View Full Version : how do trunnels hold?

02-08-2017, 09:57 AM
It's a question which has puzzled me, given the way my dowel fastened teak deck furniture tends to separate at the joints. A quick scan of trunnel related threads on this forum still leaves me somewhat confused. Perhaps it works like this? The trunnel is tapered and driven in from the outside of the hull, and then it's wedged tight on the inside. That way plank and rib are held securely together?

wizbang 13
02-08-2017, 10:31 AM
Put in dry,swells shut.Little wedgie bit on the plank side. Used on heavier scantling vessels.
Your teak deck furniture probably has 3 rings per inch,not boat timber.
And....if vessels were left unpainted in the sun, the trunnels would dry out.

Ian McColgin
02-08-2017, 10:32 AM
Not tapered. And not just any dowel.

First the wood. A wood that resists water uptake and shrinking/swelling is necessary. So no sort of oak. Black locust is absolute best.

Trennels are significantly fatter than metal fastenings for whatever size you're doing. The standard rule is about 1" diameter for every 100' LWL. For my boats, that's generally been 7/16" of 1/2" depending on the boat.

Trennels are labor intensive as the plank must be positioned tightly in place first - clamps, outside bracing, whatever it takes - and then a very exact hole drilled and finally the trunnel driven home. And then to finish the hull the proud end of the trunnel must be sawn off as the endgrain won't let it pop cleanly the way a bung will.

Finally, at the hood ends of a tightly curved plank, trennels are not so suitable for the same reason that often one can't use nails or even screws there. It takes a bolt or a rivet to hold there. Sometimes you'll see trennels in these areas with a blind wedge at the bottom and a wedge at the surface to make something about as tight as a rivet or a bolt, but that's tricky and can let water in. You'll also see wedges in some very large ship construction but those trunnels are a different engineering phenomenon.

I have used a few thousand trennels over they years when replacing bad fastenings or even plank sections. In these cases, the wood around the hole (in the plank if it's staying and in the frame) is damaged and you'd need to refasten with something larger anyway. In my old boats, just cleaning all that up makes about as big a hole as a suitable trennel would be anyway. The old dodge of cleaning to good wood and plugging the hole to then fasten with a screw really amounts to a bad trennel with wrong grain alignment and a metal core. Seems stupid to me when black locust is easily found.

The hole for the trennel should be tight. Because that's my bit size, I go 1/16" smaller than the nominal size of the trennel. Nominal as my way of making trennels (knocking square blanks through three successivly smaller holes in an iron plate to take the corners down to round) results in trennels that are actually a hair smaller than the final hole in the plate. My guess is that the real difference is the hole is about 1/32" small.

Anyway really tight fit. I find it handy to slick the trennel with a little epoxy. This will help seal the end grains in plank and frame exposed by the hole and, epoxy being so slippery, it makes driving the trennel in easy.

Sometimes the wood is so tight that as you drive the trennel in air trapped in the hole compresses and instead of the trennel bottoming out in the hole it sort of bounces. This can be solved by letting the air vent from the hole, mostly I think into the wood, by keeping up gentle rapid tapping. But if it's a real problem, I sometimes put a very fine hold all the way through the frame. Any epoxy being pushed ahead of the trennel will settle in there anyway.

But it's not the epoxy that holds the plank in - it's the tight fit.

At least for 60+ year old plank on frame cruising sailboats in the 12T to 30T size that stay wet year round, any fastening replacement is well served by switching to trennels and never having that problem again in the lifetimes of you, your children, and your grandchildren.


02-08-2017, 12:00 PM
It's a question which has puzzled me, given the way my dowel fastened teak deck furniture tends to separate at the joints.

There's a difference between a trunnel and a dowel. A dowel fits snugly in a hole, but depends on glue to keep it in place. A trunnel ("tree-nail") fits so tightly in the hole that it stays without any glue. Wedges can be used on the ends to hold it even more tightly.

02-08-2017, 03:02 PM
A friend who builds, primarily, traditional boats, has been using trunnels lately in lieu of nails for clinker construction. He saturates the hole with thinned epoxy resin, several coats while still `green', and soaks the trunnel in epoxy for half an hour or so before knocking it in. He emphasises the importance of completing the whole operation while the epoxy is still green (not cured), presumably to achieve a chemical bond.


02-09-2017, 09:10 AM
This method used to be pretty much norm on the west coast of Sweden, and boatbuilders had dedicated machines for making trunnels, I found this as an example of one

There are also smaller and simpler models, that are easy to make yourself with a chisel or a plane iron as the cutter.

The edge of the iron should be slightly skewed so that when you stop turning the tool/stick it leaves a tapered "head" on the trunnel.
You then make a similar taper on the outside part of the hole in the planking, drive the trunnel from outside and secure it with a wedge on the inside.


02-09-2017, 10:35 AM
The deck of the Kalmar Nyckel is held down entirely by trunnels, with wedges on both ends. Getting the bottom, or blind wedge right is the only tricky part. The wedge and the slot it sits in have to be sized so that the trunnel can be driven all the way home while getting a lot of compression down there in the deck beam. They don't ever come loose and the only way to remove a plank is to drill out the entire trunnel.


Paul Denison
02-09-2017, 10:59 AM
Take a look at this thread: http://forum.woodenboat.com/showthread.php?64926-I-m-finishing-my-38-cowhorn-would-like-to-help-build-the-sails

Well, I thought he was using trunnels, did a quick look and don't see them. It's a great read anyway.

02-09-2017, 01:44 PM
"Trunnels" is a corruption of "through-nails". The important detail is they need a drilled or trepanned hole all the way through.

It is an age old argument

Peerie Maa
02-09-2017, 02:11 PM
"Trunnels" is a corruption of "through-nails". The important detail is they need a drilled or trepanned hole all the way through.

It is an age old argument
Still going.
It is Tree nail. Just as wooden boxes are called treen" to the antique trade.
They can be driven blind, with the inner end split and wedged so that as it bottoms the blind hole the wedge is driven into the end, tightening it up, just as Dave Wowbox states.

02-09-2017, 05:33 PM
Some good info here