After a few years of ownership, my Yanmar 2GM20F began to have intermittant starting problems. Hit the starter, nothing happens. Hit it again, and it always started right up. I posted this issue on the Torrensen site and got this reply, which I have since implemented with great success:

The problem with aging Yanmars not engaging the starter is the wiring. Adding a relay to the starter solves the problem. You can use a continuous duty marine solenoid from West Marine or pick one up at NAPA for the purpose.

The solenoid should have four posts, two high amp, two low amp. On the low amp circuit, connect the starter key wire to one post, and ground the second post. Enaging the key will trip this circuit. On the high amp side, connect a #8 guage wire from the solenoid to the battey/alternator post on the starter motor. Then from the second high amp post run a #8 guage wire back to the post on the starter where the wire from the key originally came from.

Engaging the key will trip the low amp circuit which in turm engages the high amp connection which gives you a very healthy current to the starter solenoid.

Another thing you can do is add a remote start switch (for example near the engine) by connecting a second switch (make sure it is fused) beyween the battery side of the high amp circuit on the solenoid and the key side of the low amp circuit.