I am sure many others will chime in with a similar point of view, but,for heavy oil and/or carrydown, no effective ball goes really long and then hooks a lot in the backend. Balls designed for medium heavy oil can have more length and more backend thjan those designed for truly heavy oil.

Of course, "long" and "heavy" oil are both relative terms. You may mean "long" with respect to your father's length. I am not sure what that is. He may be just too used to his balls and he lives with their length and drive and is comfortable with them. Maybe if he does leave a lot of weak corner pins, he needs just a slight shine (NOT a lot) on one of his balls. Maybe he just nees to move a little deeper.

If you could give a little more detail on what you are now using, how it's drilled, your estimated speed, and rev rate, maybe your father's ball length, under "normal circumstances, all in order to give more relativity and to allow us (I am sure several people will respond to your request for help) to make an educated guess.

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"Those who do not remember history are doomed to repeat it."