Thor Alfa - goes faster deeper!!! - Depth / pressure questions

[slats]

G'day everyone.
Just had a great w/end sailing my Thor Alfa at my model boat clubs day out to a dive pool in Sydney Australia.

I noticed some interesting things.

1-First of all the dive pool is 6 metres (19.6 ft) deep at the deepest spot. I managed to bottom the boat and still had radio range. In Australia legal RC frequencies are 36mhZ and 40 Mhz systems. My radio is a stock Futaba Sky sport FM 4 channel 36mhz system. The range underwater I thought was fantastic, is this what you would expect in a chlorinated pool? Naturally I would never go this deep where you can't see your boat! But a dive pool is excellent for going deep.

2-I noticed whilst the boat was running near neutrally buoyant at PD or about 1 m down, beyond this depth it seemed to go negative quite quickly beyond this and at an increasing rate. I know that the deeper you go the more the pressure wants to hold you down (which is a big problem for dynamic divers at depth), but it was interesting how fast the boat went negative with minor manual pitch adjustments towards the bottom. On the way down I would just drain the tank little by little to get near neutral again. Has anyone else noticed this?

3-When blowing the tank whilst on the bottom, it took a little while for the boat to react with the sudden gain in buoyancy and the rate of ascent was at lot slower than when I have done this in shallower water. Could this be the buoyancy fighting the pressure?

4-This is the weirdest one. I started out cruising at a crawl at PD. I set the throttle in a fixed position of just crawling along. Slowly I go deeper, letting the leveller level the boat off with every half metre I descend, the boat at every stage got faster. I had a surface ship matching my speed and by the time I got to cruising around levelled off at about 4m the boat was doing almost twice its PD speed with no change in the throttle. Does anyone know why? Does the prop pushing water at higher pressure work better?

5- Had the best time ever with the Thor Alfa. It is so much fun to drive. Really seems like flying, in turns it banks over and a little bit of upward pitch, just like a plane tightens the turn up. Its acceleration is great fun. I can thoroughly recommend it. After two 1 hour sorties, no leaks in the WTC so big smiles all round.

Hope everyones well
All the best
John



Edited By Slats on 1099891100

[bcliffe]

John,

Congrats on the successfult run,

John,

Congrats on the successfult run, and good observations.

It's not so much that pressure wants to hold your model down, but more so that pressure makes elements smaller which means less buoyant.

One of the first elements to compress is the styrofoam resulting in negative buonyance at fairly shallow depths.

Because of pressure, at depth it takes more propellent to blow your ballast tank. That excess propellent will expand as the model rises, so theoretically it should go faster and faster as it approaches the surface.

With respect to the increase speed, my assumption is that the denser water due to pressure increase the thrust output of the prop. Less cavatation.

Sounds like you found a nice place to run your boat.

Cheers
BC

[raalst]

Hi, my 2 cents ]



Edited

[color=#000000]Hi, my 2 cents ]



Edited By raalst on 1099951816

[slats]

Thanks for your comments, but

Thanks for your comments, but the ballast system I have used for several years now employs an impeller pump, pumping into a sealed PVC tank. (I have previosuly written this up in the SC report). The air inside the tank compresses as the water comes in and when the tank is 60% full of water the pump cannot force anymore water into the tank as. (A good saftey feature - you can't over pressurise the tank if the pump stays on). At that stage the pressure inside the tank is 20 PSI. What stops the water from rushing back out is a clipard valve. When the value is released by servo, the water, pushed by the compressed air in the tank leaves the tank and the boat surfaces.

Your right about the pool, I don't know about North America or Europe but its pretty rare in Australia to get to use clear water like I had. Most of the time I run in freshwater lakes at periscope depth only, as the visibility and weed factor is all around.
Thanks
John.

[bradv]

Hey John,

Most likely the cause

Hey John,

Most likely the cause of the slow ascent at depth when blowing your tank is the smaller differrential between the external water pressure and that of the internal compressed air. At depth, even though the water pressure will not be enough to stop your compressed air from forcing the water out of your tank, the smaller differrential between the two pressures will reduce the rate at which the water escapes. As water is incompressible, you'll thus be losing weight more slowly. Near the surface, the water pressure will be far less, allowing the much greater air pressure to evacuate the chamber more quickly, this lightening the sub more rapidly, increasing ascent.


The dive pool sounds like a *great* place to run in! However, I think at 6m my Akula would be doing an impression of the Thresher Probably 3m is the max I'd take my little fella to. Your question regarding maximum controllable depth is an interesting one, which would make a good general question. This is what I've obtained, measuring to the top of my aerial.

Chlorine, private pool - 2m, hit bottom
Ionic, private pool - 2m, hit bottom
Freshwater lake - 1.5m, no further visibility
Chlorine, public pool - 1.2m, lost signal
Saltwater pool, private - 0.7m, lost signal
Saltwater lake - 0.05m, lost signal

Quite obviously, the more dissolved chemicals in the water, the less the signal travels as we all know. Not being an expert in dive pools, I'd suggest that they may be more lightly chlorinated due to:

1. Requirement for improved water clarity for U/W filming of divers' entry
2. Not extensively swum in by the public.
3. Swimmers aren't in the water for that long.

If this is so, it'd explain why you got such great transmission depth. Can anyone confirm/deny these assumptions?


Good to hear you got that pesky leak fixed that was plaguing you at the SubRegatta. Your Alfa is an absolutely beautiful sub to watch run

[tang]

Slats, the impeller style pump

Slats, the impeller style pump you chose .... what range of working voltage is it rated for? I have found a few pumps that could produce the necessary pressure, but all were for 12V applications. I didn't want to make a separate power source for those.

[slats]

Tang.
I have found that pumps

Tang.
I have found that pumps rated below 12 volts don't have the necessary grunt to pump sufficient water into a non vented tank. The one I use is therefore rated at 12 volts and comes from Dick Smith Electronics in Australia. Its part number is P-8907. A recent scan of their website doing a search on this indicates that they are presently out of stock.
This pump successfully pumps water into a sealed tank until its 60% full. After this the pump keeps running but can't force anymore water into the tank. You need a valve to stop the compressed air inside from throwing the water you have pump in from coming back out.
I have tried GAS, RCABS, Skips system (vented tank)+GAS backup, Aircraft retracts. All of these ballast systems work well, but for me I keep on coming back to this system - personal preference, but I love it, its so simple.
All the best
John

[tang]

Thanks for the quick reply!

Thanks for the quick reply! (WOW)

The system sounds like a great idea. What issue of SCR has your write up of this system? I'd like to see more details of this.

Thanks again.

[slats]

Tang,
see issues 52 and 57.
Hope

Tang,
see issues 52 and 57.
Hope that helps
John

[tang]

Thank you!

Thank you!