Article provided by the US Coast Guard

In 2002 recreational boaters were involved in 239 accidents involving motor or propeller strikes. Forty-seven of these resulted in death. While increased awareness of the risks associated with propeller/motor strikes will help, there are a variety of technologies that can further increase boaters’ chances of avoiding injury or death from this type of incident.

Apart from enhanced user awareness and training, propeller injury avoidance technologies can be categorized as guards, propulsion, interlocks, and sensors. Each of these types of technology is suitable in certain circumstances for particular types of boats. For example, propeller guards may be appropriate in some circumstances, but often are not effective in planing applications because they can cause hydrodynamic instability, and loss of power/speed. They also experience difficulty when boating in water with large amounts of floating or shallow-growing weeds that tend to foul the prop guards. Similarly, other technology such as rear-facing video cameras might be more appropriate for larger vessels with restricted fields of vision behind the boat, than for a smaller bow rider, or ski boat.

This article is not intended to address every propeller injury avoidance control technology. Our purpose here is to provide some examples that boaters may wish to consider. There is currently no “one-size fits all” solution to propeller injury avoidance. Boaters should review the options, and determine for themselves which devices will produce the greatest reduction in propeller-strike risk for their own unique combination of boat type and usage habits. Following is a listing of many of the propeller injury avoidance alternatives available in the marketplace today. Boaters may wish to conduct their own research, or obtain assistance from a marine professional to determine if one or more of these alternatives may be appropriate for their boat, boating habits, and boating purse.

PROPELLER GUARDS

Generally propeller guards fall into one of the four categories below. Most are capable of being retrofitted to existing boats. Propeller guards suffer to some extent from negative impact on handling characteristics, loss of power/speed, and in some circumstances can actually increase the likelihood of blunt trauma injuries, even though reducing the risk of actual propeller impact. Addition of a guard increases the physical size of the underwater propulsion body making it easier to be hit by it. Guards also create additional drag, which may result in lower top speed and fuel economy

Deflection Type (Skegs)
These devices (basically prongs forward of the propeller) have been proposed primarily to protect the “outboard or stern drive and whatever it hits” including large marine animals (manatees). These devices provide some entry protection from ahead but provide no radial or astern protection. They significantly increase the potential for blunt trauma at higher speeds and also increase the potential for entanglement. They may also block the propeller movement if dented inward by floating debris or submerged objects.

Full Cages
There have been a number of patents on these devices all of which depend on some type of metal cage surrounding the propeller to prevent human entry to the propeller blades. These devices, if properly designed, provide complete entry protection from the propeller but introduce a slightly increased risk of human entanglement in the metal mesh. Because they are larger than the propeller and present a larger frontal area under the boat they increase the potential for blunt trauma injury. They are more easily fouled than a normal propeller in the presence of seaweed or debris and block the propeller movement if dented inward by floating debris or submerged objects. They also cause hydrodynamic interference (cavitation) significant enough to cause a loss of maneuverability when on plane. The full cage creates the greatest additional drag of the various types, and therefore, the largest reduction in top speed and fuel economy.

Shrouds or Ring Guards
These devices are essentially a short cylinder surrounding the propeller which has an inside diameter somewhat greater than the propeller diameter. These devices provide protection from radial entry into the propeller but limited protection fore and aft with an increased potential for limb entrapment when entered fore or aft. Because they are larger than the propeller and present a larger frontal area under the boat they increase the potential for blunt trauma injury. They also are more easily fouled than a normal propeller in the presence of seaweed or debris and block the propeller movement if dented inward by floating debris or submerged objects. The negative impacts on speed and fuel economy are considerations with these. In addition they add significant rudder area and may affect steering torque and boat stability.

Ringed Props
This propeller design includes a hydrofoil ring around the tips of uniquely shaped blades. The blades and ring are an integral unit. These devices provide some protection from radial (side) entry into the propeller but fore and aft protection is essentially the same as for an unguarded propeller with an increased potential for limb entrapment when entered fore or aft.

