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Propeller Guard Information
What is a Prop Guard?
A prop guard is a device that fits over the open-bladed propeller on a boat. Its functions are to decrease the risk of propeller cuts for humans and marine animals and to protect the propeller from damage due to debris or running aground. Manatee Watercraft MortalityThe widespread use of propeller guards on motorboats with open-bladed propellers has been proposed by the public for a number of years as a way to "minimize" or "eliminate" manatee mortality due to encounters with watercraft. Mortality statistics, collected by the FWC Fish and Wildlife Research Institute (FWRI), over the ten year period from 1993-2003, shows that during this period of time: o 58 % of all documented watercraft deaths were caused by impact injuries resulting from the physical force of a boat traveling at a high speed referred to as blunt-force or impact trauma o 32% of all documented watercraft deaths were caused by injuries related to sharp-force trauma of contact with rotating propellers and fixed skegs o The remaining 10% of watercraft deaths was due to an indiscernible combination of the two causes stated above.
Figure
Of the 2,940 total documented dead manatees
between the years of 1993-2003, 713 recovered from Florida waters were
documented to have died of watercraft –related trauma. Two hundred and
twenty-nine animals (32%) died as a direct result of sharp-force trauma.
Propellers and skegs were the most common causal agents of sharp-force
trauma. Four hundred and fourteen animals (58%) died as a result of
blunt-force, or impact, trauma. Hulls, keels, rudders, propellers,
skegs, and other submerged features of a watercraft can cause
blunt-force trauma. The remaining 70 manatees (10%) died as a result of
a combination of sharp- and blunt-force traumatic injuries. As of 2003, there are 978,225 vessels registered in Florida alone. Boaters may think that a boat equipped with a propeller guard traveling at any speed is manatee safe, but this is not the case. If all boats had propeller guards and traveled at high speeds through important manatee use areas currently regulated with speed zones, statistics indicate that the proportion of manatee death due to impact trauma would likely remain the same or increase. Total watercraft –related manatee deaths would remain at current levels or increase in this case as well, providing no benefit to manatee recovery. Equipping all the vessels with propeller guards is also not viewed favorably by many boat owners due to the cost and concern about the effect of vessel performance. Despite the establishment of speed zones for manatee protection around the state, no single management technique can result in reducing manatee watercraft mortality to acceptable levels. Certain propeller guard designs afford protection from the cutting trauma of open blade systems on vessels traveling at low speeds, so the use of such systems in conjunction with manatee speed zones is viewed as aiding in protection from watercraft-related injury. Operating boats with open-bladed propellers is without doubt, a significant threat to the continued existence of the manatee. About 53% of the adult manatee population carries scars from encounters with propellers (FMRI assessment). The sight of a manatee with suture-like scars along its dorsal surface elicits sympathetic emotions from most onlookers. Many an ingenious inventor who has seen this sight during a trip to his or her favorite aquatic vacation spot has designed a device, at least conceptually, to prevent this from happening. How Long Have Propeller Guards Been Around?Surprisingly, propeller guards have been available to users of outboard engines about as long as outboard engines have been around. The first patent for a ring-cage type design was given to J. R. Benson in 1938. He developed his guard to protect the propeller and lower gear case (of the then low horsepower engines) from impacts with rocks and other submerged structures. By 1964, eighteen varying designs had been patented and made available to consumers. Simple ring-type guards were made as early as 1943 (B. A. Sink). Other designed guards included mesh cages, fork-tine, and semi-ducted styles. Most of these early guards were made of steel, which quickly rusted and created excessive drag for the engines upon which they were installed and used. Today, several designs have been developed that employ metal alloy or plastic construction and/or also offer more advanced engineering and simple attachment mechanisms. What kind of Prop Guards are Available on the Market?See a list of available propeller guards compiled as a service to the public. NOTE: This list does not represent an endorsement or recommendation for use by the FWC or its staff. The inclusion of advertising, logos or web site links on this web site does not constitute an endorsement by the State of Florida or the Fish and Wildlife Conservation Commission of the products or services so advertised. Well, Do Prop Guards Work?In 1997-1998, an inboard propeller guard assessment study was funded from the Save the Manatee Trust Fund. The study suggests that currently available propeller guards for inboard engines provide only limited protection for manatees and affect vessel performance.
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