The current Navionics are the Navionics + cards which have all the US and Canada preloaded ...
The Sonar Charts layer can be added to the Navionics + upon registration and update within the 1st 60 days of purchase ...
It has been reported that the amount of data already preloaded onto the Navionics + card will slow down the smaller units due to lack of processing power in the smaller units ...
It has also been reported that the preloaded maps can be deleted and then only load the lakes needed during the 1st update of the card ...
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If you're looking for a used card, Navionics Premium regional cards (discontinued) will also work in the 597 (but have also been reported to slow the unit down due to the amount of mapping data on the card) ... Un-needed mapping data cannot be deleted from the discontinued Premium cards to make them perform more efficiently ...
The current Navionics + and then delete data/reload only lakes needed wil get the best performance in the smaller 597 processor unit ...
Rickie