On 05/06/2023 09:55, James HS wrote:
> On Sunday, 4 June 2023 at 20:50:00 UTC+1, carl wrote:
>> On 02/06/2023 23:23, carl wrote:
>>> On 31/05/2023 02:24, Charles Carroll wrote:
>>>> Carl,
>>>>
>>>> I am having second thoughts about my reply. You write: “It's hard to
>>>> argue against maximising force throughout the stroke …” If I had taken
>>>> the time to be more careful about what you were saying, I would have
>>>> realised that the phrase “force throughout the stroke” refers to
>>>> power, not force. So in effect aren’t you and I talking about the same
>>>> thing — that is, maximising power, not force?
>>>>
>>>> This makes me think of Steve Fairbairn's account of the lesson he
>>>> learned from his boxing instructor.
>>>>
>>>> “An oarsman and the crew are nothing but a human hammer driving the
>>>> boat with a hit. A hit is not a mere tap-and-withdraw action. When I
>>>> was taught to hit by my boxing instructor, he took hold of my left
>>>> glove and told me to advance it steadily, and as it landed on his face
>>>> to lift my front foot and push with the other. So the beginning must
>>>> have a push in it; not merely the momentum of the body being thrown at
>>>> the water and vanishing in it, but a full-powered drive beginning with
>>>> that momentum as the blade takes the water, and carrying it through
>>>> with a drive from the stretcher.” ( “Rowing Notes,” Third Edition
>>>> 1930, p. 156)
>>>>
>>>> The hit and the follow-through are coefficients that unite in action
>>>> to produce in Fairbairn’s words “a full-powered drive.”
>>>>
>>>> Whether rowing leisurely or racing for gold, what oarsman wants to row
>>>> with less than a full-powered drive?
>>>>
>>>> Warmest regards,
>>>> Charles
>>>>
>>>
>>> Charles -
>>>
>>> The pleasure is all mine - & best to you both from Jan and me. Much to
>>> discuss privately - it's been too long.
>>>
>>> Now to business - rowing!
>>> There are 2 over-arching aspects to the net propulsive effect of the
>>> rowing stroke:
>>> 1. The amount of work that you do during the stroke which, simply put,
>>> is the sum (the integral) of the force that you apply at any instant
>>> multiplied by the distance moved in that instant by the point of
>>> application of that force (the hands)
>>> 2. The efficiency with which the work that you do is converted into
>>> useful (propulsive) work.
>>>
>>> That very simple depiction gets complicated very easily, so let's tread
>>> carefully. One reason is the sheer complexity of the inertial
>>> interactions between the rower's body (which weighs far more than the
>>> boat), & how one best to manage the relative motion of body & boat.
>>> Another reason is that the efficiency of the interaction between blade &
>>> water is far from simple, varies continuously through the stroke & is
>>> very difficult to analyse.
>>>
>>> I feel, & see, little evidence that rowing wishes to study or understand
>>> (or even acknowledge) these 2 aspects, preferring to think in stylistic
>>> & stimulating but non-scientific terms - because it is very difficult to
>>> do otherwise. Yet the time, effort & expense invested into rowing,
>>> especially at top levels, could suggest that we should do more to study
>>> those 2 factors listed above. Not to do so is like investing without
>>> understanding the significance of risk & rate of return.
>>>
>>> It's late after a busy day, so please treat the above as a place-holder
>>> & I'll return in a day or so, over a cuppa, to elaborate. But let me
>>> leave you with this thought, which I've expressed before:
>>> that swirling puddle contains all of the energy input at the blade which
>>> did _not_ move the boat.
>>>
>>> Cheer -
>>> Carl
>> Part 2: How the work that you do is dissipated, & therefore wasted, in
>> the water. Let's have some definitions:-
>>
>> 1, Newton's 2nd Law of Motion: "Force applied to a body equals the mass
>> of that body multiplied by the acceleration induced", or F = m x a,
>> where 'F' is the force(e.g. Newtons, lb force, kg force, etc) to the
>> mass, 'm' (e.g. lb mass or kg mass), and 'a' is the resulting
>> acceleration of that mass (its rate of _change_ in velocity, measured as
>> feet or metres per second per second - no repeating the per second is
>> not a typo!)
>>
>> 2. Newton's 3rd Law of Motion: "Every action has an equal & opposite
>> reaction". If I push you, the force that you apply is the force that I
>> feel &, if we're of equal mass & both standing on frictionless ice, then
>> we will each move but in opposite directions at equal velocities.
>>
>> 3. Kinetic Energy: The energy stored in any moving body of mass 'm'
>> moving at velocity 'v' by virtue of its mass and velocity. This K.E. is
>> theoretically capable of being completely recovered when bringing the
>> moving object to a halt. In mathematical terms, this is defined as E =
>> 0.5 x m x v^2/g, where 'g' is the gravitational constant (the rate of
>> acceleration of a body in free fall under gravity - ~9.81 metres/sec/sec
>> or 32.2 feet per sec per sec. To double a mass's velocity requires
>> quadruple the energy that was required to get it moving at its original
>> velocity.
>>
>> 4. Momentum: This is defined as mass times velocity - M = m x v
>>
>> It is upon these fundamental relationships that boat propulsion depends.
>> But first understand that all mechanical (including fluid-mechanical)
>> processes are to some degree inefficient - part of the work invested is
>> inevitably lost/dissipated on the way, so your blade can never convert
>> 100% of your work into propelling the boat. In fact what we can term
>> propulsive efficiency is depressingly low & with deficient technique may
>> even be 50% or less. For comparison, screw propulsion for ships tends
>> to be 60-70% efficient.
>>
>> So how is so much of your input work/energy lost? Some of it disappears
>> because, to paraphrase Newton #3, to push the boat forward against the
>> frictional drag on its hull means that something else must be pushed
>> backwards - & that something is a "lump" of water. Since energy is
>> always conserved (can't be made, can't be destroyed), whatever is moved
>> backwards has gained a bundle of kinetic energy, which is a price paid
>> that can't be recovered - it's a total waste, a dead loss, but makes a
>> lovely puddle.
>>
>> Which begs the question - how can we minimise that loss % thereby
>> increase the propulsive efficiency of our stroke?
>>
>> Let me just say that this is where momentum comes into the calculation.
>> The force applied is proportional to the momentum gained by the mass
>> against which you are reacting. So you have an increase in momentum,
>> which is proportional to the thrust, which in turn is proportional to
>> the mass of water moved by the blade & the change in its velocity
>> (assumed to start from zero at the catch). And you have a simultaneous
>> increase in the kinetic energy of that mass of water, which is
>> proportional to that same mass of water and to the _square_ of its
>> increase in velocity.
>>
>> I'll leave the there, to be resumed in my next posting. Some may
>> already see where this is heading...
>>
>> Cheers -
>> Carl
>>

