MECHANICAL SAFETY: How to Prevent Engine From Starting?

[CoryRobertLowe]

• I'm working on the propeller. How do I prevent the engine from turning over? 
• I should take out one/all of the spark plugs? Or just take off the part that hooks to the spark plug? 
• What if the battery is unhooked already? Can the engine turn over without a battery? 
• How about static causing a spark by the spark plug hole, and turning over the engine? Is that a thing?

[GaryH]

Whenever I’m doing any maintenance on my motor I have my main power switch turned off. 

 

[GitInMuhBoat]

Mercury manual says to put it in neutral and remove all spark plug leads when working on the prop.

[DinkDonkey30]

If you are concerned about the engine starting or turning over just unhook the positive battery cable. You can also unhook the negative if it makes you more comfortable. The other things you listed are impossible or next to impossible. Out of curiosity what are you doing with the prop?If trying to lock to remove they have locking tools or you can use a board and the ground to stop it spinning. Generally I hold mine by hand and can brake the prop nut loose.I only ask because your concerns with the motor while reasonable are pretty basic knowledge for those with mechanical knowledge of engine operation. You may want/need some more instruction or insight into what your project requires before you get started. I mean absolutely no offense whatsoever I just would hate for you to get in over your head if you don’t have the tools or knowledge for what you are attempting. I’m a mechanic by trade.

[CoryRobertLowe]

@DinkDonkey30,

 

Thanks. I took a small engine class, so I have some knowledge, but I just want to be 1000% confident, so I don't do anything stupid. @GitInMuhBoat mentioned what I read in the manual (I purchased the official one and read it), but I just wanted to ask online, to see if there was any other dangerous things I should be aware of.

 

I appreciate everyone on here, for your help. Thanks!

[DinkDonkey30]

6 minutes ago, CoryRobertLowe said:

@DinkDonkey30,

 

Thanks. I took a small engine class, so I have some knowledge, but I just want to be 1000% confident, so I don't do anything stupid. @GitInMuhBoat mentioned what I read in the manual (I purchased the official one and read it), but I just wanted to ask online, to see if there was any other dangerous things I should be aware of.

 

I appreciate everyone on here, for your help. Thanks!

For reference disconnecting the spark plug wires from the spark plugs will stop the motor from being able to start but does not stop the starter from being able to be engaged. To stop the motor from actually being able to turn over weather it starts or not you have to disable the starter motor circuit. To do that you can disconnect the positive starting motor battery cable, make sure the master shut off if equipped is off,disconnect the power wire at the starter, remove fuses and relays in the starter circuit, and some other more complicated and unnecessary steps. Generally the quickest and easiest is remove the positive battery cable althought any combination of the above will work like redundant saftey steps. I personally don't trust master switches. I've had them fail both open where power couldn't get where it needed to and closed where the switch doesn't shut power off. Disconnecting the postive cable guarantees power isint getting to the motor.If you don't trust that it cut power put the key in the ignition and try to start the motor nothing should happen if power is safely disconnected.As for the prop be aware it can be sharp I wear mechanics gloves when I work on mine but thats more habit than anything I've banged my hands up a lot so I try to wear gloves whenever I can working on stuff. 

[Tennessee Boy]

Interesting thread.  You didn't mention what type of motor it is.  Is it an electric start?  Do you start it with a key?  Do you have a child that might try to crank the motor while you're working on it?  My first boat would run if you disconnected the battery after starting it.  I don't think most modern outboards will do that.  

 

It's always good to be safe especially if you're not sure about something.  Personally,  with my outboard I would turn the switch off and remove the key.

[DinkDonkey30]

10 hours ago, Tennessee Boy said:

Interesting thread.  You didn't mention what type of motor it is.  Is it an electric start?  Do you start it with a key?  Do you have a child that might try to crank the motor while you're working on it?  My first boat would run if you disconnected the battery after starting it.  I don't think most modern outboards will do that.  

 

It's always good to be safe especially if you're not sure about something.  Personally,  with my outboard I would turn the switch off and remove the key.

Most engines if working properly can run with the battery disconnected after starting. The battery(s) main function is supplying power for the starter motor after that the charging system takes over supplying the electrical circuits for operation. Some engines require no batteries for example a 350 chevy with a mechanical fuel pump and magnito style ignition can be started and ran by push or pulling starting the vehicle(sprint cars). The engine when turning over creates spark and fuel supply mechanically without the need for any outside electrical battery or charging system.

[CoryRobertLowe]

13 hours ago, Tennessee Boy said:

Interesting thread.  You didn't mention what type of motor it is.  Is it an electric start?  Do you start it with a key?  Do you have a child that might try to crank the motor while you're working on it?  My first boat would run if you disconnected the battery after starting it.  I don't think most modern outboards will do that.  

 

It's always good to be safe especially if you're not sure about something.  Personally,  with my outboard I would turn the switch off and remove the key.

 

@Tennessee Boy,

 

It's a 2003 (I think) 60 HP Mercury.

MERCURY NITRO TRACKER 60 ELPTO

 

Out of curiosity, how can it start without a battery? It must be storing a charge somewhere?

 

*** I misread...please disregard this post.

[DinkDonkey30]

17 minutes ago, CoryRobertLowe said:

 

@Tennessee Boy,

 

It's a 2003 (I think) 60 HP Mercury.

MERCURY NITRO TRACKER 60 ELPTO

 

Out of curiosity, how can it start without a battery? It must be storing a charge somewhere?

