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Hydrodynamics of the Nautilus - some thoughts and observations

Nautilus, Seaview, and more

Postby TK-7642 » Mon Jul 05, 2004 6:55 pm

The front of an airplane wing curves down, yet airplanes fly up - The front of an F1 cars rear airfoil curves up, yet it pushes down - just like the bow of the Nautilus points up - and it dives down AT A POSITIVE AOA. In other words orientation to flow is all that matters (AOA). There is nothing counter-intuitive here - nothing. My explanation supports many in this thread - Bob's position as well as Jeff's. I read them all and can't recall seeing any faulty reasoning off hand. The Wright brothers airplane wing angles down at the front and points down - yet their plane flew up.
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Postby Captain Nemo » Mon Jul 05, 2004 7:22 pm

TK,

It has to do with the differences between aerodynamics and hydrodynamics. When it comes to submarine hull shapes, control surfaces, and the like, the primary factor is increased "impact" pressure, not reduced pressure "lift" generated in accord with Daniel Bernouli's theorum. Why? Because water is about 800 times denser than air is, and is virtually incompressible.

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Postby TK-7642 » Mon Jul 05, 2004 8:43 pm

Bernoulli's theory also applies to incompressible fluids such as water. This is why curved undercambered propeller blades are used on boat propellers. Viscosity is taken into account by Reynolds number in the equation. Here is some interesting info you may enjoy (cut and paste into the address line if it doesn't link):

www.uvi.edu/Physics/SCI3xxWeb/Plumbing/ ... amics.html
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Postby JWLaRue » Mon Jul 05, 2004 10:32 pm

I understand how airfoils work. The shape of the upper surface (vs. the lower) causes an area of lower pressure above the upper wing surface, causing lift.

The Goff Nautilus hull cross section is not a 'diving plane'.

The side rakers/chines do not have an airfoil cross section, rightside up or down......so there is no airfoil effect.

Which brings us to an important difference when comparing fluid dynamics in air vs. water: air is compressable, water is not.

So....what's being missed?

-Jeff
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Postby Captain Nemo » Mon Jul 05, 2004 11:43 pm

Thanks for the Link, TK. Actually, I'm pretty well-read on Bernouli: used to teach his theorum to student pilots.

In comparing the hydrodynamics of a marine hull to a propellor, you're overlooking an important variable: their relative velocities through the water. Makes a big difference. Apples to oranges....

According to your theory, the upwardly curved bow of most boats should cause them to dive. In actuality, it causes them to rise up. Surely you can see that; if not, there's probably nothing anyone can say that will change your mind.

What I've tried to share with you here is supported by many years of personal experience designing, building, and piloting aircraft and submarines; and teaching others how to do it, too. If you asked Dr. Phil Nuytten, he'd tell you the same thing.

If you, or Bob, or anyone else reading this thread chooses to disregard my point of view, that's OK with me.

Good luck with your projects. :D

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Postby Captain Nemo » Mon Jul 05, 2004 11:56 pm

Jeff,

Of course, you're right.

TK's fixated on a misconception. He'll learn.

:D

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Postby Wbnemo1 » Tue Jul 06, 2004 2:27 am

I have just one thing to add........Sorry there Bob didnt meant to insult you..hope all is well

William aka Nemo z(:{>
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Postby Captain Nemo » Tue Jul 06, 2004 2:34 am

Go ahead and insult him! He called me "the psycho sub builder!!!"

:D
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Postby TK-7642 » Tue Jul 06, 2004 2:40 am

I am a pilot as well and have been designing airplanes with low Reynolds numbers (of around 100,000, and less) with variable camber airfoils, and variable planform geometry for many years. Reynolds number includes flow speed. Not everyone must agree with me or with Bernoulli's equation - it just helps to understand how things work, until something better comes along. I choose to believe it based on my experience not because someone taught me to believe it.
The Goff Nautilus is an airfoil shape in cross section of the lateral keel chines - the same cross section airfoil the Wright flyer had, Otto Lilenthal, Bats, Pterosaurs, Flying Squirrels, Frisbees, boat propellers and hang gliders have - this shape is called an undercambered airfoil.
Hang a spoon (undercambered airfoil) loosely by your fingers and put it under a running kitchen faucet, convex part facing the running water and watch what happens. Will it be pushed away from the fast moving water or will it be sucked into the water stream as Bernoulli suggests? See and feel what happens to the spoon. According to some on this board it will be pushed out of the water stream by the water. I disagree. See it for yourself. Bernoulli's equation applies to fluids such as water. Try it at different speeds. This is how I believe the Nautilus gets sucked down in the water until someone explains why it's wrong - no one has yet. Bernoulli's equation takes into account speed, and viscosity via the Reynolds number. It applies to fluids such as air, and incompressible water.
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Postby Captain Nemo » Tue Jul 06, 2004 6:14 am

Sorry TK, but your spoon analogy is invalid.

