Banner Ad 1

Hydrodynamics of the Nautilus - some thoughts and observations

Nautilus, Seaview, and more

Postby Carcharadon » Wed Jun 02, 2004 2:25 pm

I am not a hydrodynamic engineer but with two models of the Nautilus 4 ft. and 7 ft. and with numerous speed modifications to both in my attempts to re-create the bowwave effect, as seen in the original movie with the Nautilus in ramming posture to rip the hull out of a surface vessel I can make the following observations. As others have noted the design of the Nautilus characteristically makes it want to nosedive from a surface running position. There are a number of variables related to this not necessarily in direct proportion i.e. non-linear relationships.

First the sub wants to dive in relation to its size and speed. These relationships are not necessarily proportional or direct. Taking for example a relative speed of say 50 knots the larger sub is less likely to nosedive. And while I do not believe this is a directly proportional relationship my feeling is that at some theoretical size this effect is minimal or nonexistent. So that the full scale submarine probably would not experience the effect unless of course the speed was very great.

Conversely a sub i.e. the Nautilus, at any given scale or size will not exhibit this tendency to nosedive until some theoretical speed is reached and the speed is not a relative constant but somehow varies in accord with hydrodynamics way beyond me.

This is why I believe that a full-scale Nautilus at the speed shown in the movie would nosedive. I realize of course it was a 22 ft. model towed at great speed. Also the point of thrust I believe makes a difference in the nosedive effect. A propeller at the tail is more likely to induce the nosedive effect than jets located somewhat forward of the traditional propeller. In fact if the Jets were placed very forward there probably would not be a nosedive effect. Similarly another way to look at this is to think of a propeller located completely forward at the nose and in this case there would be no dive effect either.

What I don't know is whether this diving effect persists once the sub (the Nautilus) is submerged. I think not. The reason being that there is resistance from the alligator eyes and the wheelhouse in general which would tend to overcome the downward effect once the entire submarine is underwater. Resistance at the alligator eyes would tend to raise the nose upward thus canceling the inherent tendency of the sub to submerge when on the surface. I just wish I had clearwater to observe this.

So what does this mean to the modeler? It means that for a smaller sub at a faster speed on the surface it will tend to nosedive as opposed to a larger sub at the same relative speed. (as Bob and others have noted) What does this mean for a large submarine such as Pat's mini sub? I don't think it's relevant until really really fast speeds are obtained. What does this mean in terms of trying to duplicate the bow effect with the model submarine? Well it's just one more variable that needs to be considered and adjusted.

In other words the bowwave effect without having the model actually towed is a tough endeavor. This brings me back to my original thoughts about this and that the only real way to duplicate the Disney special effect is to actually tow a model with the water cannon. However in spit of all this I think I have come close to the effect. The last time I had the sub out I was able to use the full speed of the sub in conjunction with the water cannon. But this is no easy task. Should post new video later.




Edited By Carcharadon on 1086200967
User avatar
Carcharadon
Registered User
 
Posts: 300
Joined: Fri Feb 21, 2003 9:10 am
Location: Savannah GA

Postby Bob the Builder » Wed Jun 02, 2004 5:44 pm

Carcharadon wrote:What I don't know is whether this diving effect persists once the sub (the Nautilus) is submerged. I think not. The reason being that there is resistance from the alligator eyes and the wheelhouse in general which would tend to overcome the downward effect once the entire submarine is underwater. Resistance at the alligator eyes would tend to raise the nose upward thus canceling the inherent tendency of the sub to submerge when on the surface. I just wish I had clearwater to observe this.

I can say, with great certainty, that this is most certainly the case.

I have nosedived my Nautilus, particularly during her maiden trials, to near-vertical orientation in ten feet of water. The effect is pronounced, and I'm sure that she'd very happily plow directly into the bottom whilst standing straight on her nose.

My rear dive planes overcome this problem, but it still exists, and during full speed operations she will 'suck' lower into the water with the aft planes compensating to keep her level.

