Just as Gas and Electric Drive options have distinct benefits, drawbacks and specific requirements and precautions, so too do different propulsion types. Some of the key trade points are discussed below.

General Summary

The chart below summarizes the general trades between the two options. Again, few of these criteria are absolute black & white for all ASV operations, so additional narrative is included.

For operation at low speeds - including the typical 2m/s survey regimes - prop drives are a clear winner. There are many variables to consider but when comparing the overall energy input vs. thrust output, prop drives are about twice as efficient as their jet equivalents. Pairing a prop to a DC motor has the obvious advantages of increasing the overall ASV endurance compared to a jet drive - again with everything else equal.

Jet drives are generally safer. Contact with propeller or impeller blades can broadly classify in one of 2 ways - direct (i.e. resulting from working directly on the blades clearing a blockage) and incidental (a strike not related to performing any work around the propulsion system). Both can be dangerous and result in serious injury.

Direct strikes should never occur and if they do, they result from lapses in safety protocols. When attempting to clear blockages in either type propulsion system, the operator must ensure double (minimum) or triple (ideal) isolation of control and power sources. This means first isolating the DC battery mains from the rest of the vessel, then locking-down the RC handheld (disabling manual mode) and the remote PC (disabling auto mode). Remember that ASVs have multiple control modes from different devices. Even after these control and power sources are isolated they need to be monitored to ensure they are not inadvertently enabled by someone else testing another part of the system.

In general props expose the operations and support crew to a greater risk of incidental strikes. These are the accidents that happen when working around ASVs - either for divers in the water during operation, or through the normal process of performing pre-launch checks or during launch and recovery where the ASV is being handled. This can happen regardless of the size of the system - in fact you can be more at risk of an upper body prop strike handling a small lightweight ASV like the Yellowcat than working around a larger trailer-launched system.

With intake screens properly in place on jet drives, there is no chance of incidental impeller strikes, whereas even a guarded prop can present an incidental strike hazard - the degree of which depends on the guarding configuration.

When comparing systems with equal payload and performance capability, jet drives have no props or rudder protrusions and will provide lower drafts. However this is often negated if drag-inducing sensors (such as ADCPs) are fitted to keels attached to the hull. Jet Drives can also be run aground in surf/swash zone operation with less potential for damage.

A Jet drive's shallow draft normally results in easier launches, and recoveries can be as simple as a controlled beach grounding - again assuming no keel mounted sensors are fitted - with no chance of incidental impeller contact when intake guards are fitted correctly.

This largely depends on the conditions of the study area. Although jet drives are generally thought to be less fouling than props, it largely depends on the vegetation type and density, water conditions and other factors.

Jet drives can have the upper hand in certain tall grass estuarine environments if the grass is bottom rooted and can't readily block the intake. It will likely still cause blockage due to surface floaters but likely not as fast as props. However Jets are prone to clogging in other environments such as launching through Sargassum mats, where props have a better chance of cutting their way through.

Again - some level of blockage is inevitable when operating in dense vegetation so consideration should be given to the duration before this occurs and the effect this has on the mission. Fouling may limit top speed and cause the ASV to work harder reducing endurance - but the launch may still be able to be completed without any intervention.

There is no general rule of thumb covering all vegetation fouling, but (again in general) - for ASV operation unguarded props are the least fouling, followed by guarded props, then large jet drives followed by small jet drives. And of course this ignores the implications of other trade-offs described in this section, which in reality need to be jointly considered when choosing a system.

Propellers can be swapped to change specific drive characteristics (diameter, pitch, number of blades, material, etc). Jet impellers are basically pre-tuned for the maximum efficiency of a centrifugal pump and lack the same tuning ability.

When applied to ICS ASVs, there is generally no clear winner in maneuverability and agility between a single Jet drive over a single conventional prop / rudder or pod drive system.

However, counter-rotating twin screws such as those fitted to the Yellowcat ASV give the vessel a zero turn radius with skid-steer agility - ideal for precise waypoint loitering in inspection applications.

Although hull design is a significant contributor, props contribute to a cleaner wake. Even if not severe enough to cause cavitation, collapsing bubbles from a jet drive can be noisy and adversely affect many acoustic-based data acquisitions.