On Apr 7, 9:57 pm, Brian Nystrom wrote:
wrote:
On Sun, 6 Apr 2008 21:28:05 -0700 (PDT), Norm wrote:

Greetings.

I am currently building a skin-on-frame kayak using the standard
(classic) text by Chris Cunningham. I have just reached the stage of
putting in the steam bent ribs. I realise that this stage is tricky,
especially given that he recommends that you get 50% more timber for
breakages.

I have a specific question which he does not deal with in his book.

Cunningham recommends that you start with timber 5/16 inches thick,
and that you thin the ends (maybe up to a third of the length of each
rib, depending on its length) by 1/16 inch. His instructions tell you
to take all of the 1/16 inch off one side of the rib. (In addition to
this, the ends of each rib need to be narrowed in order to fit them
into the mortices in the gunnells).

This means that when you bend the rib after steaming you have a choice
of bending it towards the thinned side or away from the thinned side.

It seems to me that it would be more likely to split if you bent it
away from the thinned side. That is, it would be better to have the
thinned side on the inside of the curve.

I have not yet broken enough ribs to know the answer to this. Does
anyone have an opinion? Or is this likely to be such a minor matter
that it will make little difference in the long run?

Yours

Norm

Bend so that the thinned part is on the inside of the bend. The thinned side has
more exposed endgrain due to the thinning, so you want that side under
compression, rather than trying to pull it apart. The longest unbroken grain is
the side that can best deal with being stretched without splintering.

While the above is correct if you're going to use Cunningham's method,
my personal opinion is that his system just creates more work than
necessary. Using 1/4" ribs without any tapering works just fine. If
you've already cut your mortises to 5/16", you're stuck, but it's
something to consider on your next boat.

One thing that is very useful is his pre-bending jig. While the ribs
won't hold the shape of the jig, it does make the ends much more pliable
and easier to insert into the frame.

For other ideas, see my Webshots albums at:

http://community.webshots.com/user/brian_nystrom-reg

Thank you for this comment, and also for the reference to your Webshot
albums.

Regarding the 1/4 inch ribs, I have been trying to follow Cunningham
EXACTLY because I have a history of minor disasters and half finished
boats. However, I have not cut my mortices to 5/16, so I do have the
choice to try thinner ribs. (There have been things that I thought of
changing but I have so far resisted).

I have used his jig. One of the problems I had with the first one I
made was that it broke apart while I was bending the first rib. I
then built another one which was much stronger and which works.
However, just last weekend I was wondering if I could make an
adjustable jig which would be suitable for keeping the different sized
ribs in till they cooled. I notice that your rib jig looks like just
such a piece of equipment. I will have a careful look at your photos
over the next few days.

I am one of those people who do not like to do things quickly and I
feel that I work at my worst when I have to make quick decisions. So
the description by Cunningham of the rib process had me worried from
the start.

Many thanks for your comments.