Thread: Box Boom Update

Beaton1,
Thanks for the explanation and photos - they helped clarify my understanding of the concept.

Maybe you could build a boom with this technique to compare with other boom designs. How else are we going to push the envelope?

BobWes

MIK just released his design for a box boom that (cleverly) makes use of excess ply in the GIS plan.

Announced on Facebook, the boom features 6mm ply sides on 58 x 12mm thick staves for a total boom width of 70mm. The depth along much of the length of the spar is 115mm, tapering down to 63mm forward and 52mm aft. I'll try to upload the plan here shortly. (success!)

GIS boom final including approx ply layout_Page_1.jpg
GIS boom final including approx ply layout_Page_2.jpg
GIS boom final including approx ply layout_Page_3.jpg

So after interpreting the new plan, I'm surprised at the dimensions. It's HUGE! Again, being a visual guy, I've taken the liberty of comparing the stock solid cross section with: Paul's, Bob's, and MIK's new design.
Booms comparo.jpg

I'm curious about the design considerations. As reported here, Bob experienced increased stiffness and decreased weight in a much smaller foot print using similar materials. Why so big MIK?

My latest box-boom (assembly completed over a year ago but, alas, not yet tested on Karen Ann) measures 45mm x 75mm, tapers to 45mm x 60mm forward and to 45mm x 45mm aft; top and bottom are 12mm Monterrey pine. It is heavier than previous models because I used 9mm meranti ply for the sides and (rather) extensive interior blocking/spacers. I intended to use this boom for Gardens of Fenwick, my balance lug yawl Pathfinder, but now plan to build Gardens' spars using the birds-mouth method (I have been yapping about this for a few months so, with my new shop nearly set up, it is time to begin milling materials and assembling some spars!). I will rig Karen Ann with this latest box-boom to see if there is any difference in performance from my first box-boom

I, too, am curious about the dimensions in Mik's rendition.

I am yanking BLEAT from long term storage Mon am and starting the new boom. I need to settle on a design. MIK's latest boom is a monster and the one he first suggested to me seems like it will be adequate. That was outside dim's of 60x43" with 6mm ply sides and 10mm solid top and bottom. This is about what I have my CIY builders doing but I don't have reports from sea trials as of yet.

Bump...Joost built his. Hoping to get an update. I am waiting for some feedback. Happy to build mine and add data to the matrix!

Mine has been stuck in the "glued up but not yet epoxied" state for months while I was busy with other things. (A friend of mine turns it over every time he comes to the house so that the dust build-up doesn't become embarrassing.) While this has been frustrating, it also may be an opportunity. The extreme stiffness of the boom in it's current state (I can sit on it without obvious deflection) and all the recent discussion on facebook have me thinking that I can probably shave a few mm off the sides before I epoxy it. That might take some of the weight out of it.

Paul, maybe your boom needs to be heavier to bring the overall center of gravity lower? Yes, I just read your heavy mast theory in your other thread.

That's possible. I just don't want to get hit in the head with it at some point. My head is hard, but not that hard.

Originally Posted by paulie

That's possible. I just don't want to get hit in the head with it at some point. My head is hard, but not that hard.

That's why I want a nice roundover on my boom edges! At lease 1/4" roundover.

Originally Posted by davlafont

Paul, maybe your boom needs to be heavier to bring the overall center of gravity lower? Yes, I just read your heavy mast theory in your other thread.

So his problem is too much weight above the gunwale when trying to get back into the boat, and you're suggesting more weight above the gunwale. Have a think about that one.

Don't forget the existing data for a range of booms - I did when I drew up the boom above. Not happy with it now

WIKI for setting up and tuning Lug and Sprit Rigs | Storer Boat Plans in Wood and Plywood

You can see they tend to centre on a more or less consistent range. Going slightly larger with the lightest ply sidewalls and 10mm timber top and bottom we dare seems like the best recipe to me.

Joost built a boom to my design above. It is too huge and too stiff. it doesn't deflect noticeably when Joost sits on it!

MIK

I've made the first cuts for Chivita's Boom Box. Based on the discussion on Facebook and here I've decided to go with 50mm x 80mm, 6mm ply on 12mm staves (okoume and Oregon respectively).

The lumber selection is based purely on what was left over from the build of the boat. The dimensions are also somewhat based on that too. The maximum ply width for my offcuts was 80mm for any decently long piece (see photo). The 12mm staves provides a decent surface area for gluing but I slender enough to bend to a tapered shape. I'll taper aft gradually to end with a 50mm x 50mm cross section purely for aesthetics. Up front there's less room to taper but I think I can go from 80 to 65mm rapidly and have a fair curve.

I'll be sure to weigh and measure flex in accordance with our established practice.

Yesterday I cut parts. Today my goal is to scarf the staves and glue the butt straps.

A final thought: I might get creative with the exterior finish, just for fun.

I cut parts to and glued the ladder frame Friday. The boom will be ~55x87 done with Eastern White Spruce staves and 1/8" Okoume ply with doublers in a couple spots.

I'm going to do a last sail with the Goat to make sure I've located all cleats and attachment points before the sides of the box go on, that way I can best locate any backing pads or reinforcements.