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11th March 2013, 02:53 PM #16
HI ALL.
If I was going about booms ... and you know what a lightweight nut I am... I would probably go 4mm ply walls but just put some extra ply for about a 200mm on the inside of the boom at the mainsheet points and downhaul point. That's 8mm - thick enough for the skin of a 25ft plywood yacht at the load points.
If using cedar I would be just a little suspect because of its wandering grain so would consider glassing the cedar with 2oz (75gsm) cloth ... just because the cedar grain goes a little bit here and there rather than being straight.
You wouldn't need to worry about glassing at all with fir. The reason for glassing the ends of the standard spars is the shapes one piece of timber is likely to split - so that is why the glass is there.
But for a box you have grain running in every direction where the holes are made because of the end blocks that close the ends of the tube. so it doesn't really need glass.
Hope this helps.
MIK
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11th March 2013 02:53 PM # ADSGoogle Adsense Advertisement
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12th March 2013, 05:10 AM #17
Thanks, MIK!
How thin a wall would you be comfortable with when using fir?
The piece I have is clear with very straight grain, 10 - 12 grains to the inch. It is rough cut and 22mm thick. If I split that with a thin-kerf blade in my table saw and smoothed the cut side (who cares if the inside face is rough?), each wall would be a minimum of 8mm thick and probably more like 9mm.
Thick enough? Or am I courting disaster?
I do have some leftover plywood bits, too, but nothing long enough. I'd have to scarf up pieces to make a boom. Too much work!
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12th March 2013, 12:29 PM #18
Mik advocated 4mm ply back when we started messing around with box booms. I didn't have any 4mm ply then - and I wasn't going to go buy a sheet just to experiment - but I did have enough 6mm ply on hand to build my boom. That boom turned out (despite less than skillful workmanship) to be light and stiff.
Not fully appreciating the physics and engineering but having built four box spars, I'm not convinced 4mm ply will make a stiff enough boom. But now, in the interests of experimentation and my own curiosity, I've decided to include a sheet of 4mm ply (meranti - sorry, Mik) when I pick up some lumber and ply later this month. I'll experiment with different combinations of materials and report the progress and results.
Retirement is a wonderful thing - I find I am able to buy tools and materials I was not able to buy when I was working. I also have time to do the things I want to do although I find I am busier now than ever before!Building Gardens of Fenwick, a Welsford Parthfinder
Gardens of Fenwick
Karen Ann, a Storer GIS
Goat Island Skiff - Sacramento
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12th March 2013, 06:45 PM #19
Hi Paul,
you can butt strap the ply. - the butt strap needs to be the same ply and its width 20 times the ply thickness. So that means 120 wide buttstraps if you want to go that way.
As far as fir thickness ... someone needs to try it to know. Normally the absolute minimum for the wall thickness of spars it taken to be 15% of the dimension with 20% more common. Though the box spars do have good gluing area and the ply sides do help prevent splitting in some directions.
We know the 12mm is pretty reliable for the sizes we are using from the masts. Would 10mm be OK? I would be guessing so ... but it is only a guess.
MIK
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12th March 2013, 06:48 PM #20
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16th March 2013, 01:52 AM #21
While waiting for my wood to dry, I've been reading Structures by Gordon. Lots of info in there on thin-walled box beams and the poor compression strength of wood and how round beams might be better than square ones in this sort of situation. Almost makes me not want to go down this road.
But the wood is dry and my weekend is mostly open. My son, whose expertise with a table saw is light-years beyond my own, will be home from school and available to help. We shall march onward. If nothing else, I will be able to provide a data point on when a box boom's walls are too thin.
With luck, the box will be made up and ready for finishing within the next two days. I hope to post pics as I go.
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16th March 2013, 07:47 PM #22
Howdy,
No need to be too paranoid. The highest stress face is the upper face of the boom which is most vulnerable me to compression.
The thing is, the top and bottom faces are the closest to the usual timber spar specification. As the depth of the beam is greater than the original, the stress will be less than the original.
