OK thanks here we go......
Please can you help with the following questions, I am keen to do the work on my small boat, which I am renovating, myself.
I am quite practical but these, for me, are sort of technical questions.
The starter motor for the boat draws 1800 w @ 12v which is I believe 150 amps.
There will be two batteries one for the starting only and one for all other ancilliaries. Both will be 100AH.
1. The alternator has an output of 55amps I am fitting a Blue Sea master switch and charging relay
so that the batteries only get what they need. Is 55 amps output OK with 2 x 100AH batteries?
More than adequate for starting. Do a load analysis of all the steady loads, radios, radar, lights... The alternator max output should be about 150% of the max static load. Or conversely, the static load should not be over 35A for a 55A alternator.
2. Should I fit a relay after the ignition switch for the starter solenoid? In fact normally only
a small wire? goes to the solenoid anyway.
Depends on what the current is through the ignition switch and how long the wire run is.
I added a
30A 12V automotive relay to my boat because the current through the starter solenoid exceeded the capability of the keyswitch, which if I recall was only 2A.
3. Should I fuse the batteries direct on each with a 100amp fuse, is it absolutley necessary?
No fuse(s) in the the starter circuit. Wire Battery+ to Switch to starter solenoid with #2AWG. Wire engine block to Battery-
One big inline fuse (60A) near the batteries in the wire to the main fuse distribution block usually located in the cabin or in the driving console.
4. Should I fuse the ignition switch/circuit (before or after the switch?) and what size fuse please?
Wire diameter is chosen based on the current that the wire carries.
The end of the wire that sources current is the one that is fused.
Fuse rating is based on wire diameter (not the load requirement).
Specific to the starter circuit: Is the solenoid the type you apply 12V to get the engine to crank, or is it the type that you apply a ground (oV) to get the engine to crank?
5. The Navigation lights will have LED bulbs and so will draw little. They like other equipment will be
wired from the domestic battery +ve bus bar to a switch panel with switches and circuit breakers.
This is how I have configured it
Switch 6, Two way 10amp CB Navigation lights x 4 or x 6
Switch 5 One way 10amp CB Anchor light
Switch 4 One way 5amp CB Panel lights (Probably LEDs inside the instruments)
Switch 3 One way 10amp CB Interior lights (LEDs)
Switch 2 One way 15amp CB Deck mount spot light (Bright LEDs 18w)
Switch 1 Push only 15amp CB Horn 5amp
I don't have a choice of Circuit breakers they came with the panel would it be a good idea to add fuses
of less amps in some cases and where?
6. The horn is only small and rated at 5 amps do you think it needs a relay?
No, but check the horn button current rating...
7. The instrument panel and gauges will be wired on the ignition circuit from another +ve (ignition circuit)
bus bar should I fuse the instruments, if so where?
My boat fuses every branch circuit except the starter.
8. Last one.....What cable sizes should I use please. For Starter motor and to +ve and -ve bus bars,
ignition switch to solenoid, general wiring nothing much over 5 amps Is it better to have a cable the next size up?....
Wire diameter is based on run length, current carried, and how tightly the wire is bundled with other current-carrying wires. Use a
wire table like this one to size the wires. As I stated earlier, the starting circuit (including the ground connection from engine block to negative poles on the batteries) requires a minimum of #2awg. I use #0awg for the Chev V8 in my boat.