Hollow shafts

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Hollow shafts

Post by topherdawson on Wed Dec 07, 2016 10:47 am

1   Hollow wooden oar shafts can be and are being made. They can be stronger and stiffer than solid shafts.

2   At some point this group will need to decide whether hollow shafts are within the skill level of community boat builders.

3   I would suggest that this means they need to be no harder to make than the boat itself.

4   I know of 5 ways to make hollow shafts:

5   The traditional timber racing oar in the 1950's and 1960's had a D shaped sitka spruce shaft, hollowed out with a spindle moulder to make a U shape on its side, and then capped with a flat forward face of ash on the compression side.

6   In the early 1900's McGruers in Scotland were wrapping two layers of veneer round a mandrel, with a longitudinal scarph joint, using the glues of the time. They could produce circular, oval or aerofoil section tubes which were used, among other things, for the struts of Vickers Vimy bombers, yacht spars, and the shafts of Everest expedition ice-axes.

McGruer hollow spar

7   Suttons of England make box section spruce oar shafts with an ash front, and then round them down to a circular outside shape. These are common in gigs. Some St Ayles use box section, only slightly rounded, with Douglas for the tension side, ash for the compression side, and Western Red Cedar for the top and bottom webs. These are light and stiff.

Suttons oar section

Click on oars then tech spec

8   Some skilled builders are using birdsmouth construction, in which square or rectangular section strips are macined to have a vee goove on one face to accept a corner of the next strip. The strips are built up to form a circular or oval cross section. The strips needed for an oar shaft taper down to a really fiddly thing you could break between your fingers so I'd say this is definitely one for the experts.

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9   Using heat, 4mm ply can be wrapped round a NACA 0030 aerofoil section. A "main spar" is needed at the point of max thickness to stop it collapsing vertically. At a chord of 200mm and a depth of 60mm at the oarlock, it is strong enough for us and very stiff. It is also light and pretty cheap in materials. I could never get the whol thing hot enough to bend but did make samples.

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10   Of all these, the box section is the only one potentially easy enough to make, and even then i'd like to take a vote.

11   Who here has made a hollow shaft? Who here thinks they are within the grasp of community boatbuilders? Remember your own skill level may not be typical.

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Re: Hollow shafts

Post by John Mitchell on Wed Dec 07, 2016 2:11 pm

I am experimenting at the moment in the construction of an oar with a hollow shaft which is very simple. The shaft and loom are made up by gluing three laminations together, the centre one being thicker than the other two. The void is formed by simply cutting out a central slot through the middle section over the region of the shaft. It is no more difficult to glue up than a solid shaft and much easier than a built up box section.

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Re: Hollow shafts

Post by topherdawson on Wed Dec 07, 2016 2:55 pm

Have a look at Don Currie's design in the Dropbox folder (link on the home page). I think he is doing a similar thing, although his central section is glued up in a previous operation.

A hollow section is one way to use denser timber without making the shaft heavier.

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Re: Hollow shafts

Post by colinmuirhead on Wed Dec 07, 2016 6:42 pm

I have made a set of hollow oars based on John Macintyre 15ft hollow oars but with a couple of modification.

Easy to make and strong, made they using skirting board so wood only cost about £10 - £20 per oars.


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Re: Hollow shafts

Post by John Mitchell on Thu Dec 08, 2016 11:39 am

The hollow does not have to be on the inside! Could be an I-beam section, either glue up or with grooves routered out from the solid. Has anyone tried this?

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Re: Hollow shafts

Post by John Mitchell on Thu Dec 08, 2016 5:03 pm

PS to my previous comment: the torsional stiffness of a box section would be superior to that of an I-beam.

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Re: Hollow shafts

Post by topherdawson on Thu Dec 08, 2016 6:23 pm

Hi Colin,

If the oars you made were stiff and light (what were the modifications?) then we'd love to see the measurements for them/

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Re: Hollow shafts

Post by topherdawson on Thu Dec 08, 2016 6:27 pm

Hi John,

Eastern and Loch Broom have made routered grooves on the outside to make an I-beam but not enough to make a big difference to the weight. I know that Admiralty whaler oars began to do this when the racing got serious. It's also well known that I-beams lack torsional stiffness which can lead to the oar pitch changing, with annoying results.

As far as I know nobody has made a really skinny I-beam for an oar shaft, maybe we should try it.

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Re: Hollow shafts

Post by Don Currie on Thu Dec 08, 2016 7:24 pm

I have a sneaking suspicion that by the time you'd made a nicely tapered I beam loom, including a router cut tapered slot, you could have just as easily built a hollow rectangular loom!

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Re: Hollow shafts

Post by colinmuirhead on Thu Dec 08, 2016 7:28 pm

topherdawson wrote:Hi Colin,

If the oars you made were stiff and light (what were the modifications?) then we'd love to see the measurements for them/

Hi Topher,

These are the oars that I plan to do the measurements on for your survey so will find out how light they are as I have never weighted them. The mod's were to the shape of the blade, handle and shaft sizes.

We did have one breakage due to a knot in the wrong place, the replacement was made out of Douglas fir that we have used to make our other oars from. The other three are holding up well and were used in a number of races at the words including some gold medal winners.

Colin

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Re: Hollow shafts

Post by topherdawson on Thu Dec 08, 2016 8:38 pm

Don Currie wrote:I have a sneaking suspicion that by the time you'd made a nicely tapered I beam loom, including a router cut tapered slot, you could have just as easily built a hollow rectangular loom!
The slots were not tapered, and to be honest they were more for the look of the thing than to save serious weight.

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Re: Hollow shafts

Post by topherdawson on Wed Dec 14, 2016 9:54 am

This is from an email from Robert Dunster of Blakeney in Norfolk:

The oars are basically rectangular in section, with rounded corners. We use kabe/thole pin, and often row in restricted waters, resulting in the need to pull oars in quickly to avoid hire boats & scenery, & found that a round section suffers damage where it is pulled over the gunwale - this resulted in failure of 2 commercial oars. Square section means the pressure is more spread out and damage does not seem to occur.

Viewed from the top, the oars do not taper until the blade is reached. This maximises stiffness.

Viewed from the side, the oars taper from the end of the blade, right up to the start of the handle. This maximises weight inboard / reduces it outboard.

The oars are constructed from identical from & back pieces, running from blade to start of handle. The section outboard of the leathered section is hollowed out (concave section, the whole hollowed area tapering) leaving a minimum of 9mm wall thickness. Front & back are separated by various 20mm thick centre pieces - 2 running full length again hollowed out outboard of the leathered section, one that is part of the handle - flat section changing to round, think cricket bat handle, and one to fill in the gap between the handle and start of the hollow section. Thus there is no joint as such between solid & hollow sections.

This explains the dimension of 61 fore & aft (nominally should be 60), and 69 vertically.

The handles are oak to provide weight inboard & are hard wearing. Not sure that they are less work to make that carving a handle from the spruce however- actually quite tricky to get right.
The based are inserted in a slot cut into the end after assembly. This was intended to be easier that carving spruce sections after sticking them to the shaft, again not sure the it actually is!
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