The tramp is too tight when you start pulling grommets out.
Honestly, I don't think having a tight tramp really makes that much of a difference on the 18 as far as performance goes, but I can't stand having a sagging trampoline. From a racing standpoint, I think at the very least, having a tight tramp adds to the psychological aspect of knowing your boat is as tight as you can make it. Plus, there's nothing worse than sitting in a puddle or having your butt drag on the water. So for my racing boat, I pull the sucker about as tight as I can get it. It sounds like a drum when I hit it with my hand. You definitely run the risk of pulling out grommets with the tramp like this, so you may want to consider adding double grommets to the center lacing. Also keep some 1" wide nylon webbing handy with a needle and thread. You can easily repair or reinforce pulled out grommets by hand sewing a piece of webbing using a few rows of bar tacks, I've done this several times.
I find the best way to avoid pulling out grommets is to tighten the tramp incrementally. Lace it up and pull it snug. Then go back and do a second round of tensioning to pull it real tight. This way you minimize overloading any single grommet. I also find that the best way to get the tramp good and tight and to minimize wrinkles or pockets is to tension the rear lacing first, then do the center lacing.