And so I begin another project designed to distract me from real life. I had not planned to do this when I built my GIS, but I have now begun construction of a hollow boxed boom for Chivita's lug sail.
Why? If I may quote myself, here's how I responded to such a question on the GIS Facebook page:
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The first thing to consider is that the original boom is perfectly adequate for average sailing conditions and experience. Untold numbers of us have done just fine with a solid timber boom of about 40mm cross-section. Some builders have experienced more bend than others and it's probably due to the wood chosen (it's hard to get really tight-grained Fir or other suitable lumber).
The box boom is driven by those that want to take the Goat to "The Next Level." A loose footed sail allows for greater range of draft adjustment; One can achieve a very rounded belly if the circumstances call for it. The price is greater complexity, greater skill level required to make any use of the capability, and--most important for this discussion--the need for a very stiff boom. It's not hard to visualize how a boom will flex more if it is acting upon a sail lashed only at the two ends while being pulled by the down haul and the mainsheet between them.
I wasn't planning to take Chivita to "The Next Level" but I have been a little annoyed at how the foot of my sail is shaped and one solution is to convert from lashed to loose foot. Also, I've got the material necessary and I've really enjoyed the online discussion.
Here are two forum threads that hash out all sorts of considerations and the results of some experimentation:
http://www.woodworkforums.com/156481-search-perfect-lug-rig-boom/
http://www.woodworkforums.com/182938-box-boom-update/
There is also a long thread on the GIS Facebook page, but I can't find a good way of linking directly to it. I'm not a big fan of using Facebook for this sort of discussion and collaboration, but it is what it is.
Here is a graphic depiction of some of the new booms being done compared to my design:
All the other builders report great stiffness (except Clint who is still building his as this is written). Joost's giant boom is based directly on Storer's recent design and he has readily admitted it is far larger than necessary. Bob and Paul had already built theirs and found them to be a bit heavy but certainly stiff. I am trying to match the original solid boom weight (or be lighter) while increasing stiffness enough for loose footed rigging. Is it feasible? I'm not an engineer and I don't have material density tables at the ready. But I'm a Powerpoint wiz and I can compare the cross section of one design to the other to see if the amount of wood similar or not:
I conclude that there is a tad more material in the new boom, but it's possible that the plywood (shown here as light tan in color) is light enough to weigh the same as a smaller amount of heavier Fir. It's not out of the question.
So today I started making sawdust. My goal is to have the boom ready for the late-September Goat Gathering I've been coordinating.
Pictures please...
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| The offcuts |
I still have most if not all the excess wood from my build. There are two large chunks of ply left from the plan that will suit this boom build well. My plan is for overall dimensions of 50mm x 80mm at the middle, tapering gently to 50mm x 50mm aft and tapering more rapidly to 50mm x 65mm forward. The side panels will be 6mm thick ply separated by 12mm thick staves of Douglas Fir. The fir is the same stock I used for my current boom, which started out as a fairly tight grained 4x4 post.
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| Remnants of 4x4 that yielded original boom |
I laid the fir to orient the grain horizontally and made a 38mm cut to get the desired width of the staves.
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| Blank that will yield three staves |
I turned the resulting blank grain to vertical to slice off three staves of 12mm each. I will need three because the boom's length is longer than the 8 ft. of the lumber. There will be scarfing...
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| Three staves yielded |
These are 12mm thick. There was talk about going thinner, but I didn't think this lumber would support being sliced much thinner without checking or warping. 12mm will give a good amount of gluing surface. Plus, I'll be rounding off the corners and 10mm won't leave much material for that.
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| The offcuts, cut |
After slicing up the Fir, I ran the ply through the saw at 80mm width. There is a short section that is only 75mm wide, but that will be integrated into the aft taper. I'm keeping the large triangle intact for some as-yet unknown future use. Hoarding has its benefits sometimes...
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| Lining up the joints |
The overall length of the boom will be 3.6m, or close to 12 ft. The lumber is only 8 ft. long so some joinery is needed. The ply will be joined with butt straps like the GIS side and bottom panels. The staves will be scarfed (not my favorite task). I want to make sure none of the joints align with others so as to minimize weak spots.
This is where I stand tonight. I would glue things tonight, but the scarf joints will take time to cut properly; I'll try to do that tomorrow. Then I can glue the scarfs and the butt straps all at once. I have two tubes of West Systems epoxy adhesive. I don't want to waste any of it so I need to have enough things to glue before breaking the seal on the tubes.
When the butt straps are cured, I can lay out my taper lines (the boom top will be flat, the bottom will curve upward) and cut the shape. That will be the guide for the staves and what ever end blocks I end up using. I wish I knew whether the tube glue will last (uncured) between steps. If push comes to shove, I've got some plain epoxy and adhesive powder filler I can mix up. These materials are all remnants from the family canoe build of 2012. Yay hoarding!
More to follow...
