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Our 303 came with an old Lowrance combined unit where the fish finder no longer works so I am thinking of pulling the whole ting out and replacing it with a mounted iPad if it fits under the windshield. Has anyone else done this and can offer some suggestions?

That might work for basic navigation if your iPad has GPS and there's good data signal wherever you go, but I'd strongly consider instead installing a modern Garmin or other brand chartplotter, preferably with an in-water transducer. First, the unit will be designed to survive and be useful in a marine environment under less-than-ideal conditions (storm, rough water etc.). Second, you will have more useful information like actual water depth, water temp, sonar displays (depending on what unit and transducer you get), anchor drift alarm, etc.. We use the sonar and anchor-drift alarm a lot more than I thought we would, and our boat is only 22'. With a larger boat those are going to be that much more valuable.

"Has anyone else done this and can offer some suggestions?"

Yep. Works great. About $200 for the entire set up. Loaded Navionics app. Tablet needs to have GPS (signal never an issue). The only downside is that tablets (with Nav) are power pigs.