I have a copy of W. E. May’s book Boats of Men of War.  It is an excellent book that I recommend.  Amongst other items in the book are a fair number of reproductions of draughts of ship’s boats.  One showing an 18′ cutter seemed to have a very small height between the top of the thwarts and the bottom of the chocks for the thole pins.  Out of curiosity, I measured each of the draughts where I could identify the chock-to-thwart distances reliably.  Here is a summary of my results:

Boat Year Length Beam No. of thwarts Double banked? Thwart spacing Vertical height between thwart and chock Horizontal distance from chock center to aft edge of thwart
Barge 1809 32′ 7′ 6″ 7 Yes 3′ 0″ 9.9″ 7.5″
Pinnace 1815 37′ 6″ 8′ 9 Yes 2′ 11.5″ 10.5″ 10″
Yawl 1799 26′ 6′ 6″ 6 No 2′ 7.5″ 9″ 10.6″
Cutter 1764 28′ 6′ 7″ 8 No 2′ 2″ 8.2″ 9.8″
Cutter 1786 28′ 7′ 1″ 8 No 2′ 3″ 9.1″ 10.6″
Yawl 21′ 6′ 5″ 6 No 2′ 4.5″ 7.9″ 11.6″
Cutter 1800 18′ 6′ 6.5″ 4 No 2′ 10″ 6.2″ 9″
Launch 1779 29′ 8′ 7″ 7 Yes 2′ 10.8″ 7.2″ 9.2″
Launch 1804 33′ 9′ 10″ 8 Yes 2′ 9.2″ 8.4″ 9.7″
Launch 1815 35′ 10.5″ 9′ 8.5″ 9 Yes 2′ 11.6″ 10.2″ 10.5″
Gig 1815 25′ 4″ 5′ 7″ 6 No 2′ 7.3″ 6″ 9.8″

Having only 6 inches between the thwart and chock seems too small, even if the rower is sitting on the far side of the boat.