A assume you are talking of the Yamaha 9.9 4 strokes, if so I have had a few of these. I never had any problem with the multipin connectors on my catamaran
but did give them a spray with contact cleaner and silicone grease from time to time. They were almost constantly wet when sailing, the connectors looked mickey mouse but worked.
You are right that the charging
output goes through the starter motor cables
so the loom does not need to take that current
. My guess is that the biggest current
draw will be to energize the starter solenoid. No idea of the current but guess in the region of 5A for a short period so the cable will not burn out. The only issue I can think of is that the volt drop across your very long loom plus the volt drop on the battery
from the starting current could cause the starter solenoid to drop out giving you a succession of 'clunks' but no start. The only other significant current draw is the electric
choke if you have that version, but I cannot believe that would be a problem.
Trailer multicore cable is usually plain copper multi strand and that will not last in the marine environment
, it will rapidly corrode inside the insulation
and prove very difficult to maintain a good connection. You need tinned copper strands and good termination.
You could always make up your own cable with cable sleeving and 7 cores of 2.5mm cable. In the UK I have sourced 2.5mm tinned copper conductor cable with a thin sheath. The other issue is the connectors. High current, marine
proof multi-pin connectors that last are not cheap
. It may be more worthwhile trying to source the Yamaha bits on ebay.