Inboard Lengths and Seating Positions

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Inboard Lengths and Seating Positions

Post by Finlay Robertson on Fri Dec 09, 2016 10:10 am

I've started this new topic because the seating convention adopted will directly impact design of the oar. Inboard length and seat spacing have to be grouped together in a common discussion, as they are intrinsically linked.

The fundamental aim is to prevent clashes between rowers (i.e. bruised kidneys!), for which I can identify 4 main solutions:

1. accepting the restricted space (not really a solution, but new clubs often endure this for a period)
2. deviation from Iain Oughtred's original plans to provide wider spacing of seats
3. shortening the inboard lengths such that they pass outboard of the next rower
4. offset seating with a corresponding increase in inboard length; this can be done instead of or in addition to wider seat spacing

The link between rower position and inboard length means that the adoption of a single oar design will also require a standardisation of seat spacing / rower positioning. This will, in turn, require modification of thwarts, seating, knees (if fitted) and footrests on some existing skiffs. Any proposal we make will require substantial boatwork to be carried out throughout the fleet - a point we need to bear in mind before extolling too heavily the virtues of a 'simple' oar design.

Discuss!

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Re: Inboard Lengths and Seating Positions

Post by topherdawson on Fri Dec 09, 2016 11:25 am

I think most of the fleet have worked out that at seats 2 and 3, inboards of about <875mm or >1125mm are needed to avoid clashing. The "short oar" option is your point 3 above; note that this is also offset seating, but in the opposite direction to long oars. Your point 4 is the "long oar" option.

How this works out in terms of oar length depends on gearing. We use a FISA gearing of 3 (coincidentally also a true gearing of 3.03) which means our present long bladed oars are 3572mm at seats 2 and 3.

If like some Australian clubs you are prepared to row at a FISA gearing of say 2.2, you can use an oar of 3.6m length (standard sliding seat sweep oar) and still have a long enough inboard to prevent clashes.

All the boats I have measured lie in the region 2.6 to 3.

Yes there are repercussions for thole pin position and footrests, but I don't think we need to put them in the rules. I think we should be advising clubs to move the seats as above, as well as adopting the new design. Since the plans show footrests which depend on floorboards and most of us have discarded floorboards, we need to be providing guidance on footrests but I don't think we need to legislate.

It would probably be just to provide a "short oar" version of the design for clubs who like short oars, and a design for "long oar" clubs which would have two long oars around 4.5m for seats 2 and 3 plus shorter ones for bow and stroke.

I don't think it would be good to have a lot of intermediate length oars around, but it could get messy.

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Re: Inboard Lengths and Seating Positions

Post by Don Currie on Tue Dec 13, 2016 7:57 pm

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Seems to me that as soon as you start trying to minimise clashes by increasing the inboard length another, simpler, option pops into view - move the thole pin inboard!  Most boats seem to have the thole pins splayed outwards, roughly in line with the angle of the top strake.  This has a few of perverse effects - it introduces all manner of weird rotations to the loom throughout the stroke, it means the tholes catch on wharf piles etc when coming along side, and it moves the pivot point of the oar outboard.  Have a look at the attached sketch.  If the thole pin is attached via a small block mounted to the inside of the gunwale, and the pin stood vertical, the pivot point of the oar is moved inboard about 69mm.  So with the rower sitting in exactly the same place, you can cut 69mm off the inboard - and with 2.5:1 gearing you can chop 172mm off the outboard.  Total of 240mm off the oar by moving the pin in 69mm.  The saving are even greater with higher gearing.
I've not yet had approval to chop into our boat yet, but it seems to me that moving the #2 and #3 pins inboard 80mm or so would be well worth trying.  There will be a natural limit to how far in you can come because you'll eventually start limiting the amount you can dip the oar in rough water, but being able to use shorter (thus lighter and cheaper) oars at the same time as having a clear gunwale and a vertical pin has got to be worth a try.  Rule 3.2 requires thole pins to  be "at the gunwale" - which is not the same as "through the gunwale", so I'd assume that the mounting I'm proposing would be legal.  It's certainly not an outrigger - quite the opposite!

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Re: Inboard Lengths and Seating Positions

Post by topherdawson on Wed Dec 14, 2016 9:28 am

Hi Don,

The "inrigger" you propose had occurred to me too, and I emailed Robbie Wightman, the convener, asking about it. I did not get a reply but I will try again.

Our (Ullapool) pins are already vertical. so we would be gaining maybe 50mm if the pins touch the inside of the gunwale, or more if we went further. As you point out, there is a limit which is the point where the underside of the oar at its lowest angle hits the gunwale.

But it's true that at our gearing of 3 we could make the oar 200mm shorter which does make it lighter and cheaper, and may well allow the bow oar to be the same length.

I can hear skiff builders all over the world breathing a weary sigh at all this work!

I will sound out Robbie and get back. Also see the next post in the oarlock thread for another idea.

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Re: Inboard Lengths and Seating Positions

Post by Finlay Robertson on Wed Dec 21, 2016 1:07 pm

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Like this? This was the original system fitted to North Berwick's 3rd boat, Blackadder, though it did not prove popular.  As soon as the boat came in for winter maintenence a few months after this picture was taken, it was replaced with the previous ('normal') system that we already had on our other two boats (pins vertical on a raised routh but through the gunwale rather than an inrigger). I honestly can't remember what the exact issues were as we didn't keep this system for long.

It would be a challenging concept to trial, since it would obviously have to be attached to a particular boat. I'm also a bit concerned that any proposals that require substantial boatwork are unlikely to be met with much enthusiasm from club bosuns.

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Re: Inboard Lengths and Seating Positions

Post by topherdawson on Wed Dec 21, 2016 5:55 pm

That is exactly the proposal Finlay for oars 2 and 3, although not needed at bow and stroke. If it is indeed unworkable it would eliminate a line of enquiry. Like you I thought that builders would be unenthusiastic.

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Re: Inboard Lengths and Seating Positions

Post by Don Currie on Thu Dec 22, 2016 12:17 am

Finlay's photo, and description of how the differing thole mountings were received by the rowers outlines the problem with making any progress at all in this world! Did the rowers use the new position in conjunction with shorter and lighter oars? If not, all they got was something different, and the old oars feeling exactly the same.
Even a "Cooper Harper" approach doesn't really move the decision making process along very much unless rowers are:
a) comparing the revised pin position in conjunction with the full consequential changes, and
b) they spend enough time with the revised arrangement to get past the "I don't like this because it's different" phase.
Looking at the photo reinforces my feeling that small inriggers would be very easy to make - mounting metal rowlocks in a small block attached to the inside of the gunwale has been a normal process for small tenders (that will come along side the mother ship) since the metal rowlock was invented - nothing in that photo looks even remotely difficult to make. It's difficult to see that making a couple of inriggers would be (for instance) more difficult than making wooden rowlocks.
The other thing that stands out in the photo is the fact that each rowing position is only fitted for a pin/rowlock on one side. I've long thought that one of the advantages of the single oar per rower St Ayles is that if you want to experiment with rowlocks, or something like inriggers, then (provided that your rowers are ambioarstrous!) you can have differing oar and rowlock systems on each side of each seat, and so very quickly change from one system to another. The ability to set a boat up for two systems and quickly change between them should allow testing in the same wind and/or wave conditions.
All in all, the photo shows a neat and clean gunwale - no wear pads, no pins leaning out to snag anything. It looks really nice to my eye, and it seems odd and a bit sad that the proposal failed to fly, or even get much of a chance to fly.

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Re: Inboard Lengths and Seating Positions

Post by topherdawson on Thu Dec 22, 2016 9:53 am

Perhaps Finlay we need to know what the problem was? Do you know who tried it, and maybe I can email them? Even if the oars were exactly the same, there should have been less chance of a clash.

Don not all oars are ambioarstrous, and with spoon blades are about to get even less so. But it would be possible to mount inriggers on a boat with pin holes also in the gunwale, and move the pins from one hole to another.

I'm willing to do this to one of our boats but the problem here is that we need to know fairly quickly if we are going to try this, because it means we'd be making a prototype oar 4.3m long instead of 4.5m.

