NK Oarlock - rigging and Physics question

[James HS]

A little knowledge is a dangerous thing!

I was on camp last week and had some flat water to play with (yippee) so I rigged up an NK oarlock to show me what my angles were and actually have some data to start looking at rigging.

The oarlock data with my original rig showed me I was achieving an overall arc of 104 degrees - happy with that as a short(ish), slightly overweight, inflexible old bloke :) (I am working on everything except height and age :))

But for the first time I had data that showed me that my arc was symmetrical around the pin (52 degrees in front and behind).

Much that i have read, and asking around has told me that I should be more in front of the pin - approx 60/40 (not seen any explanation, hence this email)

So I altered my rig by bringing my wing rigger towards me (I have an old Simms Evolution and was fairly well centred, if a bit sterward) and moved my footplate to give me room - the wing moved more than the footplate so it did not cancel out.

Lo and behold the new angle measurements (after the rather tortured re-calibration) were 60/44 (in front of the pin)

Before the change I ran 5 x 250s in two directions on the water (CERLA in Spain for the first time - no real flow but slight head wind differences) and then after I made the change I did another 5 x 250s

Lots of numbers popped out - but when I look at some consistent figures i do see an improvement in speed - as an example on one run typical splits dropped from 2:06 to 2:00

Obviously the data points are not big enough - and i would have liked to done more tests but was coaching most of the time .... but, the numbers were consistently better on all the tests i did.

So, what is they physics behind this?

My go at an explanation would be that I am bringing the blades further to the side of the boat, at a location where the blade is at its most efficient as an aerofoil (do stamp on me if I am too sloppy) - at the point where I enter the blade into the water it may not convert much direct force into motion as it is too far round, but it will allow me to efficiently bend the blade handle and store some energy from the large leg muscles and I can only assume that is a good thing.

I will also have more of an arc to catch the water and employ large leg muscles in front of the point where there is stall and therefore accelerate the boat more effectively.

I am then thinking that the point past the pin/stall point I will not have lost the stored energy of the bent blade and that I will release this at the same time as the end of the back/arms sequence is happening?

So - I guess my question is whether this ham fisted explanation is heading int eh right direction, and that the figure is not 65% in front because the blades would be coming too close to the side of the boat and therefore law of diminishing returns? and it is not 40/60 because of the fact that the legs are the most powerful and therefore best to employ first?

Or am I completely off the mark and can someone point me in the direction of a better explanation - as I am well aware that I could have gone faster because I had made a change and invested in it :)

Thanks!



James

[carl]

James -

You have conducted an invaluable experiment and, at first glance anyway,
your explanation seems pretty sound. I'm up to my ears at the moment
(duty to clients, you'll understand!), but I greatly respect your
experimental approach and I would have anticipated that kind of result.

Bravo!
Carl


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[sander]

I have gone through a similar change when I got my Empower Oarlocks, but the difference was smaller.
I am guessing your peak force angle is now negative, i.e. your peak force occurs at a scull angle where the efficiency is higher (losing less power in puddle creation).

Can you confirm that you were rowing at the same (constant) average power in the "before" and "after" scenario? Even though I believe that the physics point in the same direction as your experimental result, the effect that you report is bigger than I expected, so there may be a (partial) placebo effect where you rowed "cleaner" in the "after" experiment?

[sander]

A 5% improvement (2:06 to 2:00) seems much bigger than my calculations suggest (about 1%). Do you have the average power values for those experiments?

I would be greatly interested tosh receive the data files as exported to CSV by the NK LiNK software. Would you be so kind to send them to roosendaalsander"at"gmail.com? Will share results with you and publish them here.

[carl]

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I agree with you, Sander, that the indicated improvement is
considerable, representing almost 16% increase in net propulsive power.
We should perhaps say that there might be a smidge of experimental
error, & it would take a lot more testing, with repeated switches of the
arc settings, to establish a definite result.

Some of what James sees might indeed be, if not actually a placebo
effect, then due to a more comfortable arc distribution. Thus if the
arc starts further forward, that might better suit James's speed of
catch load application. I'm not saying James is slow to load up, but
the speed of rotation of oar around the pin will be significantly lower
at a 60 deg catch angle than at 52 deg.

That would also enable him to load up over a smaller angular movement &
so to have the load on while the blade is still foiling at high efficiency.

I'd also expect the power stroke to last a little longer under those
circumstances, helpfully changing the ratio of stroke to recovery times.

A few more half-digested thoughts about cause & effect are still
rumbling at the back of my head, but for some other time.

Cheers -

[sander]

The beauty about the Empower Oarlock is that it gathers a ton of data during the row, somif James still has the recordings on his device, we could normalize for power and look at the differences.

The most notable difference in my simulation was indeed in the stroke rhythm (drive/recovery ratio). With the increased catch angle (more towards stern) the stroke will be perceived as more heavy but the force of the drive is actually spread over a larger time, and if it enabled James to Row a wider arc, his average force may have been lower at the same power, or alternatively it could have enabled him to generate a higher power at the same average handle force.

In my experience, simply the fact that you are seeing numbers helps you row cleaner and more consistent. Especially in crew boats this could be a significant effect.

James, if you have the data, I offer a thorough analysis. I do believe that would be a small but important step to more evidence based rowing.

Better than believing that a certain sun glasses brand gives you 'free speed'.

