When does outboard power become ridiculous?

Outboard engines have their place and uses. However, a trend over the last few years has been to simply add more outboards to a hull in order to give it more power. No longer are there simply triple or quad outboard installations, but sometimes even five, six or more.

At what point does simply adding more outboards become pointless?

When the costs outweigh the benefits.

The vessel pictured is a pretty well-known photograph of a drug-runner caught in the English Channel (a “scandal” in the UK marine industry at the time). It essentially is engine and fuel tank. Now, most people are not buying boats for smuggling, but are intending to use the vessel over an extended period of time. What does adding extra engines actually do as far as performance?

Negatives

  1. Added weight (not just the engine, but all equipment, such as mounts and the weight of extra fuel).
  2. Added drag.
  3. Reduced maneuverability.
  4. Reduced safety (often, the hull was not designed for such applications)
  5. Huge increase in fuel costs.
  6. Reduced running time and range (unless larger fuel tanks were designed or installed)
  7. Greatly increased maintenance expense
  8. Difficulty in ventilating the props.

 

Positives

  1. Greater power.
  2. Greater acceleration.

 

Note that the negatives are pretty big, compared to the positives and their associated costs. Of course, these negatives can be minimized by using diesel inboard engines.

Should a vessel be equipped with two 250 hp outboards, or a single VGT 500? What about instead of three 300 hp Mercury Verados, two VGT 450s?

Unless the vessel has absolutely no room for an inboard, the inboard option will win every time.

  1. Weight: Two Mercury Verados weigh 586 kg. A single VGT 500 weighs 515 kg. Even with gearbox and drive, the weight differential is only around 100 kg.
  2. Drag: Fewer engines mean less drag. All of the time.
  3. Maneuverability: Even numbers are more maneuverable, due to less torsional pull. However, the reduced drag and greater number of propulsion options mean that a single inboard will be as good as or more maneuverable than multiple outboards.
  4. Fuel: No contest. A single diesel VGT will save enough money to pay for the differential in price very, very quickly.
  5. Range: Likewise, the range can be longer, with smaller fuel tanks.
  6. Maintenance: Though an inboard diesel, depending on engine compartment and configuration, is less accessible, it will have a far higher life cycle and require less maintenance and access.
  7. Acceleration: Multiple outboards can accellerate very quickly. But guess what. A Marinediesel VGT engine, designed for high speed craft, will accelerate faster.
  8. Service life: A VGT engine will last as much as five times longer than even the highest quality outboards.