This sounds exactly like what is happening to me. I'll get a nice hole-shot and just as soon as the bow starts to bow down, the "blowout" occurs. I do have thru-hull exhaust.
Dumb question, but what exactly are the exhaust bellows? Those butterfly-looking valves that redirect the exhaust on my captain's call? We might be getting somewhere here.... Thanks for the idea'rs, guys...
Ok, if you have "captains call" exhaust then you need to replace the bellows with a exhaust tube. This is called for in Mercruisers service manual. The tube might be there if the captains call was installed at the factory. Tilt your drive up and look underneith of it, you should see a large bellows thats the u-joint bellows and the lower one is the exhaust. The small one on the right side has the shift cable going through it. If you see a "tube" that is open and seals off when the drive is down then you need to look at a diffrent prop that prevents blow-out?