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Thread: Box Boom Update
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12th August 2014, 11:39 AM #31
Excellent! Your work is so neat. Professional even...
This is my method:
I glued the two stave and the end blocks to one side. A couple of hours later I closed it up with the other side. That was earlier today.
Are you coming to the Great Goat gathering Clint? Both Paul and myself have room for kids and Pat Daniels is bringing his family (I think his boy is 8?). The last time I saw bleat was in Mystic in 2011 when she was still raw. I'd love to see this boom for realz
Keep up the good (really good) work!Dave
StorerBoat Builder, Sailor, Enthusiast
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12th August 2014, 11:42 AM #32Senior Member
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Your methods look much like my methods. A good pile of bricks and a concrete floor are handy things to have.
You know you're making progress when there's sawdust in your coffee.
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13th August 2014, 04:33 AM #33
I'm at 9 lbs. minus whatever all this weighs:
Dave
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15th August 2014, 06:29 AM #34
Metrics
I took some measurements before doing the aesthetics:
The boom weighs in at approx. 3.4 kg (7.6 lbs on my bathroom scale). This raw wood, but after shaping and sanding. Epoxy and hardware will add to that some.
Here's my set-up:
IMG_2290.jpg
Because the lashing I used to hand the bucket is 5mm line, I decided to add these little pads to raise the reference string to be as close to parallel with the boom's surface as I could get:
IMG_2291.jpg
The most relevant metric is the downward flex of the upright boom. Here are pics at rest, with 10 kg, and with 20 kg:
IMG_2298.jpg IMG_2299.jpg IMG_2306.jpg
Result: 3mm@10 kg; 8mm@20 kg
Here is lateral flex pulling down on the port side, at rest, 10 kg, 20 kg:
(the pre-loaded gap is part flex under its own weight and part warped construction)
IMG_2296.jpgIMG_2297.jpgIMG_2308.jpg
Result: 20mm@10 kg; 37mm@20 kg
Here is lateral flex pulling down on the starboard side, at rest, 10 kg, 20 kg:
IMG_2300.jpgIMG_2301.jpgIMG_2303.jpg
result: 19.5mm@10kg; 30mm@20 kg.
So, the boom is pretty stiff. Since Joost thought he might be able to sit on his boom, I thought I'd give it a try (sort of). I weigh 77 kg. Laying hands on the center, I pushed myself upward until my feet came off the floor. The boom flexed, maybe a couple/few centimeters, but it did not buckle.
I'm pleased with the results. I just have to get finished in time for the Great North American Northeast Goat Gathering/Regatta/Rumfest/Campout (Black Tie Optional) of 2014.Dave
StorerBoat Builder, Sailor, Enthusiast
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20th August 2014, 08:35 PM #35SENIOR MEMBER
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Thank you for the photos, description and measurements of your boom Dave!
This evening (if the weather is okay) I will try to make some measurements on MIK's original version for the record and for MIK's spar database.
Yesterday evening I bought some timber (tightly grained pinewood 45 mm wide x 9 mm thick) to build a second version. I will again be using 4 mm okoume/gaboon ply for the side walls, so the width of the boom will be 53 mm. Height will be 90 mm with the front end tapering to 65 mm and the aft end tapering to 55 mm. I hope to be building the boom over the weekend and to have some measurements early next week.
Joost
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21st August 2014, 07:44 AM #36SENIOR MEMBER
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Just didthe measurements with the boom supported on its ends (10 cm from each end) thesame way as Dave has done it.
- Deflectionin vertical plane is appr. 2 mm with 10 kg and 4 mm with 20 kg. The boom easilysupports my full body weight (appr. 85 kg). Of course it then deflects morethat 4 mm but it seems to considerably stiffen up and my guesstimate is that itdoes not deflect much more than 10-15 mm or so. No cracking noises, buckling ortwisting in any way. For comparison: Dave’s boom bends 3 mm with 10 kg and 8 mmwith 20 kg.