I'm almost positive that this effect is created because of the upward rake on the sidekeels. Some form of hydrodynamics far beyond my schooling thus far is responsible.

I've noticed in all of the pictures that I've collected of other people's models (an extensive collection) that any of them that achieve a decent rate of speed will always be oriented in an upwards angle in order to compensate for this effect and to keep the prop submerged.

I've got more plans for testing in the near future, and I'll post my findings here.
Bob Martin,
RCSub homepage: http://www.rc-sub.com
User avatar
Bob the Builder
SubCommittee Member
 
Posts: 774
Joined: Fri Feb 21, 2003 9:40 am
Location: Naples, FL

Postby Carcharadon » Wed Jun 02, 2004 6:48 pm

What you describe Bob sounds like a very pronounced effect. I didn't notice the effect on my 7 ft. sub until after I increased the speed. But compared to the effect you describe, on my sub it would be called minimal. At full speed the 7 ft. will lift its tail about three quarter inch and likewise drop its nose about three quarter inch. On a 7 ft. length this is not a lot. But in trying to make the water cannon work properly it makes a very big difference. So it was because of my attempts with the water cannon that I became more aware of this effect. With the 7 ft. at the old speed if it occurred it was not noticeable. In other words my attempts at the water cannon might actually work better at the slower speed because I would have a more stable platform and could make better adjustments. I noticed the same effect with the 4 ft. when I increased its speed. In the case of the 4ft the change was more, about an inch. That's why I started using the 7 ft. since I thought it would be a more stable platform, which it still is.

After I submerge my subs they seem to level out if I let off of the down control. If I let them they would go all the way across the pond underwater.

But the effect you describe seems what I would call very pronounced. I don't understand not only the effect itself but additionally why there would-be such a dramatic difference between our subs?
User avatar
Carcharadon
Registered User
 
Posts: 300
Joined: Fri Feb 21, 2003 9:10 am
Location: Savannah GA

Postby Lundberg » Thu Jun 03, 2004 3:43 am

Hi,

I've had this experience as well, I noticed it the first time when I was doing seatrials on my R/C Nautilus. At this point I had heard that in order for a Disney Nautilus to dive dynamically I would need larger divingplanes so I had equipped my boat with extended forward divingplanes.

The extended divingplanes setup did not work as expected because they jammed against the side rakers if I don't remember incorrectly, at this point I removed the divingplanes extensions and just ran the boat for fun in the local pond.

To my surprise I noticed that the boat would submerge automatically when it reached a certain surface speed, I didn't need to use the planes at all to make her dive! The only thing the planes were needed for was to keep her leveled once submerged. Which by the way was quite tricky because I did not have an automatic level control ;-)

So, I've also found that the Disney Nautilus dives automatically without the use of divingplanes at a certain speed, keeping the propeller in the water is also a challenge but in my case I have the boat trimmed quite low at approximatelly deck's awash so it works OK.

/Bjorn
User avatar
Lundberg
Registered User
 
Posts: 59
Joined: Sat Feb 22, 2003 1:20 pm
Location: Sweden

Postby Captain Nemo » Thu Jun 03, 2004 6:43 am

Guys,

I think this "auto-diving" problem may be attributable to the weight / buoyancy / balance characteristics of R/C models which are primarily a relatively small WTC within a relatively large free-flooding hull.

My NAUTILUS MINISUB is pretty much all pressure hull, just about the way the real one would be. She's as stable as a pool table: no unruly tendencies whatsoever. To dive dynamically, I have to ballast down deep awash, run at top speed, and pitch the boat nose-down. Without flooding the forward tanks, she won't dive at all.

Once submerged and running, she's easy to level out and run straight and level.

Cut the power (when slightly buoyant and submerged dynamically) and she gradually cruises her way back to the surface in a level attitude.

Built with the pressure hull occupying the proper percentage of the exterior hull, and the Goff Disney design is a very honest boat with no surprises.At least, that's been my experience with it.

