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USCG’s Search For Prop Guard Test Protocol


Is the “cure” riskier than the disease?

Several years ago, Bob MacNeill and his wife Sandy were idling along in their inflatable dinghy in the canal behind his home in Florida when a series of freak events sent the normally cautious boat owner overboard. He had just finished casually unclipping the engine’s kill switch (a mistake) in preparation for landing at the dock and was talking to friends in another boat when an unseen wake bounced him across his dinghy and into his wife Sandy. In the awkward split second that he struggled to regain his composure, he inadvertently spun the outboard’s tiller and throttle, causing the boat to accelerate and turn sharply to port. MacNeill was thrown into the water and almost immediately struck the boat’s propeller. He says he was “lucky” if you can call someone who was run over by a boat propeller lucky; although he was badly cut, MacNeill wasn’t killed and he didn’t lose a limb.

Not surprisingly, when the Coast Guard announced it was planning to develop a protocol for testing prop guards, MacNeill quickly volunteered. Not only does he have the perspective of someone who was cut up by a propeller, MacNeill is also a yacht designer and the retired president of Chris-Craft and Carver Yachts.

While the idea of reducing propeller accidents is certainly appealing, none of the boat manufacturers, including Chris-Craft and Carver, has ever offered the devices. Nor has West Marine. MacNeill said that when he was at Chris-Craft and Carver, not much was known about prop guards — what was available or how well they worked. Even today, tests tend to be contradictory and there is a wide disparity of opinion on how well prop guards work or even if they work at all.

Richard Blackman, who recently retired after eight years as an engineer at the Coast Guard’s Boating Safety Division, says that the Coast Guard’s prop guard testing protocol is in the final stages of development and anyone — engine manufacturer or prop guard manufacturer — will be able to follow it so that any future tests and test results will be comparable. In the past, test results have varied, not surprisingly, depending on who was doing the testing. Prop guard manufacturers claim that in addition to reducing the chances of a propeller accident, their tests prove that their devices have the added benefit of improving a boat’s speed and performance. Tests by engine manufacturers, however, have indicated the opposite. Both Mercury and OMC have found the devices are liable to work only on boats moving slowly, at about 10 knots or less. The faster a boat is moving, the more likely that a guard will reduce speed and increase fuel consumption. An even larger concern for engine manufacturers has been a prop guard’s potential impact on maneuverability. As one engineer said, anything underwater with a horizontal surface will act as a planing surface and anything with a vertical surface will act as a rudder. Depending on the shape of the device, it can introduce hydrodynamic forces that will have an effect on how a boat handles.

Significant Difference

The most significant difference, however, involves the degree of protection provided by the various devices. One of the prop guard manufacturers has a dramatic video demonstration of watermelon being hacked to pieces after it is tossed from the stern of a boat into an unguarded propeller. A guard is then affixed to the lower unit and a second watermelon is repeatedly tossed at the propeller without being affected.

Some baseline testing sponsored by the American Boat & Yacht Council (ABYC), however, found that some of the guards failed to prevent injuries at slow speeds. And at high speeds, all of the guards tested were capable of inflicting significant “blunt force” injuries that were at least as significant as injuries from the prop itself. Note that the injuries were to a specially developed gel “leg” that was designed to approximate the characteristics of a human leg.

How well the prop guards work to protect a person in the water and how much they will affect a boat’s performance and fuel consumption are still open questions. Dave Gerr, the highly regarded naval architect and author of The Propeller Handbook (McGraw-Hill/International Maine), believes that any device near the propeller will interrupt water flow and affect acceleration, fuel consumption, speed and maneuverability. Other people who have studied the guards believe it is likely that some of the guards work in some situations and with some engines and boats. Several people have mentioned that the “cage” type guards seem to do a good job of protecting people in the water when a boat is moving slowly and have minimal effect on fuel consumption and maneuverability. At higher speeds however, the cage guards consistently caused significant boathandling problems and were more likely to inflict potentially fatal blunt trauma injuries. Other guards appear to cause fewer boathandling problems at higher speeds but offer far less protection to anyone in the water, even at slower speeds.

If the Coast Guard protocol is ever used to thoroughly test prop guards with different boats and engines, we may have a better idea of what, if any, protection they provide. In the meantime, everyone who was interviewed for this article said that the best way to prevent propeller accidents is to follow a few basic safety rules:

  • Make sure everyone is seated safely inside the boat.
  • Never allow passengers to ride on the bow, gunwales, or transom.
  • Slow down in heavy seas or when you encounter a large wake.
  • Avoid letting people aboard drink heavily.
  • Wear your engine cutoff switch lanyard when the boat is underway.
  • When launching or ungrounding a boat, keep people in the water away from the stern and prop.
  • Never put the engine in reverse and back toward a skier (or anyone else) in the water.
  • Don’t use an outboard or I/O’s lower unit for boarding, even when the engine is off.
  • Shut the engine off when you’re near anyone in the water.

BoatUS Editors

Contributor, BoatUS Magazine