Mechanics of oarlocks

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Mechanics of oarlocks

Post by topherdawson on Fri Dec 02, 2016 9:02 pm

1 I am using the word oarlock to mean any and all means of pivoting the oar at the gunwale.

2 They all take the oar thrust and allow it to rotate about a vertical axis to row and a fore-and-aft axis to go in and out of the water.

3 Some allow freedom for the oar to rotate in pitch about its long axis, i.e. feather.

4 Some allow freedom for the oar to slide in and out which alters the gearing.

5 Most of the St Ayles clubs have adopted systems which do not allow the oar to slide in and out, or feather.

6 Some have a single gear but others have two or three or more.

7 The survey shows that most want no feathering and a choice of fixed gears.

8 Gig oars are free in feathering and in/out. Gig rowers feather but only about 45 degrees which does not reduce windage much.

9 Sliding seat oars rotate with flats which act as stops, and they rotate 86 degrees which does reduce windage and so they can have large area blades without a drag penalty. They have buttons which the rower keeps pressed against the gate, defining the gear.

10 Pitch is the angle between the plane of the blade and a vertical line. The standard recommendation which seems to work for us is 4 degrees positive (top edge tipped towards the cox) which prevents the oars digging too deep. Ideally the 4 degrees should be kept throughout the stroke.

11 The most common oarlock system in the skiff is the pin and plate. This is fixed in/out and also in pitch, i.e. can't feather. If there are multiple holes in the plate there will be multiple gears.

12 While the plate is resting on the gunwale, the gunwale is the reference for the oar blade pitch. When the rower pulls on the oar it tends to pull the oar against the pin, and the pin is now the pitch reference. The gunwale line rises and falls and also angles out at the stern and in at the bow. Some clubs have made angled wear strips (rouths) on top of the gunwale to level this surface. Pins are sometimes vertical but more often angled out.

13 Because of all this, there is often a conflict between the plate on the gunwale trying to set the pitch, and the oar pressing against the pin. This can cause a lot of wear and clonking.

14 The oar needs to rotate freely in two dimensions, fore/aft and up/down. Sliding seat nylon swivels and metal rowlocks allow the oar to rock up and down while resting on the bottom bar of the swivel or rowlock, and accommodate the fore/aft movement by rotating. This leaves the gunwale free of wear. The axis of the swivel/rowlock defines the pitch and if the axis is vertical the pitch remains the same all the time.

15 If the swivel/rowlock axis is canted outwards as it often is, then the pitch of the blade is more at the catch and less at the finish. This makes the blade liable to pop out at the catch and be hard to extract at the finish, exactly the opposite of what is needed. Thus a vertical axis is to be preferred.

16 Another system in use is double thole pins. Gigs use this. The two pins have to be further apart than the width of the oar to allow the oar to rotate fore and aft. If the rower catches a crab and the oar is swept aft, it breaks the aft pin which is made of pine for this purpose. Because of the large gap between the pins, the oar clonks to and fro. In the St Ayles Skiff the gunwale angles in and out over the length of the boat and so the stroke oar can run out of clearance at the catch and the bow oar at the finish. There is complete freedom in feathering and in/out.

17 The original plans specify the Scandinavian system of a single rectangular pin (Kabe) with a lanyard (humliband) holding the oar against it. This system seems to work for the Scandinavians but it has many of the worst features of all the systems. Although free in feathering and in/out, it needs the humliband to be really tight otherwise the slack in the system causes clonking. If the humliband is tight then the pitch and gearing drift away from the optimum and are hard to correct. No skiff now uses this system.

18 Ullapool, North Berwick and some others use swivels made of plywood or other wood based materials, rotating on wooden pins. The weight of the oar rests on the pin which reduces friction. These work well but do not look traditional, are difficult to make, and need an additional lanyard over the oar or some other restraint to prevent the oar jumping up and out in a seaway. The oars are fixed in pitch (no feathering) and in/out.

19 The need to make the system out of wood arises from a measurement rule confirmed at an early AGM. It has not resulted in a unified system, and in my opinion (opinion alert) makes the best oarlock systems unnecessarily complex to make. Many traditional oarlocks are of metal which is locally available, hardwearing, compact and reasonably cheap.

