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Start In Gear Protection And Kill Switches
Built In engine stop switches.

What is a neutral safety switch/start-in-gear protection, and why is it different from a Kill Switch?

Start-In-Gear protection - Neutral Safety Switch:
  Outboard motors with more than 115 lb. of static thrust must have a device that prevents the engine from starting when the engine is in gear.  115 lb. of static thrust is about 3 horsepower.

Code of Federal Regulations


Subpart L: Start-in-Gear Protection

183.710 - Start-in-gear protection required.
(a) Any outboard motor which is capable of developing a static thrust of 115 pounds or more at any motor operating speed with any propeller or jet attachment recommended for or shipped with the motor by the manufacturer, must be equipped with a device to prevent the motor being started when controls are set so as to attain that thrust level, as follows.


This is required because if an engine starts up suddenly while in gear, a person could be knocked down or thrown about, or even thrown out of the boat when the boat suddenly accelerates. This has happened many times in older boats that did not have start-in-gear protection. This is not required by law in inboard powered boats, however ABYC ( has a standard that requires this in all boats with mechanical propulsion.


Start-in-Gear Protection - Start-in-Gear protection shall be installed to prevent starting an engine while in gear.

Because of this standard almost all boats, outboard powered, inboard, and stern drive powered boats have a neutral safety switch built into the engine control that prevents the engine from being started when in gear.  Generally this switch is built into the throttle/shift mechanism and  connected to the ignition electrically, and is in the off position when the engine is in gear. Thus, no electricity flows through the ignition switch when it is in the start position and the engine is in gear.  However on older boats this may have been done mechanically in the transmission.

Usually the neutral safety switch is supplied with the engine and controls so it is not the boat manufacturer's responsibility to make the mechanism. But it is up to the boat manufacturer to make sure the controls are installed correctly and function correctly.  If the neutral safety switch does not function correctly you can be required to recall the boats and fix the problem.  See Defect Notification

Mercury Wiring diagrams showing neutral safety switch.

Kill Switch:

While the neutral safety switch prevents people from being thrown out of the boat, a kill switch stops the engine if you are thrown out of the operators seat or out of the boat. technically this is called an emergency engine/propulsion cut off switch, but most people refer to it as a kill switch.

The U. S. Coast Guard has not yet made a regulation requiring this.  ABYC does have a standard for the devices and that standard could be easily adopted by the Coast Guard.  All Personal Watercraft and most outboard engines now have kill switches. Many new inboard and stern drive boats do too.  The boat building industry has adopted this voluntarily as a safety measure and as a protection against liability.  It is a good safety feature and you should install an ignition switch that has a built in kill switch.  The following is the ABYC Standard for theses devices.



This standard is a guide for the design, construction, installation and performance of a system used to disable the propulsion engine when the operator is unexpectedly displaced from the boat.

The above is a standard for the switch itself.  It does not require boats to have one, but if the boat does have one it must be built and installed according to this standard.

What this switch does is shut off the ignition.  Usually there is a lanyard attached to the ignition key, or a separate key inserted into a socket on the ignition switch,  that is also attached to the operator. If the operator moves too far away from the operator's position (about three feet) , the key is yanked out and the engine stops.  The key can then be re-inserted and the engine restarted.  Click here for a good article from BoatUS on Kill Switches; Keeping Current On Kill Switches

Below are some references that show how ignition switches are wired.

Diagrams of various boat ignition switches:
Ignition Switch Wiring
A variety of Kill Switches available from

Shift Interrupter Switch:  This is a separate switch to interrupt the engine when the boat is being shifted into gear.  The reason for this is, when a boat is in gear there is pressure on the gears and this makes shifting difficult.  To relieve the pressure as the throttle/shift control approaches neutral the shift interrupter switch momentarily kills the ignition to take the pressure off the gears.  The load is then off the engine making shifting easier.  If it did not do this, when the transmission went into gear there would be a sudden jerk and a heavy load placed on the gears, and the engine would accelerate causing it to either die or  suddenly lurch forward or backward.  This switch momentarily stalls the engine to keep it in idle. This is part of the engine package.  But as the boat manufacturer you need to be aware of it.  Also, not all engine transmission packages have or need a shift interrupter switch. It depends on the package. 

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