Before you install; manually place the FF adjacent/around the compass...see if it affects the card.* I'll bet it doesn't.* Turn the FF on.* That 3' length seems to be a catch all distance, for all manufacturers...good luck!* Installing a VHF would be another thing, it has a magnet in the speaker.*
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Boston Whaler, "MUMBLER", 24 Outrage, twin 175 HP Evinrude Ocean Pros