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Hydrodynamics of the Nautilus - some thoughts and observations

Nautilus, Seaview, and more

Postby Captain Nemo » Sat Jun 05, 2004 4:00 am

Guys,

My forward side rakers are flat rather than upswept (a result of the poor quality plans I had available when I built the boat) but I think that supports my theory.

Upswept rakers would induce a lifting moment to the bow, not a dive. In fact, I once thought the upswept bow and spur was to facilitate the ram penetrating a curved ship hull; but someone (Will or Harry?) told me the designer's real intent was to give the basic hull a built-in ascent characteristic that would bring her back to the surface in the event the dive planes failed.

Looking at the camber, one might think the "lift" would be on the bottom side of the curve: but hydrodynamics and aerodynamics differ there, especially at the slow speeds we achieve.

What we experience is mostly what's called "impact lift" resulting from molecules attacking an inclined surface in motion, and pushing it up (as opposed to aerodynamic lift resulting from reduced pressure generated by accelerated fluid flow velocities, in accord with Bernouli's Theorum). So in the case of the upswept NAUTILUS bow, that characteristic will lift the boat's nose if anything.

If there was some other characteristic of the basic NAUTILUS design that was producing the "auto-dive" phenomenon, the lack of an upswept bow on my NM would enhance that effect, if anything. But no: she's as tame as an old mare.

I still come back to the differences between a NAUTILUS that's essentially dry inside; versus one that's a cylindrical pressure vessel inside a larger free-flooding hull. All that water inside doesn't actually weigh the boat down when the whole thing is submerged; but its MASS does figure into the equation when underway. It's a factor with enclosed wet subs like the REEF RANGER, too.

The inertia of all that water weight has to be overcome when initiating a maneuver, and the momentum it imparts once underway must be countered to reverse the effect. Sounds to me like, when speaking of a free-flooding NAUTILUS replica, if a diving trend get's started as speed increases, it will be enhanced by the mass of water in the bow of the boat.

As Bjorn points out, this is not a problem if it enhances the boat's "divability"; but it might be if, as someone else said, the trend continues until the sub is pointing it's tailfins at the sky. That's nice if that's what you want to happen; but if it's happening on it's own, you have a serious lack of control.

Another thing to consider is the open scupper drains that lead into your hull, and the open ballast vents in the bottom: these all let water flow into the hull interior. Underway, all kinds of gremlins could be joining the party that don't meet the eye at first glance. But again, these conditions would effect a free flooding replica, but not one with a dry pressure hull taking up most of it's interior space.

The jury is still out, but I'm inclined to stand by my original assessment that this is somehow related to the characteristics of a cylinderical pressure vessel inside a larger free-flooding hull; because that seems probable to me, and it also seems supported by the differences we observe between the performance of dry versus wet replicas.

Mi dos centavos, amigos... ;-)

Pat
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Postby Carcharadon » Sat Jun 05, 2004 6:42 am

While I don't claim to be an expert, I have spent a lot of time at my "test pond". I've made three functional RC subs all working on the same principle of Jet propulsion. Those of you who have seen the videos will probably agree that I have a lot of "testing" experience. The first sub I made, a seven footer, I lost in my test pond in about 20 ft. of water. It was gone, sitting on the bottom. I was fortunately able to retrieve it. After that experience I was paranoid about losing a sub. To avoid this problem I made several design changes in the next seven footer. I installed a gas system, which worked great. I would purposely sink the sub in about 30 ft. of water. I would let it sit on the bottom for a while and then just pop it to the surface. This worked great until one day the hose filling the ascent bladder came off and all the gas just bubbled to surface. Well at least I was able to follow the trail of bubbles down to the sub and retrieve it. After that incident I said to heck with the gas system, it's back to dynamic diver. Another modification I made was to put a slight upsweep on the front rakers, as Pat mentions to achieve a slight upward direction when the sub is underwater. The 4 foot sub on the other hand has straight rakers. In either case the subs function the same. Once underwater they seem to level out and just go straight. I have found however that more important is the sitting orientation of the sub before moving or submerging. In the 7 ft. sub I can make a fine adjustment; if the tail is sitting very low, thereby pointing the nose up, where I can just barely get the sub to dive using the down control, the sub will eventually surface actually breaching out of the water. This looks real cool and about a third of the sub would come out of the water in about a 45 degree angle. I could never get a videotape of this however. Conversely if the tail is too high before the dive, it will eventually dive on a slight angle to the bottom. As a dynamic diver though all I would have to do is stop forward motion and the sub would come up. So based on my observations, orientation of the rakers may have an effect but at least in my subs the sitting orientation seems to be far more important.