Kort Nozzle
The Kort Nozzle has been on larger vessels for many years but has only recently been adapted to smaller vessels in the form of a modification to conventional outboard drives. The modification involves a carefully designed nozzle, surrounding a special propeller. These devices provide protection from radial (side) entry into the propeller but limited protection fore and aft with an increased potential for limb entrapment when entered fore or aft. Because they are larger than the propeller and present a larger frontal area under the boat they increase the potential for blunt trauma injury at higher speeds. They also are more easily fouled than a normal propeller in the presence of seaweed or debris. The Kort nozzle is most commonly used on relatively slow, non-planing workboats where additional drag is not a significant consideration.

PROPULSION ALTERNATIVES

Centrifugal Pump Jet Outboard
These drives use a centrifugal pump to draw water through an intake grate, which is essentially flush with the bottom of the boat, it forces the water out at high velocity through a nozzle. A conventional outboard power head drives the pump impeller with the pump unit-nozzle replacing the lower unit-propeller. All of the major outboard manufacturers offer such a pump jet outboard. These devices extend only a few inches below the transom and therefore provide excellent propeller injury protection. However, maneuvering astern and off-throttle steering ability are severely compromised and propulsion efficiency is reduced by 25 to 30%. Additionally, the water intakes are easily fouled by seaweed and debris.

Axial Pump Jet Outboard
This is a replacement device for the propeller on outboard drives and consists of an axial flow pump enclosed in a ducted housing. The propeller on the conventional outboard is removed and the pump impeller attaches directly onto the propeller shaft; the forward, neutral, and reverse functions are retained. The ducted housing attaches to the lower unit. These devices provide good entry protection in all directions, but have an increased potential for entanglement. They produce a marginal reduction in astern thrust compared to a normal propeller. Top speed and fuel economy are reduced due to increased drag. They also are more easily fouled than a normal propeller in the presence of seaweed or debris.

Inboard Jet Drive
These systems have the engine and pump inboard and the propulsion nozzle outboard. The water intake grate is flush with the bottom of the hull and the nozzle moves or uses moveable deflectors to obtain steering, reverse, and/or stopping. The nozzle is located above the bottom of the transom. These devices eliminate all drive system components below the transom and therefore provide perfect propeller injury protection. However, maneuvering astern and off-throttle steering ability are impaired. Top speed and fuel economy is decreased when compared to an engine of equal power driving a propeller. Additionally, the water intakes are easily fouled by seaweed and debris.

INTERLOCKS

Emergency Shutoff Switch
Outboard and sterndrive manufacturers provide automatic lanyard operated kill switches either on the engine, the associated control box, or as a remote mounted stand alone switches. When properly used, these devices cause the engine to stop immediately when the operator moves away from the operating position, as might occur during a fall overboard. These devices are practically fail-safe when properly maintained, but must be attached to the operator to work properly. They are not effective in reducing the risks to passengers other than the operator.

Ladder Interlocks
Various interlock switches are available to either sound an alarm, shut off engines, or both. When installed at boarding ladders, boarding gates, and other places where persons may be entering or leaving the water, these devices can be effective in alerting the operator that someone is at risk of a propeller related injury. As with all devices they must be maintained and periodically tested to ensure their continued proper operation.

Start-Up Time Delay
This type of device consists of a timing device and an alarm that is audible in the vicinity of the boat’s propellers. It is intended to provide a warning to people in the water that the engines will be started, before allowing power to be applied to the starter motor. The time delay and warning alarm are on the order of 7 – 10 seconds. A bypass switch is available to allow operating the engines immediately in an emergency situation. In this way those in the water have time to get aboard, or away from the area before the propellers can start thereby reducing their risk of injury.

SENSORS

Electronic
Several manufacturers have recently begun marketing a variety of electronic sensors. In general these sensors require a small monitoring device to be worn by protected individuals. These devices are in turn monitored by the surveillance system. If a protected boater is at risk, the devices are capable of shutting off engines, sounding alarms, and even broadcasting the vessels position to authorities if assistance is required.

Other Sensor Technology
Other sensor technologies are in the developmental stages. These technologies involve using sensors to detect thermal energy from a human in the water in proximity to the propeller. Again, once detected, alarms can be sounded and engines shut off.