If you put only the positive camber of the spoon against the stream, it is drawn into it: but that's because there is nothing acting on the backside to counter it, and such is not the case with a submarine You've fooled yourself! ;-)

Place the entire spoon into the center of the stream and angle it to a neutral angle of attack: you'll know when you're there because the downwash off the trailing edge will be vertically downward: perfectly aligned with the flow of the water from the faucet, and not deflected one way or the other. Now the spoon has no tendency to move in the direction of the camber: the effect is neutralized by the identical flow on the opposite side. Why? Read on:

AERODYNAMICS 101: An airfoil generates lift because air molecules traveling over the cambered side of the wing must travel a greater distance in the same amount of time as those traversing the shorter distance along the flat underside of the wing. To do that, the molecules moving over the camber must move faster. According to Bernouli, the faster the flow, the lower the pressure: and that's how an airfoil generates lift at a neutral angle of attack.

In the case of the spoon, or the side fins of the NAUTILUS, the distance along the positive side of the camber is the same as along negative side, because it's not a true airfoil shape, but rather, a flat surface formed into a curve. Thus, in the water, when the angle of attack is neutral, the flow and pressure on either side of the curve is the same, and without a pressure differential there can be no lift produced. But let's take the experiment a step further.

Now, with the entire spoon in the stream, angle it to a positive angle of attack, and it is pushed away from the cambered side by impact pressure, as I've been saying it would be.

Similarly, because the NAUTILUS side fins are a flat plane angled up at the aft side raker; and the distance along that structure is the same above and below; in a level angle of attack, the slight upward angle of the forward portion of the fins does not comprise a true airfoil or employ Bernouli's principle to generate inverse lift that drags the nose downward; instead, it generates a lifting force derived of increased impact pressure beneath the fins.

Moreover, the upsweep of the NAUTILUS side fins is not a graceful curve like a camber at all: according to the plans, the side fins are straight from the trailing edge of the fins all the way forward to the aft side raker; there the fins make an angular change upwards of about ten degrees with almost no curve involved at all. So again, it's not a cambered airfoil shape , and therefore it can't generate lift in accord with Bernouli's principle.

But just for the sake of clarification, let's imagine a scenario were the upswept fin vector occuring at the aft side raker was the center of lift of this alleged camber; in that event, it would exert it's "negative lift" only a very slight distance forward of the boat's C/G, and be overwhelmingly opposed by the stabilizing forces generated by the far greater surface area and longer moment-arm of the straight portion of the side fins extending aft; not to mention the uplifting forces on the underside of the upswept bow itself.

Thus, even if the side fins were a cambered airfoil (which I have just shown they are not); and if they did generate negative lift (which I have just shown they do not); they still would not induce a dive in the manner TK describes, because the opposing forces would be far greater.

There, TK, now you've had it proven to you. The side fins of the NAUTILUS do not comprise an airfoil, and are not generating inverse lift that induces a dive when the boat is level underway. Case closed. Anything beyond this and you've got to start paying me for instruction time. ;-)

You guys who have NAUTILUS models that are diving uncontrollably from level trim need to look elsewhere for the gremlin. It aint the upswept bow.

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Postby Captain Nemo » Tue Jul 06, 2004 6:29 am

And just for kicks, consider this:

Aerodynamically speaking, the wing shape that generates the most lift is one that is long and thin: i.e., where the span is tremendously greater than the chord line, like what's seen on a high performance glider.

If we consider the side fins of the NAUTILUS as lift generating planes, they would be the exact opposite of what's been proven to work: i.e., all chord line, and no span.

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Postby PaulC » Tue Jul 06, 2004 8:46 am

Guess I'll just have to build one of these boats and see for myself!