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Postby Sub culture » Sat Jun 05, 2004 11:56 am

Captain Nemo wrote:Guys,

My forward side rakers are flat rather than upswept (a result of the poor quality plans I had available when I built the boat) but I think that supports my theory.

Upswept rakers would induce a lifting moment to the bow, not a dive. In fact, I once thought the upswept bow and spur was to facilitate the ram penetrating a curved ship hull; but someone (Will or Harry?) told me the designer's real intent was to give the basic hull a built-in ascent characteristic that would bring her back to the surface in the event the dive planes failed.

Looking at the camber, one might think the "lift" would be on the bottom side of the curve: but hydrodynamics and aerodynamics differ there, especially at the slow speeds we achieve.

What we experience is mostly what's called "impact lift" resulting from molecules attacking an inclined surface in motion, and pushing it up (as opposed to aerodynamic lift resulting from reduced pressure generated by accelerated fluid flow velocities, in accord with Bernouli's Theorum). So in the case of the upswept NAUTILUS bow, that characteristic will lift the boat's nose if anything.

If there was some other characteristic of the basic NAUTILUS design that was producing the "auto-dive" phenomenon, the lack of an upswept bow on my NM would enhance that effect, if anything. But no: she's as tame as an old mare.

I still come back to the differences between a NAUTILUS that's essentially dry inside; versus one that's a cylindrical pressure vessel inside a larger free-flooding hull. All that water inside doesn't actually weigh the boat down when the whole thing is submerged; but its MASS does figure into the equation when underway. It's a factor with enclosed wet subs like the REEF RANGER, too.

The inertia of all that water weight has to be overcome when initiating a maneuver, and the momentum it imparts once underway must be countered to reverse the effect. Sounds to me like, when speaking of a free-flooding NAUTILUS replica, if a diving trend get's started as speed increases, it will be enhanced by the mass of water in the bow of the boat.

As Bjorn points out, this is not a problem if it enhances the boat's "divability"; but it might be if, as someone else said, the trend continues until the sub is pointing it's tailfins at the sky. That's nice if that's what you want to happen; but if it's happening on it's own, you have a serious lack of control.

Another thing to consider is the open scupper drains that lead into your hull, and the open ballast vents in the bottom: these all let water flow into the hull interior. Underway, all kinds of gremlins could be joining the party that don't meet the eye at first glance. But again, these conditions would effect a free flooding replica, but not one with a dry pressure hull taking up most of it's interior space.

The jury is still out, but I'm inclined to stand by my original assessment that this is somehow related to the characteristics of a cylinderical pressure vessel inside a larger free-flooding hull; because that seems probable to me, and it also seems supported by the differences we observe between the performance of dry versus wet replicas.

Mi dos centavos, amigos... ;-)

Pat

I take it you have a cylindrical pressure hull in your Nautilus mini sub?

After all that octagonal shape won't be anywhere near as good at withstanding hydrostatic pressure.....!

Sounds like you're suggesting a larger cylinder as opposed to a weeny one and the use of bouyancy foam.

The latter seems to be common practice in the US.

In the UK most sub modellers prefer to build a larger cylinder to provide bouyancy.

Only problem is larger cylinders require stronger material to withstand crush depth, innit!

Plus you need a lot of lead to ballast the boat down to the waterline.

No probs with a little boat, but with a biggun it's major pain the back...literally!

Andy
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Postby JWLaRue » Sat Jun 05, 2004 12:55 pm

Interesting idea about the relative sizes of the cylindrical WTC....

Here's another data point: I've built r/c subs 'both' ways, in this case Type VII U-boats and with either sub properly trimmed for surface and submerged conditions, there is no 'enhanced' or increased tendency to dive with one vs. the other. They also have open areas under the saddle tanks which is similar to the ballast grates of the Nautilus.

I think the static submerged trim of the Nautilus may be the culprit if it doesn't sit horizontal.....