20 An ideal system would:

Allow the oar to rotate fore/aft and up/down with little friction or wear.
Constrain the oar in/out to define a gearing (most clubs want this)
Hold the pitch constant throughout the stroke, without feathering. (most clubs want this)
Prevent the oar from jumping right out if hit by a wave.
Allow rowers to quickly bring the oar into the boat to avoid hitting a harbour wall or another boat.
Be reasonably easy to make locally or at least cheap to buy.
Be removable so that a cover can go on the boat.
Look reasonably traditional.


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Re: Mechanics of oarlocks

Post by Don Currie on Sun Dec 04, 2016 8:10 am

Your notes above are a great summary Topher. We started out with plates and pins and the clonking and loose fit just did my head in - like a car with square wheels. For a while we used a single pin, retaining line and round oars with conventional leathers. Round/elliptical, hollow oars are a pain to make, and don't feather all that well in the sizes required for a St Ayles, so we went to rectangular looms and a small keeper plate that holds the oar against the pin. With a very narrow wear strip on the underside of the oar the oar references solely against the pin - nice and quiet, and very little friction.

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Re: Mechanics of oarlocks

Post by Don Currie on Sun Dec 04, 2016 8:16 am

The post above was intended to have some photos, but computers are a bit of a mystery to me - I'll try to work out how to post photos and try again tomorrow.

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Re: Mechanics of oarlocks

Post by topherdawson on Sun Dec 04, 2016 7:07 pm

In the row of icons along the top of the Quick Reply box, the 13th one from the left is "host image" which lets you choose an image stored on your computer. Then from memory you need to click twice more on options whose name I don't remember, a bit clunky I found.

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Re: Mechanics of oarlocks

Post by topherdawson on Wed Dec 14, 2016 9:42 am

New idea: How about making the top of the pin hemispherical and resting the weight of the oar on it?

There are two ways to do this. The first is to have a cap plate fastened to the top of the oar, overhanging the front face, with big dimples in it for the gears. There is a keeper plate attached to the oar by spacers, to contain the front of the pin. The oar does not touch the gunwale at all.

The more radical way would be to cut a slot right through the oar, long enough to span all the gears. The cap plate with its dimples would cover the slot. Changing gear would involve lifting the oar enough to place another dimple over the rounded top of the pin.

The pin could be wood as now, or metal. It might be necessary to reinforce the oar at this point on the fore and aft faces, but remember it is solid oak at this point.

The second option would place the oar C of G right over the pin, minimising friction and wear. It makes the oar geometry independent of the curving gunwale.

The dimples could be lined with plastic or just be hard wood.

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Re: Mechanics of oarlocks

Post by Don Currie on Mon Dec 19, 2016 7:33 am

The hemispherical dimple top plate is worth trying - it would mean that pins would have to be accurately cut to length and there would need to be some way of accurately holding the pin at the gunwale - just relying on a taper would probably not be accurate enough.  The plate (hereafter known as the Dawson Dimple Plate!) could be just plywood and the dimples cut with a modified spade drill (photo of one I made many years ago attached)  With the oar supported on the top of the pin you'd end up with a nice clean gunwale, and the wearing parts would be easy to replace.  I like the idea of being able to change gears "on the run" , and with a 25mm pin you could probably use dimple spacings of about 35mm before the lands between the dimples got too fragile.[You must be registered and logged in to see this link.]

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Re: Mechanics of oarlocks

Post by Don Currie on Mon Dec 19, 2016 7:39 am

Meant to say that I prefer the idea of a keeper plate on the fore side of the oar rather than a slot through the oar - makes it much easier to replace any wearing surfaces and\or easier to adapt to a change to a different pin diameter. If you want the oar to be balanced perfectly over the pin there's nothing in the rules to say the loom has to be straight - just incorporate a kink in the loom at the scarf between the heavy timber of the inner and the lighter timber of the outer section......... that would look really funky!

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Re: Mechanics of oarlocks

Post by topherdawson on Mon Dec 19, 2016 11:16 am

Funky is right!! Might be too funky for the easily offended aesthetic sensibilities of the rowers! But I like it.