Try building a properly weighted "dry hull" R/C model and I believe you'll find she runs about like mine does.

Hope this helps.

Pat
VULCANIA SUBMARINE: Home of the Nautilus Minisub and 20,000 Leagues Diving Suits
User avatar
Captain Nemo
Registered User
 
Posts: 119
Joined: Sat Mar 08, 2003 12:16 am
Location: Hawaii

Postby Sub culture » Thu Jun 03, 2004 10:57 am

Captain Nemo wrote:Guys,

I think this "auto-diving" problem may be attributable to the weight / buoyancy / balance characteristics of R/C models which are primarily a relatively small WTC within a relatively large free-flooding hull.

My NAUTILUS MINISUB is pretty much all pressure hull, just about the way the real one would be. She's as stable as a pool table: no unruly tendencies whatsoever. To dive dynamically, I have to ballast down deep awash, run at top speed, and pitch the boat nose-down. Without flooding the forward tanks, she won't dive at all.

Once submerged and running, she's easy to level out and run straight and level.

Cut the power (when slightly buoyant and submerged dynamically) and she gradually cruises her way back to the surface in a level attitude.

Built with the pressure hull occupying the proper percentage of the exterior hull, and the Goff Disney design is a very honest boat with no surprises.At least, that's been my experience with it.

Try building a properly weighted "dry hull" R/C model and I believe you'll find she runs about like mine does.

Hope this helps.

Pat

Fascinating!

It sounds to me like static forces are overcoming the hydrodynamic forces with your 'large' model.

Andy
'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
User avatar
Sub culture
SubCommittee Member
 
Posts: 2814
Joined: Fri Feb 21, 2003 6:55 am
Location: London, UK

Postby Bob the Builder » Thu Jun 03, 2004 11:47 am

A thought for Carcharadon and Pat "the psycho sub builder" Regan...

Are your side rakers flared upwards near the front of your sub, or are they perfectly flat? It seems to me that if I remember pics of both of your subs correctly, the side keels are flat (which makes more sense to me anyways). It is still my thoughts that this innate need to dive is created by the flared side keels of the Disney version.

Pat's sub:
Image

Carcharadon's:
Image

Mine (note the upswept side keels, beginning at the forward planes. The front exit for the ram is actually approximately 5/8" above the centerline of the model, as per the original filming miniature!):
Image




Edited By Bob the Builder on 1086279300
Bob Martin,
RCSub homepage: http://www.rc-sub.com
User avatar
Bob the Builder
SubCommittee Member
 
Posts: 774
Joined: Fri Feb 21, 2003 9:40 am
Location: Naples, FL

Postby JWLaRue » Thu Jun 03, 2004 1:06 pm

...but from a hydrodynamic perspective, the upswept side strakers should cause the bow to want to lift.....

Perhaps there is some other cause for the observed behavior of the models?

-Jeff
Rohr 1.....Los!
User avatar
JWLaRue
SubCommittee Member
 
Posts: 3813
Joined: Wed Aug 31, 1994 6:00 pm
Location: Annapolis, MD

Postby Sub culture » Thu Jun 03, 2004 3:20 pm

I theorise that the shape of the hull may make an inverted hydrofoil in the water, when the boat is on the surface. Thus causing the problem with self-submerging.

The side rakers may well produce some uplift, but not enough to overcome the hulls inherent problem. They're more likely to produce a lot of drag in the water.

There are probably a number of factors contributing to the problem, and it will require an emperical process to elimate methinks.

It would be an interesting study to run a Nautilus minus the side rakers to observe the effect this has on the boats running characteristics, then add in rakers at the rear, observe, then try with front rakers only.

This should narrow the problem down somewhat.

Any takers?

Andy
'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
User avatar
Sub culture
SubCommittee Member
 
Posts: 2814
Joined: Fri Feb 21, 2003 6:55 am
Location: London, UK

Postby Bob the Builder » Thu Jun 03, 2004 4:18 pm

Oh sure! I've already cut off my sidekeels with a jigsaw and duct taped the whole thing back together just in case....

