Shape of oar blades

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Shape of oar blades

Post by topherdawson on Fri Dec 02, 2016 1:53 pm

1   As can be seen in the cover photo there is a huge variety of blade shapes in St Ayles skiff oars.

2   The measurement rules currently specify flat blades. This was to make them easier to construct.

3   They also ban "cleaver" or "chopper" type blades, on aesthetic grounds.



4   Most clubs have gone for long narrow rectangular sea type blades with parallel sides.


5   Some have gone for wider shorter rectangular blades.

6   The advantages of long narrow parallel blades are: robustness, ease of construction, will work at various depths of immersion.

7   The disadvantages are: less efficient since the tip is operating at a different gearing to the root, much more vertical movement of the oar needed between fully in and fully out of the water, contributes to a lot of out of balance weight since for the same effective gearing the oar must be longer, and its lower tip is prone to catching in the sea.

8   Choppers are adopted by sliding seat clubs because they can get a large area of blade quickly in and out of the water with minimal vertical movement. The top and bottom edges of the blade are parallel to the water surface as they pass through it. From the survey it is clear that clubs don't like the appearance.

9   The tulip or Macon shape is more traditional and universal among gigs. Its maximum width is back from the tip which makes it easier not to accidentally catch the lower tip in the sea. There is less up and down motion needed between fully out and fully in than the long sea blade.


10   Curved blades have significant hydrodynamic advantages. Since the tips are curled towards the aft of the boat, the blade does not stall at the catch and generates significant lift from the flow across the blade as the blade moves outward from the boat. The rower feels this as a more solid "bite' in the water with less slip. In the middle of the stroke the water is less able to escape round the end of the blade and so mounds up. At the finish the water is travelling outwards out of the blade and the curve directs this flow more behind the boat.



11   Traditional curved or spooned blades are carved from the solid, a skilled and time consuming process and therefore an expensive one. But with modern plywood it is very simple to make a curved blade which is much more robust than the traditional solid spruce one:

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Re: Shape of oar blades

Post by topherdawson on Thu Dec 08, 2016 9:10 pm

Recently I made a curved blade laminated out of two layers of 4mm ply. It was very robust but it weighed 385 grams. The I thought of a way to make a curved scoopy blade out of one layer of 4mm ply.

The two halves put together like a closed book and joined along a slightly curved seam with cable ties. Then they are opened and pressed open with a clamp. As they are opened they bend into a scoop shape. Then the joint is epoxied to keep them open. This first one sprang back more then I wanted but it is very stiff in bending, being slightly veed, and weighs 240g.

Although the photos don't make it look likely, both blades are the same width (190mm) and length (620mm).





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Re: Shape of oar blades

Post by Rory Cowan on Sat Dec 10, 2016 4:38 pm

What exactly is the definition of a 'cleaver' or 'chopper blade?  I understand that we are permitted to have blades that are symmetrical and for best efficiency (and safety) when rowing in a seaway it is arguably important to be able to clear the blade as far above the water as is needed to ensure that the recovery stroke keeps clear of the water.  To achieve this for any given blade we need to look at the path that the blade takes from aft to forward during the recovery stroke and maximise the clearance above the water,  Clearly the outboard end of the loom of the oar will be above horizontal with the handle on or near the rower's knees - this means that for a parallel sided blade the nearest part of the blade to the water will be the section nearest the thole pin.  So what about skewing the blade on the oar so that the lower edge of the blade is parallel to the water in this position thus giving best clearance.  So we can have a symmetrical blade skewed on the loom - or does that make it a chopper or cleaver?

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Re: Shape of oar blades

Post by topherdawson on Sat Dec 10, 2016 5:25 pm

Yes Rory, it rather does. I have long narrow rectangular blades on my Flashboat, which are curved, cleavered as you suggest above, and have a strip of carbon fibre down the tension side. They are thus three times illegal under St Ayles rules.

The required symmetry has to be about the major axis of the oar.

I think the ban on cleavers was purely for aesthetics although as you point out they do allow the largest area to be put in and out of the water with the least vertical movement of the oar.

But the recent club survey did definitely reject cleavers so I think on aesthetic grounds they are out. What an aesthetic lot we are!

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Re: Shape of oar blades

Post by topherdawson on Wed Dec 21, 2016 12:02 pm

Another experiment with 4mm ply. The grain crosses the blade this time and the central seam has 12mm of curve cut away at the neck and 15mm at the tip. The halves are cable tied together while flat together, and then opened like a book. The green string is tightened to bend the blade into a curve and then the whole thing gets a coat of resin. After it sets there will be a wide fillet of resin and filler down the inside of the seam.


