactly what is done on modern airliners.

OK.  So enough about airplanes... but hold that thought.  The keel of a sailboat is amazingly similar to an airplane wing.  It is "flying" thru the water, and is required to provide lift, to keep the boat from sliding downwind.  The analogy is really very good.  So then, what keeps the water from flowing over the bottom end of the keel, from the high pressure side to the low pressure side?  Nothing...

Thus enter the winged keel... put up a fence.  However, since each side of the keel is alternately the high pressure side and then the low pressure side as you tack, the keel has to have a fence on both sides,  explaining the now-familiar shape.

The design is so effective that significantly less ballast is required, the force being replaced with more effective lift.  Thus the weight of the keel can be reduced.  And the wings serve as a good place to stash lead, down there at the very bottom of the keel, so the draft of the keel can be reduced.  And then, since everything in nautical design is connected to everything else,  the wetted surface of the boat is decreased because less total weight is being carried.  And the boat goes faster.

It really is a good idea.

So then why then did winged keels take so long to appear?

See the First and Second Corollaries of Salnick's First Law.

Winged keels?