How much wear do you think would be due to the offcentre mass of the oar, compared to the wear due to the rowing? Our wooden pins do show wear at the bottom of the swivel where the weight of the oar bears, but we are getting several years out of each pin.

The taper of our pins (2.5 degrees on each side) does locate them pretty accurately in the vertical, is firm in use and only occasionally hard to extract, which we do every row to get the cover on.

You don't need a taper reamer to make the hole in the gunwale, you just make an oversize hole, line it with epoxy, and put in the pin covered with parcel tape and a layer of vaseline. When the glue sets you remove the pin and clean up, thus ending up with a matching hole and sealed end grain.

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Re: Mechanics of oarlocks

Post by topherdawson on Mon Dec 19, 2016 11:21 am

Don Currie wrote:The hemispherical dimple top plate is worth trying - it would mean that pins would have to be accurately cut to length and there would need to be some way of accurately holding the pin at the gunwale - just relying on a taper would probably not be accurate enough.  The plate (hereafter known as the Dawson Dimple Plate!) could be just plywood and the dimples cut with a modified spade drill (photo of one I made many years ago attached)  With the oar supported on the top of the pin you'd end up with a nice clean gunwale, and the wearing parts would be easy to replace.  I like the idea of being able to change gears "on the run" , and with a 25mm pin you could probably use dimple spacings of about 35mm before the lands between the dimples got too fragile.[You must be registered and logged in to see this link.]

That's a neat way of getting a round dimple. It could be epoxy lined using the pin end as a mould, and vaseline to get it off. I think we are going to need three gears at about 50mm centres to span between gearings of 2.6 and 3 and I see Anstruther rows at 2.5!

The lands between the dimples will need to accommodate vertical rotation of the oar, but probably the necessary shape will become obvious in the making.

Anyone know any good ways to make a ball ended pin on the lathe?

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Re: Mechanics of oarlocks

Post by Finlay Robertson on Wed Dec 21, 2016 4:33 pm

Another fabulous summary, Topher. It makes a great starting point for discussion!

I can offer a wee correction regarding the North Berwick system you mention in Point 18. You state that the weight of the oar rests on the pin which, once upon a time, it did – the oarlock featured a blind hole which sat over the pin. However, there were a couple of problems with this. In use, the entirety of the load was placed on the end of the pin; I’ve tried to show this in my (very poor) diagram – sorry, it’s the best I can do with Microsoft Paint! This caused the pin to wear very quickly (and that’s really saying something, since they were made of Lignum Vitae) which allowed more play and exacerbated the problem further. It also resulted in a fairly high bending moment on the pin (force*length of pin).

I proposed a slight modification to the design which we adopted a couple of years ago. By extending the pin and boring the hole in the oarlock all the way through, the point load was removed and replaced with a uniformly distributed load. This reduced the wear, and also halved the bending moment (force*0.5*length of pin). The weight of the oar is borne by one or more sacrificial plywood or nylon (when we’re not racing) washers, which also allow adjustment of the height.
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The oars themselves have wooden ‘stops’ under a heat-shrink ‘leather’ to prevent them sliding out beyond the intended limit, though the gearing can be manually reduced (for confined spaces and harbour entrances, for example). We’ve also experimented with having multiple stops, allowing several gearings to be achieved with the same oar. I do, however, concur that this is certainly not a traditional system; rather, it was an attempt to recreate traditional metal crutches into a design compatible with the measurement rules.

There’s clearly a concern to maintain a traditional appearance, which will be fairly constraining if we decide to be bound by it. In my opinion, there are really only two ‘traditional’ systems – double pins or a single pin with a humliband. You say that the latter system is no longer used, which surprises me – I’m sure I came across it at least once this year, though I don’t recall when.

I can’t help but wonder if some adaption of the humliband system could satisfy most or all of your bullet points in Point 20. Has anyone tried using bungee chord rather than rope? It could do a lot to mitigate against some of the problems listed in Point 17 and, if combined with some sort of stop system, allow fixed gearings. I’m thinking of something like this:

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Re: Mechanics of oarlocks

Post by topherdawson on Wed Dec 21, 2016 5:49 pm

Someone, perhaps on this forum, or perhaps in an email to me, did say they used bungees and said it worked. Sorry I can't now remember who, but if they read this they can do a post.

