Andrew,
In case you are unable to find a copy of the 3GM30F service manual, which should have exploded diagrams of both the original Yanmar-suppied relay and starter motor, or in case these original components were replaced by different after-market components, you could simply treat the solenoid/starter motor combination as a "black box" with two electrical current inputs (to solenoid and starter motor, respectively), one electrical current output (engine and/or battery ground) plus a single mechanical output. In this situation you are only concerned with the solenoid current input. Take the one wire that runs from the wire harness to the solenoid's only external connection point, cut it and connect the harness end to the low current side of the relay you are planning to use, while connecting the solenoid end to the high current side. Then make a reliable, ample current connection between the battery plus and the remaining high current post of the relay while attaching a reliable ground wire to the remaining low current post of the relay and you should be all set.
Before putting in a relay in line with your solenoid, however, you may want to take into consideration that:
(1) your solenoid is itself a relay, meant to operate on the relatively low currents that can be provided by relatively long and thin engine harness wires;
(2) if it fails to do its job properly, the "black box" either has developed internal problems or, more likely in a marine environment , not enough current is being provided by the harness because of corrosion problems and/or poor connections upstream; an
(3) therefore the extra relay is either not going to help at all or only provide temporary relief (till the corrosion and/or connection problems become bad enough to provide insufficient or intermittent current to power the relay).
If and when these future problems arise, you have now managed to add quite a few more wires and connection points that can give problems and thus need to be maintained and checked; thereby violating the time-honored "KISS" principle of cruising.
After having operated our Hunter Legend 43 in the tropics for well over a decade, including the use of temporary extra solenoid relays if I was either too lazy or pressed for time to try and solve the real underlying problems, I fail to see any long-term justification for such relays unless it can be clearly shown that either the manufacturer underdesigned the engine start system (e.g. with regard to harness wire dimensions) or that later modifications and additions (e.g. power-hungry pedestal devices) severely degraded the amount of power available to the harness.
Best of luck;
Flying Dutchman
