I grow my vegetables on my rooftop due to no light at ground level. I use a soil-less medium and grow watermelons. I only get one watermelon per plant. The plant stops growing when a watermelon forms then the plant dies when the watermelon is ripe. I can not answer your question as I don't know. How did you melons do. Did they all ripen.

I can tell you this that a watermelon vine can support a 22 pound watermelon hanging from the edge of the roof. The picture in the link below is ar 12-15 pounds

Here is a link that might be useful: https://plus.google.com/photos/111099372377958308731/albums/5864651368775846321/5896464462669765506?banner=pwa&authkey=CI763rvAj8eLAg&pid=5896464462669765506&oid=111099372377958308731

I'd like to know too.