+ A boat is safe in the harbor. But this is not the purpose of a boat. 
Marlin Ledin sailed 3,000 miles across Lake Superior last year on a small sailboat. A few weeks ago he sent along some thoughts on our moorings based on his experience. They turned out to be rather timely. Yesterday afternoon a storm from the west pushed against our 13’ Whaler with such force that it pulled its 80 pound iron mooring from it’s rest and dragged it across the shoreline. Lake Superior 1, Rabbit Island 0. 
There is all sorts of tackle out there to make mooring better for you and the boats. In your situation I would use three strand Nylon 5/8" line with 10-20 feet of 3/8" chain, and a “rubber snubber”. Use a large swivel shackle–3/8" would be nice–and make sure you have metal thimbles where the rope meets the swivel and chain. This is overkill, but overkill equals zzzzz’s. You could take all this a step down to ¼" (chain, shackles, swivel) and probably be fine too, and you’d pay less. I might also recommend dropping your mooring further out from the shore this year. Proper scope for great anchoring is 7 to 1. That’s seven times the depth. 10 feet of water equals 70 feet of anchor rode. With a bomber mooring and chain, snubber, and swivel shackles you could probably get away with 5 to 1, especially since your boats are light. And don’t forget your swing circle. 70 feet of rode equals a circle 140 feet in diameter.

That would be the best combo I can think of. The next best thing relates to having a better anchor point, which basically means more weight on your iron rims.

Over and Out.
Marlin Ledin, Bayfield, Wisconsin