These plans are for an add-on dagger board and sail to convert a rowboat to a sailboat.
From the plans:
Equipped with this inexpensive portable sail rig any rowboat becomes a sailboat. While dimensions given in Fig. 2 are for a small boat having a beam of 4 ft., the length of the thwart may be varied to suit the boat at hand.
With the exception of the rudder and tiller, which should be made of oak, 3/4 in. pine will do for the leeboards and thwart.
Begin by making the thwart. Two pieces of 6-in. stock, cut to the proper length and mitered 22 degrees, are held together with a notched cleat fastened to the underside with 1 1/2-in. brass screws as in Fig. 1. The forward piece which rests on the breast hook, is attached to the thwart with a large T-hinge. The barrel of the hinge should be fitted with a removable pin so that the assembly may be taken apart easily for storing...