Three Simple Rules For A Perfectly Trimmed Sub

Building a model submarine is not a project for the faint of heart, the impatient, or the untalented. There is a reason that you don't see these at every lake.

Building a model submarine is hard.

It is, honestly, a hobby reserved for the elite of the RC community. On top of the aesthetic building of the boat itself; the painting, the weathering and excruciating attention to detail, there is the functional aspect of making everything work. Access must be created for the interior of the boat. Concession must be made for the installation and removal of the watertight cylinder. Control surfaces must move freely and throughout their entire range of motion.

On top of this, there is the fact that everything must be waterproof. Not a drop of water can enter the hallowed sanctuary of your expensive and complicated electronics. Linkages seals must not only seal out the water, but allow free and unrestricted actuation of the linkages themselves. The main prop shaft seal must, likewise, keep out the water while allowing free rotation without binding.

Now, on top of all of this, there is another and potentially more important aspect to sub-building. That is the proper trimming of the sub.

An improperly trimmed boat will perform poorly, be difficult to operate, and could ultimately end up contributing to the complete loss of the boat at the bottom of some murky pond or lake.

On the flip side, a boat that is properly trimmed out can handle like a dream, offering up control that is intuitive, smooth, and safe.

Getting that proper state of trim is of vital importance and is the area of the build that more people struggle with than any other. It's like magic, science and art all rolled into one. People will try to trim out the boat only to have it mysteriously pitch over on its side or even upside down. It may sit perfectly when surfaced and on its nose when submerged. It might sit perfectly when submerged, but crash dive when suddenly slowing or applying reverse thrust.

Over time, I have developed what I like to call "Bob's 3 Rules" for trimming out a model submarine. If followed properly, with patience and dedication to detail, you'll end up with a well-trimmed boat that will be fun to operate.

Before we get into the rules, let's look at the two materials we use to trim out a model submarine.

The first material is lead (or other heavy weight). This comes in various forms, but the one that I like the most is lead pellets. You can put them in a plastic bag and massage them into almost any nook or cranny. Add some resin and you have a removable ballast weight that perfectly conforms to your hull. You can also use lead ingots, lead strips, fishing weights, bolts, nuts, or anything heavy and dense. Just make sure you use water-friendly material. The last thing you want it to have streaks of rust pooling in the bottom of your boat and streaming out the drain holes of your model.

The second material is foam, or more specifically, closed-cell construction foam. You can get this from any home hardware store and it usually comes in pink or blue sheets of varying thicknesses. It's easy to cut, easy to sand, it's cheap, and weighs virtually nothing. Be sure to use closed-cell foam that is rigid versus the pellet-style foam that is easy to compress. The reason for this is that water pressure is relentless and can crush your foam if you ever end up at greater depths, resulting in a loss of flotation, and potentially the death for your submarine.

With those materials in hand, let's check out the procedure that I use to trim out my boats:

RULE #1: Cylinder goes low:

Room in your model submarine is typically at a premium unless you're building a huge boat. Remember that lead is dense and easily conforms to your hull's shape. If you mount your cylinder lower in the hull, you leave more room for foam above (which takes up more space). This rule is not iron-clad, but makes the next two steps far easier if you follow it.

Dropping the cylinder below the centerline typically means that an intermediary dogbone shaft or universal joint will be needed in order to bring the output of your cylinder drive motor to the level of your propeller, but the slight added complication will make trimming much easier later on.

It is worthwhile pointing out at this juncture, even though it may be common sense to some, that keeping the weight of the materials of your model that sit above the waterline in surfaced trim will have many positive benefits in the performance and trimming of your boat.

First off, your ballast tank needs only to be large enough to offset the weight of the components that you are trying to bring above the waterline. For example, if your upper hull weighs in at 500 grams, you'll need a ballast tank of at least 500mL displacement or 500 cubic cm (this is part of the wonder of the metric system, by the way, in that 1mL of water weighs exactly 1 gram and also takes up 1 cubic cm of space.). A heavier superstructure means you need a bigger ballast tank to bring it above the water. Typically, modern boats need much smaller ballast tanks than older fleet-style boats as they have a much smaller freeboard.

The advantage to having a small ballast tank is that the energy required to empty and fill the tank is less (resulting in better battery life), and the time needed to do so is likewise smaller.

Also, as the superstructure rises above the waterline, it lowers the center of buoyancy. If you are running into issues where your submarine trims out well when submerged, but rolls over when surfaced, it is likely a combination of not enough of a righting moment through your use of trimming materials and a heavy superstructure. Additionally, your submerged stability is likely low and you'll experience a lot of prop-induced roll while underway, particularly in modern, single screw boats.

Keep the top part of your model as light as possible.

RULE #2: Foam goes high. Ballast goes low:

The overarching idea behind trimming your boat is to offer static stability to the model in both surfaced and submerged trim. The way to achieve this is to give your model a righting moment (in engineering terms) that will want to correct the boat's relative position until it rests at neutral, level orientation.

In order to do this, you will want to make this righting moment as large as possible.

Think of it in terms of a wrench. If you have a small righting moment, with the model's center of buoyancy and center of mass close to one another, it is like using a wrench with a very short handle. Turning a nut with that wrench will be difficult and you won't be able to apply as much force as if you had a longer handle. In model sub terms, you want to put as much distance between the center of buoyancy and the center of mass as possible, so that the same force can create a proportionately larger righting moment.

In order to achieve this, you put your foam high up in the hull, and you put your lead low. Don't take the easy road and stuff foam in any open space available. If it isn't high up, don't use the space.

As you can see from the illustration on the right above, the ability for the model to right itself is proportional to the horizontal distance between the center of mass and the center of buoyancy. The larger the vertical spacing between the two, the larger the horizontal spacing when it is moved out of a level orientation.

RULE #3: Surface trim first, then submerged

I find it easiest to trim the boat out when surfaced first, then once that trim is established, move onto the submerged trim. Others may do it differently, but hey... this is my article. They can get one of their own!

For establishing surface trim, work only below the waterline. Foam should be installed as close to the waterline as possible so that you maximize your righting moment. You can cut long strips of foam easily with a sharp utility knife or bandsaw, then sand it with a bit of sandpaper for a nice, rounded and smooth finish. I usually use either RTV silicone or two sided tape to install it in the hull.

As a rule, I usually install more foam than I think I need during the initial installation on the bench as I find it much easier to remove pieces during the trimming process than it is to install it.

Once your surfaced trim has been established, move onto your submerged trim. When doing this, you will be working only with foam and only above the waterline. Installation of foam below the waterline at this point will mess up your surfaced trim and you'll find yourself in a never-ending circle of adjustment and re-adjustment.

For beginners (or even just people who want the peace of mind) I recommend trimming your boat so that when the tank is fully vented (full of water) the boat remains slightly positively buoyant, or floating on the surface with just the top of the sail protruding. in this scenario, loss of signal, failure of the ballast system or loss of sight of the boat will eventually end up with the boat returning to the surface unless it has encountered an underwater obstacle that has trapped it. Static diving is all well and good for showing off to people on the beach next to you, but in practical application you'll never be diving without some degree of forward movement, and just a little speed and a little movement on the planes can submerge the boat, even at very low speeds.

Even with these rules in mind, trimming is still a tricky process. Following these rules, however, will help you achieve proper performance from your model submarine and allow you to enjoy operating it rather than fighting it.

If you have any questions at all, please feel free to let me know at any time. I'll be happy to try to talk you through any issues that you're having.

Stay on a level bubble, and have a great day!


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