"Ring main" of 5v USB sockets for the boat

[BabaYaga]

I think if OP tries to produce his own 5v ring system for his boat that the actual USB sockets are difficult to get difficult to connect to and as said there are tricks to the wiring. All this will force him to go for the cheap and available 12v cigar lighter socket type converter with USB socket. olewill

While I agree that buying ready-made individual good quality USB converter sockets is far easier, faster and probably cheaper than building your own....
...it is after all not THAT complicated.
The OP seems to have abandoned the idea, but for others who might be interested, this it how I did it:

First step is a 12 > 5V buck converter. I chose 10A capacity, which should cover for four or five devices charging simultaneously.

For charging up to 2,1A, each USB socket need to have the following reference voltages on the data pins:
D-: 2,0V
D+: 2,75V.
These voltages can be achieved by a couple of voltage dividers made from a few resistors on a strip board. I used resistors of 22, 27 and 33Kohm. There are calculators online to play with in order to get the correct voltages.
The voltage divider:

I am sure there are more elegant and less bulky ways to build these. But this was my first attempt, so I put these in small casings a few inches from the USB-sockets, which came with a tail of four wire cable already attached.

Bottom line: A bit of work and (for me) a lot of learning, but this home built system has charged Ipads, Iphones and other mobile phones for two years now without any problems. So not a bad winter project.

 

[[163233]]

I've got a USB double socket in the style of a lighter socket - not an adapter. Wiring was just 12v at the back and it does 5a charging, so my phone is toasty warm as I type (USB Cable type). 5v at 4a is 20w or 2a at 12v so you need wiring good for about 3a plus anything else on the circuit.
Not noticed any interference, it does light up though which is annoying.