Start In Gear Protection

Kill Switches and Built In engine stop switches

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

Start-In-Gear protection - Neutral Safety Switch:  Outboard Engines  developing 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. This is also covered by ABYC standard P-14.


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.

See 33CFR start in gear

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 (P-14) that requires this in all boats with mechanical propulsion.

Although the regulation applies only to outboards, most inboard and sterndrive boats comply with ABYC P-14 and have this feature as well.  It is usually incorporated into the the throttle/shift control, and simply cuts off the electrical current to the ignition.


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

See Kill Switch:

The following new law takes effect on December 4, 2019, so any covered recreational boats manufactured after December 2019 (HINs that end with F020 or later) will be required to have an ABYC A-33 compliant engine cutoff switch installed. See Coast Guard Boating Safety Circular Spring 2019

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 recently been directed by statute to require this.  ABYC has a standard for the devices and that standard was 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 USC Code for theses devices.

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46 U.S. Code § 4312.Engine cut-off switches (Kill Switches)

(a)  Installation Requirement. —

A manufacturer, distributor, or dealer that installs propulsion machinery, and associated starting controls on a covered recreational vessel shall equip such vessel with an engine cut-off switch and engine cut-off switch link that meet American Boat and Yacht Council Standard A-33, as in effect on the date of the enactment of the Coast Guard Authorization Act of 2017.

(b) Education on Cut-off Switches.—

The Commandant of the Coast Guard, through the National Boating Safety Advisory Committee established under section 15105, may initiate a boating safety program on the use and benefits of cut-off switches for recreational vessels.

(c)Availability of Standard for Inspection.—

(1)In general.—Not later than 90 days after the date of the enactment of this section, the  Commandant shall transmit American Boat and Yacht Council Standard A–33, as in effect on the date of enactment of the Coast Guard Authorization Act of 2017, to—

(A) the Committee on Transportation and Infrastructure of the House of Representatives;

(B) the Committee on Commerce, Science, and Transportation of the Senate; and

(C) the Coast Guard Office of Design and Engineering Standards; and

(D) the National Archives and Records Administration.

(2)Availability.— The standard submitted under paragraph (1) shall be kept on file and available for public inspection at such Coast Guard office and the National Archives and Records Administration.

(d) Definitions.—In this section:

(1) Covered recreational vessel.—The term “ covered recreational vessel” means a recreational vessel that is—

(A) less than 26 feet overall in length; and

(B) capable of developing 115 pounds or more of static thrust.

(2) Dealer.—The term “dealer”means any person who is engaged in the sale and distribution of recreational vessels or associated equipment to purchasers whom the seller in good faith believes to be purchasing any such vessel or associated equipment for purposes other than resale.

(3) Distributor.— The term “distributor” means any person engaged in the sale and distribution of recreationalvessels and associated equipment for the purposes of resale.

(4)Manufacturer.— The term “ equipment manufacturer” means any person engaged in the manufacture, construction, or assembly of recreational vessels or associated equipment, or the importation of recreational vessels into the  United States for subsequent sale.

(5)Propulsion machinery.— The term “propulsion machinery” means a self-contained propulsion system, and includes, but is not limited to, inboard engines, outboard motors, and sterndrive engines.

(6)Static thrust.— The term “static thrust” means the forward or backwards thrust developed by propulsion machinery while stationary.

(Added Pub. L. 115–282, title V, § 503(a), Dec. 4, 2018, 132 Stat. 4270.)



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 Also available are wireless kill switches that sense when a person is thrown overboard or knocked down, and kills the ignition.  These are very convenient for people who boat alone,  need to move around in the boat, or are on large boats that necessitate moving  more than a few feet from the helm.

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

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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