Wiring unit on bow

I have another silly question. Ordered a 858 today to mount on bow of my boat. From searching past posts, it seems that everyone recommends. Running two wires all the way back to the cranking battery for power. Is that necessary? Can I just hook into the power supply running up there for the tilt/trim or lights or 12 volt receptacle rather than pulling wires?
I haven't done a lot of this type of install, don't want to fry anything, but don't want to have to fight pulling wires if its not needed.
I'm gonna have a hard enought time getting Ethernet to the center console.
If it matters, the boat is an 18 foot xpress center console. Thanks.

You can hook it to any 12 volt source but you are risking having interference from other equipment. Running twisted pair to the cranking battery is by far the best way to do it. Some people use a lawn mower type battery on the bow for power. This seems to work well but you will have to be sure to keep it charged.

Hook to the battery. The trim doesn't have true power available, and most circuits have trouble maintaining voltage while the outboard is cranking.

So what is the smallest battery I could get away with if I put one up front under the deck strictly for the 858c. It would help with control in the wind anyway. As long as I can get 10-12 hours of use in between charges.

Any riding mower battery should give you several days of use. Check with Canepole and see what he is using.

I can't help you with the 858C, but can tell you that a 7 ah battery will run an 1197 for only a few hours. It took 3 of them to get through the day, so that means I would need a min of 20 ah for my finder. Maybe the specs will let you compare power consumption for both units so you can determine the min number of ah for your finder.

As Catfan said it needs to be hooked straight to battery. My neighbor is firing it right now with a 597. Every time he cranks motor it drops out and has to be restarted, wired into dash wiring. If you have the room get a good group 24 multiple purpose cranking,trolling battery plus added weight on bow.

My front one is wired to the bow panel and my console one is wired to the dash panel--both are run off of the start battery--both panels run to the fuse panel then to the battery

I guess I'm in the minority here, but I prefer to run everything through a fuse panel. My boat has a fuse panel under the console that feeds EVERYTHING, it 's fed from the cranking battery. The wires going to the bow are on a 15 amp fuse, then the electronics at the bow are on their own 3 amp fuse. I have fished all day with the aerator off and on, two HDS-8s, one 7, one LSS-2 and one LSS-1 on and never had any issues. They all stay on all the time and don't dim or even blink when I start my 90. If your electronics dim when you start your big motor you either have wiring issues or battery issues. My $.02.

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Originally Posted by Barnacle Bill

You can hook it to any 12 volt source but you are risking having interference from other equipment. Running twisted pair to the cranking battery is by far the best way to do it. Some people use a lawn mower type battery on the bow for power. This seems to work well but you will have to be sure to keep it charged.

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What do you mean by "twisted pair"? Should I run power wires from the front back to the cranking battery...If so, what size wire would you recommend?

you can try hooking the bow unit up to the fuse panel, if you get interference, THEN run some seperate/new power/grd wires. I was able to run seperate power wires from the cranking batt to the bow under the floor on my Tracker PT175 w/o much trouble but you may have troubles as you've indicated. I have my HDS 8 wired into the fuse panel at the console, it stays on the entire time I'm on the water, and does not blink or go goofy when I crank up the 60hp. My cranking battery is 4 or 5 yrs old as well.

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Originally Posted by BobberStopper

What do you mean by "twisted pair"? Should I run power wires from the front back to the cranking battery...If so, what size wire would you recommend?

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So is 18 gauge big enough to run 20 foot, actually 40 foot if I twist them, for the 858c HD DI?

The specs for your finder show a power draw of 0.78 amps with the lights off. The backlighting is done with LEDs, so the max power draw is somewhat higher. For the sake of discussion, let's call it a max draw of 1 amp.

Using this online calculator with 18 awg, 2-wire, copper, 12v, 3% max drop (recommended by Blue Sea and others for marine electronics), and a 1 amp load, you get a max length of 22.6'. If this is the right data (Bill, please confirm), then you should move up to at least 14 ga.

You'll probably fuse this line at 3 amps and may later decide to add another load, so maybe you should use 3 amps instead of 1 amp in the calculation (??). If that's the case, the calculator says you will need 10 ga. wire.

If there's any way to add a battery up front, the results change drastically. With the full 3 amp draw, the calculator says it's fine to use up to 7.5' (3.75' after twisting) of 18 ga. wire.

Thanks Bill, great info and very educational....

I hooked one of mine to the bow nav light circuit. Works fine--just have to turn the nav light switch on.

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Originally Posted by Barnacle Bill

See This

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You're right Bill. Also did that with my underground dog fence. Surround the perimeter, then twisted the pair back to my shop where the transmitter is at. Cancels the signal out at my lab's collars.

Corker is right about wire size - if it's stranded. I choose solid core when I can. My two cents.