01-19-2007, 09:14 PM
Propeller Injury Risk Reduction Propeller Guards
The term "propeller guard" is a widely misused term since there are basically four different types of propeller guards available today, as well as several alternatives to propellers. Each "guard " or alternative has its own listing of pros and cons. The various types and some of their advantages and di
vantages are listed below.
Deflection Type (Skegs)
These devices (basically prongs forward of the propeller) have been proposed primarily to protect the "motor 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 provide complete entry protection from the propeller but introduce a slightly increased risk of 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 at higher speeds. 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.
Shrouds or Ring Guards
These devices are essentially a short cylinder surrounding the propeller which has an inside diameter slightly 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 at higher speeds. 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.
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
only recently been adapted to smaller vessels in the form of a modification to conventional outboard drives. The modification involves a nozzle, surrounding the 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.
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 eliminate all drive system components below the transom and therefore provide perfect propeller injury protection. However, maneuvering astern and off-throttle steering ability are severely compromised and propulsion efficiency is reduced by 20 to 25%. 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 and lower top speed 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 and fuel economy is decreased. Additionally, the water intakes are easily fouled by seaweed and debris.
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The term "propeller guard" is a widely misused term since there are basically four different types of propeller guards available today, as well as several alternatives to propellers. Each "guard " or alternative has its own listing of pros and cons. The various types and some of their advantages and di
vantages are listed below. Deflection Type (Skegs)
These devices (basically prongs forward of the propeller) have been proposed primarily to protect the "motor 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 provide complete entry protection from the propeller but introduce a slightly increased risk of 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 at higher speeds. 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.
Shrouds or Ring Guards
These devices are essentially a short cylinder surrounding the propeller which has an inside diameter slightly 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 at higher speeds. 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.
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
only recently been adapted to smaller vessels in the form of a modification to conventional outboard drives. The modification involves a nozzle, surrounding the 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.
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 eliminate all drive system components below the transom and therefore provide perfect propeller injury protection. However, maneuvering astern and off-throttle steering ability are severely compromised and propulsion efficiency is reduced by 20 to 25%. 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 and lower top speed 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 and fuel economy is decreased. Additionally, the water intakes are easily fouled by seaweed and debris.
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