PropulsionSystems

Propulsion Systems

Design Considerations

At its simplest, a propulsion system consists of an engine, reduction gear, propeller shaft and bearings, and a propeller. Most designs feature a long rigid drive shaft to get the correct propeller angle. AAmpere (amp), SI unit of electrical current lot of attention is paid to getting the shaft and engine aligned to prevent twisting and vibration. Periodic realignment is not uncommon. The engine can suffer wear and tear because it bears the end thrust from the propeller. The engine is hard mounted to its bed, resulting in a lot of vibration and noise transferred through the hull.

Other industries take different approaches. Racing cars have long used flexible Constant Velocity (CVConstant velocity) joints, with engines of over 1000 hphorsepower. Tractors use flexible drive shafts running from a power-take-off (PTO) on the back. These shafts are extensible on splines, and use universal joints for alignment. There is little direct loading on the PTO bearings. Some vehicles also use a fully hydraulic transmission and drive.

These advantages can be obtained in a boat by using a hard-mounted thrust bearing to take the shaft thrust from the propeller, and flexibly coupling and mounting the engine.

Boats are retro because: (1) tradition; (2) hard fixed mounts are cheaper; (3) simpler is better (more reliable); and (4) tradition (i.e., mistrust of unknown technologies). All of the alternatives are more complicated, which is not what you want with a breakdown in a crisis.

The other reality is that the marine market for small boats is too small for many manufacturers to develop specialized versions of their drives for continuous duty marine applications. This is why some areas innovate slowly, and at great expense.

Twin Screws are Better

Hydrodynamic studies show that twin screws are clearly superior to a single screw. The twin-screw environment under the stern is less chaotic. This means the propeller-wake interaction is more constant and, therefore, more efficient.

Twin-screw ships are generally easier to manoeuvre. A single-screw ship suffers from stern walk, a tendency for the stern to counteract the rotation of the screw by moving sideways. Twin screws are de-signed to rotate in opposite directions, cancelling any stern walk. Also, the two shafts can be used to "twist" the ship in docking manoeuvres.

To drive twin screws, you need two main engines or one engine with hydraulic or electric drive. Twin engines are less fuel efficient.

One Rudder is Best

Contrarily, note that one rudder is better than two. Two rudders make the stern environment more chaotic again, and require a complicated steering system like on the front wheels of a car.

Unlike propellers, rudders are more likely to be a Naca shape (symmetrical). As a rule of thumb, rudder area = 0.5 * (waterline length * displacement). Articulated/deflector/high-lift-flap rudders common on work boats are appearing on yachts. They can reduce turning circles by 50%percent.

Number of Engines

From an emergency perspective, the debate over one engine or two will never be settled. Each person’sSecond comfort zone is different when it comes to managing risk. From this perspective, there are four choices:

• No backup:
  • One main engine with one propeller
• Emergency power:
  • One main engine with one propeller, and a small wing engine with a small propeller
• Redundant power:
  • One main engine with dual electric drive, two propellers and battery backup, OR
  • Two main engines with two propellers (drive systems are unimportant)

If you’re comfortable psychologically with the proven reliability of diesel engines, you will happily go to sea with one engine and one propeller, and save some money in the bargain. You will be in the excellent company of most small fishing trawlers.

If your mind baulks at the risk this represents, then, hang the cost!, you will want two engines and two propellers.

Realistically, a small wing engine with a wimpy propeller is unlikely to have enough power in a severe-weather emergency. Two main engines is the only way to provide true redundancy. Alternatively, you might consider a single engine with an electric drive – it can be configured to use the house bank as an emergency source of power.

From a layout perspective, twin engines require a larger engine room, and may dictate an amidships layout. Twins mean twice as much cost and maintenance, and duplicate charging and fuel systems (not necessarily bad). Sometimes this results in two systems in some degree of disrepair.