Totally disagree about Dart 15s, 18s being poor. They're a brilliant design for an off the beach cat that's basic and trouble free. Same goes for the Topcat and KL/SL ranges. Even the flexy Dart 16 is a good boat at what it was designed for. As a result you get the sportier but essentially similar Topper Topaz 14 and 16 cats which are fantastic trainer/fun catamarans. These boats can not be classed as slugamarns.
The Aquacat is the obvious 'slugamaran' as is a UK built boat thankfully no longer available called a 'Peanut'. 10ft long, 6ft beam, 2 part mast with sleeved mainsail and aluminium tube side stays. A tripod rig like the Blokart. They were truly awful. Built by Zygal boats in the mid 70s, they travelled at the same speed sideways as they did forward, throw in a bit of tide and it made heading for any fixed mark an absolute lottery.