Resizing Winches

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I have been sailing on a Columbia 50' for the last year or so, and recently we have replaced the old 155% Jib with a much larger 170%. The sail works great, but the old winches are pretty slick, and even with the maximum wraps we can get on the drum the line still pulls to hard on the sheet.

The owner and I were talking about replacing the winches, but aren't really sure what the best option would be. Any thoughts would be appreciated.