ONE OAR AT THE STERN WORKS LIKE A WING

Kayaks, canoes and rowing boats push water mostly by drag: the blade is pulled through the water, then lifted out for the return. Stern-sculling does something else. A single long oar rests in a notch or oarlock on the transom, its blade always underwater. The sculler, usually facing backwards, sweeps the handle from side to side, the hands tracing an infinity sign “∞”, while twisting the oar about its own axis to set the blade’s angle. The technique is still practised in several parts of the world, but it had been studied mainly by ethnologists and biomechanists. G. Roudaut (Tud Yaouank Ar Mor association, Lampaul-Plouarzel), B. Augier (IFREMER) and M. Rabaud (Paris-Saclay University and CNRS) set out to look at it as a problem of fluid mechanics.

An oar full of sensors

Their test boat is 4.74 metres long, 1.63 metres wide and weighs about 100 kilograms empty, with a long shallow keel — stern-sculling works best with a fairly heavy, keeled boat, because the oar pushes the stern alternately left and right. The carbon oar is 3.62 metres long, with a one-metre wooden blade whose cross-section is symmetrical left to right and very slightly curved.

Gauges measured the force of the hands on the handle and of the notch on the oar. Two motion sensors followed the oar and the hull, and a paddle-wheel log, calibrated by GPS, measured speed. Because the oar is light, the forces on it almost balance, so the push of the water on the blade could be deduced from the two measured forces. Two experienced scullers each took the boat on calm water from rest to the fastest speed they could reach.

A wing, then a shove

The stroke has four phases: a sweep to one side, a quick twist of the oar, a sweep to the other side, another twist.

• During the sweeps, the water meets the blade at small angles and flows smoothly over it. The blade works like a wing, and lift drives the boat. A calculation slice by slice along the blade, using a standard wing-profile program, gives a push of 35 newtons at one instant, against 31 newtons measured.
• During the twists, the blade briefly faces the flow. Ordinary drag would give only 2 to 10 newtons, but the team measured 20 to 45. The difference is added mass: the blade accelerates hard and must set a volume of water moving with it. Their estimate, about 70 newtons, is enough to explain the measured force.

All four phases push the boat forward, even if the thrust dips close to zero twice per cycle. At 1.2 metres per second, the average push was 19.4 newtons. The blade’s left and right edges take turns as the leading edge, which the authors call unique compared with propulsion in the animal kingdom, where fins and wings generally keep the same leading edge. In the wake, the vortices shed at each reversal form a staggered street that carries water backwards.

65% at cruising speed

Dividing the power delivered to the boat by the power the sculler puts into the oar gives the efficiency. It rises at low speed, peaks at 65 ± 5% around 1.2 metres per second, then falls to about 25% around 2 metres per second. For comparison, the authors cite about 75% for a marine propeller at its design speed and for the best classical rowing.

Two things go wrong at speed, they explain. To keep a good angle on the blade, the hands would have to move sideways faster than is comfortable for a human body. And it becomes hard to pull the oar back quickly enough during the twists — even risking lifting it out of its notch. Meanwhile, the hull itself nears its critical speed, where making waves becomes the main drag: the power needed rose tenfold over the range tested, and the scullers could go no faster despite producing about 300 watts.

Room for a better oar

The team tested a deliberately stiff oar to keep the measurements simple. Real scullers often prefer some flex, and the authors suggest that a flexible blade, a longer oar or a blade curved downwards could hold better angles at speed. Designing the ideal oar for a sculling boat, they conclude, “involves many trade-offs, including the power generated by the sculler.”