We do have lanyards to restrain the oar from rising out of the swivel and the swivel from lifting off the pin. You need to be quite careful about the lead of lanyards to prevent them getting chewed through. If they are near the pivot they tend to get attacked by the oar trying to act like a blunt scissor blade, and if they are not they get tight and slack during the stroke.

But the system you show still has the problem of the gunwale and the pin fighting over pitch reference, which Don's pin and keeper system and my untried ball ended pin and Ullapool and North Berwick's crutch/swivels do not. I don't think we should go backwards.

Don's system looks very unobrusive and traditional, and the ball ended pin would be unobtrusive to the point of being nearly invisible. These systems also save the expense and bother of 4 rowlocks or swivels.

I think the play in your rowlocks may have come from the offcentre weight of the oar wearing away the forward face of the top of the pin. Did the pin wear, or the swivel? Ours don't seem to wear much but what wear can be seen is at the bottom aft face of the pin.

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Rowlocks

Post by Robert Graham on Fri Dec 23, 2016 12:15 am

Rowlocks
Topher has provided an excellent synopsis of the variety of rowlocks currently in use. I agree with most of the desirable features he has listed for an ideal system (No 20).
There is an opportunity to make a number of improvements to the design and eliminate the weaknesses of some of the current systems for the benefit of existing clubs, new builds and all future rowers in St Ayles skiffs. There are four issues I would like the new design to overcome :-

1. Rowlock pin breakages
The survey indicated a need for components to be hard wearing. Rowlock pins breaking is an issue a number of clubs have suffered due to the strength limitations of wood. I think consideration should be given to a stainless steel rowlock pin.

2. Component wear and maintenance.
Wooden parts rubbing together soon begin to wear and tolerances, particularly in relation to pitch can quickly go out of spec. The most wear occurs when there is only point contact between two surfaces or an out of balance (twisting) load. To reduce the level of maintenance on components the new design should aim to have uniformly distributed and balanced loads on wearing parts.

3. Gearing flexibility
The debate so far seems to be homing in on having three settings ranging from 2.6 to 3. The extreme ends of that range can make a dramatic difference to the performance of the oar. Having only one intermediate setting, in my opinion is not precise enough to determine the optimum. It would be better if the rowlock was fully adjustable to any setting within the full range of gearing.

4.Fresh Start
Most existing oar and rowlock systems do not have all the optimum characteristics of a good design. Coastal rowing in St Ayles skiffs is a recreational activity which is growing exponentially and this  review is an opportunity to encourage even more people into the sport by ensuring their first experience is with a good oar that is easy to row.
The new design will also enable new clubs embarking on a build to have all the information necessary to make a good set of oars from the start.
Existing clubs that know their oar and rowlock systems are not competitive will have the opportunity to upgrade to a better design.
It has been stated elsewhere in this debate that arrangements will be made for the continued use of existing designs.
The new design should not be constrained by the reluctance of a few existing clubs to change from their present systems.

Attached is a rowlock concept sketch with features I would like to see in the new design :-
a. Eliminate rowlock pin breakage by using stainless steel.
b. Wooden routh tapered on bottom to contours of gunwale so that upper face is level fore/aft and port starboard from hog and seat datums.
c. Stainless steel pin mounted vertical to upper face of routh in fore/aft and port/starboard planes.
d. Oar to have a slot with plastic wearplates and a tight tolerance on the stainless steel pin to minimise clunking and maintain pitch angle.
e. Rocker shoe fitted under oar with a transverse bead in line with the pin to maintain consistent gearing throughout the full stroke. The bead uniformly distributes the support of the oar over its full width and also maintains constant oar pitch throughout the stroke.
f. Rocker shoe can be moved to any position along the oar slot to provide any specific gearing within the range.
g. Rocker shoe can be secured to the oar using velcro, rubber bands, cable ties, etc
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Re: Mechanics of oarlocks

Post by topherdawson on Fri Dec 23, 2016 10:34 am

This system does all of the things we need. I think there may be wear between the rocker plate and the routh during the stroke, as the rocker is rotated on the routh under the weight of the oar.

