Designs and prototypes

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Designs and prototypes

Post by topherdawson on Sun Jan 01, 2017 10:26 am

Happy New Year to all,

We have now got to the point where we can design and make single prototype oars.

After discussion and a whole lot of ideas being thrown into the mix, we are looking for:

1   A well balanced oar with a target UHF of 2kg.

2   Strength: I propose a max handle force of 1000 N with the compressive stress on the forward face of no more than the timber's compressive strength. Alternately, use Finlay Robertson's criterion of 2000N at the handle but use the Modulus of Rupture published for the timber.

3   Stiffness: Max deflection of 80mm when 10kg is applied 2.5m outboard of the pivot.

4   Gears: Fixed gears in the range 2.6 to 3.0 true gearing, able to be changed in the boat by the rowers to suit the conditions.

5   Blade: Curved shapes allowed, as long as they are simple to make.

6   Feathering: Our experience of feathering so far has been unfavourable, since it needed a heavy oar to be rotated against friction, and made fixed gears impossible. But right out of left field came a system which experienced St Ayles rowers (Finlay Robertson and Ali Grant) have rowed with in New Zealand and really like, which still allows for fixed gears and is low tech and easy to make. So I propose that feathering is left open as long as it allows fixed gears, allows rowers to feather or not as they please, and has low friction.

7   Placement of the pin: The longest oars at 2 and 3 can be reduced in length from 4.5m to 4.3m if the pin is located in a block glued to the inside of the gunwale. I propose we try this, so the prototype oar needs to be 4.3m long.

8   Oarlock system: We are looking for a better system than pin and plate. We want one in which the oar pitch reference comes from a vertical pin, not the gunwale. Possible candidates in no order of merit are a ply wood swivel, a metal crutch, Robert Graham's pin and tray system, a ball ended pin, Don Currie's pin and keeper, Don's feathering pin and keeper, Crail's pin and keeper, and any other system not yet seen.

9 There is a small pot of money available for materials for prototype oars. If any of these oars turns out to be usable, then the clubs which adopt them would be expected to repay the cost to the SCRA.

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Re: Designs and prototypes

Post by topherdawson on Sun Jan 01, 2017 10:43 am

I'm proposing to make an oar to Design 4 on the oar comparison spreadsheet. It is 4.3m long, has Western Red Cedar flanges fore and aft, 4mm okoume ply webs, 75mm by 54mm section at the oarlock, solid hardwood (ash) plug in the inboard. The blade will be the developed 4mm ply Macon 700mm long by 190mm wide.

Building method will be:

Glue hardwood inboard to cedar flanges.
Glue this assembly to the bottom ply web.
Close the box with the top web.
Add reinforcement strips and blade.

This ought to fit the criteria and will be tested for stiffness.

The oarlock system will start off with a featherable ball ended pin. This is totally untried and may be a disaster, but if it is I will revert to Don's featherable system which we know works.

I hope others will design and/or make prototypes, from which we can choose the best.

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Re: Designs and prototypes

Post by topherdawson on Mon Jan 02, 2017 10:39 am

Robert Graham from Dundrum has contributed the following by email:

Hi Topher

I see you are starting to home in on the design specification in your New Year Eve's post.
I would like to comment on some of your 9 points.

No. 1. The target UHF of 2kg should be at the mid range gearing of 2.8.

No. 2. Strength. I think both of the proposed max handle force of 1000N/2000N are too high. A lower figure would give you more latitude to achieve the design UHF. You mentioned somewhere in the forum about Olympic rowers peaking at 750N. In practice I believe the average "b#m off the seat" force in the St Ayles is 300-400N.
In my opinion the UHF target is the main objective and should be on a static balance basis. I have come to the conclusion from this review that some clubs must be using pitch to dynamically balance their oars which explains why strong rowers dont have a problem with an out of balance oar but novice or less able rowers would find it uncomfortable.

No. 3. Stiffness. As well as a stiffness spec in the horizontal direction of force I think deflection at the end of the blade in the vertical plane due to the outboard weight of the oar should be specified. Should not be a problem with a light oar but would suggest a max of 80mm.

No. 4. OK

No. 5. I am very impressed with the way you have made the plywood spooned blades. However I do not believe in using pitch to balance a St Ayles skiff oar because it reduces efficiency. Adopting spooned blades as a standard design would increase the need to consider pitch in rigging the oars which is an unnecessary complication if the oar is statically balanced.

