Unbalanced Handle Force

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Unbalanced Handle Force

Post by topherdawson on Wed Dec 14, 2016 7:35 pm

1 This is the downwards force which a rower has to exert on the handle 150mm in from the end, to lift the blade out of the water.

2 If it is too large, rowers call the oar "heavy" or unbalanced".

3 Some people including me think that a balanced oar is one where the forearms and hands of the rower just balance the UHF when they are holding their arms relaxed. Allowing the hand and arm muscles a time in the stroke to relax lets the blood circulate and makes the whole experience more enjoyable I think.

4 Weighing my forearms and hands on a kitchen scale gives 3kg but my wife's are 1.5kg. Maybe 2kg is a compromise?

5 Robert Graham of Dundrum told me in a conversation that 5 pounds (2.3kg) is about optimal.

6 It seems to me that using heavy wood in a solid inboard and light wood, possibly hollow, in the outboard, we can design for whatever UHF we want. What does everyone else think?

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Re: Unbalanced Handle Force

Post by Ian Mills on Fri Dec 16, 2016 9:47 am

Crail UHF:

We measured our UHF at 200 mm as requested, but as you suggest above, I think 150 mm in from the end is a more accurate position.
Ours measured at about 4kg, but if measured at 150 mm, this would be a bit less.
I think I will re-measure at least one oar if I have time over the weekend.

Referring to Rory's use of the Cooper Harper scale in the 'Gearing' thread (which I think is a great concept and could be very useful for us and could be added to the survey?) the UHF of an oar is one part of the 'feel' of an oar, and the way it behaves in general.

To answer Topher's question in the other thread – I weighed my fore arms, out stretched but slightly flexed (in the same way as in the recovery phase of the stroke) and they weighed around 3kg.
To me, our oars feel perfectly balanced. There is no excessive downward force required on the recovery to keep the blade skimming nicely just above the water.

But, yes – this may not be optimal for smaller people.
However – I have only noticed problems handling the oars with our very smallest rowers, who would be better off with smaller oars anyway. (I am referring to a 14 year old girl who clearly needed to push down far too much.)

Part of the 'feel' of the oar is also the way it is thrown around by rough water (in the recovery) and the way it takes the catch. Both of these aspects need an oar with a bit of 'heft' to them.
I suspect that the UHF of modern carbon oars is much lighter than ours, but they do not have to contend with such rough conditions as us (usually).

If you catch the blade on a wave in the recovery, a very low UHF is not a good idea as the blade would be sent skywards more easily.
At the catch, similarly, it helps to have a little weight there to help drop the blade in with no effort - Just by releasing the hands upwards, rather than a conscious physical effort to raise them.

So, I would caution against very low UHFs for optimum handling.

I'll try and have a play with our oars again this weekend and report back!

Ian

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Re: Unbalanced Handle Force

Post by topherdawson on Fri Dec 16, 2016 10:23 am

Even with a UHF in the region of 2kg, the oar when released will drop in quickly. The folded ply spoon shape is vee'd vertically so if only partly immersed it tends to dig down when the pressure is applied until fully immersed.

Another "feel" factor is the polar moment of inertia. A heavy balanced oar will have more of it than a light balanced oar, and muscular effort is needed to accelerate and decelerate the oar movement in the horizontal plane.

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Re: Unbalanced Handle Force

Post by Ian Mills on Fri Dec 16, 2016 11:16 am

Yes, point taken.
I think Polar Inertia is really what I am referring to in my 5th paragraph above.
As you point out PMI and UHF are separate issues.

P.S.
For what its worth, I think our Polar Moment of Inertia is fine as it is.
But yes, our UHF could be a little lighter. (But not a huge amount.)
This could easily have been improved by lighter blades and/or solid and less rounded inboard.

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Re: Unbalanced Handle Force

Post by topherdawson on Fri Dec 16, 2016 5:38 pm

After wrapping chain round the handle of my oar to reduce the UHF I became more aware of PMI. Adding 1kg of chain did not increase the PMI by much so I think that the reason I became more aware of it was that my perception of the "weight" of the oar was a mixture of the vertical UHF and the horizontal PMI. When I took some of the UHF out of the equation I became more aware of the PMI.

What comes out of this is that making the outboard lighter is better than making the inboard heavier.

Your point about lighter blades and solid inboards is exactly that.

We have a set of carbon/glass oars cast off by Inverness Rowing Club and tomorrow I'll measure UHF and stiffness.

