Some powerboats do have gears, but these are the exception rather than the rule.
The difference between boats and cars lies in the way in which the power generated by the engine is translated into movement of the vehicle. In a boat the engine turns a propeller which pushes water backwards. The reaction to this rapidly moving stream of water pushes the boat forward.
If the engine and propeller are well matched there will be sufficient power to turn the propeller, even when the engine is running slowly.
If the boat is large it may take some time to accelerate, during which water can be seen streaming away from the stern. Have a look next time you are on a ferry, you’ll see churning water at the back of the ship, even though the ship has yet to move.
In a car the wheels can turn only if the car moves, in contrast to the ferry, but it takes a lot of power to accelerate from standing.
Unfortunately, internal combustion engines do not generate much power when they are running slowly, so if the engine were connected to the wheels without a gearbox, the inertia of the vehicle would stall the engine.
The gearbox allows the engine to turn rapidly, generating power, even when the wheels are moving slowly. If it were not for the ingenuity of gearbox and clutch designers, the internal combustion engine would have had no future in road vehicles. In contrast, steam engines generate a lot of power from a standing start so steam locomotives can pull away without a gearbox.
On loose surfaces such as sand, a car’s wheels can turn without the vehicle immediately moving too.
This is somewhat similar to the ferry, in that sand is thrown backwards as the wheels rotate. However, more sand does not immediately rush in to take its place, so the wheels are likely to dig themselves into the sand until the car is embedded to its axles.
Speedboats experience a huge amount of drag. Typical full-speed drag force is a quarter of the weight of the boat, which is like driving a car up a slope of slightly over 25 per cent.
Speedboats have to be low geared to overcome this drag, and a multi-speed gearbox would make very little difference for low-speed acceleration. The drag is so large that any gear change would have to be extremely fast or the boat would slow down too much during the changes.
Because propellers slip through the water when the boat is starting up, there is no need for a clutch, the water acts like one.
It is possible to change gear on a boat by changing the propeller for one of a different pitch. A lower pitch gives better acceleration, and allows you to pull larger loads, while a higher pitch gives better top speed, if the waves are small, which reduces drag. However, the typical change that might be useful on a boat is less than the difference between adjacent gears on a car.
The speed of a car is proportional to the engine speed for a particular gear. This is not the case in a boat because the propeller can “slip” in the water, whereas a car tyre stays stuck to the road. In all engines an increase in revolutions means an increase in power, up to a certain point.
Most of us will have accidentally taken off from traffic lights in a car in third gear. The number of revolutions in third gear is much lower than that in first gear, so the engine does not generate enough power to move the car and it stalls.
This shows that a low gear is essential in a car for power at low speed. But if you open the throttle fully in a boat, the propeller spins freely in the water, the engine reaches a high number of revolutions and the boat moves off without stalling.
The boat’s single gear is designed so that the propeller works most efficiently within the engine’s operating range. There is no need for additional gears.
In a boat, the drop in power while changing gear results in a large drop in speed, because the resistance in water is much greater than on a road, so a boat cannot pass through a gear change as easily as a car can.