No. 6. No comment

No. 7. Pin position. The inboard pin proposal, if adopted, would require the majority of the fleet to change their oarlock system. It is generally accepted that long oars are more successful in races and from the oar comparison spreadsheet the mean length of 2 and 3 is 4500mm. The challenge as far as the oar is concerned should be to try and achieve the design objectives at what is recognised as a good racing oar length (4500mm). A 4500mm oar can still be used with an inboard pin system so I would appeal to you to reconsider your proposed prototype oar length.

No. 8. Oarlock system. As well as constant pitch referenced off a vertical pin the other desirable feature is constant gearing/balance throughout the full stroke movement.

No. 9. No comment

No. 10. The target weight of a 4500mm oar should be 5.4kg

I think it would be a real challenge to make an oar to the spec of design two in the comparison spreadsheet but it would be awsome if you managed it.

Regards
Robert

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Re: Designs and prototypes

Post by topherdawson on Mon Jan 02, 2017 12:31 pm

topherdawson wrote:Robert Graham from Dundrum has contributed the following by email:

Hi Topher

I see you are starting to home in on the design specification in your New Year Eve's post.
I would like to comment on some of your 9 points.

No. 1. The target UHF of 2kg should be at the mid range gearing of 2.8.

This varies a bit with gearing, and with the light design it varies more than the existing designs. At 3.0 the UHF would be about 0.3kg more than at 2.8.

No. 2. Strength. I think both of the proposed max handle force of 1000N/2000N are too high. A lower figure would give you more latitude to achieve the design UHF. You mentioned somewhere in the forum about Olympic rowers peaking at 750N. In practice I believe the average "b#m off the seat" force in the St Ayles is 300-400N.

Finlay uses 2000N but in conjunction with a Modulus of Rupture calculation which in my view overestimates beam strength. I use 1000N in conjunction with the compressive strength figure for timber which concentrates on the place wooden beams fail first. As a result Finlay and I come to similar numbers, and North Berwick's oars have broken during sprints with inexperienced but strong rowers.

How did you do your calculations? What values of stress did you use? Our oars have not broken so may be too strong and heavy.

In my opinion the UHF target is the main objective and should be on a static balance basis.

I agree.

I have come to the conclusion from this review that some clubs must be using pitch to dynamically balance their oars which explains why strong rowers dont have a problem with an out of balance oar but novice or less able rowers would find it uncomfortable.

I've never thought that oar unbalance was a problem while the oar was in the water, as the blade buoyancy cancels it out. The 4 degree pitch is to stop any upwards force by the rower at the handle causing the blade to bury too deeply, but as has been pointed out elsewhere this may not be necessary. Blade unbalance during the recovery stroke is the problem, and as you say a major one.

No. 3. Stiffness. As well as a stiffness spec in the horizontal direction of force I think deflection at the end of the blade in the vertical plane due to the outboard weight of the oar should be specified. Should not be a problem with a light oar but would suggest a max of 80mm.

This is a whole new ballgame. I agree that lack of vertical stiffness can make an otherwise usable oar unpleasant to use or even useless. How would you measure this deflection? How would you get a zero position from which to measure the deflection?

No. 4. OK

No. 5. I am very impressed with the way you have made the plywood spooned blades. However I do not believe in using pitch to balance a St Ayles skiff oar because it reduces efficiency. Adopting spooned blades as a standard design would increase the need to consider pitch in rigging the oars which is an unnecessary complication if the oar is statically balanced.

The spooned blade is intended to help the UHF by being very light, and help the rowing by being more efficient (less slip, less turbulence). It is not intended to affect the balance while in the water. Its vee cross section may help to maintain the correct blade immersion during the stroke. Its buoyancy will be low but with a low UHF I hope that will not be a problem.

No. 6. No comment

No. 7. Pin position. The inboard pin proposal, if adopted, would require the majority of the fleet to change their oarlock system. It is generally accepted that long oars are more successful in races and from the oar comparison spreadsheet the mean length of 2 and 3 is 4500mm. The challenge as far as the oar is concerned should be to try and achieve the design objectives at what is recognised as a good racing oar length (4500mm). A 4500mm oar can still be used with an inboard pin system so I would appeal to you to reconsider your proposed prototype oar length.

This proposal is a trade-off between extra work installing the blocks and the weight and expense reduction of 4.3m oars. If a 4.5m oar could be made the same weight and UHF as a 4.3m oar it might be marginally faster, but we know it can't be made as light. Polar moment of inertia and UHF have an adverse effect on performance so making it shorter may not reduce performance. We do know that making it shorter and lighter improves the experience for smaller rowers.

I do agree that the average length of oars 2 and 3 is 4.5m but that is because they have to be that long to avoid clashes. It does not mean that clubs would have necessarily have gone for 4.5m oars if they could have used shorter ones.

