Rowing/swimming backstroke: optimal angle of oar/arm

[nomadreid]

TL;DR Summary

Hydrodynamics: which is better in (a) rowing in water, (b) backstroke swimming when the oar or arm, resp, is in the water: almost parallel to surface of water, almost perpendicular, or another angle?

First, obviously the two cases (in a rowboat or swimming in a pool) differ significantly, but I suppose that the resultant velocity forward of the boat or of the body, resp., for a given force, would both depend in a similar way (although I am ready to be corrected) on the angle which the oar or the arm makes with the surface of the water. Assume no current in the water, and other simplifying assumptions as needed to make the problem tractable. (This is partly out of curiosity, but mainly in advising someone how to improve her backstroke. As I have no training in hydrodynamics, I have my prejudices, but they need not be aired here.)

 

[berkeman]

I don't know about the oar case, but in backstroke (and the other swimming strokes), you use a "sculling" motion to optimize the hydrofoil lift of your hands (not much propulsion comes from your arms, which you can show by trying to swim the "fists" drill). The sculling motion is used to try to keep your hand accelerating still water throughout the stroke, as opposed to accelerating the same piece of water through the stroke (which produces very little lift after the initial acceleration).

Check out this video and other resources online about backstroke swimming technique...

 

[nomadreid]

Thank you, berkeman. I have been analyzing your answer and the video to try to figure out the exact movement, and it appears that the flexibility of the arms, as well as the restrictions that the construction of the shoulder puts on the movement of the arm, plus the rotational movement that you say optimize the thrust of the hands (it being a surprise to me that you don't get much from the arms themselves), makes the backstroke quite a different case to the rowing of the boat, even though the word "sculling" comes from the rowing case. I need to keep analyzing this and other sources to come to enough of a conclusion to be able to advise on backstroke technique. Maybe some other kind member of PF will add something about the rowing case.

 

[anorlunda]

I do not envy you your task @nomadreid. Combing physics with human physiology is notoriously difficult.

But I do have some years of rowing experience, that may or may not help.

The number one factor in rowing is oar length. The longer the better.

Efficient rowing depends more on the time profile of forces during the stroke than it does on angles. One example is shown in the picture (I prefer to plot it with time on the horizontal axis). Through the stroke, shoulders, arms, back, and leg muscles all contribute at different times within the stroke.

 

[nomadreid]

Thank you for your helpful reply, anorlunda. If I may, a few questions:

I prefer to plot it with time on the horizontal axis

Your graph has stroke length versus force; where does time come into it?

The longer the better.

But if I understand your graph correctly (which I probably don't), it seems that there is an optimal length, around 60 cm.

Efficient rowing depends more on the time profile of forces during the stroke than it does on angles.

OK, I will have to search details on the time profile , sounds interesting. But even though the angle is a minor factor, is it insignificant? That is, if you had an experiment whereby length of the oar, the time profiles, and all other factors except the angle were kept the same, would there be a significant difference in the forward velocity of the boat?

I swim more than I row, but I also row (but not competitively) occasionally, so your help is most appreciated.

 

[anorlunda]

Your graph has stroke length versus force; where does time come into it?
But if I understand your graph correctly (which I probably don't), it seems that there is an optimal length, around 60 cm.

I interpreted stroke length in the first plot as the distance moved relative to the beginning of the stroke. Thus it changes with time. Here's a better time-based explanation.

https://www.rowinginmotion.com/stroke-efficiency-explained/
In this figure, the horizontal axis is time. You have to read the linked article for a full explanation.

OK, I will have to search details on the time profile , sounds interesting. But even though the angle is a minor factor, is it insignificant?

I was commenting only on rowing. I can't comment directly on your question about the angle while swimming.

In rowing, if the end of the oar is only just below the surface, then the oar angle is determined by the height of the gunwhale above the water and the length of the oar. Hence, the longer the oar the better. Short oars which might give you a 30-45 degree angle between the water and the oar are horrible for rowing. (Paddling is entirely different than rowing.)

 

[berkeman]

I swim more than I row

Is there a reason that you asked about the backstroke specifically? The freestyle stroke has more freedom to exploit all of the skulling motion options (versus the simultaneous bilateral butterfly stroke and breaststroke, and the constrained on-your-back backstroke).

I am continuously fine tuning my freestyle stroke (and to a lesser extent the other strokes), since I first discovered the sculling stroke thanks to Stanford coach George Haines in a Masters swimming clinic many years ago. After I attended his seminar, I bought one of his books on swimming technique (sorry, I'm not able to find it on Google or Amazon right now), and I changed to the sculling stroke the next day and took a full minute off of my mile workout time. Amazing and so real.

