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Hydrodynamics of the Nautilus - some thoughts and observations

Nautilus, Seaview, and more

Postby Captain Nemo » Fri Aug 13, 2004 10:50 am

Tom,

In the video clip I watched, the sub was not underway. It was sitting still and then submerged on ballast. That's a static submergence, not a dynamic dive. So the nose down effect was not induced by hydrodynamics, it is occuring under static conditions, and is therefore a factor of weight and balance.

Actually, the CG of most NAUTILUS replicas I've seen (mine included) is slightly forward of the salon windows; yours is the only one I've seen where it's aft. Probably due to variables in the systems, weight and buoyancy distribution, etc. Whatever works, eh?

And I tend to agree: shifting Bob's CG aft could compensate for the nose down dive, but then it would sit tail low on the surface when at rest.

I think it comes down to the variables between weight and buoyancy of the front half of the sub; and the difference in those variables between a pressure hull type submarine where the wheelhouse is dry, and a model where the wheelhouse is flooded. The latter lacks the buoyancy to offset the weight of the structure, and that's why models nose down.

With the NAUTILUS MINISUB, I can sit still at a level attitude on the surface, flood the tanks, and submerge on an even keel; the boat will descend on an even keel, straight down, and not pitch down during the descent. That's because my foredeck and wheelhouse structures are filled with air, and that buoyancy offsets the structural weight.

Models where the hull is primarily a freeflooding shell with a smaller cylindrical WTC inside don't share that same additional buoyancy offsetting the weight of the wheelhouse structures. Thus, while sitting on the surface the boat might be trimmed level. But when water fills that foredeck and wheelhouse area, she drops at the bow.

It seems to me this is pretty clear from the video clip Bob has on his website. She settles nicely; the tail starts to drop, even. But when the water covers the wheelhouse, down she goes nose first. Check it out. It looks like that's what's happening to me.

I would think the way to go would be for Bob to try to add some buoyancy to the area of the wheelhouse, that wouldn't come into play until the boat was submerged. Maybe if he sealed off the wheelhouse floor (and maybe also part of the foredeck) and somehow made it a dry chamber inside? That way he'd have the additional buoyancy where it seems he needs it, when he needs it: i.e., when that part of the boat is underwater.

VBR,

Pat
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Postby Carcharadon » Fri Aug 13, 2004 11:51 am

Pat, it's hard to armchair these problems and solutions and theories without actually getting your hands on the various models. (We should all have a Nautilus RC meeting in Hawaii.) However, in my case with the two subs there are subtle differences; the wheelhouse on the 4 ft. is solid except for a space for the receiver battery. The wheelhouse on the 7 ft. is filled with foam. Both subs exhibit a nose dip concurrent when maximum speed is reached. But in both cases the wheelhouse is above water before the nose dip. Both subs will continue moving forward on the surface with a slight nose dip until the down control is activated, which is jet pumps pushing the nose down further at a steeper ankle. Then the whole sub slips under as you've seen in the various videos. The nose dip on the 4 ft. is slightly more pronounced than on the 7 ft. I only became aware of this while trying to get the water cannon to work right. Any variation too high or low affects the net result. So the assumption I make is that I can extrapolate my experience to other models but this assumption could be totally invalid. The ideas put forward by you and others on this topic I find very informative, even the ones I don't understand. In any case apparently an interesting topic for others as well. Tom
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Postby Bob the Builder » Fri Aug 13, 2004 11:56 am

Carcharadon wrote:
Captain Nemo wrote: 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?

I started my Nautilus out with a 12V, 7Ah battery less than half the size and weight of my current one. The same tendency was exhibited then as now. I'm currently running a honkin' 12V 12Ah battery.

I just had her out on a fully charged battery with Greg Sharpe last Saturday. I tried some high-speed runs with a dry tank just to see how fast I could really get her. I maxed the throttle, threw up the trim for a little extra 'oomph', and let her fly. With my new rear dive planes, I commanded 'bow up' on the planes (and consequentially, aft down). My model ran with a distinct upward pitch of around 15 degrees or so and with the prop buried I got up some good speed. I wish I could have taken some video, as I estimate her at a good four to five miles per hour and I got a great bow wave.

Interestingly enough, after she built up speed, the nose eventually dropped yet again, completely overriding my fully raised aft planes, pulling the bow down and lifting the prop out of the water.

This leads me to believe that whatever forces are working upon the model do not have anything to do with anything above the waterline, as the model is trimmed level and the only factor working upon it is speed and hydrodynamics.

Pat,

I think you could be right, to a certain degree. Now that I've watched more carefully my boat submerging, it looks now to be sinking on an even keel after the wheelhouse is flooded. If you watch my .avi again, you'll see that what looks like the nose sinking drastically has a lot to do with the model actually rotating clockwise as she sinks. When you are standing right there, you'll see that she actually hits bottom flat.
Bob Martin,
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Postby Captain Nemo » Fri Aug 13, 2004 8:08 pm

Bob,

You know, I thought it might be rotating, but I couldn't be sure from the video. To me, though, it looks like's it's pitching nose down, and in the last second of the tape we see the bottom for rakers hit first, and then the tail drops to the bottom a moment later? No?

Interesting and informative rap, guys.

Pat
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