[physic] friction

The part that is needed to speed up a boat is the propeller connecting to the engine and the part that is needed to speed up a car is the tires connecting to the engine.

To speed up a car, a force is needed to exert on the surface of the road by the car's engine so a force is exerted on the car by the road surface to make it move. If friction is increased on the tires and the connections between them and the engine, then more force can be exerted on the surface road by the engine using the tires, and more force is exert on the car to make it move. Therefore the system is more efficient

To speed up a boat, a force is needed to exert on the water molecules by the boat's engine so a force is exert on the boat by the water molecules to make it move. Same as above, if friction increases here at how the propeller is made and the connections between it and the engine, then the system is more efficient.

Both the car and the boat are the same. The parts that are needed to speed up need to increase friction. On the other hand, the parts that are not needed to speed up the boat need not to increase friction because no force is exert by them on the road surface/water molecules to make the car/boat moves. But reducing the friction allows the force exerted by other substances on the car/boat to be smaller, making the car/boat moves easier, because this force is acting against the force making the car/boat move.