.What type of ballast for a big WWII sub?

[skiprch]

Howdy,
I noticed that otw has a wtc that I would really like to have but can't spend that much. All I really need is the ballast and was wondering how the OTW system worked? Do they just pump water in and out of the tank? If so how does the air get removed? Or does the chamber just fill up partially and compress the air?
thx!
Walt

[JWLaRue]

Re: .What type of ballast for a big WWII sub?

Hi Walt,

The OTW dive module pumps water into and out of the ballast tank and uses a solenoid check valve to retain the current water level in the tank (i.e. no pump bleed through). The air in the ballast tank is allowed to vent into the two dry chambers of the dive module. The ballast control module includes a high and low cut-off for the water level in the tank....which prevents the water from getting high enough to vent into the dry space.

-hope this helps!

Jeff

[Guest]

Re: .What type of ballast for a big WWII sub?

You can copy this system using a geared pump instead of the Shurflo type pump OTW use. This offers no real advantage other than it allows a little more flexibility in packaging - or in other words, geared pumps are available in different sizes.

A geared pump may also be a little slower at pumping large volumes, however you can parallel pumps up if you want a faster dive time. A good make of geared pump is Kavan.

In place of a solenoid actuated check valve, you can use a servo actuated pinch valve. This essentially pinches up a section of silicone hose. It's simple, cheap, and will effortlessly hold back the flow of water.

The OTW dive module uses custom electronics to control the pump. You can use a simple two way switch, based either on a solid state device like a Subtech unit, or a pair of microswitches actuated by the same servo that is used to control the pinch valve. Instead of probes to indicate the water level, you could use a float switch to cut the feed to the pump when the water level reaches about 80% of the volume of the tank.

An alternative is to forget about the vent to the dry space, and pump into a sealed tank. If you're going to go this route there are a few extra precautions you need to take. The tank must be cylindrical or spherical and of sturdy construction, as it has to withstand about 50 psi on a regular basis, and it must remain completely air tight. Lexan or PVC tubing of reasonably thick wall is fine.

The end caps of the tank should be firmly fixed to the cylinder. You can bond them together with an acrylic glue or a strong mastic adhesive like Sikaflex. If you also run studding between the endcaps, this will make the tank close to bombproof.

The tank must be of larger volume. This system can fill the tank to about two thirds full, as that is generally the limit of the pump. The OTW system allows you to fill to about 80% full, because the air compresses into the dry spaces. As a consequence the tank will take up more room in the model.

You must use a geared pump or a centrifugal pump designed for higher pressure. The Shurflo nautilus pump used in the OTW modules cannot pressurize beyond one atmosphere, and at 15psi it's flow rate is basically zero. This is one of the reasons the tank is vented- it keeps pressure to within a few psi, which the pump can easily manage.

The pressurized tank system offers a couple of advantages. Humid air is not cycled through the dry spaces, and the fore and aft compartments remain unpressurized, thus there is little strain on the cylinders seals.

Finally, either system results in a ballast tank with an exposed water surface. This can result in slosh- the unwanted movement of water inside the ballast tank which plays havoc with the boats stability. This is countered by fitting baffles inside the tank, which slows down the movement of water. Most tanks have at least one baffle.

Ensuring the model is stable with weight down low, also helps stability. Endeavour to make your tank as short and fat as possible- short and fat minimises slosh. The perfect shape for a ballast tank with an exposed water surface is a sphere. Unfortunately spheres are tricky to make, and a rather inefficient shape to stow in a submarine hull, so it's rarely used.

[skiprch]

Re: .What type of ballast for a big WWII sub?

thx fer the replys guys!
I am so glad I did not sell my Engel Gato now! I am now remembering how beneficial it is to think about a idea and to try to make it work.
I had a idea about the ballast for my Gato. Could you take a inflatable membrane and put it into a tube. the ballast tube would have holes in the bottom to allow water to pass in and out. As the membrane inflated by taking air from the "dry" areas of the WTC it would force the water out of the ballast tube and raise the boat. Maybe? lol
thx!
Walt

[JWLaRue]

Re: .What type of ballast for a big WWII sub?

