The first thing i decided to do was to build the bracket which will attach to the end of the dolphin striker bolt. The DS bolt will go through it. The rear section of the understay will attach to the back of it, and angle upwards to the rear crossmember and the aft end of the spinnaker tube. Then the front section of the understay will attach to the front of the bracket. (or understay backbone) two cables will attach here. One will lead directly under the jib, furler and forestay, and the other will lead through a spreader extending down from under the forestay, and forwards to attach at the tip of the pole, underneath the spinnaker. My earlier post has pictures of a similar design.
Right. so i had to make the bracket. The first thing i did was to determine the target distance for the bracket to protrude underneath the front crossbeam, assuming it would be immediately above the DS strap. (putting it below would require drilling a 9/16 hole through a 1.5 inch aluminum strap. too risky) The objective was to maximize the distance down to increase the amount of vertical downward pull on the forestay, while minimizing the extent to which it would protrude under the beam, to avoid hitting many waves. Taking into account the deflection rates of the selected spinnaker pole, i plotted the curve of force vs. angle of shroud, and decided on 10 1/2 inches depth of DS strap, and roughly 10 inches down for the understay backbone.
Having arrived at that measurement, i was able to plot the triangle created by extending the rear understay to the rear beam from the DS backbone. I then determined the angle to be approximately 8.5 degrees.
I took 2 pieces of 1x1/2 inch steel bar from my inventory. I bent one of them to the 8.5 degree i needed. This way, i can use the old staymaster adjustable shroud i have in inventory. I the took the second piece, made it shorter than the first and placed it on top, so the angled section extended aft of the stacked part. Then i welded them together all the way around. I grinded the weld filets smooth, and drilled a 9/16 hole in the center (for the DS bolt) and holes in the front and back for the fore and aft sections of the understay.
