The handles of a pair of oars should overlap by 4 inches, that is, the inboard portion of each is 2 inches longer than half the distance between the rowlocks. The distance between the rear edge of the rower's seat and the rowlocks should be about the distance between the inside of the rower's wrist and the inside of the elbow.
An oar is a lever with its fulcrum at the oarlock. The oarlock's position relative to the oar is set by the location of the button, the ring that keeps an oar from sliding through the oarlock. Shaw and Tenney (Orono, ME), one of America's premier makers of oars, recommends a ratio of 7:18 for all boats designed for efficient rowing, from skiffs to shells. To achieve this, the inboard portion of the oar must be 7⁄25ths of the overall length of the oar. So, for example, if the distance between rowlocks is 42 inches, the distance from the end of the handle to the button should be (42 ÷ 2, + 2 =) 23 inches, and the overall length of the oar (23 ÷ 7, × 25 =) 82 inches or 6 feet 10 inches. Stock oar sizes from 6 to 10 feet are in 6 inch increments, so round up to 7 feet.
For a utility boat not really made for efficient rowing, such as a dinghy, round down instead of up. Longer oars will do no good and just be more cumbersome. But in any boat, if an oar continually pops out of the oarlock, the oar is too short.
The point at which the oar would balance if it were supported on a narrow rail should be on the outboard side, but within a foot of the button.
Platt Monfort's way of making an oar out of ordinary dimensioned lumber:
Shaw and Tenney's lovely website:
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Last revised: 20 October 2014.