Currently the mean length of bow oars is 4322mm so we could make oars 1, 2 and 3 the same. Stroke oars are a mean length of 3994, say 4m, so we can't use a 4.3m oar there.

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Re: Inboard Lengths and Seating Positions

Post by Don Currie on Thu Dec 22, 2016 7:09 pm

No problem at all to make a spoon ambioarstrous with the vertical retainer plate and spacer system - ya just put a narrow wear strip top and bottom. We did that for the prototype hollow square oar. Minimal extra work and weight. 4.3M is a really nice size (we use that for our bow oar) - feels much nicer than the 4.5s we use at positions 2 and 3. It would be nice if we were able to end up with a ship set being 3x 4.3M ambioarstrous oars and only the stroke being the odd one out. Hanging out for more feedback from Finlay.....!

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Re: Inboard Lengths and Seating Positions

Post by topherdawson on Thu Dec 22, 2016 8:30 pm

Did you do a 4 degree pitch (top tipped towards the cox) and if so where? Sliding seat and fixed seat rowers seem to use 4 degrees a lot, and it stops the blade going too deep.

I can see your system makes swapping sides easy and I'm attracted to the idea that we could have 3 oars the same.

Can you fill in a measurement form for your hollow oars? Do you remember what the width and depth at the oarlock were, and what the strip thickness was? Scaling off your pin and keeper drawing it looks like 70x80 or so. What was the UHF?

I found an error in the design spreadsheet which made the hollow oar look better than it actually was. Current best design is width 85, depth 57, walls 7 which is a bit scarily thin, it will need wee bulkheads to stop it squishing. Total weight for that one is 5.4kg, UHF 2.0kg.

Best solid trapezoid is width 75, depth 54, total weight 5.2kg, UHF 2.3kg. Not much to choose between them now.

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Re: Inboard Lengths and Seating Positions

Post by Finlay Robertson on Thu Dec 22, 2016 9:42 pm

The issues certainly weren’t exclusively because of the inriggers; the oars had a placeholder pins-and-plates system that had a tendency to rock and certainly put people off. There were also problems with wear, which quickly started causing damage to both the oar plates and the gunwale. Don’s system to permanently engage the oar against the pin might reduce this, and Topher’s method of supporting the entire weight of the oar on the pin would remove it entirely. However, we added in sacrificial rouths to take the damage, and also fix a separate issue...

Prior to adding in the rouths (which raised the oars an inch or so), I think that one of the biggest problems was that the oars – particularly in 2 and 3 – were just too low down; it introduced the continual problem of clashing with the rowers‘ knees, and made them particularly susceptible to catching crabs (especially in a seaway). I imagine a lot depends on just how high a particular boat's seats are mounted, and also the types of oars - our Macon blades are more susceptible to catching the water than narrower ones. Lowering the seats might go some way to getting around the problem, though this – again – is extra work for club bosuns! It might be worth considering ways of raising the inwale on a permanent routh to increase its height above the gunwale, though this would rather spoil the clean aesthetic that Don so admires!

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Re: Inboard Lengths and Seating Positions

Post by Don Currie on Thu Dec 22, 2016 10:21 pm

So many questions Topher! No we don't use a 4 degree pitch on the pin - my understanding of the reason for the sliding seat brigade using an inclined pin is that with the rowlocks so close to the water, and thus the immersed oar still being at quite a flat angle, the handle ends up being below the rower's shoulder. When the rower puts the power on they tend to pull the handle slightly upwards, which drives the blade down. The pitch angle works against this tendency. With the higher gunwales of the St Ayles the oar is angled down more, and from the photos I've looked at most St Ayles rowers are pulling the handle through at close to shoulder height. So I'm not convinced that we should just follow the slidy seat mob in this case.
I filled out an oar measurement form and emailed it to you on the 14th. Perhaps check your emails again, or have you already consigned me to the spam folder! So dismissive! Oars are 70 by 60 at the lock, with an 8mm hardwood wear strip full depth in way of the pin.
We use 6mm strakes top and bottom for the square oars. No problems to date, though the oar is solid in way of the thole pin (and probably 150mm beyond). The fore and aft strakes are much thicker, but they need to be to make the whole thing easier to glue up. Really thin strakes are a bit wayward before they are glued up - using a backing board over them during glueing helps tame them. All written up in the article I sent you.
I don't want to be struck by lightning for suggesting this, but perhaps the top and bottom strakes could be made from plywood - 4mm Gaboon should be fine. Go crazy. Nice consistent material, cheap and available, tack it all together with staples 'till the glue is dry, so not many clamps required, and only the front and rear strakes would need to be made out of expensive spruce or the like.

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Re: Inboard Lengths and Seating Positions

Post by Don Currie on Thu Dec 22, 2016 10:43 pm

Thanks for the reply Finlay - that goes a long way to explaining why the club went away from the inriggers. Have a look at the sketch I did a few posts earlier - is a "routh" the "half round oar pad" in the drawing? I'm not familiar with the term "routh". And yes, I tend to agree that if we were all starting again we'd put the seats quite a bit lower so that all rowers other than the very tall would require a packer (cushion) under their back side. The alternative is some sort of pad on the gunwale in the vicinity of the pin, but with a bit of care that can be made to look quite nice. I become very prepared to sacrifice the aesthetic in favour of the practical when faced with a long row in rough water!

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Re: Inboard Lengths and Seating Positions

Post by topherdawson on Thu Dec 22, 2016 11:46 pm

Don Currie wrote:So many questions Topher!  No we don't use a 4 degree pitch on the pin - my understanding of the reason for the sliding seat brigade using an inclined pin is that with the rowlocks so close to the water, and thus the immersed oar still being at quite a flat angle, the handle ends up being below the rower's shoulder.  When the rower puts the power on they tend to pull the handle slightly upwards, which drives the blade down.  The pitch angle works against this tendency.  With the higher gunwales of the St Ayles the oar is angled down more, and from the photos I've looked  at most St Ayles rowers are pulling the handle through at close to shoulder height.  So I'm not convinced that we should just follow the slidy seat mob in this case.  
I filled out an oar measurement form and emailed it to you on the 14th.
 
Sorry Don, that seems to have gone astray. I'd be very grateful if you could send it again.

Perhaps check your emails again, or have you already consigned me to the spam folder!  So dismissive!   Oars are 70 by 60 at the lock, with an 8mm hardwood wear strip full depth in way of the pin.
We use 6mm strakes top and bottom for the square oars.  No problems to date, though the oar is solid in way of the thole pin (and probably 150mm beyond).  The fore and aft strakes are much thicker, but they need to be to make the whole thing easier to glue up.  Really thin strakes are a bit wayward before they are glued up - using a backing board over them during glueing helps tame them.   All written up in the article I sent you.  
I don't want to be struck by lightning for suggesting this, but perhaps the top and bottom strakes could be made from plywood - 4mm Gaboon should be fine.  Go crazy.  Nice consistent material, cheap and available, tack it all together with staples 'till the glue is dry, so not many clamps required, and only the front and rear strakes would need to be made out of expensive spruce or the like.  

I did once make a box section oar with ply webs and it worked fine. If the boat can be ply why not the oars?
I've had a couple of radical phone calls with a Dutch oar maker in Finland and also the designer of the St Ayles Skiff Iain Oughtred which I'm going to post in the New Design forum.

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Re: Inboard Lengths and Seating Positions

Post by Finlay Robertson on Fri Dec 23, 2016 6:57 pm

Another thing to remember when considering the pitch angle of the pin is that a St Ayles Skiff trims down by the stern when in motion – I calculate a magnitude of around 2.5 degrees based on the submergence of all but the top 3 strakes at the stern and the submergence of only 1 strake at the bow. (More-or-less consistent with a cursory assessment of some photos I looked at online.) Hydrodynamically, this is on account of the low pressure generated by the curved stern of a skiff – a feature that narrow slidey-seat (and, indeed, fixed-seat) rowing shells lack. As such, they remain at a more-or-less level trim regardless of speed.

Assuming that the pins of a skiff are considered to be vertical when respecting the keel as the horizontal datum, the oar blades will actually be entering the water at an angle of around 2.5 degrees from the vertical with the boat in motion.

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Re: Inboard Lengths and Seating Positions

Post by topherdawson on Fri Dec 23, 2016 8:07 pm

Hi Finlay,

A couple of other things; the new oars will have a lot less UHF so the force pushing the blade under will be less.

If we use spoons shallowly vee'd in cross section they will tend to self level because if just the lower half of the spoon is immersed when the pull starts, it will pull down into the water until the upper half makes an equal and opposite upwards force.

For these two reasons plus your 2.5 degrees inclination of the keel, we can forget about the 4 degrees which may make the oars ambioarstrous.

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Re: Inboard Lengths and Seating Positions

Post by Finlay Robertson on Tue Dec 27, 2016 6:42 pm

I've had some feedback from our bosuns with regard to North Berwick's previous 'inwale' as pictured above. I've tried to put together a summary; not sure how helpful it really is in identifying the specific issues that led to its rejection, but here it is:

  • The purpose of the inwale was to enable the vertical mounting of pins without having to drill through the gunwale.
  • We already had vertical pins drilled through the gunwale of one of our existing boats with the rotating oarlock system, but the individual responsible for the new skiff wasn’t entirely confident about drilling through the gunwale – thus the new ‘inwale’ system.
  • There was no systematic trial, but anecdotal feedback indicated widespread discontent with the new system. There’s no consensus as to any one specific problem, but the fact that the pin was inboard of what rowers were used to was at least part of the issue.
  • The decision was taken to standardise the previous rotating oarlock system on all three of our boats, as this system was almost universally popular

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Re: Inboard Lengths and Seating Positions

Post by topherdawson on Tue Dec 27, 2016 7:38 pm

Thanks Finlay for researching this. I think it is worth trying and we may discover that we don't like it, but the potential upside is (a) we can use lighter less expensive 4.3m oars and (b) the bow oar can be the same as 2 and 3.

For what it's worth the weakening of the gunwale with a diagonal hole was one of the reasons I went for a spaced gunwale in our boats.

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Re: Inboard Lengths and Seating Positions

Post by SHUG on Fri Jan 13, 2017 12:32 pm

I have followed the debate on the forum with interest mingled with some concern since I don’t subscribe to the idea that a “referendum” on oars will actually lead to a good design. At worst it may perpetuate all the oar design faults, which may already exist. So, I’m going to stick my head above the parapet and check the arithmetic of oar design for the Skiff. So, here:
Gearing: From hydrodynamic limits, the maximum speed of a skiff is about 5.5knots or 2.8 metres per second.
The range of movement of the rower’s hands during the Drive is about 1metre.
At 32 strokes per minute the total stroke sequence takes about 1.9 seconds and if that is split evenly between Drive and Recovery (which is debateable) then the time spent in Drive is 0.95 seconds.
This means that the rower’s hand velocity in Drive is 1/0.95 metres per second or 1.05m/s
So to match the boat speed to the hand speed we use the gearing of the oar.
So, the first stab at gearing is: boat speed/hand speed =2.8/1.05= 2.66
Oars are not an efficient means of propulsion and are generally about 80% efficient due to slippage, vortice shedding etc so lets add on 20% to allow for that and this brings the gearing to 3.2. For simplicity lets make that a nice round 3 which is a commonly used gearing.
Oar Length: For centreline rowing the starting point is the distance from the oarlock to the centreline of the boat. Typical measurements are as follows ( please excuse Imperial units): Using a gearing of 3.
Oarlock to centreline
Stroke 35 inches so oar length is 35+(3x35) = 140 inches or 11.6 ft  
No2 40inches    40+(3x40) = 160 inches or 13.3ft    
No3 39inches   39+(3x39) =156 inches or 13ft
Bow 34inches   34 +(3x34) = 136 inches or 11.3 ft

Typically the rower’s hands are placed about 12inches apart so an additional 6inches should be added inboard to retain the rower’s centre of effort on the skiff centreline.
This makes the rounded up overall oar lengths for centreline rowing to be
Stroke 12.1ft  No2  13ft 9inches No 3  13ft 10 inches, , Bow 11.8ft
Or for practical use the skiff could have two 12ft oars and two 14ft oars.
Or in metric units: two at 3658mm and two at 4267mm

Compare with survey values here: [You must be registered and logged in to see this link.] comparisons November 2016.xls?dl=0
Bumping: This  “problem” was first perceived by Ullapool but is not seen as a general problem by other Clubs .It has led to a series of misconceptions and design problems.
To avoid bumping, offset seating is one solution although increasing the seat spacing is also quite practical. The difficulty is that if you move the rower’s position inboard by 1ft you have to increase the outboard oar length by 3ft to maintain the gearing. So for a  1ft inboard increase in oar length we end up with a 4ft overall increase in oar length giving Stroke, 16ft, No2, 18ft, No3 18ft, Bow 16ft.. These long oars create design problems in terms of stress distribution in the oar shafts, outboard weight etc.
A misconception is that the longer oars give a greater length of stroke “in the water”. What happens with longer oars at the same stroke rate is that the arc through which the oar moves is reduced and this reduces bumping. If you wish to achieve the same hand movement as you have with shorter oars then you have to reduce the stroke rate. Then you are back where you started but the loom of an oar rather than the knuckles of the rower behind you will bump you.
To give you the arithmetic of this With the boat moving at 2.8m/s , gearing of 3 and a stroke rate of 32per min, the relative motion between the oarlock and the oar blade in the Drive is 2.66metres. For an outboard length o 3.2metres  (14ft oar) the arc is about 50 degrees.
If for the same conditions the outboard length is 4.1metres (18ft oar) the arc reduces to 37degrees. The only way of retrieving the 13 degrees difference is by keeping the oar in the water for longer with a lower stroke rate.
Now this scenario may suit some rowers where long slow powerful strokes can achieve the same Drive as shorter strokes at a higher stroke rate. The problem with the slow stroke is that the Recovery time is longer and the skiff will slow down more between strokes and this may reduce the average speed.
So I hope this has given food for thought. My arithmetic may not be perfect but you can do your own calculations along the same lines as I have set out.
Discuss!!!! Shocked

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Re: Inboard Lengths and Seating Positions

Post by topherdawson on Fri Jan 13, 2017 2:39 pm

Hi Hugh,

Thanks for this long post which contains many points. I will comment in the order you raise them. I'm going to concentrate on oars 2 and 3 which in the "long oar" staggered layout are causing the most hassle.


I have followed the debate on the forum with interest mingled with some concern since I don’t subscribe to the idea that a “referendum” on oars will actually lead to a good design. At worst it may perpetuate all the oar design faults, which may already exist.

We are not taking part in a referendum but we are discussing what we've agreed are targets for a better set of oars than existing ones, and what can be done to achieve these targets.

So, I’m going to stick my head above the parapet and check the arithmetic of oar design for the Skiff. So, here:
Gearing: From hydrodynamic limits, the maximum speed of a skiff is about 5.5knots or 2.8 metres per second.
The range of movement of the rower’s hands during the Drive is about 1metre.

Note that even with spacing the seats out more than the plans show, this is about the distance between seat centres, so the handle of the oar has to reach into the space occupied by the next rower aft.

At 32 strokes per minute the total stroke sequence takes about 1.9 seconds and if that is split evenly between Drive and Recovery (which is debateable) then the time spent in Drive is 0.95 seconds.

We (UCRC) are racing at 24 to 28 strokes per minute and it's possible the recovery takes longer than the drive.

This means that the rower’s hand velocity in Drive is 1/0.95 metres per second or 1.05m/s
So to match the boat speed to the hand speed we use the gearing of the oar.
So, the first stab at gearing is: boat speed/hand speed =2.8/1.05= 2.66
Oars are not an efficient means of propulsion and are generally about 80% efficient due to slippage, vortice shedding etc so lets add on 20% to allow for that and this brings the gearing to 3.2. For simplicity lets make that a nice round 3 which is a commonly used gearing.
Oar Length: For centreline rowing the starting point is the distance from the oarlock to the centreline of the boat. Typical measurements are as follows ( please excuse Imperial units): Using a gearing of 3.
Oarlock to centreline
Stroke 35 inches so oar length is 35+(3x35) = 140 inches or 11.6 ft  
No2 40inches    40+(3x40) = 160 inches or 13.3ft    
No3 39inches   39+(3x39) =156 inches or 13ft
Bow 34inches   34 +(3x34) = 136 inches or 11.3 ft

Typically the rower’s hands are placed about 12inches apart so an additional 6inches should be added inboard to retain the rower’s centre of effort on the skiff centreline.
This makes the rounded up overall oar lengths for centreline rowing to be
Stroke 12.1ft  No2  13ft 9inches No 3  13ft 10 inches, , Bow 11.8ft
Or for practical use the skiff could have two 12ft oars and two 14ft oars.
Or in metric units: two at 3658mm and two at 4267mm
Compare with survey values here: [You must be registered and logged in to see this link.] comparisons November 2016.xls?dl=0
Bumping: This  “problem” was first perceived by Ullapool but is not seen as a general problem by other Clubs .

If "bumping" or as I have been calling it, clashing, is not a problem why have so many clubs gone to oars just long enough to overcome it? Rowers from your club (who shall be nameless) have told me that you do have a clash problem.

It has led to a series of misconceptions and design problems.
To avoid bumping, offset seating is one solution although increasing the seat spacing is also quite practical. The difficulty is that if you move the rower’s position inboard by 1ft you have to increase the outboard oar length by 3ft to maintain the gearing. So for a  1ft inboard increase in oar length we end up with a 4ft overall increase in oar length giving Stroke, 16ft, No2, 18ft, No3 18ft, Bow 16ft.. These long oars create design problems in terms of stress distribution in the oar shafts, outboard weight etc.

You don't need to have oars as long as this. Scale drawings of the boat and seat layout and inspection of the spreadsheet you quote  confirm that as long as the inboard is more than about 1150 or less than about 890 there won't be a problem, if you also space the seats. At a FISA  gearing of 2.8 this translates to an overall length of more than 4.37m (14.3ft) or less than 3.38m (11ft). At a FISA gearing of 3 the oars need to be more than 4.6m (15ft) or less than 3.56m (11.6ft). This depends a bit on how long and narrow your blades are, because the true gearing is calculated from the middle of the blade area to the middle of the hands.

Don Currie, Rory Cowan and I are currently making prototype oars for positions 2 and 3 which are 4.3m long. We have achieved this saving in length by (a) deciding to move pins 2 and 3 inboard by 70mm and (b) by making the blade have a centre of effort 300mm in from the end of the blade.


A misconception is that the longer oars give a greater length of stroke “in the water”.

Agreed.

What happens with longer oars at the same stroke rate is that the arc through which the oar moves is reduced and this reduces bumping.

What you're saying is that to reduce clashes you are taking short strokes. Anyone can prevent clashes if they limit their stroke, but it limits the ergonomic efficiency of the stroke.

If you wish to achieve the same hand movement as you have with shorter oars then you have to reduce the stroke rate. Then you are back where you started but the loom of an oar rather than the knuckles of the rower behind you will bump you.

No because the loom of the oar at the beginning of the stroke angles forward and so misses the body of the next rower. I think reducing the stroke rate from 32 is probably beneficial. All the top boats at the Worlds were noticeably doing long fairly slow strokes and getting a lot out of each one.

To give you the arithmetic of this With the boat moving at 2.8m/s , gearing of 3 and a stroke rate of 32per min, the relative motion between the oarlock and the oar blade in the Drive is 2.66metres. For an outboard length o 3.2metres  (14ft oar) the arc is about 50 degrees.
If for the same conditions the outboard length is 4.1metres (18ft oar) the arc reduces to 37degrees. The only way of retrieving the 13 degrees difference is by keeping the oar in the water for longer with a lower stroke rate.
Now this scenario may suit some rowers where long slow powerful strokes can achieve the same Drive as shorter strokes at a higher stroke rate. The problem with the slow stroke is that the Recovery time is longer and the skiff will slow down more between strokes and this may reduce the average speed.

I think implicit in your remarks is the admission that you are limiting the stroke length to avoid clashes and stroking faster as a result. I think we should be offering the clubs a solution which does not artificially limit the stroke length. I think by carefully discussing this and designing accordingly we have in fact gone some way to solving the problem. I am finishing today a solid and easily made oar which looks as though we can hit the 2kg UHF target.

So I hope this has given food for thought. My arithmetic may not be perfect but you can do your own calculations along the same lines as I have set out.
Discuss!!!! Shocked

SHUG

Topher

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Gearing Oar Length and Bumping ensuing discission

Post by SHUG on Sun Jan 15, 2017 2:39 pm

To respond to each of Topher’s points in turn:

Note that even with spacing the seats out more than the plans show, this is about the distance between seat centres, so the handle of the oar has to reach into the space occupied by the next rower aft.

For the skiff I used in these calculations, the spacing between the seats is 1.05 metres and that leaves a useful clearance for an average rower with a 1metre hand stroke

We (UCRC) are racing at 24 to 28 strokes per minute and it's possible the recovery takes longer than the drive.

There are two points here. You are probably using a lower stroke rate because, with longer oars, you simply have to. With longer oars , for the same stroke rate, the arc of movement is reduced. The only way to remedy this is by reducing the stroke rate. I agree that the Recovery takes longer than the Drive but I didn’t want to introduce another controversy into the discussion. It is simple to adapt the arithmetic accordingly. This raises another parameter into the debate. In that different crews will prefer different stroke rates and this feeds directly into the choice of oar length


You don't need to have oars as long as this. Scale drawings of the boat and seat layout and inspection of the spreadsheet you quote  confirm that as long as the inboard is more than about 1150 or less than about 890 there won't be a problem, if you also space the seats. At a FISA  gearing of 2.8 this translates to an overall length of more than 4.37m (14.3ft) or less than 3.38m (11ft). At a FISA gearing of 3 the oars need to be more than 4.6m (15ft) or less than 3.56m (11.6ft). This depends a bit on how long and narrow your blades are, because the true gearing is calculated from the middle of the blade area to the middle of the hands.[/i]

Agreed. I also agree that there should be an accepted definition of gearing. I was making the point that to maintain the gearing with longer oars, for every increase in length, L, inboard you obviously have to increase the outboard length by  Lx gearing . It is probably clear that I don’t agree with your “Bumping Analysis. With longer oars, if you try to achieve the same arc of movement, and bumping has been a problem, you will hit the rower in front of you with the oar rather than with your knuckles.

I think implicit in your remarks is the admission that you are limiting the stroke length to avoid clashes and stroking faster as a result. I think we should be offering the clubs a solution which does not artificially limit the stroke length. I think by carefully discussing this and designing accordingly we have in fact gone some way to solving the problem. I am finishing today a solid and easily made oar which looks as though we can hit the 2kg UHF target.

I admitting nothing and you are incorrect to assume that the stroke length is being artificially limited. For the skiff on which my arithmetic is based, the seat spacing has been increased to avoid the bumping problem. The stroke rate I chose was somewhat arbitrary but is commonly achieved when the crew is pushing hard towards the finishing line which is when earlier oar decisions are really put to the test.

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Re: Inboard Lengths and Seating Positions

Post by john mcintyre on Wed Jan 18, 2017 7:09 pm

All

I wonder if the following helps, forgive me if you already know it. Assuming the oarsman leans back and reaches forward the same amount and the gearing remains the same for both long and short oars then this is how the angles of the blade change as the length of the oar changes

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Re: Inboard Lengths and Seating Positions

Post by Sean Watters on Wed Jan 18, 2017 9:50 pm

I was trying to find a similar diagram to post. With longer oars the angle of the arc is reduced, but the actual movement (given the same gearing) is pretty much identical to a short oar. Boats with long oars aren't keeping their blades in the water for longer to make up for a 13 degree difference. They don't need to. They've already pulled the same distance, albeit with a narrower angle.

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Re: Inboard Lengths and Seating Positions

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