If their plastic they can break off (under the water line) and flood the boat. I have metal and once a year check them.
P/s use a check valve on the thru hull fitting, I put them on all mine.
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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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