im MartinB

I've been doing some significant changes to my 31 Sonic. Ground down both drive bullets... took off a lot of material and fared them up. Brought both drives to a nice pointed edge.. this affected the feel a lot.. very crisp feeling after this was done. Removed roughly 3/8" hook from strake to strake. Sharpened all the transom lines. Removed 1/8" hook from lower strake ends. Smoothed out bottom of both trim tabs.. heavy orange peel. Took every thing out of the cabin of the boat that was loose or could easily be removed including the quite heavy modular center sliding cushion. All said and done.. Top speed was unchanged. Top speed stuck at 85. So far the only thing that's been noticeable was the sharpening of the leading edge of the drives. I only know this because I intentionally did this last.
Then I spent some time measuring the toe of the lower units. I found 28 1/2" space forward and 27 1/4" at the rear. This had me excited. I added and inch to 28 1/4" and saw no gain, but the boat did seem to react a little less to trim, and top speed was achieved with less total trim. To my way of thinking, if the 2 drives are not exactly parallel they will be adding transom lift which will in theory give you a wetter bow. I plan now to apply the tension pushing the rear of both drives together and then adjust them to be perfectly parallel.

im MartinB

So I finally got it back on the water with the drives set to exactly parallel. Talked with the guys at Hering and they said my 5 blade props spinning out would be pushing the water towards the center and I should go slightly toe out to compensate. I used a strap and with a fare amount of tension the drives didn't move in either direction. I do have all new hardware so maybe Merc has improved on their transom assemblies. Anyway no change in speed but my slip improved by a decent amount. Same speed but 200 rpm lower. Conditions were not as favorable the lake was not very choppy and I had a 3rd person on board so maybe running the exact same top speed means I did improve slightly.

Twin O/B Sonic

Great info, I love it!

Thx for taking the time to post.

Validates what I found decades ago.