Like all compromises it is debatable and I think should remain open at the moment. I'm proposing to test it with a prototype but that does not mean it is set in stone. I hope others will comment and when we have data from the prototype we will know more. I hope that we make other prototypes to compare with.


No. 8. Oarlock system. As well as constant pitch referenced off a vertical pin the other desirable feature is constant gearing/balance throughout the full stroke movement.

Agreed. This is covered by point 4 specifying (multiple) fixed gears.

No. 9. No comment

No. 10. The target weight of a 4500mm oar should be 5.4kg

Even at 4.5m, Design 3 has a weight of 3.53kg. At 4.3m Design 4 shows 3.38kg. I think the weight should be as low as we can make it, because since I'm too lazy to calculate Polar Moment of Area, weight is a way to approximate PMI and that should be as low as possible.

I think it would be a real challenge to make an oar to the spec of design two in the comparison spreadsheet but it would be awsome if you managed it.

Agreed!

Regards
Robert  

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Re: Designs and prototypes

Post by Finlay Robertson on Tue Jan 03, 2017 3:43 pm

I’d just like to add to the point regarding strength. I’m inclined to agree with Robert Graham that the peak handle forces that occur during a stroke must be around the 400N level, but if this were the maximum force ever experienced then a 41mm square spruce cross-section would theoretically be adequate at the oarlock – clearly this isn’t the case. The truth is that the highest bending moments occur during sudden dynamic loadings when a rower catches a crab or, worse, misses the catch and puts the blade into the water when they’ve already started the drive. The proposed handle forces specified by Topher and me (1000N if you assume failure at crushing modulus or 2000N if you assume failure at bending modulus) are not representative of a real-life quantity – they simply allow an approximation of the worst-case bending conditions that must be resisted. Mine are empirically derived based on known breakages and proven designs; I’m not sure how Topher came up with his but the two approaches agree almost exactly. I’ve switched to using the latter for consistency.

Alternatively, the matter can be viewed this way – 500N is the assumed peak handle force during a powerful stroke, and a safety factor of 2 is applied to account for sudden peak loading (4 if you use the less conservative bending modulus rather than crushing).

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Re: Designs and prototypes

Post by Finlay Robertson on Tue Jan 03, 2017 4:07 pm

Here’s a bit of a controversial point that I think we need to consider carefully.

Many of us on this forum – myself included – are enthusiastic supporters of the more intricate oar designs that offer great performance (and sometimes cost) benefits in return for slightly increased build complexity (‘if you can build a boat, you can build a hollow oar’ etc.). However, we need to remember that this is a somewhat controversial position!

The fact remains that there is some concern over build complexity and movement away from the perceived ‘traditional’ oar. 52 of the 62 clubs polled rated build simplicity as ‘desirable’ or higher. I don’t want to get too embroiled in the politics of this, but I’m concerned that if we concentrate exclusively on the most innovative designs, this divide of opinion will make a vote on adopting them as standard unlikely to pass at the next AGM.

Just because we manage to produce some excellent, lightweight oars doesn’t mean that opponents of complex designs will suddenly change their minds. (If it did, we wouldn't have been so quick to reject carbon fibre!) As such, I think we need to broaden our scope beyond simply making the best oar possible; rather, we need to prototype the best oars within some additional constraints to build complexity and possibly cost.

For example, consider a pair of prototypes; both with the same length, gearing, oarlock, blade and strength (or deflection) but with one utilising a hollow design and one optimised for construction from a single length of timber. They could then be compared like-for-like, and feedback could be gathered to assess whether the wider rowing community thinks the better performance justifies the increased complexity.

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Re: Designs and prototypes

Post by topherdawson on Tue Jan 03, 2017 9:24 pm

Hi Finlay,

Yes this is pretty crucial. It would be a shame to miss the target by getting too complex.

We started with a skiff which needs long oars, or short oars. Most of the clubs have gone for long oars but a very simple, traditionally authentic solution would be to limit oars to 3.3m or even 3.0m.

I suspect people would complain like hell because there is a perception that long oars are faster but they sure aren't traditional in this boat.

If we go to long oars then the only way I can see to make the balance better is (a) to make the outboard and blade as light as possible and (b) make the inboard of a dense timber since lead is not an option. I don't think that making an oar from one piece of wood is going to balance well at 4.5m or 4.3m if we go for inriggers unless the inboard becomes gigantic (as some traditional oars have).

The simplest possible oar design I know from two pieces of wood is Design 1, which is a solid cedar outboard and a solid hardwood inboard, so one scarph joint. It has a UHF of 2.2kg at a gear of 2.8, or 2.3kg at 3.0. Making it 4.3m reduces UHF by about 0.1kg. Doing it in spruce the UHF creeps up a little and it's more expensive.

