"船的载货量" can be translated as "ship's cargo capacity" or simply "cargo capacity of a ship" in English.
Here are some example sentences for context:
The ship's cargo capacity is an important factor in determining its profitability.
(这艘船的载货量是决定其盈利能力的一个重要因素。)
We need to know the cargo capacity of the ship before we can plan the shipment.
(在规划运输之前,我们需要了解这艘船的载货量。)