Hi Tom,
I ponder over the affectivity of outboards v diesel, 4 stroke v 2 Stroke all the time, the guys running Diesels seem to be catching fish all the time but is it the motor or are they out there twice as much being more dedicated to the sport?
My experience with 2 strokes and Harmonic's has lead me to the conclusion that you just have to be where the fish are in the first place to raise them. Our Surtees is running a Evinrude ficht that is somehow still going after a thrashing 13yrs later. It trolls at about 6.5-7.5 knots and 9-10l an hour.
It took me a few hours on the water but the first fish I ever hooked on it was trying to eat the bungee 6ft from the transom, You could have gaffed it! We had a double up this year that also had a fish right in behind the boat so they obviously don't mind the sound, but I definitely wouldn't call the boat a fish magnet. I say all this as I'm currently looking a getting a 4 stroke bolted on the back! The hours on the water are just some much more comfortable without the old two strokes banging away in your ears.
I had a little stabi with a Mercury Opti for last summer, only did one day on it out the Manukau but bagged a fish slow trolling a livey so they mustn't mind the Optis either.