The APEX Blade Design

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Dreher oars and sculls are made by Durham Boat Company, Inc. We are located in the U.S. and have been making carbon oars and sculls as well as seats, footstretchers and riggers since 1989. Most of our oars and sculls are sold in North America but we have a couple of agents in the UK and Germany and we also sell our carbon components to other builders around the world. You may have seen our Dreher oars and sculls used by U.S. crews at Worlds or Olympics.

Our newest competitive blade shape is the APEX blade shape. The noticeable features are a smooth back surface with all elements in line with the water flow past the blade, a thin blade made to FISA minimum thickness with smooth rounded edges, a smooth continuous transition from blade to shaft and a smooth, polished surface.

The improvements that users have found with the APEX blade shape are: (1.) A very easy release even in rough conditions – power can be applied right to the end of the stroke without concern of getting caught at the finish. This is best demonstrated by rowing full pressure oars squared. (2.) A blade that stays fully immersed at a constant depth by using the water displacement volume of the bulge where the shaft attaches at the top as a depth limitation through buoyancy. (3.) Speed improvement compared to other big blades.

Speed improvement was the original reason for pursuing this blade shape. Hydrodynamic theory tells us that we can make the boat go faster if we can more fully optimize the lift forces that are generated by the blade moving through the water.

Blade Theory: The blade is a hydrofoil in fluid dynamic terms that generates a lifting force perpendicular to its direction of motion when it is put into the moving water and also a drag force in line with the shaft. The lift force should be much greater than the drag. So if you are designing blades you want to find a way to design for lots of lift and little drag. The problem with designing blades is that most foils like an aircraft wing are designed to operate in a narrow range efficiently. However, an oar blade is constantly changing orientation and velocity throughout the stroke as the tip of the blade moves from boat speed at the catch to a stalled position at its furthest travel about 1 meter further out from the boat at the 90-degree position. The blade then reverses itself and accelerates back about 0.3 m. toward the boat at the finish. It traces a pattern as seen from above that looks somewhat like a comma. This type of deceleration, stopping and change in direction does not occur in aircraft or any other common situation so no data or theory that can directly be applied to oar blades is available. We just do not know what the motion of the blade does to the lift and drag. However, we do know that lift and drag forces are generated by the blade so we can work to maximize the lift and minimize the drag to make the blade more efficient by making the blade move through the water more efficiently. Areas to consider are shape, size, thickness, surface, edges, attachment to the shaft, angle to the shaft and stiffness.

At the catch and finish the velocity of the blade relative to the water is maximum and a small round or delta shaped blade shape would work well. At the mid point of the stroke where the blade stops and reverses direction a large blade shape would work best. However we have initially stayed with the familiar big blade shape for the APEX.

To prolong lift and minimize drag, the shape, thickness and radius of the edges of the blade are important as well as how smoothly the blade mates with the shaft. By optimizing these features you can help the blade generate lift efficiently and longer and minimize drag by delaying the onset of air entrapment which releases suction on the back of the blade. The Apex blade is made as thin as possible just above FISA minimum thickness with rounded edges and a smooth transition to the shaft and a polished as molded surface all in an attempt to present a hydro-dynamically smooth shape to the water and to preserve the lifting force as long as possible.

But it is clear from the theory that the back of the blade, not the front is the controlling surface and that surface should be free from anything interrupting the water flow. This suggests that lift could be helped and drag reduced by moving the tapered bump on the back surface that starts from the shaft attachment point so that it is parallel to the water flow allowing the water to flow uninterrupted over the surface.

The shaft is reinforced in the pulling direction by selectively placing unidirectional carbon strips top and bottom to improve bending resistance. This results in a slightly elliptical cross section on the tapered shaft. A longer 20-cm sleeve is used to take advantage of the carbon adjustable handle. The shaft and adjustable handle is not new in design and is nearly identical to what we have had since 1991. 

It is difficult to give absolute reliable technical data on blade performance. The reason that it is hard to measure is that the improvement in speed performance is so small that it is probably beyond the margin of error of the instruments used to record the data. Our protocol for speed testing is to do multiple passes on a sheltered 250 dead water stretch of the river, which abuts our facility. We used a maximum speed comparison with 5 or 6 different types of blades of our own and another makes and then repeated using each set of sculls twice. Many variables are involved even under the best conditions. Only after many tests using different people can you hope to see a trend. (See below minicam apparatus on CII Smoothie Sculls and Dreher APEX Scull – close-up)

Over our 250 meter course in a single we row to warm up and then would take three strokes to build and during the next 10 to 15 strokes we would observe the max speed in M/Sec. Using a NK Speedcoach. In our tests with the APEX blade shape we saw max speed values of 0.02 to 0.03 m/sec. faster than our big blade BB2198. APEX blades are the fastest we have tested and we attribute that to optimizing lift from the smooth back surface because the contour of the blade is identical to our BB2198. Our customers have been our best test pilots and their word of mouth endorsements have made the APEX the quickest customer acceptance of any blade shape we have ever introduced.

Recently, we have gone one step further to investigate if a deeper and rounded or delta shape would further enhance the APEX concept of a high lift/low drag design. Making a deeper blade may further reduce the onset of drag and the rounded tip may present a reduced projected surface at the catch and finish.

For the prototype we took a standard APEX scull blade and added approximately 4 cm to the bottom and then rounded the edges to approximate a delta shape resulting in approximately the same area. With these prototypes subjective tests have been made since mid summer. The “round” APEX moves the center of pressure down 1 cm. further effectively allowing you to row “deeper” while maintaining roughly an equivalent surface area. Initial results have been surprising in that although wider the blade can still be easily rowed squared and releases effortlessly. The “round” Apex prototype rowed best with 1 or 2 degree pitch. No data has yet been gathered as to speed relative to the current APEX.

During testing of the APEX sculls, Bob Dreher made up a rig that attached a small micro camera to the oar shaft so that you could see the effect of the oar moving relative to the water. It is interesting to see the effect of water flowing past the blade. There is very little disturbance on entry and very little evidence of air entrapment or separation from the backside with the APEX shape.

by James Dreher (11/10/00)

 

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