The gear adjustment is infinite which allows fine adjustment but if it is to be adjusted while the rowers are out on a trip (not necessarily while rowing) there needs to be a clear way to tell what gear it is in, with a scale or markers.

If it were possible to have notches or holes which were at 40mm centres then one could have 4 gears in the range 2.6 to 3.0, so two intermediate gears. If one could have gears at 30mm centres, then 5 gears. If the pin is stainless then its diameter will be less than the wooden pins, say 16mm?

If the above system were modified to have the slot in the oar capped by a ply wood plate with dimples to accommodate a rounded hemispherical end to the pin, the weight of the oar would rest on the pin and no rocker plate would be needed. Gear spacings of 30mm would allow 14mm walls between dimples which ought to be strong enough.

The plywood dimples could be lined with epoxy and if the ball ends were smooth there should be little wear.

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Re: Mechanics of oarlocks

Post by Robert Graham on Fri Dec 23, 2016 8:54 pm

The slot in the oar provides good stability in the fore/aft plane in both the drive and recovery phase of the stroke.
However, having a gearing system based on a spherical ended pin and dimpled plate covering the slot has some inherent weaknesses.

1. It is generally agreed that as far as overall oar weight is concerned the lighter the better. The proposed new trapezoidal design of oar is 4500mm long, UHF of 2kg and a weight of 5.4kg. There is not much downward force to keep the dimple on the pin and a number of situations could easily knock the oar out of gear.

2. The oar would be raised off the gunwale/routh on the pin so as well as a shear stress there would also be a bending moment which may lead to the pin breaking if it is made of wood. This is not a problem if the pin is stainless steel but its smaller diameter(16mm) would increase the pressure and wear on the dimple.

3. The 4 - 8 degree pitch on some club's oars will increase the chance of the dimple lifting off the pin.

Having the oar supported on a rocker shoe addresses these concerns and enhances stability.


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Re: Mechanics of oarlocks

Post by topherdawson on Fri Dec 23, 2016 10:49 pm

Good to be discussing details. John Macintyre also thinks the dimple will jump off the pin so I have added a screw to act as a barrier between gears:
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At Ullapool our gear notches under the oars are only 15mm deep so the oar only has to jump up 15mm to change gear, but it seldom does even if we leave uncleated the lanyards provided to prevent a wave lifting the oar off the pin.

Elsewhere on the forum Finlay suggests that if the pins are at right angles to the keel, the keel rises going forward at 2.5 degrees so adding a pitch angle may not be necessary.

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Re: Mechanics of oarlocks

Post by Finlay Robertson on Thu Dec 29, 2016 3:47 pm

I wonder, Don, if you could share some pictures of your feathering system here? I know it's not really pertinent to what we're talking about here, but Topher was asking me about it and I think it's worth sharing. I know I took some photos of them when I was in NZ but I seem to have mislaid them!

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Re: Mechanics of oarlocks

Post by Don Currie on Thu Dec 29, 2016 8:06 pm

Hey, good timing Finlay!  Just last night I finished a long email to Topher, describing the development of the "feathering square" oar.  Some photos attached.  For those who've not seen it, it's essentially a standard vertical plate and spacer oarlock system which,  instead of being mounted directly on the oar, is mounted on a plate that is hinged along the long axis of the oar.  The oar loom is free (within pre set limits) to twist as is required to feather.  There are a few tweaks in the geometry to make the feathering effort very low.  

I'll declare my position right now - I'm a huge fan of oars that have the ability to feather.  Note I'm not saying you feather all the time, but living as I do only 3 degrees of latitude from the "roaring forties"  we do a lot of rowing in winds of 15 to 20 knots, and the associated waves.  Using oars that feather just makes life so much easier.  You can row with a lazy, low stroke and if the blade hits a wave it just skips over the top and you carry on.  Note that when the conditions are nice you don't need to feather at all.  But having the ability to feather when required really transforms enjoyment of rough conditions, as well as opening up options for shorter, wider, blade shapes.[You must be registered and logged in to see this link.]
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Re: Mechanics of oarlocks

Post by Don Currie on Thu Dec 29, 2016 9:29 pm

I should have said in the post above that I'm not 100% sure that we ought be moving towards a feathering oar system for the proposed one design. I do think however that whatever rowlock system we move towards ought be able to accept a feathering option for enjoyable open water rowing, or as a racing option later on. At this stage I'd just like to bust through the "feathering is bad" sentiment that seemed to be apparent from the survey results. Yep, high friction, no limits to rotation systems like a conventional round loom are not good with oars as large as are used for the St Ayles. But that doesn't mean feathering is inherently bad, it just means we haven't figured out a good way to feather yet. The system above isn't perfect, but it is good enough to make me think that it's worth spending time to develop a good option.