That's the nice thing about the book, it is a gradually growing perspective.
If top face was thin it depth if beam less instinct would tend to make us think it is scary.
But the opposite is happening .
MIK
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17th March 2013, 07:33 AM #23
<sigh/> No progress so far.
Today has been a reminder about something I always seem to forget: the most useless object in the known world is the basement workbench in the average suburban home.
I know it looks useful. But when you actually try to use it, it is always buried in stuff. Sometimes it's stuff that needs fixing. Sometimes it's stuff that someone couldn't figure out where else to put. Sometimes no one in the house remembers how it got there. Perhaps the garden gnomes carry things in and dump them on the workbench in the middle of the night.
At any rate, even if the time since the work surface was last clear can be measured in minutes, it will be lost under a pile of wreckage when you actually want to get something done, and it will take hours to clear it off again.
The surface is now clear and I was even able to unbury the table saw (which had baskets of laundry and some brewing equipment on it, for which I refuse to take responsibility). I'm hoping both stay clear during dinner and I can use both this evening.
If I see a garden gnome carrying anything toward the basement door, I'm setting the dogs on him.
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17th March 2013, 07:01 PM #24Senior Member
- Join Date
- Feb 2013
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- New London, Minnesota
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Work Bench
I'm setting up a temporary work bench in the basement so I will have two places to collect unimagined quantities of useless junk.
Must buy WCR staves tomorrow and get started on centreboard and rudder.
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29th March 2013, 07:23 AM #25Rusty Member
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- Feb 2011
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- San Diego, CA
- Posts
- 236
If it's any encouragement, I love my solid ovoid DF boom except for the weight. It's plenty stiff enough, but it is reminiscent of a baseball bat when it hits your head, and in very light wind sailing dictates the sail shape a bit too much for my tastes from the weight of the boom on the clew.
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29th March 2013, 12:04 PM #26
Things are warming up a bit here so I've been slowly returning to work on my Pathfinder.
First things first: A new spar bench is under construction. Three sawhorses, two 2x10x10' planks, some 1x4 edging and two layers of plywood (3/8" and 3/4") for the top.
DSCF6423.jpg
Shimming the planks to even out the end-to-end joint, adding the edging and installing the plywood top will be completed tomorrow.
I've milled the 12' top & bottom pieces to 12mm x 27mm, as well as cut and milled the plugs and blanks for cutting the spacers.
DSCF6412.jpg
These plugs are much longer than what I used on the GIS boom but weight is not a concern for the new boom.
Once the bench is done I can begin assembly. My best guess is that this boom won't be fully assembled until early next week, given the family gatherings going on this weekend.Building Gardens of Fenwick, a Welsford Parthfinder
Gardens of Fenwick
Karen Ann, a Storer GIS
Goat Island Skiff - Sacramento
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1st April 2013, 09:02 AM #27
After getting the spar table to a functioning state (topped with 16' of 3/8" ply), I dry fit the top, bottom, plugs and spacers for the boom. The photo is looking forward along the boom. There is some sanding and fitting to be done, but once I'm satisfied with the fit, I'll glue this assembly together and prepare the 9mm plywood sides (need to butt join the ply to the needed length).
Building Gardens of Fenwick, a Welsford Parthfinder
Gardens of Fenwick
Karen Ann, a Storer GIS
Goat Island Skiff - Sacramento
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2nd April 2013, 11:34 PM #28Senior Member
- Join Date
- Jul 2011
- Location
- Finland
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- 131
Do you have any extra plan for those holes? Or are they there to reduce weight?
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3rd April 2013, 12:31 AM #29
There is no particular plan for the holes. They are intended to reduce weight by however a small amount.
Building Gardens of Fenwick, a Welsford Parthfinder
Gardens of Fenwick
Karen Ann, a Storer GIS
Goat Island Skiff - Sacramento
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3rd April 2013, 03:30 AM #30
Dimensions to build?
I've recently bought the douglas fir for my mast and there's going to be enough left over to build box spars. What is the current consensus on a design for a fir box yard and boom?
-W
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