Julian Coates wrote to the Editor after reading a draft the article about driving buses.

** John Coates - the Grandfather of Sarah, Ellie and Isabelle Coates - see photo on page 10). He was one of the inspiring forces behind the building of the replica Greek Trireme. When designing the boat, John borrowed oars from MCSBC, to check measurements using the first mock-up section of the Trireme in his back garden. A section of Trireme is now housed in the River and Rowing Museum at Henley. Well worth a visit! Two Monktonians, Pete Askew and Adam Howard were in its first crew. See Newsletter 1987.

Blade Shapes, Ancient and Modern

Nowadays, unless we are yachtsmen, our ideas about oars revolve around competitive sport rowing, generally on rivers or lakes. Speed is the thing and usually over distances up to only two kilometres or so. Rowing as a sport is about two centuries old, but oars have probably been in use for getting about in boats (and ships) for about four millennia. At sea as well as inland on rivers and lakes, boats were with few exceptions built and shaped to be useful for carrying people and goods. In such craft, very broad and heavy compared with a modern scull or eight, great speed under oar has not been attainable, sustainable nor particularly sensible, as anyone who has tried to row hard for at all long in a work-a-day dinghy will know.

Oars at the start of competitive rowing in the early 1800s were just like those used in the fast Cornish gigs, lightly built for taking pilots out to ships entering the English Channel or in naval gigs for ferrying admirals to and from warships in harbour. Cornish gigs have been rowed from Cornwall across to Brittany and Ireland. To cope with the sort of rough water met at sea, oars have to be pulled deep and strokes have to be shortened. In most sea conditions getting the oar out of the water with little delay, even when it happens to be buried in a passing wave is more and more vital (and less certain!) the rougher the water. Sea oars have to be tough; harbours are often crowded places and oars are often needed to fend off. In the past they also had to be cheap so they were made in one piece out of single poles usually of ash, a tough but heavy timber. They therefore had narrow straight blades which, because in deep short strokes the blade would move more up and down in the water than in long shallow strokes, is not such an inefficient shape as one might think. Blade cross-sections were lozenges (diamond shaped) or oval. Sea oars have been well discussed by Eric McKee in Working Boats of Britain (1983, Conway, London, ISBN 0-85177-277-3)

There were exceptions; oared warships, being long, narrow and heavily manned with many oars, could reach speeds of 7 or even 9 knots for short sprints. Their oars seem to have been made either in two pieces of spruce with wider blades rivetted to forked ends of the shafts or in one piece with narrower blades. Two-piece oars were used in ships where each oar was rowed by one man as in triremes, and their blades seem to have been elliptical in shape, quite short and broad. Oars in the larger polyremes were multi-manned, made of ash and more massive. In later galleys of medieval times up to the eighteenth (and even the nineteenth) century with up to 5 men to an oar, blades were long, narrow and cambered. In most kinds of oared warships oarsmen were seated facing about 20° outboard (as well as aft!). In calm enough water and when a spurt was called for, the catch could therefore have been about 45° forward of athwartships and the finish not so far aft of athwartships, as in latter day fast seagoing gigs and in modern racing practice whether on fixed or sliding seats. Cambering blades of oars carved out of a single pole weakens them considerably but less so if they are of modern materials.

In the two hundred years in which sport rowing has developed, the shape of blades has changed, though very slowly until well into the last century. This a bit surprising considering the difference between the short dipping sea stroke and the long and shallow river stroke. Camber was adopted fairly soon and there was a general tendency over the years for blades of sporting oars to widen; money was evidently available to pay for the amount of wasted high-grade timber that had to be cut away in carving out the whole oar from a single, fatter pole before the days of waterproof glues. However, G.C.Bourne (1861-1933), a sport rowing guru in his time, was very guarded about increased breadth of blade (though not on any account of expense). In his very thorough book, A Textbook of Oarsmanship (1925, reprinted in 1989, Toronto. ISBN 0-920905-12-9) he spends only 20 lines or so in the 370 pages of the whole book on blade shape. On page 184 he writes, a little tersely:

"As for the shape of blades, I can only say that Mr Ewing McGruer is clearly right. The correct blade is that which has its area concentrated near the tip. Barrel-shaped or coffin-shaped blades are wrong in principle and have nothing to recommend them."

That, so far as it goes, sounds like commonsense and indeed blades for river rowing were for many years widest (if only a little) at their tips which were square. With the recent arrival of synthetic resins and Kevlar, breadths could be and have been increased much more than Bourne would have approved. This change was probably in recognition of the lesser distance travelled in the water by the neck compared with the tip, by an amount directly proportional to the length of the blade. It led to shorter but wider shapes, to maintain blade area and limit slip in the water. Such blades were often barrel-shaped.

McGruer, whom I knew, was a fine, thinking boatbuilder but I doubt if he would have claimed to be an expert in hydrodynamics. No one to my knowledge has even yet done much useful work on the study of the flow round an oar blade. That flow is certainly very complicated indeed, as every rower will have observed. It is made particularly so because the blade is only just below or partly in the surface of the water. Nearly everything about an oar and its blade is a compromise between many conflicting factors.

However, there are two general points about lifting surfaces in fluids which help to indicate a few features likely to be good or bad about the shape of oar blades. Lifting surfaces are relevant to oars because the thrust sustained by a blade is analogous to the lift of an aero- or hydro-foil, even though the angles of attack of an oar blade in the water increases during the course of a stroke to be large enough to cause the blade during much of its latter part to 'stall', to use an aeronautical term.

The first of these general points about lifting surfaces (or foils) is that a foil like a glider's wing, with a great span across the air flow but a narrow chord or wing breadth, is the most efficient. Thus, as already mentioned, narrow blades are sensible for sea oars worked with short, up-and-down strokes because the flow of water is mainly across their blades. The same point can be applied to river oars but in another direction. At the start of a stroke with the catch well forward of athwartships the blade will move tip-first in the water at a speed decreasing to zero when the oar is athwartships. Thus, because at the start of the stroke water is flowing over the blade from tip to neck, lift will be more efficiently generated if the blade is as wide as possible to increase its span across the flow if only in a limited way. Another advantage of the catch being well forward is that the speed of blade needed to enter the water without shock is less than it would have to be if the catch were nearer athwartships. This allows the rapid build-up of swing and pressure to be smoother.

The second general point is about camber. Camber allows the flow over the tip (if it is towards the neck) to be deflected more gradually as it is directed round the curve of the camber than it would be if the blade were flat. That can allow the flow along the back of the blade to contribute to the lift, or thrust, of the oar instead of breaking free and being, as it were, lost in eddies. It follows however that camber in a blade is only useful when the catch is well forward of athwartships. This second point is significant because, as experiments with strain gauges on oar shafts have shown, rowers tend to pull hardest on their handles soon after the start of strokes, the pull diminishing more or less steadily, particularly as their arms bend, to be zero at the finish.

In recent years the appearance of oar blades has changed quite dramatically but in a way that agrees with these two general points. The shape has become a cambered rectangle set on the shaft at an angle so that its upper edge can be in the water surface when the blade is just immersed. The breadth is as large as is practicable while enabling the blade to clear the water as the feather comes off before the catch, and for rotational balance about the axis of the shaft between blade pressures and handle pull during the stroke. The length of blade is sufficient to provide a big enough area to limit slip during the latter part of the stroke when the blade is stalled anyway so that the water is pressing virtually on the concave face of the blade only. Such a shape is practicable only by the use of modern synthetic materials. It does have a logic to a degree hitherto lacking in oars for calm water sport rowing and it will be interesting to see how blade shapes change in future!

John Coates - October 2006