>
> I am waiting for the next instalment ......
>
> but - my reading so far, is;
>
> we are aiming for the boat to have as constant a speed as is possible - because this reduces 'drag' and therefore is the most 'efficient' use of the input.
>
> The blade efficiency (and possibly biomechanical force production capability) vary through the stroke cycle - I think I remember that the blade efficiency is greatest at the beginning and the end of the cycle, and biomechanically we are probably (muscle cross section) looking at the most force production availability in the early portion of the drive (up to perpendicular with the pin, when most knees are down and hip swing part way through?)
>
> What I am going to postulate ('cos I only learn by having my thinking picked apart) is that even within this cycle, with these caveats, there is a dis-benefit to over-doing the acceleration early in the cycle (drag, fluid dynamics, blisters, tendons etc) and more benefit to an application of force that maximises the efficiencies here, and then holding that force through the stroke to the end where it becomes more efficient?)
>
> So I guess, to crudely summarise, I am a "hang and hold" rather than a "hang and bang".
>
> James
>

Forget the rower for the moment, & just consider the relationships
between thrust, momentum and kinetic energy:-

1. The equal & opposite reaction force between blade & a mass of water
is proportional to the rate at which momentum is given to that mass &
the rate of change of its velocity (its acceleration).
2. The kinetic energy given to that mass (which is energy not available
for boat propulsion) is proportional to that mass of water & the square
of its velocity
change.

