Length, beam, draft, displacement & key hull coefficients explained in a practical way.
If you strip a ship down to its essentials, what you’re left with is a handful of numbers.
Length, beam, draft, displacement — and a few coefficients that don’t look like much at first. But in reality, these are the values that quietly control almost everything. Before resistance calculations, propulsion estimates, or stability checks even begin, these parameters are already defining what the vessel can and cannot do.
What makes them useful isn’t just that they describe a ship. It’s that they allow comparison. A bulk carrier and a patrol vessel might be completely different in purpose, but once reduced to the same parameters, their design intent becomes much easier to read.
This is not meant to be theoretical. The goal here is simply to walk through these parameters in a way that makes them usable — especially if you’re trying to understand your numbers or make quick estimates.
“Length” sounds straightforward, but in practice, it rarely is.
You’ll come across different definitions — length overall (LOA), length at waterline (LWL), and length between perpendiculars (LBP). They don’t give the same number, and more importantly, they’re not interchangeable.
For most calculations, LOA isn’t particularly useful. LWL or LBP is what actually shows up in resistance estimates and empirical methods.
Length does more than define size. It influences how the vessel interacts with water, especially in terms of wave formation. That’s why longer vessels tend to operate more efficiently at higher speeds.
Beam is simply the width of the vessel at its widest point, but it carries more weight than it seems.
A wider beam generally improves stability. The vessel resists rolling more effectively, which is especially important for cargo ships. On the other hand, increasing beam also increases wetted surface area, which leads to higher resistance.
So there’s always a balance — stability on one side, efficiency on the other.
Draft is the vertical distance from the keel to the waterline.
It directly determines how much of the vessel is submerged, which in turn defines displacement. More draft means more underwater volume, and therefore more weight.
But draft is also operational. Too much of it, and the vessel simply cannot enter certain ports or pass through shallow waters.
Depth is often confused with draft, but they describe different things.
Depth is measured from the keel to the main deck. It relates more to structural design and safety margins than to buoyancy directly.
The difference between depth and draft gives you freeboard, which plays a key role in seaworthiness.
Displacement is the total weight of the vessel, expressed as the weight of water it displaces.
It isn’t an independent value — it comes from the combination of length, beam, draft, and hull shape.
If you need to calculate it quickly, you can use the Displacement Calculator.
Almost every major calculation in naval architecture depends on displacement in some way, which is why it’s rarely treated casually.
The block coefficient gives a quick idea of how “full” a hull is.
A high Cb means the hull is boxier and carries more volume — typical for tankers and bulk carriers. A lower value indicates a finer, more streamlined shape.
You can calculate it directly using the Block Coefficient Calculator.
Even without drawings, Cb alone can tell you a lot about what a vessel is designed for.
While Cb looks at overall fullness, Cp focuses on how volume is distributed along the length.
This becomes especially important in resistance and speed considerations.
You can explore this further using the Prismatic Coefficient Calculator.
The midship coefficient describes the shape of the vessel at its widest section.
Higher values indicate a more rectangular section, which is common in cargo vessels. Lower values point to more curved shapes.
You can calculate it using the Midship Coefficient Calculator.
The waterplane coefficient reflects how full the vessel is at the waterline.
It has a strong influence on initial stability and how the vessel behaves in waves.
You can check values using the Waterplane Coefficient Calculator.
None of these values exist in isolation.
Increasing beam affects stability and displacement. Increasing draft increases displacement but may introduce operational limits. Adjusting coefficients changes resistance behavior.
Ship design is not about optimizing a single number — it’s about balancing all of them.
These parameters form the basic language of naval architecture.
Once you get used to them, you can look at a small set of numbers and quickly understand what kind of vessel you’re dealing with and how it’s likely to behave.
In practice, they’re rarely used alone. They’re part of an iterative process, where each adjustment influences the others — sometimes in ways that aren’t obvious at first.
The concepts discussed here are standard in naval architecture and are covered in established references such as: