Here's a close up of my laminate trimmer. The base of my standard Porter Cable router was just a bit large for cutting these port openings. Unless you have lots of flat cabin side real estate on your particular boat use a laminate trimmer. The base of a laminate trimmer is nice and small and it worked very well, on our boat, when used from the outside. The cabin liner on the inside interfered with the base this is why I chose to cut from the outside.
Basically, once adjusted for proper depth of cut, the guide bearing rides along the surface of the template and the blade cuts the fiberglass. It's quick easy and neat but NOT clean.
I advise having a shop vac on board for this job. No matter how you try and contain the dust it makes a HUGE mess. One other tip is to remove your interior cushions before doing this job..
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