Outboard motors are the primary engines for many small power and motor boats, but they can also act as supplementary or trolling motors. Choosing the right motor is about identifying one that matches the size and seaworthiness of your boat and the kind of boating you want to do.
Size and Power
Any boat has a maximum horsepower it can use without risking a loss of control. Most boaters want a motor as close to the maximum power as possible, since an underpowered boat is slow to escape storms and other hazards, but there are exceptions. A big motor might weigh too much for a small boat, especially if you want to load it up with a lot of people and gear. Generally, the larger a motor is, the more powerful it is, but the relationship is not exact. Some small outboard motors are more powerful than larger ones.
Electric or Petrol?
Petrol motors are usually more powerful, but they are also noisy and produce exhaust fumes. If the outboard is supposed to be your boat's primary source of power, then you probably need petrol. If maximising power is not your goal, consider electric motors. Many anglers use two outboards: a large petrol-powered motor for getting out to the fish and then a small electric outboard motor for trolling.
Smaller and simpler than their four-stroke counterparts, two-stroke engines accelerate faster, so they are a popular choice for small machines like portable outboard motors. Older two-stroke engines leaked large amounts of unburned fuel, a major problem for both efficiency and pollution, but newer designs use precisely timed fuel injection to eliminate leakage. It is worth asking whether a two-stroke uses direct fuel injection, especially when buying a used outboard motor.
Four-stroke outboard motors are larger and require more complex maintenance than two-stroke motors, but they usually are more fuel-efficient and provide more torque for the same horsepower. That means they are a better choice for larger boats and for going longer distances where fast acceleration is not that important.
Outboard motors come with propellers, but they do not always come with the best propeller for your boat or for that particular type of motor. The shape of the prop and its pitch both change how fast the motor turns at a given throttle level. If the RPM at full throttle is too high or too low, the motor does not function well. Finding the right prop requires some trial and error.