:^)
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Postby Captain Nemo » Tue Jul 06, 2004 8:47 am

Anyway, nuff about what the problem aint...about what it might be:

Bob, I went back and read your posts describing what your boat does. Sounds like you've had a built in tendency to submerge when level right from the start, without having to pitch the boat down. It shouldn't do that. If anything, it should plane.

I've been trying to get the point across that, based not only on hull and fin shape, but also largely dependent on the hydrodynamics involved at the speeds you're travelling, the possibility that your boat is being "sucked" down into the water by something akin to negative hydrodynamic lift is remote in the extreme. The greatest forces acting on your hull impart impact pressure that "push" the boat one way or another; the NAUTILUS crashdives well when deliberately pitched down so the lateral fins and deck hit the relative flow with a negative angle of attack; but the configuration of the hull and fins is such that it should negate any tendency to submerge automatically from a level attitude when underway.

Where you described your C/G relative to the Salon Windows sounds right to me.

So what else might it be? Again, I come back around to possible dynamic variables affecting the weight and balance of a free-flooding hull. Could be that, while underway, water is being rammed into the open ballast vents and scupper drains, rising up inside the hull via ram pressure, and thereby increasing the weight of the boat, causing it to ride lower.

You mentioned it would do that until it started to go down and then the tail would lift far enough for the prop to break water. Also that it went vertical tail-up when submerged once. Thinking of how your hull is made, with cast upper and lower halves, I'm wondering if a bubble of air from the wheelhouse area might have shifted aft when the sub pitched down and gotten trapped in the area of the skiff socket? That might produce the conditions you describe; depending on how your hull is bulkheaded and vented.

Just a thought...

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Postby Captain Nemo » Tue Jul 06, 2004 8:51 am

PS: if that were the problem, a possible fix might be to close off the ballast vents and limbering holes from inside, and put some vent holes inside the skiff socket.

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Postby Bob the Builder » Tue Jul 06, 2004 11:17 am

Wow! I take a couple of days away from the board and miss out on my whole Hydrodynamics course. I'll flunk the final for sure!

Pat, I've carefully designed the interior to allow as much trapped air out as fast as possible. The skiff is actually a solid piece, with the hard angle of the deck aft of the skiff slot filled in. All of the deck grating is practical, composed of 1/16" stainless steel mesh. The only place that air seems to get trapped slightly is in the wheelhouse, and even then it's only because my vents there are only (2) 1/16" diameter holes disguised just above the 'alligator eyes' and below their shrouds and they vent a little slower than everything else. On a side note, it makes for a really cool double stream of bubbles for a few seconds when she gets just under the surface. I don't think that trapped air is my problem.

And I use the term problem loosely here. I want to clarify that I find my model infinitely easy to drive, very responsive, and almost self-navigating. The only reason for this, however, is the addition of my third set of planes aft of my propeller.

Before this modification, I could still experience the pleasure of diving my model, but only at lower speeds. I didn't use the dive planes at all in those early days. A little forward throttle, and she'd slowly drift under the surface in a beautifully controlled dive. Kicking the rudder in helped pull the aft end down to keep her upward oriented if I needed to.

Now that I've finished the mod, I can use full throttle and the APC will keep her dead-level, with only a minute porpoising effect that occurs at full speed only.

My intent in posting this effect in this string was simply to let other Goff Nautilus builders know what I've experienced. Apparently this is a common problem, as at least three other people have experienced the same phenomenon with their models. Coincidence? I think not.

I'd consider closing in my ballast grates, except that I like having eight monstrous vents to let my trapped water out of my hull when I pull her from the water. You can imagine that a (nearly) six foot long model with two inches of free space around the entire length of the WTC will hold a lot of water. She weighs a ton when she's first pulled out!

I guess my motto is "if it ain't broke, don't fix it!" I can't imagine how my model could work better (other than a bit more speed... you can never really have enough, right?), so I'm very happy leaving it as it is.

She's currently drydocked while I do some tweaking to my WTC mounts, my battery holder, and I install a new high-output LED lighting system. After I get her underway again, I'll taker her out to Langford Lake and shoot some more video so everyone can see what I'm talking about.

This has been a great thread, (most of the technical jargon partially over my head, but educational nonetheless).
Bob Martin,
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