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Re: Shape of oar blades

Post by topherdawson on Tue Dec 27, 2016 7:45 pm

I have made a document with instructions for the 4mm ply blade:
[You must be registered and logged in to see this link.]

It is pretty easy to vary the dimensions but the current blade is nearly 700mm long and 190mm wide at the widest, so about 3.7 times longer than wide.

My hunch is that it is strong and stiff enough to be self supporting if the shaft ended halfway along it, but it would be stronger and more knock resistant if the forward face of the shaft was wrapped round the convex face of the blade.

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Re: Shape of oar blades

Post by john mcintyre on Thu Jan 12, 2017 6:49 pm

All

I know we are not aiming to produce the ultimate sea rowing oar but rather a design that combines factors such as 'nice feel' for the greatest number of rowers, not just for the strongest rowers, and some blend of aesthetics and ease of construction. Nevertheless it would be nice to understand what factors combine to give high efficiency. i.e. oar length (why?) blade area, blade camber, planform etc.

I am NOT sure I understand anything / much? despite having done a good deal of reading and some computer programming so what definite knowledge can anyone else offer?

Here is one of the reports I found when I was making the long oars for the Ullapool boat.

Rowing Blade Design using CFD (Master of Science Thesis), which can be downloaded here: [You must be registered and logged in to see this link.]

Most of what I have read in the engineering literature is for sliding seat shells so I wonder how the results change for our slower boats (and longer oars)?

I would like more information about these boats to use in modelling so I towed Ulla with its crew aboard using a spring balance and hand held gps to measure the skiffs drag curve. It would be nice to do this again - or better still - if any of you have the time to do it - to use a gps and load cell to collect a more accurate drag curve(s)

It would also be nice to log the boats speed over a stroke using gps. Perhaps by ensemble averaging.

This would tell us a good deal about the skiffs and allow us to better model and optimise the oars.

John (Ullapool)

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Re: Shape of oar blades

Post by john mcintyre on Fri Jan 13, 2017 2:40 pm

Here is the drag curve I measured for Ulla. I towed the skiff with its crew aboard with a long light line (about 75 m) on a calm day and allowed the speed to stabilise before reading the spring balance.

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Boat Drag

Post by Rory Cowan on Mon Jan 16, 2017 10:46 am

Hi John,
This is useful data and will be more so when we come to look at rudders in the same light.  It seems that the fore and aft trim of the boat is very important and small changes in cox's weight make a large change to the draught aft and hence the rudder wetted surface area - which is very critical when dealing with drag.
Were your test points for increasing speed followed by decreasing speed?  Lastly it seems that from your plot the drag increases at around the 2.8 m/s point which is what we would expect.

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Re: Shape of oar blades

Post by Finlay Robertson on Mon Jan 16, 2017 9:39 pm

I think we also need to remember the feedback we got from the survey, which made it clear that the majority of support was for flat (i.e. not spooned) blades; whilst still bearing the curved designs (such as Topher’s above) in mind, I do feel that we need to give the more popular flat blades at least as much attention – as I’ve said on other threads, there’s no point in developing excellent designs that may in the end simply lack the popular support to be carried through a vote.

Flat-bladed oars are sufficiently hydrodynamically different from spooned blades for us to need to take care with our technical references. Furthermore, the smaller stroke angles of a St Ayles Skiff as compared to a rowing shell mean that our oars work mostly in the drag-dominated regime rather than the lift-dominated (blades acting as baffles rather than as hydrofoils).

Of principal interest to us is the shape of the blade – whether it is high aspect ratio (long and thin) or low aspect ratio (short and broad). When considering which is more efficient (i.e. has less slip, and therefore grips the water better), we must be careful not to be mislead by the popular low aspect ratio Macon and Cleaver designs – these rely on their curved profiles for their efficiency, not simply their planform shapes. For a flat blade in a free flow, a high aspect ratio of a given area actually grips the water better than the same area of a low aspect ratio blade. (This obviously assumes that both are sufficiently well submerged not to ventilate.) I suspect it’s because there's more flow separation on account of the greater length of the blade edges.

There’s a link to an excellent paper below, in which flat plates of different aspect ratios have been tested in a wind tunnel. The graph in Figure 3 illustrates my point quite well.

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Re: Shape of oar blades

Post by topherdawson on Mon Jan 16, 2017 10:43 pm

Hi Finlay,

Thanks for the reference. Agreed a high drag coefficient is important.

