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Hydrodynamics of the Nautilus - some thoughts and observations

Nautilus, Seaview, and more

Postby Captain Nemo » Tue Jul 06, 2004 4:35 pm

Bob,

OK, the way you describe the deck being vented eliminates the "airbubble under the deck" idea.

Then, the auto-dive phenomenon still sounds like a change in vessel weight and C/G, characteristic of free-flooding hulls with a WTC inside.

I re-submit the possibility that, while the boat is underway at full speed, water is entering the hull through the forward deck limbering holes and perhaps the forward ballast vents, too.

Under ram pressure, this additional water ballast is rising upwards inside the hull, increasing weight, and causing the boat to settle to a deeper waterline.

When the additional water ballast starts filling the deck area, we have a forward shift in C/G because the naviform foredeck and wheelhouse have greater volume than the aft deck does.

This forward shift in C/G engenders a slight nose-down attitude (wouldn't take much at full speed, and might not be readily perceptible from shore) which imparts a negative planing attitude to the straight portions of the side fins and upper surfaces of the hull, initiating a dynamic dive. (Note: this is not a function of "negative lift" produced by Bernouli's reduced pressure effect; it is resultant of increased impact pressure above the planing surfaces.)

As this slight diving trend continues, the top surface of the deck goes awash, and it, too, becomes a negative planing surface, driving the boat down still further.

Without stern planes to counter the effect, the trend continues to produce what, for lack of a better word at the moment, I'll call a "crash dive".

The stock quad planes are reminiscent of what was state-of-the-art design theory for the 1860's; but there are many drawbacks: potential for "slop" in the linkages to produce unequal control forces; and the short / unequal moment-arm distance of the fore / aft plane pairs from the C/G minimalizing / unbalancing control surface efficiency, to name two of the best reasons why designers eventually switched to stern planes.

As you've mentioned, the auto-dive phenomenon is controllable with the use of stern planes; and I'll add that, while your arrangement with an additional plane aft of the prop is most efficient, hinging the trailing section of the side fins themselves to turn them into a pair of stern planes, will also work pretty well.

As an alternative for those who don't want stern planes: a mechanized, R/C actuated, "sliding weight" device that enables the C/G to be adjusted slightly fore / aft while the boat is underway, could compensate for the dynamic variation in C/G, too; but it would add weight, complexity, and be more difficult to control. I've never liked such devices, but some people do use them. Could work in this case, so I just thought I'd throw it out for others to consider.

HTH

Pat
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Postby boatbuilder1 » Tue Jul 06, 2004 7:59 pm

have you watched the dvd special features it shows the models used under water and the lit up models used for this seaquence it was a model that was only have built from the center line up and mounted on a board and was towed behind a small motor boat
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Postby JWLaRue » Tue Jul 06, 2004 11:50 pm

For a wet hull sub with a WTC, any possible 'ram effect' of water due to increased speed will not increase/change the internal weight of the sub....it's already full of water! (...and water being incompressable means that you can't pack more in....). If anything a 'ram effect' as described where the water enters the lower forward ballast vents and pushes up and back would cause the hull to lift.

Bob....a question: is it possible the 'ram effect' is pushing water into your ballast tank and compressing the remaining *air* in it?

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Postby Captain Nemo » Wed Jul 07, 2004 6:17 am

Jeff Wrote:

"For a wet hull sub with a WTC, any possible 'ram effect' of water due to increased speed will not increase/change the internal weight of the sub....it's already full of water!"

Not so in this case. When a hollow Disney NAUTILUS model is surfaced, the compartments above the waterline are dry inside: wheelhouse, deck, and a bit of the top section of the hull.

My thought is that water, under motion-induced ram pressure, is entering the deck limbering holes and forward ballast tank grates, and thereby raising the internal water level up slightly higher than the external waterline, within those enclosed areas. This adds weight to that part of the boat, and causes the boat to settle and submerge.

