Freeboard Calculator
Calculate a boat's freeboard from hull depth and draft. Get the remaining freeboard above the waterline in meters, feet, inches, and as a percentage of the hull depth.
Freeboard Results
What is boat freeboard?
Freeboard is the vertical distance between the waterline and the upper reference point of a vessel, such as the deck edge or gunwale. It represents how much of the hull remains above the water at a particular loading condition.
Freeboard changes when the boat's loading condition changes. Adding weight generally increases draft and therefore reduces the remaining freeboard.
How is freeboard calculated?
For a basic vertical hull calculation, freeboard is the difference between hull depth and draft.
$$Freeboard = Depth - Draft$$
- Freeboard: Vertical distance remaining above the waterline.
- Depth: Vertical hull dimension to the selected upper reference.
- Draft: Vertical distance from the baseline or keel reference to the waterline.
Example of freeboard calculation
Suppose a boat has a hull depth of 1.8 meters and a draft of 0.8 meters. The freeboard is:
$$1.8 - 0.8 = 1.0\ m$$
The boat therefore has approximately 1.0 meter of vertical distance between the waterline and the selected upper reference.
Why is freeboard important?
Freeboard affects how much reserve vertical height a boat has above the waterline. It is relevant to seaworthiness, deck wetness, buoyancy reserve, flooding risk, and the vessel's ability to operate safely in different loading and sea conditions.
How does draft affect freeboard?
Increasing draft directly reduces freeboard when the upper hull reference remains unchanged.
$$\Delta Freeboard = -\Delta Draft$$
For example, if loading causes the draft to increase by 0.10 m, the basic geometric freeboard decreases by approximately 0.10 m.
What percentage of the hull is above the waterline?
A simple way to express the remaining freeboard is as a percentage of the overall measured hull depth.
$$Freeboard\% = \frac{Freeboard}{Depth} \times 100$$
Similarly, the draft percentage can be calculated as the portion of the measured depth that is below the waterline.
What is the difference between freeboard and draft?
Draft describes how far the boat extends below the waterline. Freeboard describes how far the selected upper reference is above the waterline. They describe opposite parts of the vessel's vertical position relative to the waterline.
Does freeboard change with passenger loading?
Yes. Additional passengers and cargo increase displacement and usually increase draft. As the boat sinks deeper into the water, the available freeboard decreases.
Does freeboard vary along the boat?
Yes. Boat decks and sheer lines are often not perfectly horizontal. Freeboard can therefore vary from the bow to the stern and from one location to another. A single freeboard value is a simplified measurement at a particular reference location or design condition.
Is this calculator suitable for determining safe loading?
Not by itself. Freeboard is only one part of vessel loading and stability. Safe loading also depends on displacement, buoyancy, stability, trim, hull geometry, free-surface effects, flooding arrangements, and the vessel's certified capacity.
What is reserve freeboard?
Reserve freeboard generally refers to the vertical capacity remaining above a vessel's normal waterline before a defined upper reference is reached. The exact engineering and regulatory definition can vary depending on vessel type and applicable rules.