This subject comes up a couple of times a year in this and other tug/trawler forums. Since I have the "notorious' Nordic Tug, I'll add my 2 cents.
Nordic Tugs, a semi displacement hull with hard chines, does have a notorious rep for hull slap. The first night on the mooring, I knew I had to do something. Canvas wave slap preventers, which I made up, do work (picture attached), but a pain to deploy, and a safety factor when anchored out (not to mention embarrassing). Putting 4" of sound deadening foam panels inside the hull muffled the slap, but did not eliminate it. I took the plunge and filled in the chines... a combination of bonding in CoreCell foam panels, covered with a couple of layers of carbon cloth and layers of epoxy resin. Problem solved, with no impact on performance, lift, or wave deflection. I know several other NT owners that have filled in chines. The chine area filled in on my 18 knot top-end hull is several inches aft of where the water meets the chine and going forward for about 48". As the hull lifts up above 7-8 knots, the filled area is out of the water.