I can't be bothered to properly calculate PMI so I will take total weight as a proxy for it. If we have done all we can to reduce the weight of the outboard then we need enough weight inboard to optimise the UHF.

The only way I know to measure the PMI is to put a spring of a known stiffness against the handle and count the frequency of the natural oscillation. Not something I think we can get people to do!

If we imagine what physical properties a rower could feel if rowing with their eyes shut, they would be UHF, PMI, stiffness, some sort of hydrodynamic number like blade area times drag coefficient, and something related to how fast the boat was going and how hard the rowers were pulling and what gear they were in.

I think the best we can do is optimise a design for UHF, stiffness and total weight, using a blade of around 1000 cm^2 (in the middle of the range) and efficient shape.

Currently I think both the solid shaft design and even more the hollow shaft design are better, in aggregate, that any of the oars for which we have data. We may receive or evolve better designs yet.

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Re: Unbalanced Handle Force

Post by Ian Mills on Fri Dec 16, 2016 6:06 pm

I've just checked our blade area and it is about 975 cm sq, so similar to what you propose.
It was based on the Portobello shape and is a stretched ellipse (created in Adobe Illustrator), chopped to length (if that makes sense) so it slightly tapers in towards the tip after the widest point.
Eastern also use a similar shape.
Soon I'll have time to put some photos etc in a drop box folder...

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Re: Unbalanced Handle Force

Post by topherdawson on Sat Dec 17, 2016 2:08 pm

I've just measured a carbon/glass oar cast off by Inverness Rowing Club and it's geared at 2.3 which reflects that fact the sliding seat rowers move their oar handle a lot further and faster than we do.

The total weight is 1.1kg, less than a quarter of our average weight, and the UHF is 0.4kg! So even at a tenth of the UHF we have become used to, they manage the catch.

I was interested to learn that the deflection was 90mm so it is no stiffer than ours.

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Re: Unbalanced Handle Force

Post by Ian Mills on Sat Dec 17, 2016 5:14 pm

Yup, very interesting.
Sadly, I've been deluding myself that there was some benefit to having ridiculously over-engineered oars! Crying or Very sad

Out of interest - roughly what length is it and do you know what boat it has come out of?

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Re: Unbalanced Handle Force

Post by topherdawson on Sat Dec 17, 2016 5:22 pm

It's 3770mm long (it's on the spreadsheet). Looks pretty cheap, i.e. a fairly old club oar, much used, probably more glass than carbon, a sweep oar but not sure what boat it came from. Most of their sweep oared boats are 4's.

Your oars are very nice and definitely better than most, so don't be sad!

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Re: Unbalanced Handle Force

Post by Rory Cowan on Sun Dec 18, 2016 6:55 pm

For the table of data what distance is the UHF measured from? We measured from 200mm. -

On the matter of PMI, in a previous life we used to match helicopter rotor blades on the basis of weight, age and second moment - we got the second moment quite simply by hanging the blade vertically, from the root bolts, letting it swing, and timing the oscillation - it worked well.

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Re: Unbalanced Handle Force

Post by topherdawson on Sun Dec 18, 2016 7:54 pm

My fault, originally I though 200mm was a good approximation to where the hands act on the oar but later realised that 150mm was a better fit. Average inboard is 1220 so the 50mm difference will make 4% difference in the UHF which is not serious.

That's a good way of measuring PMI which I was unaware of, but I think people have gone out and measured oars and it's too late to ask for PMI at this stage. I think total weight will be a reasonable proxy for PMI, i.e. if a blade is heavier it is also likely to have a greater PMI.

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Re: Unbalanced Handle Force

Post by Rory Cowan on Sun Dec 18, 2016 8:43 pm

I would think so for PMI but the UHF will have a relationship. Subjectively we couldl refer to that as 'heft'

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Re: Unbalanced Handle Force

Post by topherdawson on Sun Dec 18, 2016 9:01 pm

Yes and this is where it gets tricky. Our oars are hollow and pretty good but still have a UHF at the bow where I row of 3kg. Wrapping 1 kg of chain round the handle made it much nicer for balance, but I then became aware of a rather ponderous feeling that the PMI was high. I had not been aware of it before, and the chain did not increase the PMI much. Maybe 10% if that. But previous to the chain being added I had mentally rolled together the UHF and the PMI and in effect the UHF had masked the PMI. Hard to untangle!

Making the outboard shaft and blade light reduces both UHF and PMIso it is crucial, and I believe we should also keep the oar length to a minimum for the same reason. Coigach and Dundrum don't, so we may not be able to agree on that.

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