The ram pressure isn't compressing more water into an already-flooded space; it's forcing its way into the remaining air-filled compartments above the surfaced waterline, and the air in those compartments is escaping through the top vent holes Bob described as the internal water level rises. No compressibility barrier there at all.

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Postby Sub culture » Wed Jul 07, 2004 6:32 am

I don't see how, at the low speeds these boats are travelling at, that air compression is an issue. Must be talking about a fraction of a PSI, if that.

I reckon the best way to control a nautilus is with a gimballed prop, similar to the way a helicopter tilts the angle of it's blades.

That way the thrust is directed in the direction you wish the boat to travel, plus the prop is right at the rear of the boat where it can produce the maximum moment of force.

A slidable weight would be very useful for low speed underwater cruising. Details for constructing one are well executed in Norbert Bruggens 'Model Submarine Technology'
'Why are you staring at an empty pond?'

Want to dive your boat in crystal clear water? Then you had better Dive-in- http://www.diveintomodelsubmarines.co.uk
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Postby Bob the Builder » Wed Jul 07, 2004 11:56 am

Pat,


What you were talking about would definitely be a possibility, except that the same phenomenon occurs even when the hull is completely flooded and the sub has a full ballast tank.

I'd venture that my opened scupper vents aren't the culprit either, as I have foam packed pretty tightly against them (painted black, of course) that will probably limit the flow through them.

I'm thinking more and more that the ballast grates are to blame...
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Postby JWLaRue » Wed Jul 07, 2004 12:31 pm

"For a wet hull sub with a WTC, any possible 'ram effect' of water due to increased speed will not increase/change the internal weight of the sub....it's already full of water!

Not so in this case."

Ahhh...my bad. I thought this was a submerged condition problem.

Seems like a simple test then would be to temporarily cover the forward lower ballast grates with tape....run it...and see what happens......

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Postby TK-7642 » Thu Jul 08, 2004 1:16 am

Just for the record the spoon is pulled into the water stream just as Bernoulli's principle predicts. When both sides of the spoon are placed into the stream at a positive angle of attack the the spoon is strongly pulled further into the water in the exact same direction that all my previous posts have stated an undercambered airfoil would be. Both sides of the spoon push in the same direction as usual. Both vectors are in the same direction. There are no new discoveries here. The bow or the Nautilus is pushed down just as Bob says he has seen.
As I have stated before at a slightly negative angle of attack around -2 deg according to wind and water tunnel tests the undercambered airfoil shape has no lift. No new laws of physics need to be created here. I do not need to adjust my position.
The Nautilus is a complex shape and no doubt has many forces applied to it in many ways.
Having "long thin wings" is called high aspect ratio, and this is not needed to lift an airplane. Earlier last century there were full size circular flying pancake airplanes with very low aspect ratio that flew often. Kelly Johnson's SR-71 has fuselage lift from its very low aspect ratio (lateral keel) "chines" at landing speeds according to him and his aeronautical engineering teams. Lifting bodies have no wings at all (X-24, HL-10 etc.) not even (lateral keel) chines like the Nautilus. High aspect ratio long thin wings are not a requirement for lift. You don't even need wings to create lift.
Undercambered airfoils are very well understood in the water and in the air. Undercambered curved airfoils have been in use by humans as lifting surfaces for thousands of years in the form of the lateen sail of a boat. There are few "impact driven" flat sails in the modern world of sailing. There are few "impact driven" flat wings lifting planes into the air - even though they would be much cheaper and easier to make. Flat wings are very inefficient for creating lift, and impacting molecules are not enough - not for airplanes or slow turning ship propellers. At low speeds airplane flaps create the time tested, curved undercambered airfoil for greatest lift during the most dangerous part of flight -landing.
Nature chose a curved undercambered airfoil not an "impact driven" flat one as the most efficient for flight, and has been lifting animals into the air for 200 million years with curved undercambered airfoils. The laws of physics, nature, and my position remain unaffected.
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Postby Captain Nemo » Thu Jul 08, 2004 7:44 am

BOB THE BUILDER WROTE:

"What you were talking about would definitely be a possibility, except that the same phenomenon occurs even when the hull is completely flooded and the sub has a full ballast tank.