One of the fine-tuning tips that I discovered a couple weeks ago is to over-emphasize the hip rotation at the end of your freestyle stroke, when your hand is pushing hard past your hip. If you use your kick and torso to add in a strong hip rotation, you can use that to add to the strength of your hand hydrofoil push. It's taken about a second off of my 50m length times so far. Great stuff!

EDIT / ADD -- This improvement is aided by an emphasis in my weight training to add more lat/tri pulldown sets on the weight machines. That gives me extra strength to use in that final part of the sculling stroke past my hips.

 

[Klystron]

[snip]
One of the fine-tuning tips that I discovered a couple weeks ago is to over-emphasize the hip rotation at the end of your freestyle stroke, when your hand is pushing hard past your hip. If you use your kick and torso to add in a strong hip rotation, you can use that to add to the strength of your hand hydrofoil push. It's taken about a second off of my 50m length times so far. Great stuff!

Thanks for introducing lower body motion to this thread. I swim laps wearing heavy fins. Inverted freestyle -- essentially sitting -- provides a powerful kicking position. I move so fast inverted, I only need hands and arms alongside my hips for direction and light sculling. I also find that hip rotation adds power to the leg stroke.

 

[nomadreid]

Thanks to anorlunda, berkeman and Klystron for the interesting and helpful replies.

First, anorlunda, I have read the link which explains the graph. The analysis is interesting, but of course starts from the reader knowing how a typical stroke is supposed to go. But nonetheless a very good explanation of stroke efficiency. Your comment about the angle depending on the oar length (and of course the height of the gunwhale as well as the position and height of the rower) and the desirability of long oars tells me that the angle to the horizontal should be small.

Next,

Is there a reason that you asked about the backstroke specifically?

Yes, as I mentioned, a friend, observing that I swim relatively well, asked me for advice on her backstroke. (She made the usual three errors: (a) ability in one sub-field translates into ability in other sub-fields, (b) that ability of an activity translates to knowledge about the ability, (c) knowledge of an ability translates to ability to teach that knowledge.) But I am actually not very good in backstroke, so I turned to this forum, which has been a wonderful resource for my questions about all and sundry. I prefer freestyle, so your comments on hip rotation in freestyle are of interest to me for my own improvement, showing the value of spin-offs. (Attention, funding agencies!)

 

[anorlunda]

The more I think about it, the more convinced I become that rowing is not relevant to swimming. It will just distract you from the real task.

 

[nomadreid]

The more I think about it, the more convinced I become that rowing is not relevant to swimming. It will just distract you from the real task.

Yes, upon examining the two different approaches, this seems to be the case. But each one is interesting in its own right. The various posts have put me on the right track, though, for which my thanks to all the posters.

 

[Klystron]

This thread reminds me of a science oriented Japanese television show popular throughout South Asia circa 1970's. Premise: a highly competitive female swim team hires a young physics graduate to coach the best swimmers to improve lap times. The prestige of the club is at stake!

Doctor Swim -- not distracted by the bevy of bathing beauties in skimpy outfits -- and the upstart plucky heroine "Yuki" apply various formula to improve performance including the then-new concept of weight training alongside Okinawan martial arts. In an Archimedes moment following a narrow defeat by a rival richer team with questionable morals, Dr. Swim realizes that, while both are fluids, air is less viscous than water offering less resistance to a swimmer.

Dr. Swim trains Yuki to achieve maximum speed in the water and to launch her body into the air while kicking off from the side of the pool from a turn. In the season finale Yuki and the teammates she mentors shock the competitive swimming establishment by launching into the air after a turn, gliding horizontally above the water -- filmed in graceful slow motion -- shaving precious milliseconds from their times. Winning the overall competition with the flying technique restores the honor of their club/school with Dr. Swim offered a position teaching hydrodynamics at University with Yuki as star pupil.

[I tried this technique in my twenties. Momentarily gliding above the water feels super-cool but the splashdown and re-entry likely mitigated any advantage from making a proper turn and remaining in the water. ]

 

[berkeman]

[I tried this technique in my twenties. Momentarily gliding above the water feels super-cool but the splashdown and re-entry likely mitigated any advantage from making a proper turn and remaining in the water. ]

Compare that to the current fastest turn and glide technique -- angle down from the wall, glide and dolphin kick underwater for as far as you can before surfacing. Can you think of a couple of reasons why that is faster than trying to turn above the water?