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COPYRIGHT Tim Lovett © July 2004 

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Noah's Ark... Held together by wooden nails?

Not quite as silly as it sounds. Ships were built that way. 

"Trunnels" are round or multi-sided wooden pins (literally "Tree-nails") used to hold wooden parts of a ship together. Trunnels were still used even when nails were available because they were cheaper, and because metal exposed to salt water promoted rotting of the wood. Trunnels were commonly 25 to 40mm (1 - 1.5 inches) in diameter, usually in a harder timber such as Locust. Water seeping into the endgrain of the trunnel caused expansion, improving the seal. Wooden wedges were sometimes driven into the trunnel to increase the grip, then the protruding fastener was sawn off flush with a handsaw.

The Trunnel or Tree nail. Noah's not-so-high-tech fastener?

Used correctly, trunnels (treenails, pegs, dowels, pins) have advantages over metal nails. The mass production of steel nails from the mid 1800's made joining faster and more convenient, without the need to predrill. But a properly designed timber structure can be successfully held together by the humble trunnel, provided there is a large enough wood area to work with. 

Membrane Elements.

Top candidates for the trunnel are the laminated membrane elements forming the bottom, sides and top of the Ark. This cheap, quick to make, low-tech solution has no corrosion problems and no worries about hitting metal while drilling. Wood is much cheaper than bronze too, in any era. 

The Bottom

As Noah's Ark undergoes hogging and sagging loads applied by the waves, the bottom and roof will be in alternating tension and compression. They must act as a single piece. This is achieved using multiple layers of planks joined by trunnels. The pitch will act as an adhesive which effectively turns the entire slab into a glue laminated beam. However, an initial conservative estimate will rely solely on the dowels.

How efficient is the system?  

Adjacent planks must have enough overlap to secure with an adequate number of trunnels. If the diameter is too large the remaining plank area is inadequate, too small and a greater number will be needed, too close together and the planks could spit. This defines the amount of plank overlap required. 

 

In this situation the trunnels on the extreme left and right of the joint carry the most load, while those towards the middle carry less. We will initially assume the central pegs carry only 50% load, but this assumption will need to be checked.

Notice in the diagram below that the inner planks put the trunnels into double shear, effectively doubling the cross-sectional area of each fastener. Top and bottom planks are limited to single shear.

The design is based on 4 layers of planks, with at least 3 fully joined planks active at any cross section along the Ark. This requires each plank to be 4 times the length of the overlap as shown below. Hence a stack of 4 planks will have only 3 continuous planks passing any plank end. This gives a maximum of 75% efficiency, compared to the strength of a single-piece plank set.

However, the planks would also be overlapping transversely, like bricks. Each plank could be laterally connected to 2 others, with the adjacent set of planks staggered so that there is always 7 out of 8 planks taking the load. Maximum efficiency is now 88%. It is not inconceivable to carry the multi-member advantage even further, so that a single joint might be spread over 12 or more planks. This would harness more than 90% of the total plank area in resisting the axial loads caused by wave bending moment. There is strength in numbers.

 

Trunnels (Tree Nails)

 

 

Dowel Diameter

40

mm

Dowel Parallel Shear Strength

3

Mpa

Dowel Perp Shear Strength

9

Mpa

Plank Thickness

100

mm

Plank Width

400

mm

Plank Tensile Strength

7

Mpa

Plank Perp Tensile Strength (splitting)

0.5

Mpa

Plank Long Comp Strength

10

Mpa

Plank 100% Load

280000

N

 

 

 

Dowel Shear

 

 

Dowel CSA

1256.63

mm2

Dowel shear force (double shear)

22619.47

N

Total number of layers

4

 

Extreme plank single shear adjustment

75%

 

Inside dowel loading ratio

50%

 

Average dowel efficiency

56%

 

Average dowel shear force per plank

      12,723.45

N

No of dowels to equal plank tensile

            22.01

 

 

 

 

Parallel Shear Check

 

 

Area Ratio > 3 ?

3.1 

 

 

 

 

Hole Crushing

 

 

Hole Crush Area

4000

mm2

Hole crushing force

40000

N

 

 

 

Plank Splitting Length

 

 

Code splitting criterion (7 diams)

280

mm

 

 

 

Assume dowels at split distance

 

 

Min plank overlap

              3,080

mm

 

3.08

m

Min plank length

12324

mm

 

12.324

m

 

 

 

Max Joint efficiency 

              0.88

 

1 dowel hole substracted…

0.9

 

Final efficiency

79%

 

Notes

The ultimate solution is many planks and many small diameter dowels, so it's a tradeoff between efficiency and amount of work.

The adhesion of the pitch has been ignored, but even a weak glue could be very affective over such large areas. 

