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Saturday, September 05, 2009

Revisiting the Bow Pulpit Design (again)


Not sure what it is. Seems the more you stare at a CAD drawing, the more you want to change it. I wonder if design engineers have this problem.

Well, one of this fall/winter's big projects will be the bow pulpit. Factory finished Westsail 42's had two style bow pulpits. Early production boats had their pulpit hinge bolted to flat plate brackets surface mounted to the hull, just underneath the caprail. Later production boats had the mountings thru-bolted to the top of the caprail. In both cases the pulpits were fabricated from 1 inch tubing for the upper rails, 2 inch schedule 40 pipe as the perimeter structure, and 1.5 inch angle iron for cross members. All stainless steel.

Between the two original designs, I prefer the second model as it produces a platform that is more flush with the top of the caprail. The anchor chain does not have to go "up and over" the caprail. With the second design, it can go "up" to the bow roller. Therefore, my latest design iterations have started with this second model. Of course, I want to make changes, which are listed as
  • integral rubstrake for the anchor chain
  • place to mount chain stoppers
  • any small tweak that might improve strength
  • single person/wrench ability to install/remove
I do have a copy the original 30 year old drawing of the "revision 2" pulpit sent to me by Bud over at World Cruiser, so I started a CAD drawing based on that. "Feature" changes to that drawing include
  • four attachment points, instead of three
  • forward attachment points act as rubstrakes for the anchor chain. Also can have chain stoppers mounted
  • use of fiberglass grating instead of teak planking.
So, after showing the factory original drawing to my stainless fabricator, we came to the conclusion that the reason for using angle iron and schedule 40 pipe was likely due to the fact that it was the more readily available material of the time. Nowadays, stainless tubing is easily available in various sizes, pre-polished in 600 and 800 grits in both 316 and 304 grades. So, with that, the drawing has been updated to use 2 inch and 1 1/4 inch tubing (instead of the schedule 40 pipe and angle iron) with comparable wall thickness. Furthermore, round tubing for cross members in such a structure will give more strength over angle-iron and may be cheaper to use (comes pre-polished and requires fewer welds).

Finally, we have a tentative plan on how to proceed. The pulpit build will happen in three phases:
  • The first phase will be fabricating just the "platform" with no "upper rails" and no mounting tabs, just the basic tubing structure. Then it will be test fitted to the bow of the boat so the size and positions of the mounting tabs can be dialed in. The fiberglass grate for the top of the platform will also be cut and test fitted to determine where the grate mounting tabs should be placed.
  • The second phase will see the pulpit mounting tabs, the mounting tab backing plates, and the forestay/bobstay plate fabricated (all probably jet-cut), then welded to the platform. It will be taken back to the boat for a test fitting/mounting. Backing plates will be set and the platform will be thru-bolted.
  • Finally, if everything matches and fits ok, the platform will be removed to have the upper rail tubing fabricated and welded. The pulpit should then be ready for final installation.
Piece of cake, as they say. As for timetables, maybe by spring time next year we will have a bow pulpit installed. Of course, I reserve the right to change my mind at any time!

As for the stern pulpit, that should not nearly be as complicated.


Click here to see a more detail drawing of the bow pulpit platform.
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