There are a few types of propulsion units in common use, each with different characteristics suited to different circumstances.

• Shaft Drives
  

  
  Shaft driven boats are commonly found in use as work or ski boats. This type of drive unit comprises a propellor attatched to a straight shaft, which goes through a water-tight seal on the hull and connects to the inboard engine. The inboard engine(s) powering the boat can therefore be placed further towards the bow, allowing the boat trim and weight distribution to be improved.
  
  

• Outboards

  
  Almost always petrol powered, outboard engines are probably the most popular installation in smaller boats, including sailing dinghies and yachts, as well as powerboats.
  
  Available from 1 to well over 300 horsepower and in single and multiple engine configurations, outboards are versatile, and their power to weight ratio is good.
  
  The fact that the drive unit and engine is self contained is also a distinct advantage, as this means they are compact, leaving more space in the boat, and allows for easy access for servicing and repacement.
  
  

• Outdrives
  

  
  The fact that outdrives are part-inboard part-outboard combines advantages from both types of drive. They allow the weight of the engine to be positioned further forward than an outboard, whilst the moveable nature of the drive leg allows for slightly easier steering. 
  
  The leg can also be trimmed up and down as desired to help raise or lower the bow of the boat.
  
  

• Jet Drives

  
  Not commonly found on pleasure boats, jet drives work by sucking water in from underneath the engine or drive and ejecting it at high pressure through a small steerable jet at the back of the boat. Reverse thrust is obtained by the use of a bucket over the nozzle to redirect the waterflow.
  
  Jet drives can be used in very shallow water without the risk of major damage to the unit, as the power is delivered through the use of an impellor situated inside the hull.