Now halve that mass of water. Then as the same total momentum must be
imparted to that reduced water mass its velocity gain must double.
Doubling that velocity change means the velocity-squared kinetic energy
term doubles the total kinetic energy lost to that water.

Instead double the mass of water engaged with the blade &, for the same
reaction force, the kinetic energy lost to the water is halved.

Thus the bigger the volume (mass) of water with which the blade can
usefully engage the less of your input energy is lost - doubling that
volume halves the losses, etc.

While simple, this analysis is directly applicable to how you row, and
applies to all propulsion systems based on reaction from giving momentum
to a surrounding fluid.

In order to engage in/with more water, around the mid-stroke the blade
must be completely surrounded by water - water above as well as below &
around. This is achievable only by rowing the blade deeper in that part
of the stroke to ensure there is water, rather than air, above it. That
conflicts with popular disapproval of "looming", but fluid dynamics
tends to be counter-intuitive.

The popular argument against taking the mid-stroke deeper is that the
loom or shaft of the oar is then immersed & that, as the boat is moving
forwards, the shaft must then be dragging through the water. That
argument contains one huge and erroneous assumption, which is that the
turning point of the oar lies somewhere within the length of the blade
and than none of the shaft is moving sternwards. So let's examine that.

If any part of the blade is effectively static within the water, then
however much of the blade that is inboard from that point must be moving
forward, backwatering & thus generating speed-reducing drag. The
reality is that in the mid-stroke the entire blade is stalled and is
moving sternwards through the water, the outboard end moving faster
sternwards than the inboard end as the oar is rotating about an as-yet
undefined vertical axis. And that means that this axis - call it the
point of rotation WRT the water - must lie somewhere inboard from the
inner end of the blade.

This means that we can immerse, entirely without drag penalty, any part
of the shaft which lies outboard from that point of rotation, so the
blade can safely go deeper without incurring any adverse fluid drag on
the shaft & ensuring that it is fully covered. So much for those
antiquated arguments against looming!

Now scrutinise videos of great antipodean scullers, e.g. Drysdale or
Waddell. See how deep their blades go at mid-stroke. And then look for
any sign of shaft back-watering - it ain't happening.

Note that I'm dealing here only with the stalled (hence least efficient,
but inevitable & necessary) part of the stroke. We can discuss the very
different and more efficient (Lower energy losses) initial & terminal
phases of the stroke at another time.

One of the most obvious perceptions, for the rower, of rowing a deeper
mid-stroke is that the stroke takes somewhat longer to complete because
the increase in efficiency of that phase reduces the rate of blade
slippage. Unfortunately, most of us are averse to anything which upsets
our sense of how long the stroke should take, so we quite naturally tend
to over-exert ourselves in the vain hope of getting the time in the
water back down to what we're used to. And then we decide that we're
now "overloaded" or too severely geared. But the reality is that the
overloading we feel is that which we unwisely impose upon ourselves in
our effort to shorten stroke duration back to "where it should be".

The fundamental lesson is that slip is not a substitute for gearing, &
that rowing deeper in mid-stroke educes slip, increases propulsive
efficiency & demands a slightly increased time in the water. Yet most
rowers shy away from that, persuaded by the siren voices of the
anti-loomers. How strange that a sport so dependent on fluid dynamics
has so little interest in or grasp of that very subject.

Cheers -
Carl

--
Carl Douglas Racing Shells -
Fine Small-Boats/AeRoWing Low-drag Riggers/Advanced Accessories
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