Hi Adam,

I think using a Bermudan compression post below the partners of a Junk Rig mast is not necessarily problem-free.  Below a Bermudan mast, the compression post will take up only axial compression load, and it needs a certain diameter to avoid buckling (sideways flipping-out) under the compression load.

However, unlike a Bermudan mast, an unstayed Junk Rig mast takes up a bending moment that is at its maximum at the partners, and in order to have enough bending strength it needs a larger diameter there than a Bermudan mast’s compression post.    I  From the step, the moment increases linearly from zero at the step to Mmax at the partners like I have tried to illustrate that by sketching on the photo..  At point “1” at the tips of the stiffeners, the moment M = M1 = Mmax * b/a.  By measuring on the photo, b/a = (a little above) 0,7, so the bending moment M1 will be about Mmax * 0,7.

As the diameter of the compression post is that much smaller than the mast diameter at the partners, I am quite sure that the bending stress is much higher at “1” than at the partners.  I would therefore recommend that you check the bending stress at point “1”.  If it is too high, the solution is simply to extend the large diameter further downward until the stress level at the diameter change is comfortable.

Good luck with your project!

Nils

Anonymous wrote:

Hi Adam,

the problem is not the compression post/ tabernacle, but the deck strength. The freestanding mast will put forces into the deck that it was never designed for. My suggestion would be to look at cutting out part of the top layer of the deck and remove the balsa core, then lay up a solid fiberglass laminate that will transfer the loads back into the deck and hull. A lot more information would be required to design something that would work. You might also want to look at a keel stepped mast with a hinge above deck and a sliding pipe collar that gives it its strength. I have installed this arrangement on my boat Gypsy Rose and it is working quite well. I am intending to do an article on it for the magazine but not got to it so far.

All the best with the project.

David.

Hi Adam,

the problem is not the compression post/ tabernacle, but the deck strength. The freestanding mast will put forces into the deck that it was never designed for. My suggestion would be to look at cutting out part of the top layer of the deck and remove the balsa core, then lay up a solid fiberglass laminate that will transfer the loads back into the deck and hull. A lot more information would be required to design something that would work. You might also want to look at a keel stepped mast with a hinge above deck and a sliding pipe collar that gives it its strength. I have installed this arrangement on my boat Gypsy Rose and it is working quite well. I am intending to do an article on it for the magazine but not got to it so far.

All the best with the project.

David.