This oar plus Don's pin and keeper (keeping existing wooden pins) or a stainless rowlock would be the simplest offer.

The problem is that Don has also developed a simple feathering system which is nice to row with. Do you think that option is worth pursuing? You are one of only two people I know of in Scotland who have tried it!

So one can have:

short and traditional, easy to make out of one cheap bit of wood

long and semi traditional from one piece of wood heavy as hell to row with

long and semi traditional made from two bits of wood with a 4mm ply blade, pretty well balanced

long and semi traditional made from five bits of wood, hollow, better balanced.

as above but with a more complex feathering pin and keeper, nicer to row with?

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Re: Designs and prototypes

Post by Robert Graham on Wed Jan 04, 2017 5:10 pm

topherdawson wrote:
topherdawson wrote:Robert Graham from Dundrum has contributed the following by email:

Hi Topher

I see you are starting to home in on the design specification in your New Year Eve's post.
I would like to comment on some of your 9 points.

No. 1. The target UHF of 2kg should be at the mid range gearing of 2.8.

This varies a bit with gearing, and with the light design it varies more than the existing designs. At 3.0 the UHF would be about 0.3kg more than at 2.8.

No. 2. Strength. I think both of the proposed max handle force of 1000N/2000N are too high. A lower figure would give you more latitude to achieve the design UHF. You mentioned somewhere in the forum about Olympic rowers peaking at 750N. In practice I believe the average "b#m off the seat" force in the St Ayles is 300-400N.

Finlay uses 2000N but in conjunction with a Modulus of Rupture calculation which in my view overestimates beam strength. I use 1000N in conjunction with the compressive strength figure for timber which concentrates on the place wooden beams fail first. As a result Finlay and I come to similar numbers, and North Berwick's oars have broken during sprints with inexperienced but strong rowers.

How did you do your calculations? I did no calculations.
What values of stress did you use? I did not work to a stress value. To determine stress limit the outboard of a single oar was trimmed in progressive stages until it suffered a compression failure on the shaft adjacent to the blade.
Our oars have not broken so may be too strong and heavy.
The criteria I worked with were oar length, deflection, gearing and UHF. Balance was the main aim so outboard was trimmed to what was considered a safe limit of deflection and then weight added inboard to get desired UHF for the optimum gearing. Balancing was the last operation and could accommodate variations between oars due to the manufacturing process..

In my opinion the UHF target is the main objective and should be on a static balance basis.

I agree.

I have come to the conclusion from this review that some clubs must be using pitch to dynamically balance their oars which explains why strong rowers dont have a problem with an out of balance oar but novice or less able rowers would find it uncomfortable.

I've never thought that oar unbalance was a problem while the oar was in the water, as the blade buoyancy cancels it out. The 4 degree pitch is to stop any upwards force by the rower at the handle causing the blade to bury too deeply, but as has been pointed out elsewhere this may not be necessary. Blade unbalance during the recovery stroke is the problem, and as you say a major one.

0]]I thought I had found a reason for needing pitch in the St Ayles oars and if it is not for dynamically balancing then I am back to square one - totally baffled!! I had rationalised that pitch which has its uses in sliding seat rowing was being used to solve a different problem in fixed seat rowing. In my limited experience upwards force is not an issue, particularly at stroke and bow where if the boat is not rigged correctly you get rowers trying to finish the stroke with their hands up round their ears. 4 degree oar pitch I imagine would also increase the risk of blade washout at the end of the stroke and oar bounce at the oarlock.[/b]

No. 3. Stiffness. As well as a stiffness spec in the horizontal direction of force I think deflection at the end of the blade in the vertical plane due to the outboard weight of the oar should be specified. Should not be a problem with a light oar but would suggest a max of 80mm.

This is a whole new ballgame. I agree that lack of vertical stiffness can make an otherwise usable oar unpleasant to use or even useless. How would you measure this deflection? How would you get a zero position from which to measure the deflection?

0]]I think the proposed oars are very ambitious and to achieve balance will be pushing at design boundaries and vertical stiffness may become an issue that needs to be considered. I have not tested ours but one way of doing it would be to lay an oar down flat on the rungs of a ladder, brace the oar at the rowlock position and handle, against the sides of the ladder, mark the position of the end of the blade on a rung, rotate the ladder through 90 degrees to the vertical position and measure the deflection.[/b]

No. 4. OK

No. 5. I am very impressed with the way you have made the plywood spooned blades. However I do not believe in using pitch to balance a St Ayles skiff oar because it reduces efficiency. Adopting spooned blades as a standard design would increase the need to consider pitch in rigging the oars which is an unnecessary complication if the oar is statically balanced.