(that's sarcasm in case you didn't catch it....)
Bob Martin,
RCSub homepage: http://www.rc-sub.com
User avatar
Bob the Builder
SubCommittee Member
 
Posts: 774
Joined: Fri Feb 21, 2003 9:40 am
Location: Naples, FL

Postby Lundberg » Fri Jun 04, 2004 4:15 am

I don't think it's a problem, I rather think it's a neat feature, I mean it is a submarine after all and it's supposed to submerge ;-)

/Bjorn
User avatar
Lundberg
Registered User
 
Posts: 59
Joined: Sat Feb 22, 2003 1:20 pm
Location: Sweden

Postby Carcharadon » Fri Jun 04, 2004 8:19 am

Bob, it seems to me that in your Nautilus the center of gravity is well forward, especially with the battery located so far forward. Now I realize you can have the boat trimmed properly and still very the center of gravity fore or aft to some degree. Where is the center of gravity in your Nautilus? If it's forward of the salons I would think that this is probably the most significant difference between our subs.
User avatar
Carcharadon
Registered User
 
Posts: 300
Joined: Fri Feb 21, 2003 9:10 am
Location: Savannah GA

Postby Bob the Builder » Fri Jun 04, 2004 12:18 pm

My CG is located about 2.5" forward of the salon.

I wonder if the large open grates on the bottom of the model might have something to do with it?

I agree with Bjorn, though. Maybe we're overanalyzing things. Before I'd impemented my aft planes, I could get the model to dive perfectly and gently with forward speed only. I didn't even use my planes at all.

Now that I have my aft planes in place, the APC will keep her level, and she'll 'suck' downwards on an even keel until she eventually dives on her own at full throttle.
Bob Martin,
RCSub homepage: http://www.rc-sub.com
User avatar
Bob the Builder
SubCommittee Member
 
Posts: 774
Joined: Fri Feb 21, 2003 9:40 am
Location: Naples, FL

Postby Carcharadon » Fri Jun 04, 2004 12:42 pm

Well I never meant to imply that it was a problem I was merely making comments and observations. From what Pat describes the operation of the Mini Sub is very similar to the way my subs work. Furthermore from what Pat says pretty much supports what I've been saying that in a larger sub this effect is probably undetectable unless superfast speeds are employed. Now there's probably some mathematical formula that would bear this out but as I said its way beyond me. I think Andy also hit on a good idea. I'm just commenting on observations with a few suppositions thrown in. And as Bjorn says it's actually an asset especially for the dynamic diver. Now in the case where I'm trying to get the water cannon to work it presents a bit of a challenge. If the water cannon is too deep the effect is diminished and if it's too high it looks like a fire hose effect. This margin of error is small and hard to adjust. But all this I think is interesting especially if your a Nautilus fan.

O forgot to mention the cg on both my subs is just aft of the salons.




Edited By Carcharadon on 1086368800
User avatar
Carcharadon
Registered User
 
Posts: 300
Joined: Fri Feb 21, 2003 9:10 am
Location: Savannah GA

Postby Captain Nemo12 » Fri Jun 04, 2004 8:01 pm

Speaking of hydro dynamics, I haven't really seen the Disney Nautilus in close-up, are the forward divingplanes simply flat when looked sideways or are they structured like plane wings? I understand that in order to have minimum drag underwater, the diving planes have to be curved somewhat, are the planes on the Nautilus shaped like that by any chance?
280 meters.... and she's still in one piece!
-Jurgen Prochnow, Das Boot.

http://eisernwolf.blog.com
User avatar
Captain Nemo12
Registered User
 
Posts: 293
Joined: Sun Sep 14, 2003 3:56 pm

Next

Return to Television and Movie Submarines

Who is online

Users browsing this forum: No registered users

cron