Cupping is a curled up lip @ only a few degrees (around [3] I suppose). This helps to hold water on the blade when it is running out of the water a bit, and adds a theortical increase of pitch around (+1) inch.This is to simply increase efficiency with less slip. This is not to be looked at as a speed increase, but as an RPM reducer of approximately 200 RPM. Double cupping is something that is not often seen, as it is a very aggressive cup that can be up to twice the standard. It would be a good idea to discuss it with a propeller expert from a reputable shop, if one feels that a regular cupping isn't sufficiently doing the job. Double cupping can "otherwise" have a very negative effect on performance. I hope this answer is accurate.
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