Check out this site, They have apps for everything including marine. Scroll down until you get to the article in Southern Boating. One type in here is a matting you can glue in and it reduces noise by 26 db. That is what most decent industrial ear muffs are rated. I use a set that is 25db when I cut grass or weed eat and they really reduce the noise.
http://www.acoustiblok.com/marine.html
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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
