Power-to-Weight Ratio (P/W ratio)

 Power-to-Weight Ratio (P/W ratio) is the amount of power produced per unit weight of a machine, vehicle, or engine.

$$\textbf{Power-to-Weight Ratio} = \frac{\text{Power Output}}{\text{Weight}}$$

Units

• kW per tonne

• HP per tonne

• kW per kg

Affects:

• Acceleration

• Speed

• Maneuverability

Example:

• A lightweight fast patrol vessel → High P/W ratio

• A bulk carrier → Low P/W ratio

⚙️ For Marine Engines

Higher P/W ratio means:

• More compact engine

• Higher performance

• Better efficiency for high-speed vessels

Lower P/W ratio means:

• Heavier, slow-speed engines (common in merchant ships)

• Designed for fuel efficiency, not speed

Practical Example

If:

• Engine power = 10,000 kW

• Ship displacement = 20,000 tonnes

$$P/W = \frac{10000}{20000} = 0.5 \text{ kW/tonne}$$

Importance

👉 Higher P/W ratio:

• Faster ships

• Higher fuel consumption

• Used in naval ships & fast crafts

👉 Lower P/W ratio:

• Economical operation

• Used in tankers & bulk carriers

Power-to-weight ratio is the ratio of engine power output to the weight of the ship or machinery. It indicates performance capability, where higher ratio means better acceleration and speed.