[James HS]

thanks for the thoughts - I am probably slow to load up and, being aware that I should be going faster may have pushed it harder - but also it was slightly windy at CERLA and I was trying to be unbiassed but the tests were several days apart, at different times of day and I was chomping at the bit to get out for the second one having waited to find a time when there was no wind and I was not coaching :)

Sander - the oarlock stopped communicating on the second set of tests so I will dig out the data for the first set and see what is in there - I reasoned that I could not spend an hour trying to get it to re-connect properly (I had seen the angles on a windy outing that was not good enough for my 'tests' and then the communication seemed to break down) but I had impeller data and could measure splits - which I did because going faster was what I wanted!

I am now back in London and on the friendly Tideway - but with evening rowing coming back in I can do more comparisons and see if I can get some 'side by side' data and properly spend time setting up the oarlock (they paired happily but no rotational data was coming through ..... something I have more patience to explore here.

I will also then throw in that I row with quite a short set-up (287/88) whereas Kleshnev (sp) data suggests that I should be using 289/89 and do once I have embedded the initial wing move I can move on to a longer blade.

As my boat has arrived back de-rigged I will put it back to the original and try a few days of bursts at pre-change, and then a few at 'new'. Always tricky on the tideway as conditions change so much, but I am fairly good at seeking out the best bits and the best times :)

So over the next month or so I will take you up on the offer and try and supply some clean data.

I am well aware I am not a finely toned experimental subject - but if a 'belief' and kit change gets 16% then that is still worth having - though I am more that happy to see the data show a slightly worse improvement and that I am cherry picking visually recorded data :)

More to come! (And my squad have naturally had their interest 'piqued' but I want a more solid foundation before I roll out hours of rigging changes!

James

[sander]

NK has published about an experiment with a novice eight, where they used the oarlock data to align catch angles of a novice eight. The improvement they saw amounted to a 0.5 second pace improvement over 500m (from 1:41.5 to 1:40.0 at 300W). See http://www.nkhome.com/news/we-put-empower-oarlocks-in-a-collegiate-8-for-one-hour-and-this-is-what-happened/

Improved blade efficiency through rigging is not going to give you a 16% improvement. However, in my personal experience, using the feedback from the oarlock to scull more consistently and eliminate technique flaws makes me faster, but the main effect is getting more watts measured at the pin through rowing cleaner and maintaining length.

[atkin...@gmail.com]

James:

I think you are pretty much on the mark.

My ROWING model <http://www.atkinsopht.com/row/bowangle.htm> indicates that there seems to be a speed advantage related to tight catch bow angles. That is, blades at the catch as close to the boat as is feasible.

I must admit that I cannot explain the physical reason for this, except to say that it may effect a better match of maximum effort to the ninety-degree oar position; see <http://www.atkinsopht.com/row/forcpeak.htm>.

Nice work and best regards,
Bill

[sander]

Bill,
Your calculations show a 1% speed gain for a 10 degree shift. That is a smaller effect than that reported by James.

It would be great to redo the experiment in calm weather and normalized to power. I have an Empower Oarlock and can get lots of data from it. I will need to think about the best experimental setup to obtain the required 1% precision. I definitely cannot hold power within 1% but it may not be necessary.

I would probably do a few 250s in each setting, at different stroke rates and power settings, trying to keep total length constant.

Currently my thinking is that other effects (rowing comfort, drive length or total arc, the speed of loading the blade, force drop at the finish) are influenced by moving the footstretcher. And I suspect these effects may be bigger than efficiency gains.

[James HS]

The other one(for me) that felt really significant, is that in my original set-up as I came forward for the catch my knees had a habit of opening up (due to lack of flexibility) with the left knee coming approximately 300-400mm away from the centre, and shins not reaching vertical. I have been working hard on that through pilates - but it is there in my initial set-up.

Once I changed the footplate/rig setup I noticed that I seem to be getting to front stops with a much better compression (not vertical, but more) and with much less knee deviation - and therefore way more comfortable, stronger drive location and WAY less need to set the boat up between strokes.

I frequently get cramp in my Adductor (longus I think) and this has been put down to bad pelvis stability at the catch .......

So My feeling is that much of what I am doing in my set-up is actually addressing biomechanical instabilities and inefficiencies in my set-up that are happily fixed by looking at the angles.

Over the next month I will endeavour to get more data points and may well find there is less improvement, or more improvement driven by better biomechanics.

At least I have a tool to start getting data - though have initially found the oarlock slightly tricky to set-up and calibrate (already broken 3 calibration tools and so am going to design and 3d print a more robust version - I broke the new more robust NK version!)

Thanks for all the comments - most insightful.

James

[Dr. Valery Kleshnev]

It was proved many times that optimal (not too short, but not too long as well) catch and finish angles improve efficiency, eg., see http://www.biorow.com/RBN_en_2011_files/2011RowBiomNews04.pdf
I think the following factors contribute to it:
1. Heavier gearing at longer catch angles help to increase force quicker and make it using acceleration of the rower's mass (using legs, but not trunk or arms). Heavier gearing means slower handle velocity at the same blade velocity, which gives a rower more time to accelerate CM.
2. Hydro-lift force, which is 100% efficient and works mainly at oar angles sharper than 40-45 deg (RBN 2007/12).
3. Heavier gearing also means higher force and its impulse at lower mechanical power (not other way around, which was mentioned here). Lower mechanical power means lower energy consumption from the rower.
4. Muscle physiology (Hill law - 2007/09), slower contraction velocity might be more effective and comfortable for muscles.
5. Psychology - feeling of doing smart things improves motivation and increase efforts.