Since we are assuming a rather conservative cross-grain shear of only 3 times the parallel shear, the risk of parallel shear is easily checked. We simply need a parallel shear area in excess of 3xCSA per shear plane. At 40mm diameter and 100mm plank thickness the area ratio is 4000/1256 = 3.1. So 40mm is a good choice. But a shorter, fatter dowel could fail in parallel shear, so it is not helpful to go much bigger in diameter. 


References

1. Old Ways of Working Wood: The Techniques and Tools of a Time-Honored Craft. A.W. Bealer, Barre Publishing Massachusetts.

Trunnels (Tree nails):  "Before the era of mass produced nails the main fastening device for the frames of houses, mills and bridges was the wood peg, usually designated a trunnel, which is a corruption of the ancient term treenail. Most trunnels used before 1840 were cut square then driven into the round peg hole of mortise and tenon with a maul or mallet. The use of trunnels practically disappeared after 1840 in house building, but the builders of wooden bridges, particularly covered bridges, used them to pin the timbers together almost until World War 1. Bridge makers used mass-produced trunnels, round instead of square, but preferred over spikes and bolts because wood in wood held up longer under the vibration and tension of constant traffic by iron shod horses and heavy wagons." (Ref 1, p224) 

2. http://home.att.net/~ShipModelFAQ/smf-qTrunnels.html

"Trunnels" are wooden pins (literally "Tree-nails") or dowels that are used to hold wooden parts of a ship together. Trunnels were used instead of metal nails both because they were cheaper, and because metal exposed to salt water promoted rotting of the wood... trunnels were commonly 1 - 1.5 inches in diameter. 

3. http://www.hamilton-scourge.city.hamilton.on.ca/tech&sup.htm

Planks were held in place with iron C-clamps until they were permanently fastened to the frame with a treenail or trunnel fastenings (locust pegs about 1 1/4" in diameter).

4. http://www.hamilton-scourge.city.hamilton.on.ca/glossary.htm#Treenail

Treenail. [trunnel] A round or multi-sided piece of hardwood, driven through planks and timbers to connect them. Treenails were employed most frequently in attaching planking to frames, attaching knees to ceiling or beams, and in the scarfing of timbers.

5. http://www.wcml.org.uk/gmb/shipw.htm

THE SHIPWRIGHT: In this print, the shipwright is seen standing at the stern of the ship on a scaffold. The holes were bored with an auger and the wedges driven in with a wooden trunnel. They were then levelled with a saw. The shipwright is shown with his tools, his saw, his auger, his axe and punches of different sizes at his feet ready for use.

6. Wood and Timber as a Marine Material. http://web.usna.navy.mil/~phmiller/en380/wood.doc

7. http://www.davistownmuseum.org/bioDrew.htm

..."Special attention was given, in the beginning, to the manufacture of what was known as Treenail (treenail, pronounced trunnel) Augers, used in the building of wooden shipping and differing from the regular Carpenter's Augers in the matter of length of the twisted section and also in quality.  They were used for boring holes in the oak timbers that formed the frames of ships, preparatory to fastening those timbers together with iron bolts or with tree nails" (literally Tree Nails, or wooden pegs, just as houses were framed).  "The nature of this work demanded long Augers and the best quality of steel and workmanship, to perform their work and withstand the severe service required of them"

8. http://www.tollway.com/swift/info.htm

Also involved in her longevity is the fact that she was completely "trunnel" fastened (locust tree-nails) so there were no fastenings to corrode or plugs to leak and start soft places.

9. http://www.town.kennebunkport.me.us/Public_Documents/KennebunkportME_Comp/00075FB4-70E903AC

It took many skills to build a ship, and virtually all of the labor was done by experienced craftsmen.  Carpenters, sailmakers, blacksmiths, caulkers, painters, and adzemen were only a few of the skills required by the yards.  These were not easy jobs, but they were jobs a man could be proud of.  To be considered the best trunnel-borer, plank-liner, or rigger was a mark of distinction.

10. http://www.shipwrecks.nf.ca/news/news3-2.shtml

In the early shipyards of outport Newfoundland long iron nails were often hard to come by and vessels were trunnelled with wooden nails, similar to a dowel. The trunnel extended completely through the plank, the seal and the timbers, and this technique provided extra strength. However, Eileen Lake was fastened with wooden spikes that reached several inches into the timber. Spikes were quicker to pound into the planks than trunnels and were easier to make. It was a shortcut that probably hastened the demise of a fine ship.

11. http://www.daviscoltd.com/nams/Documents/nvic7-95.pdf

Glossary. Guidance on Inspection, Repair, and Maintenance of Wooden Hulls. Treenail - (Trunnel) A wood dowel used as a fastening; often fitted with a wedge in the dowel end to hold it in place. Dense wood such as locust is used for the dowel.

12. http://www.kastenmarine.com/phinisi.htm

The traditional Indonesian sailing vessel the Pinisi was built using blind trunnels (like dowels) between adjacent planks. The hull planking was laid up first - like most timber boats outside of the European frame-first method.