On powering the boat, with an outboard from that era, I had some experience with the OMC line you may find useful, if you want to put on a motor from that time. I bought a 16' Glastron at the end of 1976 (not a GT-150). It was rated for 100 HP. I would have put the Evinrude 85HP on it, but the dealer was out, so I went with the 75HP. The top end speed was fine, above what I would cruise at. Early the next summer, when we first used it for skiing, I was terribly disappointed in the pulling power of the 75. We changed props, from 21 down to 19-pitch, which had the engine WOT maxed out at 5800 rpm at light load, and lost perhaps 2 mph top speed, but the low-end pulling power for skiing remained less than satisfactory.
Two years later, OMC filled in the big HP gap in the V-4 lineup between 85 and 115 HP, adding a 100 HP model, and I upgraded. What a difference! While that little 10 HP difference in top-end power added little to maximum speed, the low-end torque was a night/day improvement over the 75. That 3-cylinder 75 began its history as a 50 HP engine, improved over the years to 75 HP by carburetion. The displacement was 49.7 cu.in. The V-4 series (85, 100, 115, 130 or 135 HP) block had a displacement fully double (99.7) that of the 3-cylinder 75, which gave the engine that enormous pulling power at low rpms. Something to think about when you pick a power plant, depending on what you want to have.
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