How can you find your max useful load ?
I doubt if you are going to overload an 18 footer with a cabin with people in the cockpit, you will run out of room to work the sheets first. If you are out cruising for a week with 5 others, carrying water and provisions...The other wild card is the wind/wave action, boat heel depending on sail plan and how tender the hull is comes into play. Without seeing the hull and doing the usual seaworthiness numbers to get an impression of what they''ve built, there is not likely an answer. I watched a San Juan 21 turtle in high winds because it had two crewmembers out on the deck together, taking in the laundry, and when she heeled, they both grabbed the boom and mast, and over it went.