archboard

"You can punch your fist through the archboard; the last dozen feet of the stringers and shelf are soggy like a bad potato.

The day after they reached 'The Corner' he would inspect the ship from breasthook to archboard; he would exercise them aloft and at the guns with a watch in his hand.

The geometric line which stretches from the wing transom to the archboard is called the counter; a large vessel may have two such counters, called a lower counter and a second or upper counter.