The spooned blade is intended to help the UHF by being very light, and help the rowing by being more efficient (less slip, less turbulence). It is not intended to affect the balance while in the water. Its vee cross section may help to maintain the correct blade immersion during the stroke. Its buoyancy will be low but with a low UHF I hope that will not be a problem.

The reference datum for pitch on the Dundrum oars is the level surface of the water and the objective was, at the catch, to have an oar and oarlock system that presented the blade at zero degrees and for that pitch to be maintained throughout the full drive phase of the stroke. Would a spooned blade with a vee cross section not surely mean that the bottom half of the blade goes into the water at a negative angle and pitch becomes an extra essential component of the rigging set up to get an efficient catch? I think pitch is part of the "black art" of sliding seat rowing but is an unnecessary complication in the St Ayles skiff. Flat blades are more conducive to community boat builds and provided they are not too wide, strength and balance should not be significantly compromised.

No. 6. No comment

No. 7. Pin position. The inboard pin proposal, if adopted, would require the majority of the fleet to change their oarlock system. It is generally accepted that long oars are more successful in races and from the oar comparison spreadsheet the mean length of 2 and 3 is 4500mm. The challenge as far as the oar is concerned should be to try and achieve the design objectives at what is recognised as a good racing oar length (4500mm). A 4500mm oar can still be used with an inboard pin system so I would appeal to you to reconsider your proposed prototype oar length.

This proposal is a trade-off between extra work installing the blocks and the weight and expense reduction of 4.3m oars. If a 4.5m oar could be made the same weight and UHF as a 4.3m oar it might be marginally faster, but we know it can't be made as light. Polar moment of inertia and UHF have an adverse effect on performance so making it shorter may not reduce performance. We do know that making it shorter and lighter improves the experience for smaller rowers.

From the oar comparison spreadsheet, the Dundrum oars are the heaviest but with the lowest UHF. There was a trade off between heavy weight with higher inertia forces and balance but the outcome has been that one set of oars can be used by young and old, big and small, novice and expert, at the optimum setting in any sea conditions. If anyone has any doubt to the validity of this statement they would be very welcome to come and give them a try.

I do agree that the average length of oars 2 and 3 is 4.5m but that is because they have to be that long to avoid clashes. It does not mean that clubs would have necessarily have gone for 4.5m oars if they could have used shorter ones.

Like all compromises it is debatable and I think should remain open at the moment. I'm proposing to test it with a prototype but that does not mean it is set in stone. I hope others will comment and when we have data from the prototype we will know more. I hope that we make other prototypes to compare with.


No. 8. Oarlock system. As well as constant pitch referenced off a vertical pin the other desirable feature is constant gearing/balance throughout the full stroke movement.

Agreed. This is covered by point 4 specifying (multiple) fixed gears.

No. 9. No comment

No. 10. The target weight of a 4500mm oar should be 5.4kg

Even at 4.5m, Design 3 has a weight of 3.53kg. At 4.3m Design 4 shows 3.38kg. I think the weight should be as low as we can make it, because since I'm too lazy to calculate Polar Moment of Area, weight is a way to approximate PMI and that should be as low as possible.

I agree. Any oar that is under 5.4kg and balanced will be a good oar but a challenge to make.

I think it would be a real challenge to make an oar to the spec of design two in the comparison spreadsheet but it would be awsome if you managed it.

Agreed!

Regards
Robert  

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Re: Designs and prototypes

Post by topherdawson on Wed Jan 04, 2017 6:30 pm

Hi Robert,

Thanks for that. 4 degrees pitch is sliding seat advice which we tried and as it seemed to work we carried on with it.

Like many aspects of oars and rigging it is coming under the microscope in this forum and may in fact not be necessary. I now see that our oars have a lot more down force i.e. UHF pulling them down into the water, and a lot more buoyancy in the blade than sliding seat oars. Thus our oars are more aggressively seeking their own level than sliding seat oars.

I think zero pitch would probably be fine and could make the oars OK for port or starboard, i.e. ambioarstrous.

The vee of the spoons is shallow, maybe 160 degrees, but my hunch about it is this: When the lower half is immersed and the rower pulls, it will pull downwards, but when all of it is immersed the effect will be neutral, which all seems fine. Similarly if the lower edge strikes a wave during the recovery, there will be a little bit of lift.

Vertical stiffness of the shaft is I agree important. Because wide shallow shafts are stiffer horizontally for their weight, there is a temptation to make the shafts too shallow and I've already done that once.

