Ulrich,

It looks like you are trying to create a 3" diameter boom with a foot
rope tunnel (I hope that is what you mean by keep).

Assuming that this boom in not mid-boom sheeting, and that you are not
planning to vang it very hard to flatten the main. Let me make some
observations and recommendations.

The nature of such a structure is such that the material in the center
does very little. A spar of the size you are considering, could easily
loose the center 1" of material - almost - the conditional is because I
do not have the final dimensions of the keep (tunnel).

If you removed material so that the remaining material was no less than
3/4" everwhere with the exception of directly under the tunnel where it
could be a little less for just that tangent. The structure is best
left solid about 3~4 diameters from both ends this is largely for
fittings and fastening.

That said, you can take a lot of wood out of that bottom piece and maybe
some out of the two upper parts. I am guessing that you all ready have
the material. If I were making this spar, I would not make it round. I
would make it tall enough to accommodate the tunnel without reducing
the loaded section very much.

Over very many years, my father and I assembled many such spars some are
still in use today and the last was layed up over 25 years ago.

We did it much the way you seem to be headed. We would cut staves and
route the tunnel and hollow the center then laminate the spar (later
with epoxy) and finally plane the final shape.

Best of luck

Matt Colie
Lifelong Waterman, Licensed Mariner and Perpetual Sailor

Ulrich G. Kliegis wrote:
After 35 summers in a pretty rainy corner of the world (Kiel, Baltic
Sea), the old boom has done its job, building a new one has started.
Now, weighing the raw timber in my hand on one side and comparing it
to the old boom, the question for hollowing the new one to reduce the
weight arises. It will be put together from three parts, two adjacent
1.5" * 3" stripes with the rail for the keep (?) routed out, and one
3" * 1.75" in landscape-format below, overall length 11 feet.

How deep and over which extent would you take out material, using a
router?

Thanks in advance for all helpful comments!

U.

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