Engines don't rely on a stored charge to run. So long as the engine can generate its own electrical current when being turned over it will be able to provide spark to run and give or take how the electrical system is wired or if it has a mechanical fuel pump can supply itself with fuel.The stored charge of a batteries main purpose is to supply power for the starter motor.  For simplicity think about the ford modle A. No battery required. When you cranked the motor over by hand crank attached to the engines crank shaft it started moving the mechanical components of the engine. The distributor starts turning when the crank spins and starts creating electrical energy needed to create spark. The fuel pump also being mechanically driven starts to supply fuel. Once you spin the motor fast enough and you get air fuel and spark to the motor it starts. All this is done mechanically without the need of batteries or mechanical starters. Even up until the mid 2000s most diesel engines didn't require batteries to be started or run. With a diesel you don't have the need to stop spark because they don't use spark. Combustion comes from heat and cylinder psi. So to stop a diesel you have to take away fuel or air. Most diesels cut the fuel supply through the mechanical fuel pumps. 2 stroke Detroit also had flapper valves that choked off the intake. They had a bad habit of "running away where they would run on oil scavenged from the crank case so cutting fuel wouldn't help because a diesel can and will burn oil and anything else it can get hot enough to combust so you have to take away the air supply. All engines require 3 things to run Air, fuel and spark/combustion. To stop them you have to remove one of those 3 things.

[DinkDonkey30]

Another example is a magneto style distributor. You can spin a magneto distributor gear by hand and it will absolutely generate electricity. Roughly 20,000 volts. That's with just you holding the distributor in one hand and turning the gear in the other. I don't recommend doing this as it hurts like hell but it's an old joke mechanics played on new people unfamiliar with how they work and what they are capable of. No battery or other sources of electricity needed.

[GaryH]

Let’s dumb the whole question down. You’re going to be removing your prop and possible doing some maintenance. Turn your main power switch or disconnect your battery. Turn your key on and see if anything happens. If not then remove your key and get to work. 

[Rockhopper]

4 hours ago, DinkDonkey30 said:

Most engines if working properly can run with the battery disconnected after starting. The battery(s) main function is supplying power for the starter motor after that the charging system takes over supplying the electrical circuits for operation. Some engines require no batteries for example a 350 chevy with a mechanical fuel pump and magnito style ignition can be started and ran by push or pulling starting the vehicle(sprint cars). The engine when turning over creates spark and fuel supply mechanically without the need for any outside electrical battery or charging system.

And when the apocalypses happens, those will be the only vehicles we are driving around in!

[Darth-Baiter]

PROPS to the OP for putting safety first.  

[DinkDonkey30]

1 hour ago, Rockhopper said:

And when the apocalypses happens, those will be the only vehicles we are driving around in!

That’s one of the reasons I love my square body. Simplicity, new tech is great but it’s hard to beat reliability and style. 

[Rockhopper]

31 minutes ago, DinkDonkey30 said:

That’s one of the reasons I love my square body. Simplicity, new tech is great but it’s hard to beat reliability and style. 

It's a '79?  I am confused because the thinner upper side trim looks like an '80, but the grill itself looks like a '79. I know there were round eye '80's, but the grill isn't 1980.

[LonnieP]

I just changed the prop on my tracker with 60hp mercury last week. Never considered the engine starting on its own while I worked on it. The key was out of the ignition though.

[GRiver]

My grandfather was a stickler for safety…….. He would pull plug wires off and disconnect batteries when working on stuff. Plugs wires to make sure if, turned over by hand, it could start. Batteries disconnected to make sure the starter could engage when you didn’t want it to. 
He would say  “ I would rather take the extra steps now, than be sorry later”.  

[DogBone_384]

On 3/25/2025 at 1:14 PM, CoryRobertLowe said:

Is that a thing?

 

You’re over thinking it.  Leave your main power off and the key out of the ignition switch and the motor in neutral.

[DinkDonkey30]

18 hours ago, Rockhopper said:

It's a '79?  I am confused because the thinner upper side trim looks like an '80, but the grill itself looks like a '79. I know there were round eye '80's, but the grill isn't 1980.

It's an all original unrestored 79 k10 scottsdale. 44k miles

 

[Susky River Rat]

Not saying these aren’t good practices but, if the boat is in neutral the prop is not connected to the motor in anyway. There  shouldn’t  be anyway that the prop would be engaged to the rest of the motor. A jet does not have a gear box and runs directly off the drive shaft.  
 

 

[Way2slow]

Depends on the motor ignition system on whether it will start without a battery or not.  Some generate their own spark, some require a battery.  If you unplug the main cable going to the engine while running and it doesn't shut off, it doesn't need external power to run.  If you do that and it keep running, you can't turn it off with the key either, without reconnecting the harness, the key switch grounds out the ignition system to shut it off.

On any properly operating motor, if the key switch is off, it can't be started by turning the motor over manually. The key switch is either supplying the voltage to the ignition system to make it run, or removing the ground that's keeping it from running.

 

So, if it's in neutral, and the key switch is off, there's no way on god's green earth you can start a motor messing with the prop, even if it happens to be in gear, if the switch is off and working properly it can't start. I've been messing with them for over 60 years and have never and will never do anything but make sure the prop free spins in both directions (it's out of gear then) before taking the nut off.   A Mercury will free spin in one direction even when in gear but will make a clicking sound doing it.  The Evinrude/Johnsons will not free spin in either direction when in gear.