Just saw a pic of your boat on another thread Sitka..nice machine! There is no way, with a boat that size, that 115 hp and 8 hp outboards would equate to a stern heavy situation, that would cause your porpoising problem. No way in the bloody world. Look around any boat yard and show me ANY new 17 or 18 footer that isn't equipped with AT LEAST 100 hp these days, if not upwards of 150 or 175. Forget the weight of the 8..its neither here nor there. I try to stay pretty openminded here Sitka but there are times that I just can't hold back :wallbash: so sorry for the outburst. It is plain bogus advice in my opinion and doesn't help you at all to suggest that the outboard (s) you have are too heavy for that boat. What are you supposed to do now? Sell the boat or sell the big one and buy a 75 or 90 hp? Go ahead and do that and I virtually guarantee the porpoising will still be there.
I don't know if you have fixed the problem or not but if its not the obvious of incorrect trim angle on the 115, I would bet with all the pennies in my savings, it is a small hull defect that can be easily fixed. I would take it back to the dealer, if you haven't already, and they can ask for factory assistance if need be to correct it. In my humble opinion, I don't believe trim tabs are the way to go either. Trim tabs are normally used on heavy boats to (a) get them up to plane faster and (b) once on plane, to adjust the angle of the bow to optimize bow angle for either planing trim or sea conditions. If you've ever boated in open sea conditions (ie west coast Vancouver Island or the Charlottes for instance), you will appreciate how trim tabs can be adjusted to help soften the ride.
I once had a 26' Bayliner with a 396 that was an absolute pig to get on plane. I like pigs generally so don't get me wrong there but this one was getting me down. Depending on the sea conditions, it would sometimes seem to take forever. That old sow would raise the nose with full throttle and literally minutes would go by before the nose evened out (well seemed like minutes!). Installed the trim tabs and that baby would lift..smell the salt air and drop down ever soooo nicely :thumbup: Oh I love pigs! Trim tabs are expensive, ugly on most boats and I think in your situation are totally unnecessary.
Like I said in an earlier post (and maybe I was just plain unlucky) but I have had 2 boats (among too many), a 15' Harbercraft prop and the 20' North River jet I now have that had porpoising problems that were cured by small adjustments made to the stern of the boat. The main point I am trying to make is and this is from my experience only (so take that for what its worth) ...YOU DO NOT HAVE A WEIGHT DISTRIBUTION PROBLEM! I would bet my whole life's experience in running boats (from 12 footers to 85 footers) on that. I don't know at what speeds you tested the boat and at what point it started to porpoise but, in the experience I've had, porpoising motion (once the boat was on plane) increased with either sustained running at one speed or with an INCREASE in speed. In fact, what would start out as very mild porpoising would increase in intensity when the speed remained the same over time or if speed was increased to the point that you'd have to throttle back and start again. As a side note, I had a truck once that although it didn't 'porpoise' as such, built up a harmonic motion (light bouncing) in the front springs on speeds between about 25 mph and 29 mph, that would increase in intensity if you drove too much at that speed. Increasing or decreasing speed below or above 25-29 would make it go away but its clear the harmonic motions exhibited by porpoising boats are not limited to boats.
I'm sure alot of us have seen those boats just pounding..err porpoising over the water. I just shake my head at the harmonic motions of those boats and feel for those people because they don't know how easy it can be to fix the problem. Don't be discouraged Sitka..get it checked out by the dealer or the factory if need be and it will be fixed. That I can guarantee and you will be a much happier boater :beerchug: