STRONGBACK DISCUSSION etc.

[CapnK]

Been thinking on this, and will be until I start doing something about it...

I took Franks post to mean that the ply - used as a stiffener - would be *on edge* (its laminates placed in a vertical orientation, like end-grain balsa). Each individual layer of wood being, in effect, the vertical component of an "I" beam. The fibers of the wood in the ply would be more-or-less vertical if it were installed that way. Laminated together flat the fibers would be horizontal, and function more like a leaf spring, right? Seems like that would be so...

Point loading: Exactly - I'm thinking "wide" in order to spread the point load of the mast base out.

The mast base pad on Katie measures a bit over 7.5" in diameter, but this sat on only a total (strongback + bulkhead edge) of 3.5" of structural support from belowdecks. (Not to mention that someone at the factory neglected to install the diagonal braces on this particular Ariel...) By making the beam 8" wide, I should see a lot more support from below.

I saw a different reaction than what you did when the deck was relieved of the mast burden. Mine has remained in the same slightly depressed shape, it didn't spring back up like yours did. I likely had more compression, due to my missing diagonal braces, though. In fact, I see evidence under the side decks that they flexed to a point which caused minor cracking - zoiks! So I am very much interested in spreading the load as far and as wide as I can, so that more of the deck/hull structure shares the burden.

I've given a lot of thought to different metal tubings as a brace. The ultimate would be 2" aluminum round tube, bent and welded in a shape just like that of the underdeck and hull shape, clear down and around. It would be as light as possible, very stiff and structurally strong, I think you could even do away with the entire bulkhead if you wanted, and it would look nicely "techno". It would not be cheap, however - I'd have to sell both CrewDogs and at least 1 kidney to be able to afford that! I think that if you welded up several square tubes, bent to fit the underdeck curve, you wouldn't need to fill them with anything other than some type of anticorrosive agent. But that would be complex, probably not cheap, and probably heavier than a wood or composite beam. Perhaps an I-beam made of flat plate, the top of it curved to mate the underdeck would work well, and be fairly easy and cheapest for a welder to make up. Maybe a wide box-shape, something like that. I don't weld, though, so someone else will have to pursue that route...

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This weekend, I am going to take some scrap ply that I have - it's 1/2" or 5/8" - and rip it into 2" wide strips, then screw them together to make a 6-8" wide "test beam" - no curve, just a flat piece as long as the strongback will be. I'll put it up on some bricks or something, and jump up and down on it, maybe jack up my Toyota and set it down on it, see if I can destructively test it to get an idea of how strong such a construct might be. Kinda like that TV show.

If it proves strong enough, then next week I'll make one to fit the boat, tab it in similar to how the original strongback was tabbed (quick and easy), and then get some galvy pipe to support it with from belowdecks. At that point I'll be able to put the mast back up for some testing. It won't be hard to remove if it:

a) either doesn't work, or

b)works great, and just needs to be put it in properly - bedded in mishmash and glassed all over.

[ebb]

Hmmmm, sounds like fun.
338 miraculously (knock on wood) has its balsa in decent shape. There was not much deterioration under the mast step. It may be that the balsa inside yours is tuna fish. If that is so the repair is easy (I did it!). Just cut the top off over the whole area where the mast sits and replace it with solid lams of xmatt. Gain a little strength there.......oh, and weight!

It's a toss up on the weight of a structure, tho some are a lot lighter, like carbon fiber. Even glass and foam can get heavy. I know, I keep creating that problem.

It's gonna be what you 'see' is right. It's already been done exactly as imagined. Just hasn't been installed yet. My vast experience has been that it nearly always is the first vision you had to solve the problem. Then you embroider or doubt it. Notwithstanding Edison's 10,000 trys at inventing the lightbulb.
Later.......

[frank durant]

Kurt...Quick note on the plywood thing. By 'on end' or 'vertically' ...I mean..as if you were putting the sheets on a wall,not a floor. Multiple layers 'on a wall' so to speak are extremely strong. Just incase I was confusing. OK OK..I know I'm confusing...

