Choosing Your Watertight Cylinder
You've got the model, but now you need to power it! But where to start?
There are a lot of choices out there... different manufacturers, different sealing systems, different ballast systems. It's all so confusing!
Let's try to break it down to make the decision an easier one for you:
First off, figure out what sort of room you have to play with. This is dictated by two things: your available length and your available beam. As subs are (typically) long and narrow, length is rarely a big issue, so start with the beam. Take your widest part of the hull and subtract about an inch. This should allow for tapering of the hull and also leave room mid-ship for the installation of ballast in the keel and foam around the perimeter of the cylinder.
Okay, so we've figured out what sort of diameter we're looking for. What next?
Are there any features that you need or want to have? This is most often dictated by the favored ballast system, as that forms the foundation for the cylinder in many cases. There are a lot of options out there, and I've created a handy reference page for you in my FAQ section that you can visit here. The most common systems are the pressure pump system, such as the OTW Dive Modules, the gas-based system, such as the older D&E cylinders, the RCABS system, such as the ones available from RC Sub Workshop in Japan, and the piston tank system that Engel of Germany favors. I offer a hybrid system that was engineered by the very talented Mr. David Merriman, called the SAS (Semi-Aspirated-Snort) system. It has an open ballast tank and vents the air to dive. I then uses an air pump to suck air into the tank from the electronics section, switching to external air when the snorkel breaches the surface. Slick, and relatively simple!
Think about the seal system for the endcaps on the cylinder. Most cylinders use an internal o-ring to seal against the inside face of the tube. These work well, but can be difficult to open at times. OTW uses an o-ring that seals on the face of the cylinder. This makes for easy access, but requires internal threaded rods to provide pressure on the end cap and seal. Bayonet seals are a good compromise, but are harder and more expensive to manufacture. Engel of Germany uses this system on their newer kits.
You should also look at the seal system for the linkages. The two main types are a cup seal and a bellows seal. The bellows seal is basically an accordion-looking rubber assembly that seals both on the end cap and the linkage itself, compressing and extending with the movement of the linkage. Cup seals seal on the outside circumference of the linkage rod. They provide less binding, are smaller, but are more prone to leaking over time.
Which leads us to another consideration, which is the ballast tank size. The purpose of the ballast tank is to bring the submarine from "surfaced trim" (with the boat sitting as far out of the water as it does for surfaced operation) to "negative trim", which is where the boat displaces less water than it weighs, therefore sinking. The difference between those two states is the volume of water, of the weight of the submarine hull that sits above the waterline.
Or, to put it more simply, your tank needs to be able to displace as much volume of water as the part of your sub that sits above the waterline in surfaced trim. Confused yet? Let's try this... weigh your upper hull, ideally just the part that sits above the waterline. If you can't get exact because of how your hull is split, take an educated guess based on the upper hull weight. Okay, now we have a weight. We'll use grams in an example, because the US imperial system of measurement is just plain stupid (get with the rest of the world, already... geez). Let's say your upper hull weighs 1000g and we guess that the part that stick out above the water is about 40% of that, or 400g. Thanks to the miracle of the metric system, we know that 1g of water is 1mL, so we need a ballast tank that will hold 400mL of water. Make sense? Factor in a safety margin and you'd be at around 500mL, or half a liter (or about 17oz for you metrically-challenged folk).
Okay, you've got your wish list. What's next?
Pick your vendor. You know what you're after, but who are you going to buy from? Just as I spoke about in the "Selecting your Kit" blog from earlier, this is a big decision. Your vendor of choice should provide excellent customer service, be prompt in communications, and have an excellent description of their product so that you know exactly what you're buying. The choice of vendor will dictate your style of cylinder and, in many cases, the size of the ballast tank they offer.
If all else fails, reach out to the experts, or at least the experienced. A good vendor will be able to help you select exactly what you need. Better yet, reach out to the people that run them. The forums are a great place for this. If there are shortcomings either in the product or vendor, people will tell you honestly.
Best of luck!