Buoyancy was given a complete overhaul in version 3.0, which is the reason for me writing this guide in the first place. Buoyancy is now determined through the actual amount of water displaced by the grid, just like the real-world; albeit with a few minor tweaks to work within SE. Unfortunately, grids require a vent to calculate airtightness at all, this is a temporary requirement and should be fixed soon.

Airtightness is the most efficient way of getting your boat to float, as it has the greatest buoyancy force compared to it's weight (which is zero :o). After that, light armor reigns supreme in being the most efficient block for flotation for the same reason. For reference, one light armor block can carry 2 others on top without sinking. One airtightness cell can carry 3.

I really recommend watching this video for getting a basic grasp of how buoyancy is determined. The channel is great for learning anything naval related.


You don't need to know the mathematics behind it, but it's totally possible to do it now.

Like I said in the previous section, buoyancy uses fluid displaced and is completely calculable now.

To explain it as quickly as possible:

Fb=ρ×g×((Vb×Cb)+(Va×Ca))
Where
Fb is buoyancy force in N
ρ is fluid density in kg/m^3 (/wdensity)
g is acceleration due to gravity in m^2/s
Vb is volume of water displaced by blocks in m^3
Va is volume of water displaced by airtightness in m^3
Cb is the block coefficient (0.042 Large, 0.2 Small)
Ca is the airtightness coefficient (0.35 Large, 1.0 Small)


Figure 1. Block Volume Example

An armor block for example takes up exactly 2.5m × 2.5m × 2.5m of space. Multiplying the length, width, and height of the block will output the total volume of that block: 15.625m. We already know the fluid density and gravity at the planet, so it's as simple as plugging it into the equation. The block in question is also large grid, so the coefficient is 0.1. The coefficients are necessary to provide a balanced water experience, without it, nearly every block floats.

Fb=1024×9.81×((15.625×0.042) + (0×0))
Fb=6585.6N

A vanilla light armor block has a mass of 500kg. You can calculate the force from gravity by just multiplying the mass by the gravity.

Fg = m×g
Where
m = mass of the object in kg
g = acceleration due to gravity in m^2/s

By plugging in the mass of 500kg and 9.81m^2/s acceleration, we get a gravity force of 4900N. Notice that the buoyancy force is greater than the gravity force, this means that the block will float with the capability to lift 1685.6 more newtons.

Just a reminder that you don't need to calculate any of this. There are LCD scripts that will automatically show how well your grid floats.


Figure 1. BuoyancyLCD Script

A number greater than one hundred percent indicates that the boat floats. Less than one hundred and you're guaranteed to be sinking.

Drag in the Water Mod is not really calculable by hand, but by explaining it I hope to give you a better understanding in how it functions.

In the real world, drag force is calculated with the following equation:

Fd = 1/2 * ρ * v^2 * Cd * A
Where
Fd is Drag Force in Newtons
ρ is Density of the fluid in kg/m^3
v is Velocity
Cd is Drag Coefficient
A is Area

In-game, I assume Cd is always one, so it is not part of the equation. The equation I use in-game is

Fd = 1/2 * ρ * v^2 * A
Where
Fd is Drag Force in Newtons
ρ is Density of the fluid in Kg/m^3
v is Velocity
A is Area

As of 3.10, water drag uses the bounding box of the grid to calculate the area for each direction. This means that a long ship will experience more drag sideways than forwards. Additionally, this introduces the ability to create rudders. Torque is not created by drag as of now

Building boats in SE isn't all that difficult provided you understand how buoyancy works. The most important part is keeping the boat's center of mass below or close to the boat's center of buoyancy. If the center of mass is well above the center of buoyancy, the boat will be very prone to tipping.

Also, if you find your boat is tipping forward, your center of mass is too far forward. Move heavy blocks to the back of the boat to correct the behavior.


Figure 2. Center of Buoyancy relationship

Please do not resort to increasing the buoyancy multiplier when your grid fails to float- the grid is too heavy and the buoyancy is in-fact working as expected. It is an engineering problem that you need to solve.

A forewarning to submarines:
Grids now experience pressure from the water around it, if a block cannot handle the pressure, it will groan and take damage that is defined in the command /wcrush. Blocks that are neighboring or inside of airtight rooms do not experience pressure, as they are sheltered from the water. Many blocks will begin to fail at 300 meters depth, heavy armor can withstand up to 500 meters. If you want to go further than that, you'll want to install a mod or go as fast as you can. Blocks will only take damage until they are no longer functional.

The new buoyancy opens lots of new opportunities for submarine ballast systems. I recommend skimming through each type to figure out what's best for you.

To keep a submarine from tipping/flipping, the center of buoyancy should be above the center of mass.

Link for information on some real-world ballast systems[www.subcommittee.com]

The collector ballast is still a thing even in Water Mod 3.0. Place a collector underwater and you can see that it collects water in the form of ice. The water displaced by the collector decreases, and therefore the buoyancy of the collector decreases. With the collector's buoyancy decreasing, the ship's overall buoyancy also decreases. You can use a collector or ejector to throw out the ice, where it will turn into water after touching it. One unfortunate downside to this method is that the mass in cargo updates infrequently, meaning that it will take a while before the game notices that you're trying to sink or rise.

This form of ballast is brand new with Water Mod 3.0, opening a door to an airtight compartment will cause the compartment to no longer be airtight, making the submarine sink. The same goes in the opposite direction.

Another abstraction by the water mod; any gas in a gas tank is considered to be water. Essentially, an empty gas tank displaces the most water and has the most buoyancy while a full gas tank is zero. This is the exact opposite of how the previous Water Mod version behaved.
You can use this as a method of balancing a submarine or boat by transferring gas between two different tanks.


The block weld ratio is also used to determine buoyancy, therefore if you were to unweld a block, it would float less. You can see this by welding and grinding an armor block, notice that the block moves slightly up or down depending on the weld ratio, where it will eventually sink.

Compartmentalizing your ship's lift tanks will improve the likelihood of you winning a battle. By compartmentalizing, or separating each room into many smaller rooms, less buoyancy will be lost when one room ruptures.


Figure 3. Bulkheads

Look at the white armor panels that are used to separate the lift tanks into individual smaller ones, this will prevent the entirety of buoyancy being lost on once side if a block is removed or destroyed.

Most boats in the real world use compartmentalization too, as seen in the image below.


Figure 4. Real-world Bulkheads