[ebb]

OK, so if you do jigsaw the 2 inchers out of ply and glue them up face to face - you will end up with a lot of short grain. Even more short grain in cut curves. Couldn't agrue that that plywood structure is particularly strong at all. Nix that. Plus it's too much work - and wastes expensive ply.

Plywood bent flat, two inches built up of 8 layers of 1/4" marine ply wil be about twenty times stronger than that on edge stuff. A 2X8" made like this might not flex at all. But what are the loads? You will be cancelling any flex with interior supports. In 1/4" you might prebend the 8" strips - taking some stress out of the lamination. A little hot water, a hot day, and a black tarp.
Maybe prebend DRY it in the form for a few days. but overbend it, ie a smaller radius so that they might relax back to the curve of the form.

Could glue up 6 pieces of ply, let it set up hard, cut out the center, put it back in the form, layin some pvc foam with epoxy gel in the hole and glue on a top and bottom of 1/4". Don't think it would act exactly like a composit - it might be stiffer - but it would be lighter. Don'i know that the saving would be worth it.

What's the matter with overbuilding the compression beam anyway???

[frank durant]

Ain't nothing wrong with overbuilding !! I know that in 'i-beam',floor joists or roof rafters that 7/16th OSB on edge with a 2x3 top/bottom holding it, go WAY farther on spans than I figure it should/would....but the engineers say yep.On your big arch I can see where it would be too much waste etc....but the average guy supporting his 'stock' under mast support would find great additional strength by simply laminating a nice piece of mahogany 3/8th ply to each (for/aft) side. Way easier than the steel bolt on in the manual and looks better too.

[tha3rdman]

I dout there would be much waste, the arch wouldn't have to be continuos per ply, just dont pile up the joints in line with each other, the glue is stronger then the wood anyhow. If I were to do it I'd likely do it in 1/3 - 1/2 - 1/3 . . . plys put em in a vac bag, and wala. If you wanted to over kill the joints, scarf them.

Edit: As Frank mentioned the Tj's (the osb capped by dim lumber) can and do span further and hold more weight on the longer span then their relitively sized solid wood joists.

[ebb]

Thing is Epiph wants a 2" deep beam that is 8" wide. The width is a valiant attempt to gain some mass, so that the unusual shallow beam will not want to bend. As I understand it, the original concept is to create a number of mini verticals inside the 2X8 to get that on edge stiffness in the width of the proposed bridge.

The problem is to create a non-flexing structure in a 2" X 8" parameter.

I'm absolutely sure that you cannot cut 2" wide vertical arcs out of ply and glue them side to side to make a 2" X 8" curved beam. You can ofcourse, but the resulting pieces are essentially 85% SHORT grain. And the finished piece no matter how much epoxy glue will not be very strong The only way to get the full benefit of ply is to glue it up in a short stack conventionally.

A solid 2" X 8" glued up mahogany plywood beam wouldn't weigh much more than the 8# of the original white oak one. The beam is less than 4' long. There isn't very much to bend here.

But it might bend.

But it WILL NOT BEND if the 2" X 8" beam is supported at the ends AND WITH
ONE OR TWO COLUMNS. A well constructed strut that has a very solid base.

You could put a compression pole right under the mast down to the keel and get away with no beam at all. Done all the time.

The original beam is 4 1/4" in the middle tapering to 2 1/2" on the ends. It is 2 3/4" wide. It is a very efficient, nice looking piece as designed. In no way could it be bent by the mast. Unless it rotted.

The only way the cabin top flattens (and it is really minimal: 1/2" maybe a bit more, limited experience here) is that the wood structures in the boat settled (and/or the balsacore in the composit cabintop rots) - with help from above. The beam remains unchanged. The bulkhead, beam and vertical struts were all carpentered DRY into place by the factory. Designed to move in time!!!

Dry means no glue was used (in 338). When anybody renovates in 2006, you want to glue in a monolithic composit solid mast support. No fooling around this time.

Can we get Epiph's 2" minimum??? No problem!