I think we may have missed the boat for measuring it in existing oars, although your ladder suggestion is ingenious and ought to work. Part of the problem is that oars with a heavy blade have a bigger vertical stiffness requirement I think, and I can't figure out a way to separate them. Your ladder suggestion couples the weight of the blade with the vertical stiffness, as the load is effectively the blade.

The whole point of making prototypes is to see if the design models are actually as good as they look, and all bets are off until we make them.

There is little difference in weight and UHF between the solid and hollow so I'm swithering a bit about which to try first. Finlay has recently posted that if the design is too complicated people will be put off.

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Re: Designs and prototypes

Post by Finlay Robertson on Sun Jan 08, 2017 8:13 pm

To address some of the complexity issues discussed above and in the e-mail thread with Robbie, and to make sure we strike some sort of balance (pun fully intended), I'm going to put some of the more elaborate ideas for a 1kg UHF oar I've been sitting on aside for now and aim for simplicity. I'm still playing with the numbers, but I want to see what I can come up with that does not require anything too fiddly to build; certainly there are plenty of clubs out there rowing with oars that could be made a lot lighter through better design optimisation but without modifying the basic build technique.

It's only by allowing the chance to compare the 'simple' and 'complex' options that clubs can decide whether they think the more involved builds are worth it for the improved performance. ("Is that saved 0.5 kg really worth the change from solid to hollow oar?" That's not for any of us to decide, but it's certainly something we can put to the test with the rest of the rowing community!)

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Re: Designs and prototypes

Post by topherdawson on Thu Jan 12, 2017 3:57 pm

UHF of 1kg! I want to see this. If it looks doable it will be well worth making, and I've come to the conclusion that this is exactly the point when we should be making wild and wacky oars which may well break. The SCRA is paying for the materials and we are not making sets of 4.

This is the moment when daring experiments may reap a big reward, so I urge you to complete the design and let us all have a look at it.

My solid oar is nearly done, glue setting on the inboard/outboard scarph as I type this. It was pretty simple to make and soon we will know how well it works.

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Re: Designs and prototypes

Post by Finlay Robertson on Sun Jan 15, 2017 8:04 pm

Okay then, Topher - if you think such things are worth pursuing I'll keep it alive for now. Still working on it, but I'll share on this forum in due course. Just to keep you guessing, the 3-word hint is "De Haviland Mosquito"!

I look forward to hearing some feedback on your solid oar design; it will be good to see if it validates the spreadsheet.

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Re: Designs and prototypes

Post by topherdawson on Mon Jan 16, 2017 9:22 am

Hi Finlay,

I have posted a preliminary report on the solid oar Design 2 here:
http://sasoo.forumotion.co.uk/t39-test-results-from-prototypes

All I know about the Mosquito is that it was a monocoque wooden construction, although what the wing spars looked like I don't know. Apparently the tailplane spar failed under static load at 80% load, so they had to strengthen it, but that was more efficient than making it oversize.

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Re: Designs and prototypes

Post by Don Currie on Tue Jan 17, 2017 8:47 am

Crikey Finlay, so all we know is the oars will have 2X1500hp engines with stub exhausts, so might be quite noisy in use! And they'll be glued together with casein glue, so might be a bit unreliable when wet! Or will you use Aerolite glue? And the crew will all wear leather helmets, and say things like "tally ho", and "bandits at 12 o'clock" a lot. It's going to be quite a sight and sound!!!

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Re: Designs and prototypes

Post by Finlay Robertson on Sun Feb 05, 2017 4:04 pm

Close, Don, very close!

I wanted to see just how far I could push the design within the bounds of the existing rules and current preferences, as indicated by the survey and existing designs. I’ve got two designs to share with you; both have a true gearing of 2.93 which is just a tad above average. The survey indicated that curved blade designs do not seem to have much support from the wider community (macon and cleaver blades being rated undesirable/terrible by 57% and 84% respectively), so pending wider feedback on Topher’s design I’ve gone for a flat blade. The looms are rectangular in cross-section (by far the strongest shape, as it minimises the stresses in the extreme fibres). A ratio of 2:1 is used, as more than this risks running into problems with lateral torsional buckling. (Note this is valid for solid sections, but as far as I know untested for laminated designs...!)

