“Load” is one of those terms that gets thrown around in the marine industry, and the meaning is often blurred. What, exactly, is meant buy load, and how does it impact vessel performance.
Load can refer to either:
- Engine load
- Vessel load (tied to displacement)
These two things are also tied together.
Most high speed vessel are designed with a planning hull. In other words, the hull is designed to lift itself out of the water when power is introduced. It is a question of physics and opposing forces. In absolute simplest terms, the hull is experiencing:
A force downwards. This is displacement, or the weight of the boat. If more weight is added, the greater the force necessary to lift the boat.
A force backwards. This is resistance. As the propulsion system pushes forwards, the resistance increases. Additionally, anything sticking in the water, like the drive, rudder, propeller, or spray rails, produces drag, which increases resistance.
A force upwards. This is buoyancy.
A force forward. This is propulsion.
There are other forces and impacts on performance, such as trim of drives, wind, intakes, etc. The above are the most important.
So, as is evident, vessel performance is a question of balance. In order to make the boat move forward and up, enough power needs to be provided to overcome these forces. There is no magic or voodoo involved. It is simply a question of physics. Depending on the extent of the problem, a solution may or may not be able to be found.
So, on vessels with performance problems, the first diagnostic is usually weight. More weight means more displacement means the engine must work harder to make the boat plane. This is engine load. This is also why naval architects often pull their hair out at design changes. Any additional weight will impact performance. Put a marble floor in a recreational yacht? Performance will suffer. Extra guns and ammunition on a military boat? Performance will suffer. Add 10 additional passengers on a water taxi? Performance will suffer.
Load on the engine. Load on the hull. In both cases, speed, maneuverability, and overall performance take a hit.