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Understanding the Danger of Propeller Strikes

Ian Fortey by Ian Fortey Updated on March 29, 2021. In

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Often when discussing boat safety one aspect is overlooked. Boat propeller safety is something any boater needs to be aware of. 18% of boating fatalities are caused by propeller accidents. For that reason, understanding what a propeller is and how it works is important.

Because of hydrodynamic forces, a person in the water come in contact with the boat’s propeller. This can happen far faster than you might think. Even a strong swimmer may succumb to these forces.

Let’s take a look at some basic precautions to keep in mind around boat propellers. Also what to do in case an accident does happen. Propeller safety should be a core part of boating safety.

How Do Boat Propellers Cause Damage?

A typical three blade propeller can spit at anywhere from 1200 RPMs to 3200 RPMs. This is heavily affected by engine horsepower and propeller size. With a prop running at 3,200 RPM, the number of propeller strikes is astronomical. If you were in direct contact with the spinning blades, you could be hit 160 times in a single second.

Hydrodynamic effect could pull a full sized person under a boat very easily. In doing so, the propeller could impact your body, head to toe, in about one tenth of a second. Now you can see why propeller-related injuries are fatal. You have no response time once you have made contact.

Boat propellers are made of a variety of materials. Typically alloys of aluminum and stainless steel. Some include nickel and bronze. Because of the speed and torque, this is a dangerous mix. A boat propeller can easily cut through a turtle’s shell in open water. A large enough boat could destroy a smaller boat.

When a swimmer or passengers overboard come into contact with a propeller, damage is severe. Lacerations to flesh are impossible to avoid. The risk of death from immediate trauma is present. Likewise, blood loss or infection are possible. A number of victims of propeller strikes have had to undergo amputations.

What is “The Circle of Death?”

The name sounds dramatic but for good reason. This is what happens when a boat operator falls overboard. If steering control is lost, the boat may start moving in a circle. This is because propellers spin either to the right or the left. Without human control, the boat will naturally turn in that direction. This action is called prop walk. The danger is that if you are in the water, the boat will circle back. You are at risk of being hit by the boat and the propeller.

Luckily, these kinds of injuries can be prevented.

How to Avoid Propeller Injuries

In the event of a man overboard situation, you need to respond quickly. Turn the boat towards the person who has fallen in the water. This will direct the stern away. As soon as you have done this, shift your engine into neutral. This will stop the propeller from spinning. Your most important job is to eliminate the propeller as a danger. Keep your eyes on the passengers who went overboard at all times. Never reverse a boat to try to pick up someone who has fallen overboard. Always turn around to come towards them again. If you turn too wide or miss, turn again. This is key for propeller safety.

There are safety tips you can follow to minimize damage from propeller strikes. These include things like:

  • Propeller guard. These devices can be purchased to partially or almost completely encase the propeller. They look like small, round cages or rings. If a person or body part is drawn towards them, the propeller guard can block contact. Full propeller guards have a wire cage over the front. Propeller accidents could still occur but they are much less likely.
  • Kill Switches. These are often called engine cut off devices by the U.S. Coast Guard. These devices can be affixed to a PFD. They are the size of a small keychain. Everyone on board could be outfitted with one. When a crew member goes overboard, a sensor alerts the engine. It will shut down automatically to prevent injury.
  • Boat operators can wear lanyard kill switches. These attach the boat to the operator’s life jacket or wrist by a line. They are linked to the engine. If they move too far from the helm, the line pulls free. The kill switch will then shut off the engine.
  • Anti-feedback steering. This can help prevent the “circle of death.” Anti-feedback steering resists prop torque. As a result, if your hands come off the wheel, it will not spin. That means prop walk is not a concern. The boat will not begin to turn dangerously. It will continue on a straight path.
  • Wear life jackets. These safety devices are essential. Combined with other safety equipment like a cut off switch and sensors, these save lives. All passengers should have one.
  • Limit alcohol consumption. If you’re operating a boat, the safest thing to do is not drink at all. Especially near swimming zones and with passengers on board.
  • Be aware of your position. Avoid swimming zones at all costs. Stay a safe distance from things like dive flags.
  • Be safe when boarding. Keep the engine off until everyone is on or off the boat.
  • Do a full boat walk before starting the engine. Head from bow to stern and check the water. People in the water near the propeller can be hard to see at the helm.
  • Ensure no one is near the swim platform, swim ladder or boarding ladders while the engine is on. Any locations where they might fall overboard. These are areas where swimmers are most likely to go.
  • Keep passengers in safe and secure locations. Ideally, every passenger should have their own seat. Don’t allow passengers to ride on the bow, transom, the seat backs and the gunwales. Hitting rough water can cause accidents.
  • Make sure all passengers know exactly where the propellers are.
  • Keep one passenger on prop watch. This person can maintain watch around the propeller area in case there is a person in the water. Even with propeller guards, the propeller area needs to be monitored. Swimmers can appear from nowhere.
  • Take a boating safety course. Every boater should learn how to properly control their vessel on the water. Google boating safety courses in your area. They are typically short but well worth the time and effort.

Should You Sharpen Boat Propellers?

Some boaters are actively sharpening their props. This is not necessary and can ruin your prop. If you alter the edge, it will affect how it propels your boat. If done correctly it can increase speed. This would only be relevant for racing. Normal boaters should never need to do this.

If you have damage to your propeller, you can look into filing it. Perhaps if you hit rocks or a log and caused nicks and scratches to appear. These could be filed smooth. But the blades themselves should not require any sharpening. This will only increase potential harm in the event of an accident. Also it further wears down the prop. Eventually it will need to be replaced because it’s unable to do the job properly.

If your propeller has been very badly dulled by an accident, have it repaired. A professional propeller shop can handle this type of work. Repair or replace the damage.

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