Tree Nails

This is a fairly complex subject, but to make things simple lets take a 1/4" grade 2, mild steel wood screw, with a tensile strength of 1,000 pounds. This is driven through a plank and into a 1.5" wide frame. It occupies 1/6th of the width of the frame face. Now try to size a piece of wood for the same strength.

When it comes to cyclic loads, a screw or through fastener can be used to hold the wood so tightly together that only self destruction of the surrounding wood will yield the fastener. This generally isn't the case with a trunnel, which is lucky to just marry the two pieces, let alone apply substantial clamping force over a wide load range.

In short to get the same tensile strength from a piece of say white oak, carved into a trunnel, you'll need a dowel about 1" in diameter (2/3's the width of the face of the frame), which if driven into a 1.5" frame will likely split it or at the very least compromise the strength of the frame.

 

I see.
So a trunnel has to hold the entire shear force by itself, while the metal fastener (privided it is a screw or clinched type) uses the friction of the two pieces of wood against each other to lessen the shear force?

 

I'm not sure how you extrapolated that, but a screw or through fastener has a mechanical advantage over the trunnel as well as much higher physical attributes.

 

Most of my experience is with old school post and beam and decorative pieces. Things like locking scarf joints that don't use glue at all and wedged feather tenons blind or not. My next project is a hundred and thirty year old house of windows and doors, Pauls is likely a hundred and thirty year old boat, so pick your poison Ben.

What I have seen over and over is that these old structures that are assembled using all wood fasteners have no wear in the joint and in most cases, if your life depended on it, there's no getting a tree nail out one its been sledge hammered into its final resting place. A properly made joint intended to use a tree nail doesn't concentrate the sheer stresses onto the fastener, a more simply made joint intended to use a metal bolt, screw, drift pin, whatever, seems almost always to concentrate the sheer onto the fastener. Go figure why using one type fastener in place of the other in a similarly designed joint doesn't work. Its classic apples to oranges. At least with terrestrial structures, I'm not so sure its much different with ocean going craft tho. Two very differing ways of putting something together each intended for the type of fastener available.

Its not just the direction of the joint, I recall its also the angle of the holes and if there blind or not that is what sets a scarf intended for tree nails apart from one intended for a bolt. Its been a long time but once upon a time they did this more often than not and a lot of those old boats are still around today. I think what the advent of metal fasteners really did for something like boat building is it allowed for smaller pieces of wood to be joined just as effectively although sacrificing some longevity in the joint. Dissimilar materials rubbing against one another are bound to cause wear in one or the other. Makes for lighter faster vehicles rather than the massive vessels of antiquity.

as I might have already mentioned, my own build with incorporate metal fasteners in key places like planking which is definitely a disposable item, Also there is an insurance issue ( gotta love the insurance industry eh ) ole Lloyds demands x amount of metal.

 

Dan, you are describing static structures, which for the most part don't have to deal with falling off a 30' wave, surfing down it's face then plunging into the next wave's back just to repeat the sequence. A dynamic structure that is constantly stressed, relaxed, twisted, bent and hounded by wide pressure variances, is a considerably different thing then a corner post, rim joist interface that can stand motionless for a half a century.

 

Side note:
Is trunnels short for "tree nails" like Fo'c'sle is short for fore castle?

 

See post #2, this thread.

 

No! Trunnels is for treenails.. Fo'c'sle for forecastle

 

Sorry I should have said,"Is tree nails pronounced trunnels like fore castle is pronounced...
It was an etymological question about if they had the same root.

(I'm still trying to get used to t'gan'sl )

 

well in the end your point about using a more modern fastener is right on for a number of reasons, but the flexibility issue isn't one of them, although I guess I did focus on static structures in that last so its an easy to assumption otherwise. I've kinda made a career out of antiques restoration and reproduction and it seems that the more dynamic a construct needed to be the more likely it was to be held together by pins and puzzles rather than nuts and bolts. Take the basic wagon wheel out of the 1800s or older, most were all wood except for the band and the edges of the hub. The whole thing was one gigantic example of a puzzle piece that avoided fasteners like the plague. The hoop held the tenons in place but they avoided metal fasteners completely. Same holds true for old stage coaches and train cars. There is a restoration going on down the street of an old Denver trolly and you can really see the difference in how fasteners act by looking at say the skylight and how it was all mortise and tenon together VS other parts that were just nailed. The skylight is actually in pretty good shape but anything simply nailed is a complete rebuild. I just spoke to them yesterday. Another good example is old wooden bridges and a five second search came up with numerous boats being restored using the original techniques including treenailed planking.

http://tugster.wordpress.com/2008/10/

so while I"m not disagreeing that a new build should incorporate modern techniques which allow for lighter more efficient construction there is something to be said for methods that held there ground for centuries going back beyond written history

cheers
B

 

Is this a short cut to the part you meant?

Cheers,
Angel

 

As a rule wooden structures that rely on trunnels have much larger dimensioned structural elements then those design to be fastened with metal.

To use your wagon wheel as an example the metal ring is what makes the whole thing work, other wise every portion of the wheel's elements would have to be bigger to compensate for the lose of integrity.

Simply put, trunnel assembled wooden things require huge pieces of wood, which is weight that has to be paid for and assembled. One of the biggest benefits of metal fasteners is the ability to use just enough wood to get the job done, without reserving "meat" for the trunnels. Take a look at 1880 yacht design and compare similar sized 1920's yachts and one of the major differences, is huge reductions in weight, mostly because they can scale lumber for it's use, not it's fastening system. Of course other things happened in this era, but HMC yachts would be a classic examples. They continuously evolved lighter and more clever manufacturing techniques and pocketed the material saving differences as well as beat competitors on the race course.

 

yerp
which is why I mentioned

the bulk needed to do a proper treenail is most definitely an issue

often if I'm just doing a piece for myself or a client who whats an exact reproduction I am happy to play with treenails still, but from what I can see of modern boat building techniques the use of standardized metal fasteners is a must if the build is anything resembling a modern design.

anyway I think we are actually agreeing with one another and just using different perspectives to do it.

cheers
B

 

Agreed Dan, though I too enjoy the quaintness of some of these old methods.

 

On heavy double sawn frames they are admissible. they are known to be weak, but due to the mass of wood, it is a effective way to go, with the steel fastening. By itself for a whole vessel it will be wrong to use only trunnels
Lister