23 January 2013

Bevels, Bevels, Bevels

Wow.

Sometimes, not often, 'wow' is the only appropriate word. Long time readers of this blog know that I wow fairly easily, but this time is different. I really mean it.

For the last week or so, I've been trying to figure out how to fit a new breasthook into Cabin Boy's bow. His original breasthook, installed before launch, looked okay, but when it came time to finally install the inwales, I realized it was wrong. All wrong. Wrong angle in the boat, wrong angle to the inwales, and -- most surprising -- too thin.

I figured that the breasthook should be the same thickness as the inwales. This sounds reasonable, until you look more closely at the geometry. Generally speaking, the breasthook lies flat across the boat, like the cross piece in the picture below.

Breasthook lies flat across the boat; inwales slope outwards.
But the inwales follow the flare of the sheer strake. Therefore, the breasthook must be at least as thick as the diagonal running from the top corner of the inwale to the bottom corner. But it's actually more complicated than that.

If you want the inwale to fit neatly into the breasthook, the outside edge of the breasthook must be slanted at the same angle as the top of the inwale.

How can this be? Does the breasthook need to be steam bent so it's higher in the middle, slanting down to the inwales and strakes on each side?

Even a steam lover like me wouldn't try that. No, the traditional way to solve this problem is to make the breasthook even thicker, and then plane a 'crown' into its top, so it is higher along the centerline, sloping down to the wales and strakes.

With all that in mind, I made a small sketch to get it clear in my mind.

Breasthook geometry
The crown in this drawing is greatly exaggerated, but illustrates the basic idea. The dotted line shows the required size of the timber needed for the breasthook. It must be thick enough to fit next to the strake (bottom right), and also for the crown (upper left).

Assuming a more reasonable crown of about 1/4", I figured I needed a piece of wood about 1 1/4" thick for my inwales.

Rather than test this theory on mahogany, I decided to cut a test breasthook out of a 2x12 hunk of Douglas Fir I had in my junk box. First task was to machine it to the correct thickness. I suppose I could have done this with a hand plane, but my new planer made the job a heck of a lot easier. Just look at the pile of shavings!

2x12 Douglas Fir planed to 1 1/4" thickness.
That was the easy part. The next question was, how to shape this hunk of wood into a piece that would tightly into Cabin Boy's bow. I came up with a few wacky ideas, but was saved from a long process of trial and error fitting by an old shipwright from the UK who built and fished the last sea-going coble out of Sunderland, in 1963. Something like this, probably:

Coble - stoutly-built fishing boat from the NE coast of England
That seems like a long time ago, but he still remembered the procedure (in detail!) and was kind enough to share it with me. So following his instructions as best I could, I first laid the machined wood across the sheer strakes, right up against the stem.

Marking the shape of the strakes.
I then ran a pencil along the inside edge of the strakes, on each side. To make life easy on myself, I wrote 'top' on the top of the timber, 'bottom' on the bottom, and, just to be sure, port and starboard on each side. I didn't want to get confused half-way through the process!

Here are the markings on the underside:

Shape of the shear strakes near the bow
The eagle-eyed among you will see that the strakes don't run into the stem at the same angle. They took quite a beating several nasty nights on my voyage, when I cared more about getting the storm anchor down as quickly as possible more than Cabin Boy's poor planking. In fact, the port side is more than a little beat up, so it's not surprising that the shape is not symmetrical.

But my dirty little secret is that it was never symmetrical.  Not even when the strakes were brand new. My boatbuilding skills weren't quite up to that. That is why, if you are smart, you will take your measurements from the boat, not your lofting, and not assume your flawless joinery resulted in perfect symmetry.

The next step was to cut the timber to size, making sure all the sides were perpendicular. Perpendicular sides are critical, because we will be 'lowering' the timber into the boat, by cutting various bevels. If the sides are perpendicular, the top of the timber will be the same shape and size as the bottom, so when it's 'lowered' into the boat, the top will fit as snuggly as the bottom did.

Timber cut to size, with perpendicular sides.
To take the rest of the measurements, I needed to somehow hold the timber in exactly the right position. This required 4 hands, but I only had 2 handy, so had to cook up some sort of trick. I'm sure this isn't the traditional way to do this, but it worked.

What I did was make a fairly stiff 'platform' out of masking tape. I ran the tape as tightly as I could (without distorting the sides of the boat), and used enough to make it rather firm. Then I just laid the timber on top and taped it in place with a few extra pieces.

A bit kludgy, I know, but if you know a better way, I'd love to hear it!

'Platform' made from masking tape
With the timber suspended firmly in place, I used a small 'gauge' that I cut to size myself.

Gauge for marking bevels
This gauge allows you to directly mark the bevel needed to fit the timber to the flare of the sheer strake. You hold it flat against the strake, and against the back end of the timber. The length and thickness aren't critical, but the width is. With the correct width, the gauge will run off the side of the timber 1/4" below the top, leaving enough wood sticking above the strake to plane your crown.

When you are done, it should look something like this, only in focus.

Timber with bevels marked
Then it was time to cut the bevels. This is not a job I'd fancy doing with a hand tool, though I'm sure it could be done. It was the first time I'd tried to cut a bevel with my bandsaw. I tilted the table and lined up the angle by eye. This required stepping back a bit; I clamped the piece to the table so it wouldn't slide off.

Lining up bevel by eye
My ancient and inexpensive old Craftsman bandsaw was more than up to this task. Naturally, I dropped the guides before actually making the cut. (Actually, I forgot, but it worked anyway!)

After cutting the bevels on both sides, and a bevel to fit the slope of the stem, here's how it fit:

A perfect fit.
Here's another pic that shows the stem bevel:

Piece must be beveled to fit stem

Not to waste too many words on it, it fit perfectly! The bevels were exactly the right angle and the piece was about 1/4" proud of the strakes, which should be enough for the crown. No trial and error. Just a few boatbuilding tricks that probably date back 200 years.

In other words, wow!

This is about half the work of fitting a breast hook. Next time, fitting the inwales and planing the crown.


Next Episode: Inside Bevels

2 comments:

  1. I hate to say this, but, I think you should fit the inwhales before the breast hook. No need to panic, just re cut the bevels to accommodate the inwhales. If you look at other boats you will see what I mean. This makes a much stronger joint. Otherwise you would only have a weak butt joint. Traditionally the inwhales are let into the stem, to give a greater contact area which share the loads more equably.
    Another method of making a breast hook is to laminate several strips of thin timber, soak in boiling water for a bit and glue them together in a U shape jig. This imitates the use of 'grown' timber in days of old. The resultant laminated piece can be machined as you did before. I have even used contrasting colour wood to give a very pleasing look. If it looks good varnish it. If not, paint it!
    Don't forget two more knees at the stern, quarter knees.
    Good building, enjoy, Jerry in sunny Cumbria.

    ReplyDelete
    Replies
    1. I probably should have made this clearer, but this was just a test piece, to see if I could cut the right bevels and make it fit. It's also not done, yet. I intend to let the inwales into the breasthook in the next step. Then I need to find or glue up a proper piece of wood for the real breasthook.

      Delete

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