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Author Topic:   Small Boat Wiring; c.1980 Mercury Outboard Tilt Systems
ericflys posted 07-18-2011 05:42 PM ET (US)   Profile for ericflys   Send Email to ericflys  
After looking at Boston Whaler Montauk's and Outrages for several years, I just bought my first one, an 83 Outrage 18 with a 1983 Mercury Black Max 150 with a rather unusual remotely mounted (under console) trim pump/fluid resevoir. The wiring on the boat basically looks like a tangled mess and am hope to get some answers/opinions prior starting my project of completely rewiring the boat. My questions are:

Should a fuse/circuit breaker be installed in the positive line between the engine and start battery?

I have a ignition switch with a push to choke, should fuse/breakers be installed in any of the wires going to this switch?

Should there be a circuit breaker between the remote trim pump and battery? Do I need fuses/breakers anywhere else in the trim system(ie between the pump and throttle mounted trim switch)? Should the trim system be wired to the start or house battery? Should it be wired directly (always hot) to the battery or through the battery switch?

Does anyone have a schmatic showing how the wiring is supposed to be with the remote trim system I have? Also, of note, in addition to the trim switch on the throttle, I have another switch on one of my panels that just raises the engine, but doesn't go the other way, what would the purpose of this be?

One final question, on my internal fuel tank, underneath the rear most access plate, there is a ground wire attached there but it is not attached to the boat anywhere (it's just a loose wire), should the wire be run to my negative buss bar under the console or where should it go? Is the fuel tank supposed to be grounded anywhere else?

Thanks, for any help!

jimh posted 07-18-2011 06:10 PM ET (US)     Profile for jimh  Send Email to jimh     
In classic Boston Whaler boats you typically see all metallic components of the fuel system are bonded together with a 10-AWG wire with green insulation. A transom-mounted below-the-waterline bronze electrode of about 2-inch diameter is also connected to the bonding system. The bonding system was typically not connected to the battery negative in the original configuration. It is common nowadays for boats to connect the bonding conductors of their fuel system to the battery negative, which is also connected to the outboard engine, and to omit the special transom electrode.

The primary battery power distribution to the engine starter motor from the cranking battery is typically not fused or otherwise protected against over-current because the engine starting current is very high, often several hundred Amperes, and finding an appropriate circuit breaker would be expensive. One could install such a circuit breaker, but I cannot say I have ever seen it done in a small boat.

In most outboard engines there will be an under-cowling fuse which will provide over-current protection to the branch circuits that are fed from the outboard engine's primary power circuit, and typically no additional fuses are installed. Check with the manufacturer of your outboard motor to see if they have published installation guidelines and instructions for installers to follow to determine if more fuses should be added.

If you need to learn more about small boat power distribution you might read my article on the topic and may find it helpful. The article is available from the REFERENCE section at

Boat Electrical Circuits and Wiring Practices
by James W. Hebert

ericflys posted 07-18-2011 07:04 PM ET (US)     Profile for ericflys  Send Email to ericflys     
Thanks for the reply jimh, your post and the linked article are excellent. I am very greatful for your site. My expertise is with wiring aircraft, so I am fimiliar with DC electrical, just not with what are the accepted practices on boats.

If anyone has some additional info relating to my other questions on the start switch, remote trim, and which battery the trim should be on, please post!

L H G posted 07-18-2011 07:43 PM ET (US)     Profile for L H G    
From my experience with Mercs, all fuses required are furnished as part of the self-contained engine electrical system. No external fuses are ever used.

There is always a 20 amp fuse under the cowling for the starting/ignition circuit. There is also a 20 amp fuse on the power trim circuit, but on the older remote system, I am not sure where it is.

I have the complete service/wiring manual for the remote trim system, and can send you a copy if needed. 1984 was the last year that older remote system was used.

I have the 1988 version of your engine, with the integral power trim system, a big improvement. current=Scan0018.jpg

ericflys posted 07-18-2011 09:38 PM ET (US)     Profile for ericflys  Send Email to ericflys     
L H G, Thanks for the info. Buying a service manual is on my list, but I would like to take you up on your offer. If you could send me the service/wiring manual for the remote trim, it would be appreciated. If it's in an electronic format, please send it to

Anyone have any idea on my question on which battery (house or start) the trim should be wired to? Should it be wired directly to the battery so it's always hot or should it go through my battery switch so its off when the switch is off?


jimh posted 07-19-2011 12:41 AM ET (US)     Profile for jimh  Send Email to jimh     
If there is an electrical device that is external to the outboard engine, like the electrical pump that operates the engine hydraulic tilt system, the power feed to the electrical motor needs to be fused, and it should not be wired directly to a battery without some sort of over-current protection.

Usually any device associated with the outboard engine should run from the same battery as the engine. When the engine is running it can supply current for running loads like the electrical motor for the hydraulic tilt system.

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