VERY common problem on QMs and QM control panels --- HIGh resistance in the primary circuit wiring ... including the primary circuit's engine mounted FUSE holder (normally located on the port side rear of the engine on the flywheel housing).
The fuse is the most common high resistance culprit - corroded connectors on the fuse holder. Corrosion in any of the connectors along the whole primary circuit can cause the HIGH resistance, hence voltage drop. Other 'famous' sites: the starting push button - build up of carbon deposits on the contact surfaces - clean or replace. Any high resistance will develop low voltage and HIGH amperage requirements and the high amperage will 'char' the contacts of the keyswitch and panel start 'push button'.
Suggest you first clean or replace the fuse holder assembly. Then starting with the key switch check the resistance / voltage drop across each component, and wire until you get to the solenoid primary connection - replacing or cleaning each component as necessary. You will probably have to do this checking/cleaning/replacing on a regular basis as the WHOLE control panel and primary circuit is NON-tinned wiring. For 'emergency starting' to deliver correct voltage to the solenoid install a jumper + push button between the (+) HOT terminal on the starter and the primary connection to the solenoid ... and when the Yanmar control panel (yet again in the future) corrodes and develops high resistance, simply locate the emergency starter button and energize to start because in an emergency you dont have the time to 'futz around' tinkering with corroded and high resistance components or even cobbling together a 'jumper' (or screwdrive) across the solenoid terminals to engage the starter.
For best results .... Rebuild the WHOLE control panel with tinned wiring and tinned connectors, etc. if you want to completely remedy the situation on notorious Yanmar control panels .... and put a remote 'push button jumper', all set and ready, to 'energize' the solenoid + starter as 'extra insurance'.
