Hydrodynamics of the Nautilus - some thoughts and observations

[captain nemo]

Bob,

OK, the way you

[color=#000000]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]

[boatbuilder1]

have you watched the dvd

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

[JWLaRue]

For a wet hull sub

[color=#000000]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]

[captain nemo]

Jeff Wrote]

[color=#000000]Jeff Wrote]

[Guest]

I don't see how, at

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'

[bob the builder]

Pat,


What you were talking about

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...

[JWLaRue]

"For a wet hull sub

"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......

-Jeff

[tk-7642]

Just for the record the

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.

[captain nemo]

BOB THE BUILDER WROTE]

[color=#000000]BOB THE BUILDER WROTE]

[carcharadon]

Andy, this is some info

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"

[captain nemo]

Bob,

More questions]

[color=#000000]Bob,

More questions]

[bob the builder]

My Nautilus is ballasted negatively.

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]http://www.subcommittee.com/forum/icon_smile.gif[/img]

[captain nemo]

Bob said]

[color=#000000]Bob said]

[captain nemo]

I am reviving this thread

[color=#000000]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]

[carcharadon]

That's not hydrodynamics at work;

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?