Cylinder size

[billyd]

How do you know what size cylinder to use for a given boat?

[RCSubGuy]

Cylinder size is not the most important factor. Provided you can fit your electronic gizmos in there, the cylinder can be as tiny as you want. What is important is the ballast tank size. I have a very in-depth article talking about this that you can see on my cylinder page, in my book and on my website.

How Do I Know How Big To Make My Ballast Tank?

The job of a ballast tank is to lift out of the water everything that needs to be above the water. A submarine can dive, surface and otherwise operate perfectly fine with no ballast tank whatsoever. It will, however, be unable to bring any superstructure up out of the water for surfaced operations. In order to do that, the sub needs to get lighter, and submarines do this by replacing water from their ballast tank with air.


So, how do you know how much air you need?



Assuming that the density of the materials your submarine is made out of is primarily plastic or fiberglass, we are close enough to water density that calculations can ignore these small differences. With that being the case, calculations are much simpler.

If your boat is split at the waterline (as many WW2 fleetboats are, for example), then you can simply weigh the upper hull. This is the weight of water that needs to be purged in order for you to get to surfaced waterline (plus an additional "fudge factor" for safety).

If your boat is not split at the waterline, you're going to need to take a best guess as to the weight of the upper hull. For example, if your hull is split at the midline, weigh the upper hull and take approximately 40% of that weight as the surfaced portion (or whatever portion you determine would be closest).

250 Series SubDriver ballast capacity per inch:

• 2.5 inch ballast tank diameter:
  • 4.43 cubic inches per inch of tank
  • 2.45 fluid oz per inch (106.5g)

Example #1:

If your upper hull weighs 15oz, we add 10% and round up, getting 17oz. We now need to allocate enough ballast tank to take on 17oz of water. Each section of 2.5" ballast tank in the 250 Series SubDriver can hold approximately 2.45oz of water. We need 17, so that divided by 2.45 nets us a length of 6.94". Rounding off and 7" is what we'd get.