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Re: Mechanics of oarlocks

Post by topherdawson on Sun Jan 01, 2017 10:56 am

To add to this thread, I will post an email from Don describing the progression to his system above, plus a drawing of it:

The next step along the development road for a feathering St Ayles oar was
the "Feathering Square". The flapper had shown the benefits of a feathering
action that was within contained limits, and the conventional oars had shown
that with big St Ayles oars the amount of friction to rotate the looms with
conventional leathers was just too much. So I started thinking "wouldn't it
be nice to be able to rotate an oar around its exact centre - there would be
negligable friction in that". I knew that a serviceable oar only needed to
rotate about 75 degrees in the feather axis - in fact it was an advantage to
make sure that the blade was always significantly "nose up" when meeting the
water surface. So how to "reach into" the centre of the loom to create a
pivot axis for feathering?

So I tried making an over strength sqare oar, and digging out a trough
towards the centre, and then hinging the oar along the bottom of the trough.
By that time I was using the vertical plate and spacer rowlocks for the
simple square oars, and it slowly dawned on my peanut brain that if I
separated out the action of a standard vertical plate and spacer rowlock,
and the action of feathering, then I might be onto something. So the
feathering square is basically a bracket that is hinged along the bottom of
a trough in the loom, onto which a standard vertical plate and spacer
rowlock is mounted. The first attachment shows a cross section through the
rowlock. I was working within the St Ayles rules, so couldn't use metal
hinges. So I used an old model aircraft hinge trick of a figure of 8
lacing. The drawing shows the lacing in felt pen - it's not well drawn, but
the main thing to bear in mind is the cross over of the stiching at every
turn as it passes between rowlock and oar. The photos show the lacing too.
I had to make quite a few compromises - the feathering is limited to 70
degrees - 20 degrees nose up in the feathered position. So the 12mm angled
web is 35 degrees from the vertical. I couldn't get the trough deep enough
to go to the centre of the oar without cutting too much of the oar away, so
the pivot is about 12mm above the centre of the loom. (the centre of
rotation is roughly the point where the threads cross over)

Various phots show the real thing. The rowlock bracket sits on the gunwale
pad, and its pivot is taken care of by the normal vertical plate and spacer
system. The bracket rotates with the oar, and the oar hangs off, and
rotates about, the back edge of the rowlock bracket. The whole system is
simple to make, light, and complies with the current St Ayles rules.

Because I was worried that there might still be quite a bit of friction in
the feathering action, I placed the handle forward of the centre line of the
loom. So when you push down on the handle at the end of a stroke it tends
to rotate the loom, and start the feathering action, with no work required
of the wrist. In practice this works remarkably well. Because the handle is
down below the centre line of the loom in the feathered position as soon as
the rower stops the return stroke the handle is pulled forward and this
automatically squares up the blade ready for the stroke. With no conscious
attempt to feather the oar does a partial feather with each stroke -
probably something like 45 degrees. If the conditions warrant it, you just
twist the extra 25 degrees to complete the job. The friction along the
feathering axis is so low that if the blade smacks a wave on the return that
too will just flick the oar to the fully feathered position. So all in all
the feathering square has been an unexpectedly good oar. People just plain
seem to like it. You can back water with it. You can change gear with it
positively. It just drops onto a (standard for us) single pin. You could
use a Dawson Dimple system on the rowlock. The feathering is fully
controlled by the rower.

There's still a bit of detail work I'd like to improve - I'd make the handle
offset a bit more subtle - the feathering friction is lower than what I
expected. If the rules allowed I'd use a more conventional pivot for
feathering (some sort of pin and bush system). I'd make the trough off
centre so as to make the forward side of the oar a bit stronger.