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Postby Bob the Builder » Sat Jun 05, 2004 10:11 pm

My Nautilus is dead level surfaced or submerged. I've measured accurately to within 1/8 of an inch in a testing tank.

Not sure if that has anything to do with it, as she will pull downwards from full surfaced trim at full throttle, actually diving right up to the point where the prop breaks the surface and she loses speed.

Whatever is pulling her down is below the level of the upper salon shroud (my fully, fully blown state).

The only way that this will be answered properly is to build a model that is just the dodecahedron with no salon, rakers, keels, etc. Then add the detail pieces one at a time until the culprit is found.

Looking again at the model, do you think it has anything to do with the break in the keel? There is a large flat area behind the forward section that will generate a lot of turbulence at speed (the area that houses the lower diving hatch)....
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Postby Wbnemo1 » Sun Jun 06, 2004 12:19 am

Howdy,
the original intent of Harper Goff Nautilus as he stated was to be simply this......."if for some reason the dive and trim planes failed to operate, the Nautilus would eventually surface under it's own power,this is why the upward sweep...something to that effect anyhoo!!!...Nautilus builders forget one thing it's not just the side rakers that have the approx3 degree upward sweep,it's the entire foward hull..my guess then is that either these R/C boats do not have this shape or that they aren't trimmed properly..Ray Mason sub is an example of what,if anything should be going on...his planes if going with speed lol
William
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Postby Carcharadon » Sun Jun 06, 2004 6:34 am

The three RC subs I built each vary in design slightly, and size (7 ft., 4 ft.) . However, they run exactly as I had envisioned them. And they run great, no problems. But I have noted a slight, and I want to emphasize slight, tendency for the sub to want to dip its nose. This nose dip is related to speed. The faster the speed the greater the effect. At slow or moderate speed there is no effect. Also the larger the sub (greater mass) the less the effect.

If you turn a Nautilus upside down and look at the hull sideways without the keels it looks to me like an inverted wing. I have an unfinished 7 ft. hull and if I could post a picture on this board I'd show it. There is a curved flat section on the bottom of the hull and as Andy and Jeff and others have suggested I think this results in a slight downward suction. (This seems intuitively obvious to me but it could be something else) On a larger sub, because of the greater mass and inertia involved, this effect is less prominent. I believe the stability in Pat’s Mini Sub is probably more related simply to its greater mass. (Pat what does your sub weigh?) Also in the case of Bob's sub I think the effect is accentuated because the CG of his Nautilus is very much forward, the combination of a forward CG and the downward effect I believe are accentuated in this case.

Also a question for Pat and Will! The literature that came with the 50th anniversary Disney Nautilus says that the 11 footer was powered with five car batteries I believe. And weighed 1000 lbs. I assume that that the 11 footer was a functional model? I wonder how it was controlled? Was it remotely controlled somehow? Any more information on the 11 footer?




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Postby Wbnemo1 » Sun Jun 06, 2004 1:00 pm

Those batteries were used to work the lights, propeller,and spiral speed indicator,perhaps the dive planes,not sure on those though...
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Postby Sub culture » Mon Jun 07, 2004 7:20 am

Anyone know the speed that the Nautilus 'sled' used for the surface running scenes, was winched through the water?

Andy
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Postby Carcharadon » Mon Jun 07, 2004 8:47 am

I was able to post a picture of two unfinished Nautilus hulls upside down showing the curvature in profile. The hulls are unfinished models of a 7 ft. and 4 ft. The picture illustrates how much the bottom hull resembles the cross-section of an aircraft wing. Now making some broad assumptions here that hydrodynamics and aerodynamics are somewhat similar except of course for density it is reasonable to intuitively assume that the bottom hull shape of the Nautilus can, to some degree act as a inverted airplane wing providing a slight downward thrust (or suction) in relation to increased speed. This seems to make sense to me. However, I could be entirely wrong in this.

http://home.comcast.net/~tyourk/curve.jpg



Andy I think they mention something about the 22 ft. model being towed by a truck on the 20K DVD. I think scale wise it was in excess of 300 mph. I could be wrong I'll see if I can find it again.
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Postby Sub culture » Mon Jun 07, 2004 10:40 am

Carcharadon wrote:I was able to post a picture of two unfinished Nautilus hulls upside down showing the curvature in profile. The hulls are unfinished models of a 7 ft. and 4 ft. The picture illustrates how much the bottom hull resembles the cross-section of an aircraft wing. Now making some broad assumptions here that hydrodynamics and aerodynamics are somewhat similar except of course for density it is reasonable to intuitively assume that the bottom hull shape of the Nautilus can, to some degree act as a inverted airplane wing providing a slight downward thrust (or suction) in relation to increased speed. This seems to make sense to me. However, I could be entirely wrong in this.

http://home.comcast.net/~tyourk/curve.jpg



Andy I think they mention something about the 22 ft. model being towed by a truck on the 20K DVD. I think scale wise it was in excess of 300 mph. I could be wrong I'll see if I can find it again.

