Today we are going to talk about two of the propulsion systems found in the industry – surface drives represented by the popular Arneson Drives and Jets. The article intends to give you a basic idea about these propulsion systems and what their advantages and disadvantages could be.
The Arneson Surface Drive (ASD) was developed by Howard Arneson. Except for the propeller, the whole system is located in the very back of the boat and outside the water. With ASDs the driver can affect the attitude if the boat by changing the amount of prop in or out of the water. Arneson Drives are considered to provide better steering than the fixed systems as a result of the articulated joint which allows the propeller to be directed. As its name says it – it is a surface drive, meaning the drive is outside the water. The only part which is in the water or partly submerged is the propeller. The lack of drag benefits the efficiency of the surface drives at high speeds. However, ASDs are not very good at slow speeds, and when lowering the speed, they reduce boat’s maneuverability, especially when strong winds are present. Arneson Drives create propulsion in a straight line known as positive thrust. Vessels with surface drives have a lower draft due to the lack of underwater gear
- Positive thrust
- Decreases drag by 50%
- 18% more speed
- 15% - 30% fuel savings
- Surface-peircing props
- The propeller can be adjusted while underway
- Propellers work on the surface, and with this, they reduce the noise, vibration and hull erosion
- Less mechanically complex system than other propulsion systems
- Good for shallow waters
- Tighter turns than conventional propulsion systems
- Less maneuverability at low speeds
- Reduced control in closed quarters and when slowing down
- Lots of demand on drives and props when getting from standing position to planning speed. This in combination with side forces decreases boat's stability in this transition period.
- Expensive when not in a package
- Said to be less responsive when reversing. However, the propeller design has changed and has made the ASD as responsive as conventional propulsion systems. The possibility of lowering down the propeller and fully submerge it make it as good as the shaft.
- Not safe for swimmers
Models appropriate for big boats with engines with 1800-2200 HP are ASD 14 ASD 15 and ASD 16. We can see surface drives more often with MAN and MTU engines. In combination with Caterpillar, we can notice less engine power.
In continuation, we will take a look at the Jet Drives.
By using an impeller in a tube which sucks in water from the bottom and then shooting it out a nozzle at the stern backward at a very high speed, the boat moves forward. It works in relation to Newton's Third Law of Motion -every action has an equal and opposite reaction. The jet propulsion system is installed in the aft section of the boat’s hull. When the stream of water expelled by the jet unit has its direction changed by the driver, steering is achieved. When the jet stream goes one way, the boat goes the opposite direction. This is how the driver makes the boat turn. The reverse is achieved by lowering an astern deflector into the jetstream after it leaves the nozzle. This reverses the direction of the force generated by the jet stream, forward and down, to keep the boat stationary or propel it in the astern direction (hamjet.co).
Jet drives are mainly used in small boats, but they also may power larger vessels and are used commonly for vessels designed for shallow water conditions. However, the underwater landscape should be considered, because the jet drive might suck in mud if too shallow.
- Propeller free (environmentally safe)
- Develop high speeds
- Reduced to no drag
- Very well handled in closed quarters
- Very maneuverable (opinions differ)
- Good for shallow waters
- Low maintenance requirements due to its simple structure
- Impellers are not sensitive to boats's loading which means if correctly matched, the heavy load wouldn't affect performance
- Hard low-speed handling (until the driver gets used to)
- Bad for shallow waters with soft bottoms because the drives suck up sand
- Garbage in the water can negatively affect performance
- They can stop the boat quickly but roughly
- Not very good in wild waters
- Less efficient and consumes about 30% of the engine power
Jet Drive major manufacturer
Probably the most famous producer of Jet drives is Hamilton Jet. Their HM Series is designed for commercial, yacht and passenger vessels in the range 65-195ft with power inputs from 900 to 4000 HP.
Jet drives are usually seen installed with Mercury or MTU engines.