- Deflectionin horizontal plane is on both sides 9 mm with 10 kg and 16 mm with 20 kg. Nofull body weight test done here (not sure whether the boom will survive that),but putting quite a bit of pressure on the boom makes it flex a few (2-3) cm. Forcomparison: Dave’s boom bends 20 mm with 10 kg and 37 mm with 20 kg on theportside and 19.5 mm with 10 kg and 30 mm with 20 kg on the starboard side.
Measuredthe boom’s weight again for good measure as well: 4050 grams beforeepoxy/varnish and fittings (3.4 kg for Dave’s boom).
Conclusion,it is a lot stiffer than Dave’s boom (which is not needed however) but weighs650 grams more and is more bulky.
Let’s see what the next try will yield using 9mm thick timber staves (45 mm wide) and 4 mm ply. It will be difficult to gomuch lighter I think.
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29th August 2014, 08:05 PM #37SENIOR MEMBER
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Materials
Second box boom built using 45 mm x 9 mm knot and pinhole free timber staves (pine, Pinus silvestris, 510 kg/m3) for the top, bottom, spacers and doublers and 4 mm okoume/gaboon ply for the side walls. Wood used for the previous boom is a different species of pine (Picea, 460 kg/m3).
Overall dimensions
3400 mm long (should have been 3440 but apparently made a measurement error somewhere, length should still be sufficient however), 53 mm wide, 90 mm high. The bottom end of the boom is tapering to 65 mm at the front (from 723 mm from the front end) and 55 mm at the back (from 1700 mm from the back end), as per MIK’s specifications for his version.
Measurements
- Weight is 3400 grams before epoxy, varnish and fittings.
- It should be noted that this boom includes double blocking of 45 x 9 mm for the vanghaul (200 mm long), mid boom sheet attachment (200 mm long) and outhaul track (500 mm long after the end block) and triple blocking (200 mm long) for the outhaul adjustment fittings (clamcleat + cheek block) giving 27 mm of timber to accept screws for attachments. If leaving these out, the weight of the boom would be reduced quite a bit as it would save a length of 2400 mm of 45 mm x 9 mm timber. For sure the boom would then come in below 3000 grams.
- For the record, I used more or less the same length doublers on the first boom but only 1 layer thick (meaning 24 mm to accept screws). Doublers (or rather triplers) are on purpose over length as it allows me to put the fittings in exactly the right place after I have put the rig in place on the boat.
- The boom also has 6 spacers in place in strategic places (these don’t weigh much, a total length of less than 400 mm timber).
- Deflection measurements vertically: 3.5 mm at 10 kg and 6 mm at 20 kg.
- Deflection measurements horizontally: Starboard side 12 mm at 10 kg and 25 mm at 20 kg (I did took this measurement 3 times since I found it strange that the boom did not stiffen up when higher loaded; I would have expected a figure below 2 x 12 mm = 24 mm). Port side 16 mm at 10 kg and 29 mm at 20 kg.
- Any pre-bend in the boom and deflections due to the boom’s own weight were subtracted from the deflection measured (I did the same for the first box boom as did David for his boom if I understand the pictures on MIK’s forum correctly).
Conclusions
- I will finish the second try as this one (i) should be stiff enough, (ii) has a clear weight advantage of 650 grams, (iii) is less bulky which is easier during transport (it needs to fit the storage bag for the rig) and on the boat and (iv) is better looking (esthetics) to my eye.
- Let me try to make a side by side photo of both booms. I will send this one to MIK for posting as I don’t know how to post pictures on Facebook (lazy me!). I have made a couple of pictures during the build and will also send these to MIK.
- Using 4 mm rather than 6 mm will give a slight weight advantage. With 4 mm the height of the boom may become more important , hence I went with 90 mm total boom height. Optimum timber stave width is difficult to say. It is clear that the wider the staves become, the stiffer the boom will be. I will let MIK work out that one based on the measurements now available.
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