Most modern wooden hulls are constructed with a technique called cold molding which uses epoxy and multiple thin layers to achieve the desired total thickness. This allows for complex shapes and results in a stable material that resists swelling and shrinking. Usually the inner layers are cross-diagonal and the outer layer is laid to look like traditional planking.
All good unless you don’t get uniform clamping pressure. In this case it resulted in a visible bubble with a hollow void behind and cracking. Luckily this was below the waterline with no rot so we don’t have worry about the cosmetic issues that a similar repair under the varnish would involve!
You can see the trowel marks in the dried epoxy where it never made contact with the outer layer. Next we set the depth a hair deeper to remove the dried epoxy, clean up the patch area and shape the insert.