Depending on how ruff, it can be anywhere from 1-3% of the speed but where it really hurts you is fuel burn, that can be anywhere from 5-12%.
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Tis better to be quiet and thought a fool than open your mouth and prove it!!
1991 V-20 cuddy I/O 350 volvo duo prop, 1998 15ft Grumman 9.9 Johnson 
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