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Keel rollers come in 8", 10", and 12" I believe.

12" keel rollers would be ideal but not sure if they are overkill for a 13' Whaler or not.
I don't remember what I had on my 13' but all the rest of my boats had 12" keel rollers.

See our article here:
http://www.whalercentral.com/articles...cle_id=117