I'd venture that my opened scupper vents aren't the culprit either, as I have foam packed pretty tightly against them (painted black, of course) that will probably limit the flow through them.

I'm thinking more and more that the ballast grates are to blame..."



Hmmmmm.....clarify it once more for me, will you Bob? Is the boat automatically submerging on an even keel; does it pitch nose down at any point in the dive?

I visualized this as an auto-dive that begins with an even keel submergence when the boat is running surfaced (normal waterline), and then (once it's awash) progresses to a nose down crash dive. But you can't be running surfaced when your MBT and the entire hull (wheelhouse included) are completely flooded, right?

Please describe once more: what's happening when the boat is heavy?

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Postby Carcharadon » Thu Jul 08, 2004 7:54 am

Andy, this is some info I got from Larry Brooks, concerning speed of the towed 22 ft model.


Larry says -

"All I know is that the 22-foot deck version of the Nautilus was pulled across the water with a cable by a truck. On one of the last shots they did, the driver was told to {"really goose your engine!"} so that the submarine would leap forward with a giant surge of foam. I was told by Leagues film editor, Elmo Williams, that when the truck driver shifted his vehicle into gear, well, that {'#### sub fairly flew through the water. Not only that, it also broke in half just as it got past the
camera!"}

As for the speed of the submarine in those great surface shots, I would strongly recommend you writing to Harry Hamilton. Bet HE knows. And please ask Harry to post the information for the rest of us ... I know I would like to hear what he has to say,
too!

- Larry"
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Postby Captain Nemo » Thu Jul 08, 2004 8:00 am

Bob,

More questions:

I know you said your boat sits at a level trim attitude when surfaced. What's it's static trim like when completely flooded and submerged?

In other words: if you flooded everything floodable, pushed the boat straight down underwater (with the motor stopped) on an even keel, and gently let go of it: what happens?

Does it "hover" in equilibrium on an even keel at neutral buoyancy?

Does it ascend? Descend? Pitch nose up or down? What?

Pat
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Postby Bob the Builder » Thu Jul 08, 2004 11:48 am

My Nautilus is ballasted negatively. When my tanks are full, she will slowly sink to the bottom. She sits perfectly level when surfaced, and slightly (perhaps five degrees) nose high when submerged.

Hmmmmm.....clarify it once more for me, will you Bob? Is the boat automatically submerging on an even keel; does it pitch nose down at any point in the dive?


She gets sucked down by the bow. As soon as forward speed is raised, the nose gets pulled down, and the tail gets pushed up.

I'll tell you all what: I'll finish up my LED lighting installation hopefully by this weekend and then take her out to Langford Lake to shoot some more video. Trick will be getting her to do her thing with my aft planes working... Might have to take the linkage apart and let her run like she did when I first built her.

If a picture is worth a thousand words, then surely a video will be worth a million :)
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Postby Captain Nemo » Fri Jul 09, 2004 3:31 am

Bob said:

"My Nautilus is ballasted negatively. When my tanks are full, she will slowly sink to the bottom. She sits perfectly level when surfaced, and slightly (perhaps five degrees) nose high when submerged."

If I understand you correctly (and I believe I do), when underwater with the hull and MBT completely flooded, the boat still has a slightly positive pitch angle; so then it's not water being rammed into the the hull adding weight at the wheelhouse that's causing a nose down condition and inducing the dive when underway.

BOB also wrote:

"She gets sucked down by the bow. As soon as forward speed is raised, the nose gets pulled down, and the tail gets pushed up."

Well, she's going down for some reason: whether she's being sucked or pushed, or a combination of both, is the question.

