Thy tacklings are loosed. The comparing of God to a river has led to the representation of Judah's enemies as warships (Isaiah 33:21). This causes Judah herself to be viewed as a ship—a badly appointed ship, which has to contend with one whose equipment is perfect.
The prophet's thoughts have traveled back to the existing state of things. They could not well strengthen their mast; rather, they cannot hold firm the lower part of their mast. The mast had its lower extremity inserted into a hole in a cross-beam, and required to be kept in place by the ropes.
If they were loose, it might slip out of the hole and fall overboard. They could not spread the sail; rather, they cannot spread the ensign. The ensign would seem to have been attached to the top of the mast.
If the mast fell, it would no longer be spread out, so as to be seen. Then is the prey of a great spoil divided. The word "then" is emphatic. Now the disabled ship seems incapable of coping with its enemy.
Then (after Assyria's overthrow) Judah will obtain an immense spoil (see Isaiah 33:4). Even the lame shall have their portion.