I've not put anything into the forum about feathering. I looked at the
returns from the survey, and see that a clear majority were against
feathering. That's a pity in my view, because in rough conditions a
feathering oar is a real pleasure. You don't have to lift the blade over
every wave - in fact you quickly learn to use quite a lazy and low stroke,
leaving the oar to find its own way over the waves. Having fixed limits to
the degree of rotation is critical - even when you are tired and not making
perfect feathering rotations the oar will do the work for you. I get it
that many who've tried conventional leathered round oars might not be keen
on what they percieve to be a feathering oar, but the feathering square has
taught me that there's real potential to make really nice rough water
feathering oars. I certainly intend to keep developing the current model,
and the NZ rowers are keen for me to make a full set - it seems to be a
popular oar for people once they have had a little bit of experience.

I'm more than happy for you to put as much or as little of all this into the
forum - there are no secrets about any of this, and I'd be keen to see what
other ideas new people might add to the idea (or indeed any other new ideas)

Sorry to take up so much time, hope this is of use.

All the best

Don

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Re: Mechanics of oarlocks

Post by topherdawson on Sat Jan 21, 2017 10:37 pm

I've been making the first 2 axis ball ended pin and dimples. The dimples are drilled with a Forstner bit in the bit of 18mm ply which forms the top plate, and then filled with thickened resin which is formed by leaving the ball ended pin in it till it sets.

This is the top plate (vertical in this picture but horizontal in use) with the keeper glued on to it. The keeper has a face of black polypropylene glued to it, as has the oar.

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This is the top plate/keeper assembly with glue.
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The pin will be captured by the keeper and the oar (not yet attached). This photo shows the assembly upside down. The forward face of the oar will form the other side of the slot containing the pin, and there will be screws through the 4 holes into the oar, and down through the top plate into the top of the oar.

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Re: Mechanics of oarlocks

Post by Don Currie on Sun Jan 22, 2017 8:35 am

Looks like the distance centre to centre is not equal - right hand one something like 51mm, and left hand one something like 57mm by any chance?

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Re: Mechanics of oarlocks

Post by topherdawson on Sun Jan 22, 2017 11:01 am

They are unequal, 50 and 56. My calculation goes like this:
Length = 4300. Take off 150 at the handle end and 300 for the CofA for the blade, and you get 3850.
For the 2.6 gear, add 1 and divide: 3850/3.6 = 1069 then add the 150 back so the inboard = 1219.
For the 2.8 gear, 3850/3.8 = 1013 which with the 150 = 1163
For the 3.0 gear 3850/4 = 963 which with the 150 = 1113.

Your calculation may have slightly different assumptions for the 150 and the 300, but it's very close. Time will tell if this arrangement is durable.

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Re: Mechanics of oarlocks

Post by topherdawson on Sat Feb 04, 2017 6:55 pm

Adrian Hodges of Norfolk writes:

The traditional system used in Thames Skiffs. The upright portions against which the loom of the scull works are normally known as tholes. This arrangement is usually used with half-buttoned sculls. This requires the sculler to keep an outboard directed pressure on the scull handle. Not suitable for St Ayles Skiffs in my opinion.

Paired thole pins. This is the system that we use in Blakeney. Our second boat was made with a single pin and plate system, but we have converted it to paired thole pins like the first boat. Although this system would allow for feathering the blades, the square section of the looms effectively prevents this, ( which I regret). There is inbuilt inefficiency in that the movement of the loom between the pins with each stroke must reduce the outboard arc of the oar. I have not measured this. In practice we do not find that rowers have to sit at other than a right angle to the centre line of the boat, nor do we find that they have a problem locating the oar with appropriate inboard/outboard once they have become accustomed to it. We only rarely adjust the gearing by moving the oar in or out, because it affects the trim and clearance between rowers. This system also allows crews to shorten oars in confined channels which we share with other craft. This is a frequent situation for us both at Blakeney, where there is an active sailing dinghy fleet and on the Broads where we encounter motor cruisers often with inexperienced helmsmen. Last, but by no means least, it is traditional.

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