text: Can surface drive be used with a single engine? | Search Journal
  When we released our SDS1 model at the beginning of the year, we started receiving a number of inquiries related to using surface drives on a vessel with only one engine (and shaft). Is this practical?
  The short answer is "No", and here's why:
  Propulsion is a matter of physics, with one force being applied to counteract another force.
  We always prefer to install surface drives in pairs, either two or four shafts. The reason why is that the contra-rotating propellers steer the boat straight and true. Without this counteracting force, the vessel will constantly be trying to pull either port or starbord when underway, 7 – 12 degrees, or even as much as 15 degrees, rather than maintaining a straight course. When you are talking about slow speed or medium speed engines, with large propellers, the correction is a relatively simple one to make. With high speed engines and large propellers, the situation becomes more drastic.
  So, a single engine install is complicated, from an operator's point of view.
  CIO, CTO & Developer Resources There are some drive makers in the market who have sold single-shaft surface drives. Most, but not all, of these makers are providing these drives for racing applications, for use by highly trained operators and crews. As a company, we choose not to do so, since we feel that the performance constraints outweigh any advantages in speed gained.
  What about three shafts?
  Well, the same forces are applicable, but the deviation is much less pronounced and can easily be corrected. Why install three shafts to begin with? Quite simply, it is often a matter of cost (occasionally engine room space). Three larger engines normally cost less than four smaller engines.
  So, could a differential "split" gearbox, similar to a car or truck, be installed, allowing the use of two shafts?
  Theoretically, yes. However, there are few makes on the market, and most are unproven, mechanically complicated, and require precise maintenance. Since not all of the world is developed and has ready access to good mechanical support, we essentially recomment on small, single engine vessels the use of stern drives, IPS, or outboards.