Thread: Box Boom Update

I've been thinking about doing something very similar for my build, Clint. (I'm finally getting going on it!)

I'm quite far from being a structural engineer, but I did some preliminary experimentation with a tool called the Beam Bending Applet from the Mechanical Engineering Department of the university of Utah. Try it out here: http://www.mech.utah.edu/~bamberg/re...ingApplet.html

In the end, I came up with a boom using these dimensions:

	Proposed
Boom height	55mm
Boom width	40mm
Sidewall thickness	5mm
Top and bottom thickness	10mm
cross section area (square mm)	1150

My experimentation has at least three flaws that I know of:

1. I could not account for using a different material in part of the design, so I don't know what the effect is of using plywood instead of solid wood for the sidewalls
2. 1/4" plywood is, of course, 6mm not 5mm. Not sure why I messed that up.
3. Did I mention that I'm not an engineer?

With those caveats out of the way, the modelling I did suggests that a beam with these dimensions would be as stiff as the stock design under horizontal loading, about twice as stiff under vertical loading, and weigh 71% of it's weight (excluding core spacers). That is appealing, right?

It looks like our dimensions are fairly similar. I'll be very curious to see how yours goes together.

Here is a link to more info on my experiments: https://www.evernote.com/shard/s1/sh...0b532b48189485

I am finally gluing up my new boom (18 months after cutting the parts!). Have the sides on now. Hope to get the top and bottom on tonight.

Mine will be 60mm tall by 50mm wide, tapering by 2 - 3 mm in the vertical at each end. That's for looks, not to save any weight. Sides, top, and bottom are all douglas fir, 10mm thick. Spacers are red cedar. I'll wrap one layer of fiberglass tape around each end. I may also wrap one layer at the downhaul and at the mid-boom block, but I haven't decided yet.

I'm not terribly concerned about the weight. The boom is not high in the boat and it is likely to be lighter than my original round boom anyway. I just want to make sure it doesn't fail catastrophically when I apply lots of downhaul.

It's been a long time since I took mechanics, but I seem to recall that tubing tends to be stronger than boxes?

I've just started to read up on how to rig the goat and see all the different variations in the boom sizes and construction techniques. The box boom looks like it would be similar to the square mast build and be a good match for the boat. Anyway, I was thinking about the tubular strength verses box shape this morning while lying in bed and thinking I don't rembered much from college. I then got to brainstorming how I might make a good looking tubular shape. My training is as a furniture maker so I usually think some sort of stave construction with or without a veneer, bent laminations or bent ply with veneer when it comes to curvy stuff. Came up with the idea below. No idea if it's been done before or if it would have any strength but it was fun thinking about. The veneer would probably be relatively inexpensive compared to clear solid stock and is usually high quality stuff. You could reuse the mold which you could shape by hand or if it was some high tech shape could have cut out on a CNC.

It might be time to ping BobWes on his Box Boom experience/lessons learned: https://www.woodworkforums.com/f169/search-perfect-lug-rig-boom-goat-island-skiff-156481

well, darn... I just lost my response to Cliff... let me try again...

Your proposed box-boom is similar to in size to the box booms I've built.

My first one was 45mm x 65mm, 6mm ply sides, 16mm cedar top/bottom. It weighs 2.18 kg (4.8 lbs). This is the stiffest boom I've built, too.

Subsequent booms were either lighter and not as stiff or heavier and not as stiff. My latest rendition is quite heavy (no numbers yet), built with 9mm ply sides, 12mm radiata pine top/bottom and fir plugs. It is stiffer that the first one but I am not sure that is enough to offset the weight.

For what it is worth, I plan to build a set of spars using birdsmouth construction. Because the balance lug sail specified for the Pathfinder is nearly identical to the Goat's balance lug sail, I plan to use the sail, yard and boom on both boats (I can only sail one at a time). I will need two masts. I'll share my experiences with the construction and sailing (when I get to that stage...)

Check out the wiki page Mik set up for sharing spar data and rigging info:

https://docs.google.com/document/d/1...h.q5vhwde6fblr

BowWes

...sorry, I meant Clint not Cliff...

Beaton1,

Are you familiar with birdsmouth construction?

http://www.duckworksmagazine.com/04/...uth/#remind-me

Seems that your veneer concept is along the lines of the birdsmouth method. Maybe not but it seems like it to me.

I just untaped and de-snotted the blank for my new boom this morning. It's going to be heavy but incredibly stiff. Hoping to round the edges and coat with epoxy this weekend. Then I can measure the weight and deflection.

I've seen examples of the birdsmouth construction but have never tried it myself. If looks pretty slick. I was thinking a veneered tubular shape would be easier to build and be similar to the plywood box shape construction except that you are making your own bent piece of plywood with all the grain running in the long direction and in a stronger cross section for a given weight or wall thickness.

Originally Posted by paulie

...it is likely to be lighter than my original round boom anyway.

Or NOT!*

Makes sense though, given the dimensions you cite. Never mind the finished size, the cross section of the staves add up to about 1800 mm^2 of solid wood. The solid round boom of 40mm diameter (throw away any tapering) is only 1257 mm^2 in section.

I'm a visual learner myself, so I'm attaching (I think) a graphic illustrating the comparison.

Pauls Boom.jpg

*For the benefit of those who have not seen Paulie's Facebook post, his box boom weighs about 160% his solid round...)

Originally Posted by Beaton1

I was thinking a veneered tubular shape would be easier to build and be similar to the plywood box shape construction except that you are making your own bent piece of plywood with all the grain running in the long direction and in a stronger cross section for a given weight or wall thickness.

Sorry, Beaton1, I'm not understanding your concept at all. I'm not familiar enough with veneers to be able to visualize what you are describing, so I am certain I am missing something here.

BobWess

Originally Posted by davlafont

*For the benefit of those who have not seen Paulie's Facebook post, his box boom weighs about 160% his solid round...)

Yes, it is. (9.5lb/4.3kg, vs 6lb/2.3kg for the solid.) And your explanation makes perfect sense.

Although I was quite upset at first, I'm feeling more relaxed about it now. It's still not really that heavy, it won't be high up the mast, and I can always shave off a bit if I want to.

I'm going to finish it off as is and see how it works. If I find that I really don't like the weight, I'll try drilling some holes in it. And if I don't like that, I'll make another with thinner, plywood sides.

That's the spirit! "I built it, I can un-build it!"


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