I will revise my previous stance based on your new information (above). Now it sounds like a negative lift condition: primarily at the salon window fairings.

I don't believe the upswept side fins are generating enough negative lift to destabilize the boat because their aspect ratio is wrong, their camber is minimal, and their leading edges are cluttered with big gnarly rakers that spoil the flow. (Even a little bit of ice can spoil the flow over a wing; these suckers have nine big ol' icebergs up front!) Still, I can't totally exclude the possibility that they might be generating SOME negative lift, however minor it might be.

But the underside of the salon window fairings are shaped to force the motion-relative flow through a pronounced "swoop" in a very short distance, and that IS conducive to the generation of negative lift. I believe "the major suck" is occuring at the underside of the salon window fairings.

Impact drag on the leading surface of the aft split keel could be aiding the rotation to negative pitch angles somewhat, too.

And OK, while I don't believe the cambered side keels are capable of generating sufficient negative lift to "suck the hull under" all on their own; if they are generating any negative lift at all, that downforce would contribute to the rotation as well. And I think all these trends working together could produce the observed effect.

Once the boat pitches down, the flat surfaces of the side fins, deck, and hull act as dynamic diving surfaces to force the boat under. Voila! Auto-dive syndrome!

If we're right, these tendencies should be found in other NAUTILUS replicas of similar type; and we have heard from other modelers reporting the same phenomenon.

Of course, this is all predicated on the unestablished fact that there are no minor hull deformities (wouldn't take much at high speed) that are contributing to an adverse hydrodynamic effect; but this point in the discussion, based on what I've heard so far, I'm thinking the forces described above are the most likely cause of the conditions you describe.

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Postby Captain Nemo » Fri Aug 13, 2004 5:43 am

I am reviving this thread in light of new information.

My original thought was that the "autodive phenomenon" was caused by a weight and balance problem: the weight of the flooded wheelhouse structure was not being compensated by buoyancy and resulted in a nose heavy condition that induced a dive.

To check my theory, I asked at what angle the boat would static submerge; my understanding was that Bob said it would fall with the nose slightly higher than the tail.

Based on that information, I had to say it did not look like a nose heavy condition when flooded, and we got into the possibilities of negative hydrodynamic lift generated by various aspects of the hull.

Now that I've seen Bob's short AVI clip of the model static-submerging on ballast alone, I'm convinced I was right all along.

As the tanks flood, the boat begins to settle on a fairly even keel; the tail starts to drop ever so slightly until the wheelhouse floods, and then KERPLUNK! As soon as the wheelhouse goes under, she pitches down at the bow, and hits the botton on the lower keel rakers, pitched down at a noticeable angle. That's not hydrodynamics at work; it's weight and balance.

There's your gremlin, Bob.

VBR,

Pat
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Postby Carcharadon » Fri Aug 13, 2004 8:53 am

Captain Nemo wrote: That's not hydrodynamics at work; it's weight and balance.

It is apparent that the Nautilus has a tendency to nose dip under speed. At least with small models as opposed to Pat’s Mini-Sub. Just about all who have posted operating a model Nautilus express this. It seems then that since this occurs under speed a hydrodynamic force is at work here. This behavior is pushed to the extreme in Bob's sub. In Bob's case, and as Pat suggests a weight/ balance effect is evident but I think also in combination with the hydrodynamic effect accentuating a sharp dive, sharper than most. I believe the balance part of this is related to the center of gravity which in Bob's sub is forward of the salon. That big battery is well forward. In my subs the CG is behind the salon. In the case of Pat's Mini-Sub the hydrodynamic effect I suspect, is minimal due to the large mass and inertia of his steel sub. Imagine a proportionately sized battery in the forward section of Pat's sub. As an experiment it would be interesting to see what would happen if Bob were to run his sub with a very small battery with only enough capacity for one or two runs thereby greatly reducing the weight of the battery and moving the CG back?
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