If a boat was originally set up for a single, what differs electrically on a grady set up with twins? I would likely be going with a fly by wire set-up. Ignition switches are obvious, but what else is involved? As for the mechanical part, I think I have that down.
Electrical changes needed to go from a single to twins?
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ScottyCee
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I'm wondering more about power management/ distribution at the batteries to the alternators, etc.
Well, I assume you are expecting to have a separate cranking battery for each and a switch depending on how you want to run them. Is that what you are referring to?
In theory a single battery is all you need as long as it is sized correctly. Of course, although that would work, it is not such a good solution. A lot of the electrical design depends on how you operate your boat. For example do you overrnight or hang out recreating or fishing with the stereo on and other electronic running?. You may want to run the livewell or DC fridge. Maybe you like to be out at night with lights on. Point is the accessory loads may dictate the need for a house battery or batteries and their size. The starting batteries have to be sized for the motor draw but as importantly the motor and battery cables may need to be sized. With multiple batteries, the battery switch system will probably need to change too. Battery locations as well as cable lengths may require cable resizing.
It is also possible that the draw of one of the new motors will be less than the draw of the old single motor. In that case a lot of the cabling can remain.
As mentioned, you will probably want new helm instrumentation (multi function gauges) and you may need new harnesses.
If what you have today works OK for your loads and operating mode than you are halfway there.
It is also possible that the draw of one of the new motors will be less than the draw of the old single motor. In that case a lot of the cabling can remain.
As mentioned, you will probably want new helm instrumentation (multi function gauges) and you may need new harnesses.
If what you have today works OK for your loads and operating mode than you are halfway there.
The boat I'm looking at has shore power, so it has a dedicated house system (I would think). My guess is that it already has a separate starting system like I have in our Voyager - 2 batteries. Often when I have seen the wiring system on twin GW's there is a 2nd battery switch. I don't know what this is for.
Maybe I can get GW to send me a wiring diagram for a single and twin powered boat for comparison.?
Maybe I can get GW to send me a wiring diagram for a single and twin powered boat for comparison.?
In my opinion... it doesn't matter to me what a particular manufacturer might do as that isn't always driven by "logic"... 
Now, this is just my opinion, but with twin engines and an honest need for a house system (high 12V demand when the engines are off), I would want (1) starting battery for each engine (and a crossover or at least the ability to do so with switches) and a separate house system using a larger (group 31, for example) DC battery - possibly even two DC's in parallel.
Also... don't "assume" that there is a separate house battery... probably is, but you never know till you know!
Now, this is just my opinion, but with twin engines and an honest need for a house system (high 12V demand when the engines are off), I would want (1) starting battery for each engine (and a crossover or at least the ability to do so with switches) and a separate house system using a larger (group 31, for example) DC battery - possibly even two DC's in parallel.
Also... don't "assume" that there is a separate house battery... probably is, but you never know till you know!
Be careful not to confuse AC house power with DC house power. Your shore power Is AC and just runs the battery charger and any other AC only loads.