So there’s quite a few submarines on the SP Website, right? All of which seem to use a slightly different method of underwater propulsion; some use the method of putting jet engines on the buoyant ends of pistons, so that when they extend, and the sub submerges, the jets will still be above water. Others use completely electric propulsion methods, by making custom props out of wings, and using those to generate horizontal propulsion. These are both equally great, but they’re also both equally complicated, especially for newer users.

Now, I’m not entirely sure if I’m the first to do this, (if I’m not and the technique has been addressed before, ignore this forum post) but I think I’ve come up with a much easier, much more effective method of propulsion for submarines, underwater craft, and pretty much anything that’s meant for underwater purposes. Basically, here’s the steps:

First, take a VTOL Engine, and attach it to a detacher. Next, take your VTOL Engine and Detacher, and attach them to the main craft. Make sure the VTOL Engine is only attached to the detacher, and nothing else. After that, use overload to modify the detacher to have a ridiculous detach force, like 1E+30 (a one, followed by 30 zeroes) so that the VTOL Engine basically gets launched into space at probably more than lightspeed. Next, turn collisions off of the engine via overload, so it can phase through parts of your craft. Now thankfully, space doesn’t exist in the SP Universe, so the engine will always have an infinite atmosphere to breathe in. To help with that, attach an inlet to the front of the engine. You should probably turn it’s collisions off as well, and set it’s mass scale to zero, just for good measure. If you want, you can even set the engine’s mass scale to 0 as well, if you want to increase it’s hang time in the air to probably a few actual real life years. Anyway, now for the fun part. Attach VTOL Thrusters to wherever you need them, I can’t really tell you how to do this, because positioning of the thrusters obviously varies on what craft they’re being put on.

Now all of this sounds really far fetched, but that’s because it’s SimplePlanes, real life physics and logic doesn’t apply here. Due to how it works, even though the VTOL Engine is probably Quintillions of lightyears away from your craft, it still directs power to the VTOL Thrusters, and since the engine will always be above water, combined with the fact that VTOL Thrusters can go underwater and still provide thrust, this allows your entire craft to go underwater, and still be powered by jet engines, just as long as the engine is in the air. For convenience purposes, you can set the detacher to AG-8, so it’s immediately activated the moment you spawn in. I also highly recommend that you aim the detacher UP, so it actually keeps the engine away from water.

Again, idk if this method has already been discovered prior to this post, but even if it has, I hope this helps other users with making their submarines, and other underwater craft work the way they want it to. (keep in mind, if the VTOL Engine is underpowered, you can always crank up the power via overload if you want to go faster, or slower for that matter if it’s already too fast at default thrust)