Design 1 – Laminated ‘Mosquito’ Oar
Mass = 5.10 KG
UHF = 0.73 kg
Test deflection = 50mm at 2.5m outboard, 87mm at centre of effort

This is rather an extreme design; I don’t really think there will be much appetite for such eccentric builds but Topher has encouraged me to share this nonetheless. Like the WWII aircraft, this design uses balsa cores to maximise strength whilst minimising weight. The loadbearing surfaces are Sitka spruce, located on the forward and aft faces of the oar, with balsa endgrain sandwiched between them. Despite being very soft, balsa has good shear resistance perpendicular to its grain, so the top and bottom surfaces don’t need to serve this structural function – their sole purpose is waterproofing, for which a 1mm veneer is adequate. This makes it far lighter than an equivalent hollow oar.
The blade is made from two sheets of 1mm ash veneer sandwiching a 6mm balsa core, yielding an ultra-lightweight bespoke plywood (shaving 20% off the weight of 4mm marine ply without stiffening) with a unidirectional grain orientated along the oar’s axis; this should theoretically be strong enough to allow the shaft to terminate well inboard of the centre of pressure, though I’ve not been quite brave enough to do this!


Design 2 – Conservative Solid Oar
Mass = 4.58 kg
UHF = 1.94 kg
Test deflection = 47mm at 2.5m outboard, 83mm at centre of effort

This is my attempt to produce an effective design that’s as simple as possible to make in line with my point above. The loom is made from a standard 100mm x 50mm section which makes up both the inboard and outboard parts in a single length (i.e. no scarfed inboard).The aft face is left unshaped (i.e. flat) to provide a convenient datum during mark-up and build; it also reduces the chance of imperfections introduced during the manufacture becoming stress-raisers in the tensile-loaded edge. The loom continues beyond the blade’s centre of effort out to the extreme end of the oar, providing sufficient stiffening to allow lightweight 4mm plywood to be used for the flat blade.
I’ve elected to use Sitka spruce for this design, though it could easily be re-optimised for other materials. If, hypothetically, we elected to bring forward a design such as this a possible ‘standard’ design, it would be very easy to tabulate the required/recommended principal section dimensions for a range of materials (ash, spruce, fir, cedar, radiata, etc.) and the corresponding performance (bend, weight, UHF, etc.).


The drawings are in the Dropbox folder under Oars > Oar Plans > Finlay Robertson, and the original calculation spreadsheets are in there too.

Finlay Robertson

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Re: Designs and prototypes

Post by topherdawson on Sun Feb 05, 2017 4:42 pm

These look most interesting Finlay. I will have a look at the calcs and get back.

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Re: Designs and prototypes

Post by Don Currie on Sun Feb 05, 2017 7:57 pm

That's some interesting stuff Finlay, though I'm a bit disappointed about the lack of Rolls Royce Merlins! Couple of points:
1) Now that we're getting some quite light oars it might be worth specifying whether the weight includes the rowlock elements - wear strips and the vertical plate and spacer system on the last oar I made were nearly 10% of the all up weight, so just to make sure we're comparing apples with apples, it's worth specifying whether the weights are for a bare oar, or "ready to row".
2) I'm surprised at the weight of the balsa cored oar - seems a bit high, though the UHF is spectacular. It would be useful to build at least one oar with a UHF under 1 kg to see whether you can have a UHF that is too low, ie the rower is trying to hold their arms up during the return stroke, rather than pushing down.
3) Don't know whether you've built the balsa oar yet, but if not, it might be worth priming the end grain balsa with a resin/light weight filler mix before laminating the higher density timber to it. I've not used end grain balsa myself, but when working with foam cores it's surprising how much adhesive (and weight) goes into the surface of the foam. Many people therefor prime the foam surface with a light weight filler, wait for the filler to start to tack off, then apply the adhesive and bond the surface reinforcement. I'd imagine that end grain balsa might hoover up a lot of adhesive, so a light weight primer might really help keep the weight down.
4) The numbers on that simple solid oar are pretty damn good. I'd be interested to see that idea developed more. You've got a big advantage over intuitive builders like myself in being able to squeeze the dimensions right down to the minimum right along the loom, and on the face of it the simple solid looks like it is a really good compromise between cost, complexity and ease of construction. From my distant viewpoint, I'd suggest that might be the first one to build a prototype of.
5) Don't know about the hemisphere specific software that you're using for the designs - in the southern hemisphere some of the designs appear up side down, and I'm getting a real crick in my neck reading them!

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Re: Designs and prototypes

Post by topherdawson on Sun Feb 05, 2017 8:18 pm

Great spreadsheet Finlay, although it makes my computer grunt with the effort of opening it.
Your Mosquito design is very hi-tech (WW2 instead of 1800's) but I'm not sure the balsa+1mm spruce saves much weight compared to 4mm ply which also contributes longitudinal fibres to the strength and stiffness. Noted Don's point about glue absorption too. But a nice oar!