Hi Walt,

What you have described sounds like the RCABS (ReCirculating Air Ballast System) popularized by Art Broder. A lot of folks use this system with very good results.

-Jeff

[Guest]

Re: .What type of ballast for a big WWII sub?

It's been in use in one form or another for decades. Here in the UK compressed air systems were popular in the 80's upto the mid nineties, but have fallen from favour in recent years, and these days you see more boats based around water pump or piston tank systems.

The main snag is you can't trim a boat as accurately as you can with a piston ballast tank or one based on a water pump like the OTW. With the boat in submerged trim, ideally the bag/membrane should be completely evacuated of air, as any residual air left in the bag will compress as you dive deeper, throwing out the trim of the boat. If you stick to shallow operation e.g. within a couple of feet or so, then the pressure is so slight it barely registers any noticeable change in the boat, and it's possible to run with an air bubble in the tank. However if you get the opportunity to dive in deeper waters, e.g. a swimming pool, you will notice the boat fails to maintain depth, so it's worth knowing this disadvantage I think.

With piston tank systems, there is no air bubble to compress, so you can actuate the piston for different ballast volumes to allow for different water densities or if you want to bottom the boat. With a water pump system, provided it is shut off with a valve, the air bubble inside the tank is isolated from outside water pressure, so unable to compress. Therefore you have the same level of flexibility as with a piston tank system, but minus the option of proportional control and you also have to compensate for a tank with an exposed water surface.

In a nutshell, the compressed air system is simple and fairly cheap, but you trade a little flexibility.

Every ballast system has it's pros and cons. You can sometimes get the best of all worlds if you pick and mix ballast systems, these are known as hybrid systems. An example would be to match say a compressed air system with a small piston tank. Compressed air is excellent at moving large volumes of water quickly and efficiently. Unfortunately, it's not very easy to trim and manage small volumes with it. A piston tank is excellent at this task however, and if the tank is small e.g. 100cc (4 ounces) or less, it's possible to use a servo to actuate the piston, with a little modification. As servos are incredibly inexpensive these days, you can obtain all the advantages of a proportional piston tank system without the bulk/weight and price tag.

Unfortunately no commercial dive modules exist using a hybrid ballast system, which means the builder either has to try and adapt commercial items, or scratchbuild. Not many modellers are prepared to go to those extremes, which is why you don't see many of these systems about.

If you want an example of this type of system in operation, you can take a look at the German modellers 'Pressluft' boats. These are very large boats which typically displace about 50-100kg submerged, real behemoths. The main ballast tank which is usually several litres in volume, is controlled using compressed air. The main tank is vented at bottom, and features magnetically controlled valves at the top, these are opened, allowing air to escape as the water floods in at the bottom. Two piston based compressors are set-up in parallel to pump air into a reservoir, often mounted in the bow of the boat. The air is drawn in through a snorkel mast and stored at about 90 psi. The reservoir is usually sized to allow for two blow cycles, and failsafe control systems are in place to prevent the boat diving if sufficient pressure isn't detected in the reservoir.

To surface, air is released from the reservoir via a magentic valve and regulator, the latter is there to limit the pressure to about 20psi, and into the ballast tank where the water exits out of through the vents at the bottom of the tank whence it came.

Twin piston tanks are sited either side of the C.G, and are generally volumed for about 1-2% of the boats overall displacement, which is about 1 litre (two tanks 500ml each). This allows the boat to be trimmed for differing water densities, operation of these tanks need only be carried out once in order to trim the boat to a neutral state (usually just a few ounces bouyant), and subsequent dive cycles carried out solely using compressed air.

http://www.zentrale.pressluftjunkies.net/index.php/en/

Although these are large boats, the principles used can be miniaturised and used in more modest sized boats. The biggest obstacle is the lack of an off-the-shelf miniature piston based compressor. However if you're prepared to accept a lower pressure system, the small diaphragm based compressors readily available could be used instead, and the main cylinder itself used to store air at say 8-10psi. This would limit the depth you could safely dive and surface to about 20 feet, but it removes the need for a separate reservoir, regulator, pressure switches etc. The depth limitiation will probably be enough for most modellers.