Monday, August 20, 2007

Some Advantages of a Long Catch

Technique – Some Advantages of a Long Catch
By Walter Martindale, M.P.E., ChPC, Coach Development Manager, Rowing New Zealand

“Keep your length!” “Don’t shorten your stroke!” Who among the athlete community hasn’t heard a coach yell something much this just when you’re busting your guts to finish in a sprint. Who among the coaching community hasn’t used something very much like these phrases in advising a crew about tactics associated with finishing bursts in a race plan?

Why is it so important? Why do the big, long, tall people make boats go faster than more
“normal” sized people? Here are some of the reasons (not all, because I don’t know them all) – most are straightforward, some may require some thinking to “get”.

Distance Covered Per Stroke:
That’s fairly straightforward – the farther forward1 you reach, the more of the rowing course your blades bite off with each stroke, and the fewer strokes you need to take to get to the finish line. But that’s not all.

Easier Catch Timing and Less Missed Water:
We all love to see a well-timed catch where the crew misses the smallest amount of water at the entry. Many (very many) have written eloquent descriptions of how to do this (Fairbairn, Bourne, Herberger, to name a few – heck, even I’ve had a go at this). Kleshnev wrote about this in the Rowing Biomechanics Newsletter 2007 issue 3, and 2007 issue 5. The farther you reach around to the catch, the easier it is to “match the speed of the water” with your blade, although I’m not certain if that’s exactly what we’re trying to do. We are, generally, trying to move a boat that’s in motion past stationary water (even in rowing in a current, the water is essentially stationary relative to the boat unless a blade catches an eddy). The steeper the angle of attack (the closer the blade is to parallel to the boat), the easier it is to put the blade into the water without having to accelerate it towards the stern in order to match and then exceed the speed difference between the water and the blade (in the direction of travel for the blade – remember that the blade is travelling in an arc, “away from” the boat after the catch). I.e., If you are trying to catch with the oar perpendicular to the boat, and you are travelling towards the finish line at (say) 4 m/s, to achieve any acceleration of the boat, you have to very quickly get the blade of the oar to exceed the 4 m/s that the shell is travelling, so that you can increase the propulsion on the boat. If you’re catching parallel to the boat, (nobody does), you can place the blade into water that (relative to the movement of the blade) is moving at zero velocity.. Of course, the blade is moving at the same speed as the boat at that time, but in the direction of the blade’s travel when you push the stretcher (away from the boat) the water/blade velocity relationship is zero. The difficulty with changing your blade’s direction of travel to have a quick catch at 90o is that thing from Isaac Newton – the “Action-Reaction” law – to make the oar change direction and bite the water, a person needs to have a platform on which to create the movement – you have to push on the boat to make the oar change direction. This will cause a check… If you review the velocity profiles in the momentum article also on this site, you’ll see that even with a well timed catch (you’ll have to take my word for it, the film from which this was generated may still exist, but if so, it’s in either Vancouver or Ottawa) there is a significant drop in the boat velocity at the catch (but it is still well above zero velocity – it just looks to us in the coach boat like it goes backwards because we’re usually matching the average velocity of the shell). Nobody can row or scull without this occurring unless we can have continuous propulsion.

When you reach longer, and your blade is entering the water at an angle of greater than 75 degrees (in a sculling boat) forward of perpendicular (or within 15 degrees of parallel to the centerline of the boat) you don’t need to be as quick in pressure application to start applying propulsive pressure to the water/blade interaction. This shouldn’t be construed as permission to get on the work slowly, you still need to be pushing the footstretcher and hanging on to the handles right away after (or as?) your blades enter. Some people (well, most) think it ineffective to push outwards on the water, and that the “effective” part of the stroke is that short little 40 degrees of the stroke on either side of perpendicular. It’s not – if it were, why do really tall people, who go fast in a sculling boat, scull with catches between 75 and 80 degrees around from perpendicular to the boat? Later in this paper, with more segments of the discussion out of the way, we should have a better understanding of why it is more effective (in a fast moving boat) to catch long, almost parallel to the boat. “Pinching” a boat is a myth, unless the boat is so mushy that you wouldn’t race it in the first place.

Blades move in the direction of travel, early in the stroke
If you were brave enough to put your blades into the water parallel to the centreline of the boat, the tips of the oars would be moving towards the finish line at the same speed as the boat. During the recovery, the blades move faster than the boat because they are being carried forward towards the finish line by the boat as the crew moves the handles aft during the recovery. In a real catch, which is shorter than parallel to the boat, someone who is good with trigonometry and has access to overhead video of a crew can probably calculate the angular velocity of the blade at various parts of the movement.