The increase in drag of high aspect ratio flat rectangles is backed up by Atkinson, and it would be nice to think that folk evolution had optimised the long blade shape. From the graph you reference the CD is 1.14 at AR of 2 and 1.22 at an AR of 6.

But in this reference
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Mackrossan shows graphs of Macon and Big Blade force coefficients at 90 degrees angle of attack, effectively drag coefficients, of 1.8 for the Macon and 2.0 for the Big Blade.
I'd also point out that cup anemometers work because the drag of a cup facing the flow is a lot more than the cup facing away, and presumably more than a flat plate of the same projected area. From this reference
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it looks as though the drag coefficient facing upwind is 1.4 at zero alpha, maxing at 1.6 at 40 degrees, and downwind only about 0.5. So the curve has a lot to do with it, as well as being crucial to the stiffness and the strength. I think a properly curved Macon/needle will be a lot better than a flat one.

I'm happy for others to experiment with long blades and we may need to introduce a measurement of the angle an oar has to be lifted by in order to go from fully immersed to fully clear of the water, as this has repercussions for crab catching. Will we call this Oar Dip Angle?

And would it be practical to pull an oar handle in a static boat with a spring balance and measure how fast the blade slips, as an empirical way to get to drag coefficient?

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Re: Shape of oar blades

Post by Rory Cowan on Tue Jan 17, 2017 2:43 pm

Hi All,
I seem to remember some story about a bloke in the Everglades up to his ass in alligators trying to remember that he was there to drain the swamp.  We have already discussed a wide range of thoughts and ideas for oars and oarlocks and it is appropriate that we take stock of where we have been and where we are headed.  We have a fleet of fairly simple boats all amateur built and we are setting out to standardise the oars and oarlocks.  The first principle is that these components need to be easy to make, made of relatively cheap local materials, normally timber, and not too taxing on the oar / oarlock makers.  These boats row in a wide range of wind conditions and sea states and it is important therefore to ensure that the oars are pleasant to row with under those varying circumstances.  There is no particular drive to optimise the performance of the oar (though that would be nice) since the boats race each other and not other classes (and we have seen already that we can row these boats apparently faster than the calculated hull speed).  For those reasons therefore I feel that it is better to drive at a solution which optimises the handling of the boat rather than trying to optimise speed and the physics of the oar.  Empirically here on Arran we have crews who have expressed a preference for the parallel sided (long thin) Anstruther pattern oars in most conditions other than rough water when they prefer the Macon shaped blade. That said our Macon shaped blade is fitted to a very heavy oar which could give it more drive in rougher conditions. Similarly we need also to look at the weight and 'heft' of each oar, rather than going for the lightest that there is.  After all, in rough conditions it is quite possible that crews may struggle with lighter oars purely because they do not have the weight to overcome the rough water.  I would like to see us therefore moving in the direction of making quite a few oars / oarlocks of different shapes and sizes so that we can get them out to 'the rower in the seat' and listen to what he / she prefers. We are headed there but I think we need to be more focussed on getting something out to crews to see what they think than on the very interesting, but less practical analysis of lift and drag curves.

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Re: Shape of oar blades

Post by Robert Graham on Tue Jan 17, 2017 4:03 pm

Shape of oar blades

I agree with Finlay that flat blades should be given consideration as I have concerns, as previously stated on another thread, about the pitch implications of a vee profiled blade.
I also agree that the small stroke angle in the fixed seat St Ayles means lift is not a significant factor at the catch.
As far as efficiency is concerned in relation to angle of dip and blade shape, the objective should be to get from the point of entry into the water to fully submerged in the smallest vertical movement possible of the oar handle. Bearing in mind the influence of the variation in oarlock height above the waterline (eg No1 & No3) there are a few basic principles to consider:
1. Long oars (eg 4500mm) need less vertical movement to dip the blade compared to short oars (eg 3500mm).
2. Short wide blades (eg 600 x 190) require less vertical movement than long thin blades (eg 820 x 140).

The conclusion I have come to in respect to this one criteria is a long oar with a short wide blade would be the best combination for a standardised design. It would also satisfy a number of other desirable parameters discussed elsewhere.

There have been many excellent ideas on design detail suggested on this forum and a few of these have been selected to be prototyped with a view to going forward as a recommendation for a standard design. I think there are a number of individual details that could be incorporated together to make an improved design and it would be useful at this stage to reflect on all the contributions made so far and agree the basic principles that are required in a specification. After that I think consideration needs to be given to what parameters of the design are going to be specified in the measurement rules.

