Hollow outboard shaft made from four strips of the same timber

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Hollow outboard shaft made from four strips of the same timber

Post by topherdawson on Sun Dec 11, 2016 5:41 pm

Several of us have made box section hollow shafts. From Ullapool there is John Macintyre, Dan Johnson and I. From Strangford there is Tyrone Currie. In Crail there is Ian Mills. In New Zealand there is Don Currie. Creative lot these Curries. In Perth there is Charles Colvin.

It anyone else has made one I will be glad to add to this list.

Ian Mills and John Macintyre used ash for load bearing flanges and cedar or spruce for the top and bottom. Don used the same timber for all four strips.

These shafts tend to be good and stiff, stiff also in torsion and in the vertical dimension. By varying the strip thickness the designer can adjust the design for timbers of differing density and strength, without varying the outer dimensions.

If the strips get too thin, the section can be a bit hard to glue up, and potentially if could collapse under stress. But we know that 9mm walls in a 65mm square section are stable because those are the dimensions of UCRC's present oars.

The design spreadsheet shows a dramatic improvement in UHF, for instance: An oar 75mmx63mm with a solid oak inboard and a cedar box section outboard with 10mm walls, shows a deflection of 81mm and a UHF of 1.3! It is almost too balanced. The larger depth of the section is making the oak inboard quite heavy, but even so the total oar weight is 4.82kg, quite respectably light. Again the stress is well within the strength of cedar so the extra cost of spruce is not justified.

Even if the wall thickness is increased to 12mm to allow for surface wear and tear to the soft timber, the UHF is only 1.6kg and the deflection goes down to 74mm. The total weight is now 5.05kg which is still respectable.

The same oar could be made out of spruce with the strip thickness reduced to 9mm, the UHT to 1kg and the total weight to 4.73. Doubtless there are many other light timbers which would be adequate, with the strip thickness adjusted to the properties of the timber.

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Re: Hollow outboard shaft made from four strips of the same timber

Post by colinmuirhead on Mon Dec 12, 2016 5:10 pm

I have made a set of box section oars using 12mm thk skirting board on all 4 faces. I have sent you the measurements for these. At 4.3m long the UHF is 4.2kg and a deflection of 60mm.

These oars have been well used, had 1 breakage, to do with a knot being in the wrong place.

I made the replacement with all 4 faces made from clear 12mm thk Douglas Fir and at 4.3m long the UHF is 4.2kg and the deflection was 50mm.

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Re: Hollow outboard shaft made from four strips of the same timber

Post by topherdawson on Mon Dec 12, 2016 5:32 pm

Hi Colin,

I think with skirting board pine or Douglas you could afford to use 9 or 10mm thick strips, and with a solid hardwood inboard the UHF could be reduced a bit. Deflections of 50 and 60mm are good and stiff. The box section does lend itself to different timbers.

Do you, having made them, think hollow oars are within the abilities of the average skiff builder?

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Re: Hollow outboard shaft made from four strips of the same timber

Post by colinmuirhead on Mon Dec 12, 2016 5:54 pm

Hi,

The solid sections both inboard and outboard are larch as well as the blade edges. I just left the strips thk's at 12mm as that was the stock size. By using a lower density wood for the outboard block and blade and a higher density inboard would definitely reduce the UHF.

Something I think needs to be taken into consideration is the very different rowing conditions that the skiffs are used in, calm, no wind to high wind and 5 / 6 feet swell and chop on the water. If the UHF is too low or the blade width is too much it will make it harder to row and not catch the blade in the water and possibly cause problems.

The square section hollow oars are very easy to make and should be well within the limits of anybody that can build the skiff.

Colin

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Re: Hollow outboard shaft made from four strips of the same timber

Post by topherdawson on Mon Dec 12, 2016 6:16 pm

Thanks Colin. The hollow section is so superior structurally, it may be the logical choice, so we will need good build instructions if we go that way.

Shorter wider blades may in fact be less liable to catch waves accidentally, as the long narrow ones tend to catch at their lower corner. Their length makes it a lot of vertical movement from fully in to fully out.

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Re: Hollow outboard shaft made from four strips of the same timber

Post by colinmuirhead on Mon Dec 12, 2016 6:32 pm

If the oars are the same length, the oar with the wider blade would need to be higher out of the water so that the bottom lower corner of the blade would not catch.

Example, if a blade of 200mm wide is compared to a blade of 120mm the centre line of the oar would need to come out of the water by an additional 40mm for the 200mm width blade corner to clear. Not a problem if flat conditions but in a swell this can make a difference.



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Re: Hollow outboard shaft made from four strips of the same timber

Post by topherdawson on Mon Dec 12, 2016 6:39 pm

True, but all the blades are between 900 and 1200 square centimetres in area, so as they get wider so they get shorter. Because the blades are going in and out of the water when they are angled downwards, the length as well as the width affects how much vertical movement is needed to clear the water.

Also Macons and tulips are narrower at the tip which helps make the lower edge clear the water. To answer this definitively would need a scale drawing.

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