That can't be right!

The truck would have had to have been travelling at appoximately 190mph!

My method for calculating scale speed, taken from 'Model Submarine technology'-

(Original speed/square root of model scale) x 1.75 (model factor)

Therefore

(300/2.8) x 1.75= 187.5 mph

Some truck!

Andy
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Postby Carcharadon » Mon Jun 07, 2004 11:14 am

Whatever scale speed they said it was they also said it was excessive, but the director said he liked the effect anyway and he said to go with it. I think its on the DVD in the bonus material some place. I looked but couldn't find it.


177ft/300mph = 22ft/37mph




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Postby Bob the Builder » Mon Jun 07, 2004 11:42 am

Wbnemo1 wrote:Howdy,
the original intent of Harper Goff Nautilus as he stated was to be simply this......."if for some reason the dive and trim planes failed to operate, the Nautilus would eventually surface under it's own power,this is why the upward sweep...something to that effect anyhoo!!!...Nautilus builders forget one thing it's not just the side rakers that have the approx3 degree upward sweep,it's the entire foward hull..my guess then is that either these R/C boats do not have this shape or that they aren't trimmed properly..Ray Mason sub is an example of what,if anything should be going on...his planes if going with speed lol
William

Will,


I'm quite insulted that you're calling my model "improperly trimmed"! And then you say it doesn't have the proper shape on top of it all!

Take thy dagger from my heart!

Actually, as far as I can tell, the only difference between my model and the corrected plans of yours is a slight difference in the 40' bulkhead. Mine is just slightly larger than your plans call for, but not to any huge degree. I think maybe something like 1/8" in 1/32 scale.

My bulkheads do follow the correct upward slanting design that we're talking about, and I do realize that it's the whole front end and not just the rakers that rise. That statement was not properly clarified on my part.

And in terms of trim... my model is the most gentle thing to operate in an RC sub that you could hope for. We recently had a fun run up here where about eight representatives from BC and Washington showed up. All of them remarked at how well my Nautilus performed, how steady she was, and what a dream to drive she was. These statements were not said with a "for a Nautilus" in there anywhere, but for ANY RC sub.

My statements about her tendency to dive may be taken a bit out of context here. I rather like this quirk of the model and I'm not complaining to any degree. If I were a "Nautilus Nazi (yuk yuk yuk), I'd forgo the aft planes and just let her dive on her own without ANY input from the planes.

I find it interesting that I've talked with four owners of RC Nautili (Nautiloi?) and they all say that their models have the same dive characteristics as mine. That's a bit of a stretch to say that all four models have the incorrect shape and trim, don't you think?

You just don't like me adding those planes, do you Will?

There.

I'm done ranting.

Nobody likes me. I'm going to go cry in my pillow for a while. :;):




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Postby Carcharadon » Mon Jun 07, 2004 12:53 pm

Bob, I remember an old adage I heard in school way back. Probably in an art history course. However it had to do with form and function. Looking at the Nautilus this is tough submarine to replicate in terms of form but also in terms of function. But I must say that your submarine is probably the best marriage of form and function for this sub. At least on an RC level. With all due respect to everyone else.
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Postby Sub culture » Mon Jun 07, 2004 1:00 pm

Carcharadon wrote:Whatever scale speed they said it was they also said it was excessive, but the director said he liked the effect anyway and he said to go with it. I think its on the DVD in the bonus material some place. I looked but couldn't find it.


177ft/300mph = 22ft/37mph

177ft/60 mph = 22ft/37mph, roughly.

So, according to my calculations, a 1/32nd scale Nautilus will need to cut through the wet stuff at about 18.5 mph to emulate the wave formation from the movie.

That's a fair old clip.

:O

Andy
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