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How Fast Do They Go?

Los Angeles submarine underway

It's one of the most common questions I'm asked, and forms one of the FAQs that I've created on the FAQ page of my website.

The subject of proper scale speed comes up time and time again, both from newbies to the hobby and from experienced builders.

Some might argue that you would simply take the scale speed and divide by the scale to get a scale speed. Sounds simple, right? Just take the original speed of, say, 40 knots, divide by the scale of the boat (let's say 1/96) and you get 0.41 knots. Done, right? Uh, not unless you want to wait a really long time for your boat to make her rounds. 0.4 knots is about 0.46mph (0.64 kph). Whoa. Massive turtle speed.

If we do this properly, we need to go much deeper (pun intended). There are other considerations, the most important being, what speed offers the most realistic portrayal of the original subject's impact on its environment?

Fortunately, for boats and subs, we have a perfect gauge for this, that being the water that the model moves through. By studying the effect of the ripples coming from the boat, comparing them to the effect that the full-sized boat has on the water around it, we can come up with a formula that helps to most effectively convey the look and feel of the original.

Here's what it looks like:

True scale speed is equal to the square root of the scale of the model multiplied by full size speed. Wait. What? Let's look at an example:

For a 30 knot attack sub at 1/100 scale the calculation would be 1/10 x 30 = 3 knots which is fast walking speed (3.5 mph or 5.6 kph). In calm water, that should look pretty much correct.

There are a number of things that will affect the scale look, the biggest being that water molecules aren't scaling with the sub, so they behave differently than large scale waves. Some people have taken the initiative to add a rinse-aid ejector from the front of the boat. This dispenses doses of readily available dishwasher anti-spot liquid, a formula consisting primarily of Tetrasodium EDTA, a funky molecule that basically makes water more "liquid". Not many elect to go that route, but hey, I'm educating here!

At the end of the day, the one true answer for the 'right' speed is: Whatever the heck you think looks cool. It's your boat. If you want an underwater race car in the shape of a uboat, more power to you! Looking to keep things sedate and steady? Nothing wrong with a 2mph nuclear attack sub if that's what you like.

I hope you enjoyed this little journey into submarine speed. As a fun way to finish, you need to check out this video of a German Type VII that I briefly owned a long time ago. The builder did some "performance mods" to the boat. You'll have to check it out below for the details.

Thanks for reading!

Bob Martin The RCSubGuy