You said -
“I am trying to stick with a 6 volt system, utlise a sealed lead acid battery. I am fully aware of all the arguements surrounding the use metal hydrides.”
A little clarification of what Ed (Ramius II) said - with SIMILAR POWER OUTPUT, a 6 volt motor running on 6 volts will draw more current than a 12 volt motor running on 12 volts. With this in mind, using a 12 volt system would still allow a smaller battery. Note that running a 6 volt motor on 12 volts would have the opposite effect.
“So here goes, imagine a ballast system that works along the following principles -
Diving -a reversible 6 volt geared pump pumps water into a ballast tank. The tank has a non return valve that lets air out of the tank. When fully filled the ballast tank takes the sub to periscope depth or just above it. Okay so far so good - this has been done extensively before.”
True enough - a simple pump system.
“The ballast tank - also has a snorkel that vents the tank. The snorkel is linked to a float valve that mechanically closes when the sub is completely submerged and opens the moment the sub is at a depth where the sail has just broken the surface. The snorkel is on a separate line to the non return valve mentioned above.“
“Surfacing - when fully dived at depth beyond the reach of the snorkel, the snorkel vent remains closed. To surface the 6 v geared pump runs in reverse and starts pumping out water. In doing so a vacuum forms in the ballast tank as its not vented, but the geared pump can overcome this. When the ballast tank has only pumped out 30% of the water, the boat is or should be close to decks awash, and the moment the sail breaks the surface the snorkel valve opens automatically, the vacuum is then released and the pump then empties the full contents of the tank.”
While a positive displacement pump will create some vacuum in the tank against a closed intake (snorkel), just how much remains to be seen. I’d think that the pump would need a substantial amount of power to be able to remove enough water to reach the “30%” mark. But the real problem here is that any significant vacuum in the ballast tank would hold the snorkel valve (ping pong ball?) closed with enough force that gravity alone won’t be able to allow the float to drop. Some form of mechanical actuation would be required (servo and another channel) and this destroys the K.I.S.S Principle.
In a later post you said (regarding the Albacore/Marlin ballast system) -
“I am familar with this approach and do not recommend it. If you have the slightest leak in the WTC you will find it very difficult in getting to snorkel depth to empty the ballast tank.”
As delivered, these kits use a pure pump system. With the addition of the SubTech PBU (or similar customer designed) compressed air/gas failsafe system, the customer has the best of both worlds. This is the definition of “Hybrid” system. A high volume, low current draw pump drains extremely little from the boat’s battery. Having onboard compressed air/gas allows a failsafe for that inevitable occasion resulting from fishing line/weeds in the screw. BTW - air brush propellant is not really required. A couple of pumps from a bicycle tire pump would be sufficient. After all, if there’s a problem requiring the use of failsafe, you just need to get back to the surface so you can retrieve and repair, right?
Contrary to popular opinion, a pump type ballast system does not mean that you’re limited to periscope depth operation only. I’d like a nickel for every time I’ve heard “how do you surface if the air intake is below the surface?” The answer is very simple - drive the boat back up until the intake is out of water. With a fully flooded ballast tank, I can drive Albacore and Marlin to decks awash easily. And that’s using just the bow planes. Both of my boats have APC-4 “standing alone” with no manual input from the TX.
The U.S. of A - Land of the Free BECAUSE of the Brave