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If the crack is the wood splitting (normal) I recommend you could do a repair from the back side.

One option is to place a cleat (strip of wood) on the underside that holds the two sides of the board together.

The bow locker in a Montauk uses 2 cleats to keep the boards together.

The key item is to get the 2 pieces as close together before screwing in the cleat. A pipe clamp would compress the wood and allow you time to drive the screws.

Suggest using teak or Ipe or similar wood for the cleat.

Hope this helps.