Jan 27

Written by: Tom Daniels
1/27/2009 7:28 PM 

The transom is now attached, and you may now have a little better idea about how it meets the planking.  



The edges of the transom are bevelled so that the end grain from the transom mates to the end grain of the planking.  Exposed end grain is a bad thing in a marine environment... the structure of wood is similar to a tight bundle of straws, and the end grain is the open end of those straws.  As a rule, we try to have as little end grain exposed as possible, and none exposed on the outside surfaces of the boat.  Any end grain in the boat is sealed with paint, varnish, epoxy, bedding compound... anything that seals up those straws and prevents water from flowing into the wood.  The transom / planking joint is sealed with bedding compound.

Oh, and that's a cedar transom by the way.  Why not mahogany or something a little nicer looking?  Olin Stephens specified cedar, and we listen to Olin.  Less weight up high...  

The transom is made up of a series of curved planks, and was attached one plank at a time.  This was much easier to do than building the entire curved transom in one piece and then beveling all the edges as a unit. We also used a sandwich of thick laminates to make curving the wood a little easier.



When David attached the transom, he left some extra wood along the top edge.  This gives him some wood to work with as he fairs the decking to the top of the transom.  



A power planer and a confident hand make relatively quick work of this job.



As he gets closer to his final shape, he uses a batten stretched along the deck beams and sheer plank to locate where the top of the transom should be.



The batten here says that he's right on at the sheer plank.



We attach a pair of transom knees to lock in and reinforce the connection between the transom and the rest of the boat.  Here's the port one,



and the starboard one under construction.  



It's quite the sculptural piece... not a right angle in sight.  Nice work, Joel.

There's a similar piece up at the bow called the breast hook.  



The deck beams are all installed now, and the next task is to create openings for the 2 cockpits.  

The boat steerer's cockpit opening (the aft of the boat's two cockpits) has been framed out already, and you can see it here.



The deck beams that are cut by this opening have to land on something, otherwise they'd just hang out in mid air looking foolish.  We don't put up with foolishness like that, so we have them land on a fore-and-aft beam called a Carlin.  The deck beams that come out from the side of the boat to meet the carlin are called Half Beams.  



I suppose we could just screw the half beams to the carlin, but the joint wouldn't be particularly strong.  People sit up on the side of the cockpit, they walk along the little side deck, the boat flexes and twists... anything that makes this connection more substantial is a good thing.  That's why we make sure that there's a solid mechanical connection between the half beams and carlins.  Lapped dovetails and half dovetails are the joints of choice.  

Here you can see a large deck beam with a carlin dovetailed into its aft (right, in this photo) face.  The half beam comes up from the bottom of the photo to attach to the carlin with a half dovetail.



These are all lapped dovetails, meaning that the tails don't go completely through the wood that they're socketed into.  They only go halfway through, just like a lap joint.  The dovetail that you see on the top of the half beam only goes down into the carlin to about the level of the green arrow.



The red arrow points to what's called the shoulder of the joint.  It may look like the half beam is buried in the carlin, but that's just because these guys cut the joint so nicely.  The shoulder of the half beam is actually pressing up against the outside face of the carlin.  This shoulder helps to stabilize the whole joint and it's integral to its strength.  Right now, the half beam is a little proud (i.e., it sits above) the carlin.  When it's faired in, the top of the half beam and the carlin will be at the same level.  We'll eventually lock the whole thing together with a screw through the top of the dovetail down into the carlin.  The final result is a very light, very strong joint.

Here's the area where the forward cockpit will be.



The half beams are being held in place by temporary supports there in the middle of the span.  

The next step for this cockpit is to fit the carlin.  This is a pretty big member, and  it curves to match the sweep of the boat.  In order to get that nice curve, Robin steam bends it first on a series of forms anchored to the floor.





Once it's curved to our liking, we dovetail it to the full deck beams at its forward and aft ends, and then half dovetail the half beams into it.  It's a little tricky, and the last thing we want is to screw up such a nice chunk of silverbali, but careful layout leads to happy results. 

And, yes, the breast hook, the knees, and all the dovetails and half dovetails will all be covered up by the decking.  The next person to see them will be the person replacing the deck many years hence.  Well, WE know it's there.  And now you do, too.  That counts for something.

Ok, here's a shot of the boat looking forward towards the mast step that's only included here because I love the way this boat looks.  



Karl's been sanding and oiling the inside of the boat, and it just glows. 

So, with her transom on and faired in, she's looking better every day.


Tags:

7 comment(s) so far...

Re: Nice joints that no one will ever see again.

