SNORT vs. SAS
RC submarines use a wide variety of ballast systems to submerge. This can range from simple gas systems to fully computer-controlled piston tanks.
Years ago, David Merriman brought to mainstream production a version of the watertight cylinder that used a valve to vent air and an air pump to blow ballast. This system he dubbed "SNORT".
Over the years, his design evolved and eventually became what he titled SAS, or the "Semi-Aspirated System".
What is the difference, and when would each system best be employed? Well, sit back. We're going on a tour of SNORT vs. SAS!
The SNORT ballast system is a very simple and reliable method to bring a model submarine from "submerged" to "surfaced" state. The caveat to this, however, is that with this system the model never truly reaches a state of submerged trim, but rather of "dive trim".
Yes, SNORT is actually a ballast system for a dynamically diving boat.
At no time is the model ever in a state of negative buoyancy. To dive, a vent valve at the top of the ballast tank is opened and the air is vented from the ballast tank, which allows water to flow in through the holes in the bottom. The model's buoyancy decreases, and it eventually settles into a state of very slight positive buoyancy, typically with just the top of the sail or conning tower sitting above the surface of the water. A bit of forward movement and a dive command to the planes, and the model slips under the waves.
To surface, the model is brought to "periscope depth", where the intake for the air pump breaches surface. Typically, this intake is situated at the top of a scale mast or periscope, those being the highest point of the model submarine. The command to blow ballast is given, the air pump engages, and air is drawn from the surface, through the pump and into the ballast tank, displacing water, and bringing the model to full surfaced trim.
The advantages to this system are many, the biggest of which being reliability and safety. With the model always in a state of positive buoyancy, odds are good that it will always find its way to the surface in case of emergency. As the air path is direct from intake to ballast tank, there is no chance for water to enter the dry compartments of the cylinder inadvertently, except in the case of failure of the internal hoses.
If the pump is accidentally started when the model is submerged, absolutely nothing bad will happen. The pump will simply pull water from the intake and pump it into the already full ballast tank. No harm, no foul.
The only real disadvantage is the fact that you can't statically dive (that is, you cannot dive fully without some forward movement of the model). In practical application, this is a non-issue, as no full-sized submarine would ever statically dive.
The SAS Ballast System
Just looking at the above diagram shows the more intricate layout of the SAS ballast system. With increased capability comes increased complexity.
With the SAS system, you gain the ability to fully statically dive your boat.
Diving for both SNORT and SAS is identical. The vent valve at the top of the ballast tank is opened, air escapes, and water enters. The model decreases in buoyancy and submerges.
The prime difference is how the two systems blow ballast.
With SAS, you add two components, the first being the snorkel induction assembly. This valve automatically seals the airway path when the model submerges. Typically, this intake is located as high up in the boat as possible, usually in the sail. Note that this is still much lower in the boat than the intake for the SNORT system. A float serves as a movable seal, pressing against the induction nipple as the boat lowers in the water.
SAS has two potential pathways for air to enter the pump. The first is the snorkel. The second is the air from the dry compartments of the cylinder. If the snorkel is closed, the pump pulls air from the cylinder itself, creating a partial vacuum. This air is pulled through the air pump and is then forced into the ballast tank, creating a state of positive buoyancy and the model eventually breaches surface. At that point, the snorkel opens, the air pressure equalizes, and the completion of the blow cycle occurs using air drawn from the surface.
There are two potential failure points in this system, however.
The first is the snorkel itself. If the snorkel assembly leaks while the model is submerged, water can force its way down the intake pathway.
At this point, water then reaches the safety float valve, whose job it is to protect the model in just this occurrence. Without it, water would flow easily from the snorkel valve and directly into the dry compartments of the boat. If the safety valve fails (and it is a tricky component to have work perfectly at all times), flooding of the boat is likely. Inside the safety valve, a float will rise with any water intake, sealing the pathway into the cylinder.
The primary benefit of the SAS system over the SNORT system is the fact that the operator can statically dive the boat, that is, he can dive without the need for any movement of the boat in a forward direction and without the use of dive planes.
The primary drawback is increased complexity and higher odds of cylinder flooding.
Converting from SAS to SNORT (and back again)
Fortunately, the owner of the boat can quite easily switch between the two systems if one or the other appeals to him more.
To go from SAS to SNORT, simply remove the snorkel assembly (or bypass it if you don't want to mess with your trim), and run the intake through a mast or periscope in the sail. You can then disconnect the safety float valve and remove it altogether (or keep it in the model to preserve trim). The air intake hose goes directly to the cylinder bulkhead, then to the pump's intake. From the pump, it goes back out through the bulkhead and into the ballast tank.
To go from SNORT to SAS, you add the snorkel induction valve to the base of your sail. You must also purchase and install the safety float valve. The air routing is as per the diagram above. You must "T" off the hose from the induction assembly, running the second pathway into the base of the safety float valve. The top of the safety valve has hose running from there to the dry compartment of the submarine (this can simply remain open with no hose attached if you mount it in the aft dry compartment of your cylinder).
The Gas Backup Option
Both systems can benefit from the addition of a Gas Backup System.
This system installs in any Nautilus Drydocks SubDriver, and is actuated via the ballast servo, so there is no need for additional wiring.
Set up properly, the gas backup can be manually or automatically activated (with the aid of the excellent BLM failsafe module available from the store under the Electronics section).
Basically, the gas backup uses compressed gas that is commonly used as airbrush propellant to blow ballast. The small reservoir holds enough liquid air to blow ballast a small number of times, but certainly enough to bring the boat to surface in case of loss of signal or owner panic.
Both SNORT and SAS systems are superb examples of RC engineering.
All future Nautilus Drydocks SubDrivers will come as a SNORT variant with gas backup as default with the option to add the snorkel induction and safety float valve components at additional cost.
From a safety perspective, this is the best configuration for new sub drivers or for experienced ones who want the safety factor of constant positive buoyancy. I, personally, set up all of my boats as positively buoyant, and with that being the case, the SNORT system is what I would recommend for 90% of sub drivers.