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Re: Shape of oar blades

Post by topherdawson on Tue Jan 17, 2017 5:21 pm

Hi Rory and Robert,

Finlay is a naval architect and enjoys the technical aspects of this, and I am glad to discuss them, but really the proof of the blade is in the rowing and that's no more than a week away since I glued the blade on to the shaft this morning.
I think we are in a good place (and not up to our asses in crocodiles) because as i have reported here
[You must be registered and logged in to see this link.]
we already have a simple solid oar which has a UHF of 2.6kg, total weight less than 5kg, deflection 62mm. It has a ply spoon blade which is yet to meet the water but the shaft is already good enough to meet our criteria. The UHF could be lower but I think we need a hollow shaft to do that. At some future point when all the prototypes are made we will need to put our heads together to decide which design to make 4 of.

We can put a flat blade or a curved blade of any profile on our shaft, but we need to be aware that long blades make the oar longer since the centre of effort needs to stay in the same place, so the UHF goes up rapidly.

I see from Rory's comment that his rowers prefer the Macon outline in rough weather because they need less vertical movement than long narrow blades to get them clear of the water. I agree with Robert that long blades take more vertical movement to go from completely immersed to completely out of the water, and this vertical movement needs to be minimised.

I propose that we measure vertical movement needed to go from fully immersed to fully clear of the water, and we do it at the handle end at the middle gear of 2.8. With the boat either afloat or out of the water and level, it would be easy to measure down to the seat top from the handle end in both positions and find the difference. We could call it Oar Dip Movement. It would settle once and for all which blade shape is best for not catching in waves or banging the rower's knees. Possibly it could be done with an accurate scale drawing.

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Re: Shape of oar blades

Post by topherdawson on Tue Jan 17, 2017 5:28 pm

Robert and Stuart have raised an important point which at some stage we will need to address, which is what aspects of the oar will we measure when signing off an oar?

I would suggest we need total length, (+ or - some tolerance like 10mm), distance to top and bottom gears, width?? depth?? blade max length and max width, max UHF???

I'd leave blade curvature and exact outline well alone, but I'm tempted to specify max UHF of say 3kg because we don't want our hard work undone by someone making a Stone Age oar by beefing up all the dimensions and making it out of solid greenheart.

So a bit to think of there.

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Don't forget the need for shorter oars

Post by Rory Cowan on Wed Jan 18, 2017 8:57 am

It is also quite important to recognise that younger rowers will not be able to handle long oars as easily as adults. Junior races will be subject to the same oars rules. Smaller rowers (often, but not always, the ladies) will be similar. We need at least to nod in the direction of ultralight crews and leave an option to use much shorter oars which might not measure in the conventional way we are headed.  We have a set of 12ft oars for this purpose - I think we will need to ensure that such oars will be permitted for competition as well.  Obviously crews should not be able to derive competitive advantage by skilful interpretation of the rules but I don't think short oars would fall into that category unless of course we chose to compete in seas that we had no business to be out in!

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Re: Shape of oar blades

Post by topherdawson on Wed Jan 18, 2017 11:52 am

Several clubs have short oars i.e. less than 3.5m (11ft 6 inches) and for Portsoy and Boatie Blest they are the main racing oars. I think we will need to provide a short version of the standard oar for these clubs so that they are not disadvantaged by not being able to use the spooned blades if we decide to go for them.

The biggest change we are bringing is the much reduced UHF, which is going to be a huge advantage to smaller rowers. I must say that yesterday I took out some 13 year olds with our existing long oars and they managed very well, but they would have done better with a lower UHF. We do have light 11ft oars but these S2 pupils prefer the long ones.

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Re: Shape of oar blades

Post by john mcintyre on Wed Jan 18, 2017 7:46 pm

All

I came to the same (tentative - I mean I am always doubtful of my own understanding) conclusions as Finley regarding blade shape when designing Ulla's long oars:

1. That our oars are operating at lower angles than the spooned blades of sliding seat seat shells. More so for longer oars. (I put a picture on the "Oar length" department of this forum.)

2. They mainly work as drag devices

3. In this case it pays to increase the blade length to get more edge. (See Finley's reference. Also remember how nice Inuit kayak paddles are to use, and roll with.)

I feel we should try not to innovate for the sake of innovation but have an obligation to provide oar designs that respect sea rowing tradition and aesthetics.

Its my suspicion that blades such as "The big blade" used in shells on flat water will not be so nice to use in waves.

Having said this, it seems reasonable to me for our group to try some other things, see how they work and feel to use...

John





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