An oar needs a pivot point on the gunwale, and there are three simple choices: thole pins, wooden pegs driven into the gunwale with the oar resting between two of them; metal oarlocks, U-shaped fittings that swivel in a socket, salvageable from any recreational boat; or a rope grommet, a loop through a hole in the gunwale. The pivot carries more than the rower’s pull, 1.44 times it in the worked example, and because bending stress in a pin goes with the cube of its diameter, a 3 cm pin takes about 42% less stress than a 2.5 cm one.
Part of Canoe & Rowing Boat Construction
Oarlocks and thole pins are the fittings on a boat’s gunwale that hold an oar and let it pivot.
The Pivot Takes The Rower’s Pull Plus The Blade’s Push
An oar is a lever pivoting at the oarlock. The rower pulls on the handle with one force and the water pushes back on the blade with another, and the pivot carries the sum. In the drive, the boat is pushed forward through the pivot, so the forward pin takes the load; the after pin keeps the oar from sliding aft and holds it on the recovery.
The carrier lists three fittings. Thole pins are simple wooden pegs driven into the gunwale; the oar rests between two pins, or against one with a rope loop. They are the simplest to make. Metal oarlocks are U-shaped fittings that swivel in a socket in the gunwale, found on any recreational boat. Rope grommets are loops of rope through holes in the gunwale; crude but functional.
What You Need
- Thole pins: hard, straight-grained wood such as ash or oak, shaped to a pin
- Or metal oarlocks and sockets, with bolts
- Or rope for grommets, and a drill for the gunwale holes
- Leather or rope wrapping for the part of the oar that bears on the pivot
- A stop (a collar or a leather button) on the oar, so that it cannot slide through
Method: Fit And Test
Step 1: Decide the position. Place the pivots at the gunwale at the rowing seat, where the oar has the ratio you chose in Oar Construction.
Step 2: Make the pin or fitting. Drive thole pins firmly through the gunwale, fit sockets into reinforced wood, or drill holes for grommets; the gunwale must be sound.
Step 3: Set the pin spacing slightly wider than the oar shaft at the bearing, so the oar swings freely but not loosely.
Step 4: Wrap the bearing area of the oar with leather or rope to protect against wear, and fit a stop.
Step 5: Load test by pulling on an oar from the seat, with the boat moored, until you are sure nothing moves.
Worked Example: Loads On A Thole Pin
From Oar Construction: an assumed 200 N pull on the handle, 0.70 m inboard and 1.60 m outboard. The blade force balancing it is 200 × 0.70 ÷ 1.60 = 87.5 N. The pivot carries the sum: 200 + 87.5 = 287.5 N, which is 287.5 ÷ 200 = 1.44 times the hand force.
If the pin projects 0.12 m above the gunwale (assumed), the bending moment at its base is 287.5 × 0.12 = 34.5 N·m. For a round pin of 2.5 cm, section modulus π × 0.025³ ÷ 32 = 1.53 × 10⁻⁶ m³, and stress = 34.5 ÷ 1.53 × 10⁻⁶ = 22.5 MPa. For a 3.0 cm pin, modulus 2.65 × 10⁻⁶ m³ and stress 13.0 MPa. The ratio (2.5 ÷ 3.0)³ = 0.58, so the thicker pin sees 42% less stress. Compare these figures with a bend test as in Mast Construction. The 2.5 cm pin’s 22.5 MPa exceeds the 15.6 MPa in a 4.5 cm oar shaft at the same pull, so the pin is the likelier part to break: use thick pins, a short projection, or a reinforced gunwale.
Common Mistakes
- A pin driven into a thin gunwale without a block — the gunwale splits.
- Pins too tall — stress rises with height above the gunwale.
- No leather on the oar — it wears through at the bearing.
- No stop on the oar — it slides out and is lost overboard.
Safety
Lost oars and loose pins
An oar that slips from a broken pivot may be lost, leaving the boat without propulsion. Tie each oar to the boat with a lanyard, check pins and sockets before every outing, and carry a spare oar or paddle.
What’s Next
- Learn next: Rowing Technique — using the fittings well
- Combine with: Oars & Rowing Technique — the whole propulsion system