The old rule of thumb for cleat size is 2" for every 1/8" of rope diameter. So for 5/8" mooring lines the smallest cleat should be 10"; for 7/8" it should be 14".
Boat: Research vessel for a university, retired now.
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Re: Large cleats - Where to find?
I don't blame you for not wanting Chinese. Their metallurgy is always of question. A large cleat that is under load from a stretched out nylon line can be deadly shooting back at your face.