Full size outboards don't use flex shafts. Flex shafts need to be supported all along their length to stop them being turned into a pretzel, that is what the stuffing tube is for in most cases with model boats.
Many people do run an oil/grease seal on the inner end of the stuffing tube, which is easily made by slipping a short length of silicone tube, or heat shrink tube over the end of the stuffing tube so it grips the stuffing tube firmly, and just touches the flex.
Paul Upton-Taylor, Greased Weasel Racing.
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