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  1. #31
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    Quote Originally Posted by wesesque View Post
    What is the current consensus on a design for a fir box yard and boom?
    -W
    I don't think we've arrived at consensus yet. Bob has done a few different designs but hasn't picked a clear favorite yet. Paulie is in progress (remember, you have a June deadline). I keep looking at the scrap 6mm okoume in my basement and I know there are two boom sides worth of material there (with some butt joinery) and I have some extra DF for the top and bottom. But that's low on the priority list for me.
    Dave
    StorerBoat Builder, Sailor, Enthusiast
    Dave's GIS Chronicles | Dave's Lugs'l Chronicles | Dave's StorerBoat Forum Thread

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  3. #32
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    Jul 2008
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    Fenwick, Michigan
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    I agree with Dave, there is no design consensus for a box boom design beyond "light and stiff."
    Building Gardens of Fenwick, a Welsford Parthfinder
    Gardens of Fenwick
    Karen Ann, a Storer GIS
    Goat Island Skiff - Sacramento

  4. #33
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    Jul 2005
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    'Delaide, Australia
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    Thats the value of recording info on the WIKI. It means we actually work out either the BEST way or a range of ways and it is there for examination any time.

    It is part of the reason that racing boats always improve and cruising boats just go around in endless cycles.

    To be fair racing boats go round in cycles too but they are smaller!!!

    MIK

  5. #34
    Join Date
    Jul 2011
    Location
    Finland
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    131

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    The dryfit image of the boom gives a such sturdy impression on me. I think it would last even without plywood sides. Especially if you put the spacers at strategical places like where the down haul gets attached for example

  6. #35
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    Jul 2008
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    Fenwick, Michigan
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    Engblum,

    Once it is glued up, I'll measure the stiffness of the boom frame and let you know how it compares to the completed boom (with ply sides).

    While the weather is warming, 38° F in the garage is not warm enough for gluing. Late this week or early next week... I hope...
    Building Gardens of Fenwick, a Welsford Parthfinder
    Gardens of Fenwick
    Karen Ann, a Storer GIS
    Goat Island Skiff - Sacramento

  7. #36
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    Jul 2005
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    Quote Originally Posted by engblom View Post
    The dryfit image of the boom gives a such sturdy impression on me. I think it would last even without plywood sides. Especially if you put the spacers at strategical places like where the down haul gets attached for example
    Howdy,

    Nope ... wouldn't work. The advantage of hollow type beams (or simultating them with trusses) is that they are hugely stiff in torsion with little or no weight used to gain the extra function.

    If you used an I-beam as a boom it would twist to maximise the bending (Its response to try and reduce stored energy) then would fail in compression. You need torsional stiffness to prevent this.

    I- beams and other open beams are excellent only if the load comes from one direction and involves no torsion

    MIK

  8. #37
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    Jul 2008
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    The main boom frame is glued up. Not much to tell as it went pretty smoothly - no spilled epoxy, no giant globs of goo where they shouldn't be (hey, I even put wax paper under the frame to keep from gluing it to the spar bench!), no known disasters…

    The frame will be sanded and prepped for the plywood sides. The plywood sides will be fitted and glued tomorrow.

    I didn't measure the stiffness of the frame alone and I'm not going to - even though it seemed like a good idea a couple of weeks ago. The frame is very flexible - could almost be described as "noodlely" and twisty. I will measure the boom's stiffness after the plywood sides have been added and trimmed.
    Attached Images Attached Images
    Building Gardens of Fenwick, a Welsford Parthfinder
    Gardens of Fenwick
    Karen Ann, a Storer GIS
    Goat Island Skiff - Sacramento

  9. #38
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    Quote Originally Posted by BobWes View Post
    I didn't measure the stiffness of the frame alone and I'm not going to - even though it seemed like a good idea a couple of weeks ago. The frame is very flexible - could almost be described as "noodlely" and twisty. I will measure the boom's stiffness after the plywood sides have been added and trimmed.
    I'm glad you didn't.

    The torsional stiffness of what you have built so far is somewhat similar to an I beam which is also very flexible in torsion, unlike the box beam that the boom will become when the sides are put on.

    I-beams work in architecture because most of the loads only come from one direction. Additionally they are usually prevented from twisting much because of the structure sitting on top of it.

    But I beams - and the unsided boxbeam to be that you have two main failure modes.

    1/ If overloaded the compression side will fail in compression as the upper flange (in a building or ship) collapses. This is at very low loads compared to a box beam of similar outside dimensions

    2/ If the load is eccentric and the beam is not restrained by surrounding structure then it will throw the load off the top of the beam by twisting.

    If you did load it up BobWes ... my guess is that it would have twisted over and broken all the glue bonds in your connecting structure.

    Looking forward to more data oh GIS boom-meister!

    Is this effectively boom #4?

    MIK

  10. #39
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    Thanks Mik,

    You're able to explain what I somehow knew intuitively (well, I like to think I knew it intuitively) but as far as being the GIS boom-meister... I dunno...

    This is boom #4. I'll soon be building yard #3.
    Building Gardens of Fenwick, a Welsford Parthfinder
    Gardens of Fenwick
    Karen Ann, a Storer GIS
    Goat Island Skiff - Sacramento

  11. #40
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    Apr 2009
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    Hunter Valley NSW
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    C'mon, build that boom already! Sheesh, how long do you need Bob? Its just a boom, no more than a couple of hours work there, so what's the hold-up?

    More pics, please, quick....

  12. #41
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    Hey, c'mon... I'm on that variation of "Island Time" known as retirement... Things get done at my pace...
    Building Gardens of Fenwick, a Welsford Parthfinder
    Gardens of Fenwick
    Karen Ann, a Storer GIS
    Goat Island Skiff - Sacramento

  13. #42
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    Main Boom: The 9mm sides have been glued to the frame and cut/trimmed to shape using a trim router with a flush cut bit.

    The edges need rounding over; the plywood butt joins need some sanding and fairing; and the entire boom needs sanding before it will be ready for varnish. This boom feels significantly heavier to me than the boom I did for Karen Ann (my Goat Island Skiff). That boom has 6mm plywood sides. I will weigh the new boom and measure its stiffness once it is ready for varnish.
    Attached Images Attached Images
    Building Gardens of Fenwick, a Welsford Parthfinder
    Gardens of Fenwick
    Karen Ann, a Storer GIS
    Goat Island Skiff - Sacramento

  14. #43
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    The edges were rounded over today. The butt joins still need fairing but the yard is shaping up nicely.

    This is a "monster" boom weighing 4.55 kg (10 pounds). Compared to the Karen Ann's boom (similar dimensions and construction but with lighter materials) at 2.18 kg (4.8 pounds), the new boom is quite heavy. It is also quite stiff. A 10kg weight suspended at the mid-point of the boom caused a 6mm deflection in the vertical dimension.
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    Building Gardens of Fenwick, a Welsford Parthfinder
    Gardens of Fenwick
    Karen Ann, a Storer GIS
    Goat Island Skiff - Sacramento

  15. #44
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    Jul 2005
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    'Delaide, Australia
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