I never really had an issue with porpoise before I pulled mine apart, but it had a stock 460, I did flip it but I didn't change the bottom at all I just repaired anything that wasn't perfect and smoothed the intake entry. I am curious to see how mine will act as I set my pump back a few inches to where the hand hole cover is half in half out of the boat. I have a shoe, ride plate setup. Lots of good info In here, I hope you get yours ironed out!
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I don't remember a porpoise issue when it was a 60 MPH boat either. However, when I increased the HP a little bit it started porpoising. It's not really bad until the speed catches up to the RPM. I just think the hull was designed as an efficient hull for stock power. I don't think Southwind thought that boaters would be putting big power in them especially when they have quarter stringers (though I have heard the earlier 18's had full stringers but I have never seen one). There's a lot of lift with the large strakes and the center most lifting strakes ending halfway through the intake. If you look at a CP the strakes are much smaller but the innermost strakes do go all the way to the transom. If you look at the Rogers, Hallett and Bahner hull the innermost strakes all end before the intake.
What is interesting is that the Miller hull has a bottom very similar to the 18 Southwind. There is a really nice black and blue 81 Miller out there that had big power (900 HP) that ran really good but did have a hop out on take off but it didn't really seem to porpoise. Chris at B1 did the pump and Todd at TRG did the bottom. I wondering if the bottom had a 1/16 to 1/8 hook in it to cure the porpoise deal.
I have been reading about the Southwind porpoise issue for years and have collected information about it over the years. Here's a couple things the pros have been saying:
Jack at MPD posted that with the Southwind hull you should leave 1/16" to 1/8" of hook near the back of the hull. He said these hulls like it.
Jeff Bennett posted that the Southwind and all of its derivatives are very good boats, but often porpoise. That problem gets worse as speed increases, so that high horsepower versions are especially difficult to deal with. This means from my perspective what the hull needs is more than a blueprint, but a modification, which is substantially more difficult. Greg Shoemaker and I came up with a combination that resolved the Southwind porpoise last year that involves both a hull modification as well as a bolt-on change (Cavitation Plates). If you are going to work on the bottom, you might as well get it right on the first shot. Because of the complexity of this job, you might consider giving Greg a call.