Your solid one is really good and simple. By increasing the width to 100mm and optimising the taper you have managed to do without the hardwood inboard, which makes it very simple to make. The cost, though, is high. Robbins quote 100x50 spruce at £22.56 per metre, twice the cost of Douglas and ash. An oar's worth with VAT would be about £120. And that's assuming it would come out of a 100x50 sawn piece but after planing it's more like 94x44. How would that affect the design? It has stiffness to spare.
Or what would the numbers look like in Douglas?
The small complication of making the inboard out of ash would save money and reduce the UHF.

I tried to copy the design numbers for your Mosquito oar into my much cruder spreadsheet but the stresses came out much lower than your spreadsheet. This worries me because I have a hollow oar in the workshop with the glue setting with cedar flanges 12mm thick and 46mm deep at the pin, with an overall width of 75mm. The flanges taper down to 3mm thick at the tip by 17 deep.

Time will tell! Anyway good work, and the wide shaft would work well with my ball ended pin in central slot system.

I will report on Design 4 when the glue is set.


Last edited by topherdawson on Sun Feb 05, 2017 8:26 pm; edited 1 time in total

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Re: Designs and prototypes

Post by topherdawson on Sun Feb 05, 2017 8:23 pm

Hi Don,

In Adobe Acrobat go to View then Rotate view and you can restore its Antipodeanism to normality!


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Re: Designs and prototypes

Post by topherdawson on Sun Feb 05, 2017 8:24 pm

I would say to everyone that getting light low UHF's is making a huge improvement in the pleasure of rowing as my rowers keep telling me. So if we can pull this off we will have done the skiff community a big favour.

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Re: Designs and prototypes

Post by Finlay Robertson on Tue Feb 07, 2017 11:03 pm

Ach, Don – I did the drawings that way round specifically for your benefit! Clearly file exports to the southern hemisphere don’t work the way I’d assumed.

You picked up on a point that surprised me too – the weight of the balsa oar. The ash inboard is responsible for this apparent discrepancy, as it makes up almost two thirds of the total weight of the oar. If the inboard were Sitka instead, the mass would reduce to 3.88 kg but the UHF would rise significantly to 1.23 kg.

I’ve not built this oar so your suggestions will most definitely be useful if it’s taken any further; I’m no expert on the use of marine glues and I was quite concerned at how much epoxy it might soak up! Elisa and I did load test a 70mm long, 9mm wide section of the proposed blade (loaded as a simply supported beam) as a proof-of-concept a wee while ago. Even with over 7kg hanging off it, we couldn’t get it to fail.

Topher – you’ve made a couple of really important points there so I’ve had a go at addressing them.
I’ve re-optimised the oar for ash and Douglas fir; I’m most pleasantly surprised at the performance of the latter. Here’s a summary:

European Ash
Mass: 6.88 kg
UHF: 2.28 kg
Deflection: 58 mm at 2.5m outboard, 102 mm at centre of effort

Douglas Fir
Mass: 5.23 kg
UHF: 1.92 kg
Deflection: 55 mm at 2.5m outboard, 96 mm at centre of effort

I’ve also checked that the 100x50 can be trimmed back to 94x44 without any significant issues. The deflection at 2.5m outboard increases by just 3-4mm, and there’s no impact on strength since that part of the loom has been well over-sized to increase stiffness. It saves half a kilogram of mass, too.

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Re: Designs and prototypes

Post by topherdawson on Tue Feb 07, 2017 11:19 pm

Hi Finlay,

Thanks for looking at the other timbers, as using timber as expensive as spruce as a counterweight is a bit extravagant! I wasn't actually meaning an oar wholly of ash, but an oar with an ash inboard and a Douglas or a Cedar outboard. Building the boat involves making 24 plank scarphs so an oar should be a doddle.

I'm glad 94x44 works, that would save money. I think 80mm deflection at 2.5m is OK as a stiffness target, because it is in the middle of the stiffness measurements of existing good oars. My current hollow one is 100mm so it will be interesting to see if this makes it unpleasantly soft to use.

Thinking about the way you have optimised stress near the max, a way to save UHF by reducing stiffness would be to take the section reduction closer to the pin, thus increasing the stress more in the low stress section near the pin.

I've also been thinking about the trapezoidal section compared to rectangular. I suspect for the same cross section area it is stiffer vertically. If we are designing by adjusting the size of each section until the max compressive stress is just below what the wood will stand, then the trapezoidal section will take more moment to get to that point, i.e. will be stronger. The max tensile stress will be about double but that is OK. I will look again at Design 2.

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