A boat building instructor once said, "Hey, we're building a boat, not a grand piano." Now, we know what he means. That grand piano probably doesn't need those beautiful half-lapped dovetail joins.

By Bob Easton on   1/28/2009 5:16 PM

Re: Nice joints that no one will ever see again.

Hi Tom,

The transom looks good - Olin's designs evolved with later sixes. An amazingly complex shape.

I assume there will also be blocking placed between the deck beams in the areas of most stress, such as where the winches will be mounted?

Also, will you be installing coamings before or after the deck goes on?

By Matt Cockburn on   1/28/2009 5:16 PM

Re: Nice joints that no one will ever see again.

Hi folks,
You know, you probably could get by with a lap joint and screws, but the added strength of the lapped dovetails is substantial enough for it to be standard practice on every boat (all 3!) that I've worked on. This happens in furniture too, though... sliding dovetails, mortises and tenons... it's the nature of the beast to have strong joinery that you then work to hide!

I liked the piano analogy though. David has often commented that we're building a violin. When the floors were set up along the keel, we found that they had good resonance properties, and that we could play them like a xylophone. So, maybe we're really making a floating marimba.

Matt, you're correct about the blocking. We'll install that beneath winches, the traveller, and the deck cleats. The side stays will go through the deck right down to the sheer clamp. We may also end up making a swallowtail plate like we did on to add strength there.

The coamings should go on after the decking. It's just easier to fit things that way. If you cut the decking opening a little small, you can go around the framing with a router and a straight pattern-cutting bit and get a perfect line up from the framing to the decking. Not only that, you don't have to custom fit every single decking plank to the openings... very slow work there. And not only THAT, but it's nice to have the decking laid down in long continuous sweeping strakes to get things symmetrical. .

By Tom Daniels on   1/28/2009 5:44 PM

Re: Nice joints that no one will ever see again.

Hello Tom,
Very nice work and very interesting project.
What camber (radius) did you use for the deck beams? And what siding did you use for the horn timber?

mac

By mac on   1/29/2009 7:03 PM

Re: Nice joints that no one will ever see again.

Hi Mac,
I can't remember those numbers off the top of my head. Naturally they come from Olin's drawings, but I'll check next time I'm up in Newport (I live in Stonington now but come up weekly to keep the blog going).
Tom

By Tom Daniels on   1/29/2009 7:05 PM

Re: Nice joints that no one will ever see again.

So - why doesn't the transom need to be caulked? Not a big enough surface area to move as the timber changes temperature?

By Chris on   1/31/2009 12:35 PM

Re: Nice joints that no one will ever see again.

That's a good question! In fact, sometimes seams are not caulked, even in the hull. Some european boats use what's called "tight seam construction" which is just a way of saying that they fit the planks together so tightly that water can't get in. This type of construction relies on the swelling of the wood to seal the seams. Double planked hulls are also not caulked as the overlapping inner and outer planks (with thickened shellac in between) form a waterproof barrier without a need for caulking.

I haven't asked David directly about the transome caulking, and I'll check with him tomorrow. In the meantime, at the risk of looking like a complete bonehead, here's my best guesses.
First, the construction doesn't led itself to caulking. Essentially the transom fits down into the hull planking at an angle in a wedge shape. Adding caulking puts a prying force on the transom, essentially pushing it out and away from the boat. The fasteners hold the transom to the boat, but why add a constant pressure against them? When you caulk the hull, the frames are holding the planks in place, and the caulking serves to tighten up the whole construction. The pressure on the fasteners in this case is less outward than a shearing pressure, and the fasteners can withstand this for a very long time.

Second, the seam at the transom is tight and almost invisible in this construction so a small amount of flexible bedding or adhesive compound would do a fine job of sealing the joint.

Third, transoms are often not caulked. The boats that I've built have all simply used a thread of cotton bedded in bedding compound in the seam.

You can read tons about this topic in Larry Pardy's excellent "Details of Classic Boat Construction" in the chapter on building the transom. Bud McIntosh goes over fitting mitered transoms to the hull in his book "How to Build a Wooden Boat" but isn't as thorough. And Chapelle doesn't even mention mitering the transom at all in his book, "Boatbuilding." So, there you go...

And yet, no one really talks about why they don't caulk a mitered seam.

Oh, well, more opportunity to ponder and pontificate!

By Tom Daniels on   2/3/2009 8:55 PM

Your name:
Your email:
(Optional) Email used only to show Gravatar.
Your website:
Title:
Comment:
Security Code
Enter the code shown above in the box below
Add Comment   Cancel