The closest I’ve seen anyone to catching parallel to the boat has been an overhead photo of Pertti Karppinen, from Finland, who won the 1976, 1980, and 1984 Olympic single sculls gold medals, and was reaching around to about 80 degrees (i.e., 10 degrees short of the parallel, if “0” is perpendicular to the boat.) Volker Nolte uses this photo in his presentations about rigging. The photo is printed at the right, with Volker’s permission. (This doesn’t mean that Mahé Drysdale, Nathan Cohen, Storm Uru, Marcel

Hacker, Olaf Tufte or any of the current crop of elite scullers don’t reach that far, it means I don’t have a photo of them.) In the above picture, the angle included by both sculls is about 40.6 degrees (that’s using a bit of trigonometry). For the sake of discussion, I’ll assume that the blades are at the same angle around from centreline (they’re not exactly, but close enough for these purposes), or – each blade is about 80.3 degrees around towards parallel to the boat.
After the blades are in the water – please recall that you’re trying to move your boat past the blades, not pull the blades past you – the blades still slide through the water towards the finish line immediately after entry. The amount they slide through the water in this direction depends a bit on how hard the athlete’s drive is, a bit on the overall reach at the catch, and a bit on the “instantaneous” angle of the oar relative to the boat. In sculling this effect is greater than in rowing because of the greater angles involved.

Pushing outward on the water early in the drive is GOOD to do.
What? With the blades of a sculling boat “locked” in at the catch, and with the sculler loading up the oarhandles by pushing the foot-stretcher/rigger hard against the button/wear plate, the sculler pushes hard against the water in which the blade is sliding lengthwise, in the direction of the tip of the blade. If you think of this from an “instantaneous” perspective, say at 0.111 second after the catch, and then again at 0.112 second, and again at 0.113 second, the blade is moving through the water following the tip of the blade, and the force acting on the water is perpendicular to the face of the blade. The force acting on the water is usually resolved by biomechanists into the “effective” vector – opposite to the direction of travel, and the “ineffective” vector – perpendicular to the boat. This requires a re-think – a serious re-think – well, I think so, anyway.

Action-Reaction and a wet bar of soap.
When the blades are pushing outwards on the water, a kinetics specialist would say that the water is pushing inwards on the blade. If you are very long as in the photo above, and consider the time between (oh, say) .111 and .112 seconds after the entry (1/1000 s. difference in time, not much movement has happened), the forces going outwards generated by the rower are mostly countered by the forces going inwards in reaction, there is very little actual outward motion of the blades, and relatively more forward (towards the finish line) motion of the blades. If you do a small mental flip and consider that the rower is resisting outwards while the blades are being pushed inwards by the water, you can imagine a wet bar of soap, being squeezed, and squirting out of the hand.

The boat/oars/sculler is the bar of soap, in a way, and the reaction force of the water causes the boat to “squirt” forward as the blades slide through the water. This effect becomes reduced as the blade angles get farther and farther through the stroke, and as the blade tips get farther and farther away from the boat.

Only works at “pace”.
This “instantaneous reaction force” effect of a long catch works only after the boat is moving – you need to start with shorter strokes, or there is not enough movement along the length of the scull with which to take advantage of the length and the “bar of soap” analogy. Lift along the surface of the blade is also a phenomenon that is a complex bit of fluid dynamics, but the important bit at the very long part of the stroke, when the blade is pushing outwards, is proposed by me to be the reaction force from the water acting on the blade – the blades don’t move outwards very much during the first moments of the drive, but the rower/boat motion system sure picks up speed. If this “squirting” or “bar of soap” action was not in effect, we should be able to encourage scullers to rig so that they can row short at the catch. I think that the “lift” that has been much discussed in some circles of late helps the athlete feel well anchored in the water.

Where did I come up with this?
In 2004? 2003? – whenever it was that the Rowing Canada National Coaches Conference was in Richmond, BC, Volker Nolte did a bit of a wrapping-up discussion of the dynamics of the blade in the water. In that discussion, he discussed the path of the blade in the water having three phases – Phase 1 is the early part, where most of the motion of the blade (in addition to angular motion) is along its length, and about which there has been much discussion of “lift”.

Phase 2 is the part in the range that has been historically described as the “efficient” range, where the blade is mainly pushing aft on the water. Phase three is where the tip is coming back towards the boat and the athlete has to think very hard about getting the blade out of the water. The discussion about the “bar of soap” action is almost entirely related to Phase 1. Volker contrasted the sculling/rowing race start against the speed skating race start. We take shorter strokes to remain a bit more stable and to get the boat going, reaching farther and farther around, and eventually pressing outwards on the water in Phase 1. The speed skater starts the race by running on the ice with the skate blades pushing backwards, and as he/she picks up speed, the skater then gets into a streamlined position and is coached to push outwards on the skate. The marks made on the ice by the pushing blade are very similar to the path of the blade tip in a very long sculling stroke in Phase 1. (The similarity doesn’t end there; the skater is actually gliding and pushing on water, due to the pressure from the narrow blade melting a very thin layer of ice.)

This writer is aware of one elite sculler (not from Kiwi land) who, upon seeing the overhead video of the catch length, re-rigged the boat so that the catch would be shorter, and subsequently slowed down enough to go from being a serious contender for Gold, to struggling for a medal. I know that this isn’t the best description, and that a currently practicing biomechanics professional would likely shudder at my phraseology, but, having looked at this stuff long and hard, I think this is a valid description of what’s going on. (As well – what’s a Coach Development Manager supposed to do other than stir up thought and discussion?)

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