STRONGBACK DISCUSSION etc.

[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!

[CapnK]

Again, the materials used in late-model "Katie Marie" prove to be different than that used in earlier boats...

Of course, she had no diagonal braces, so I am sure that that had an effect on what I see with her OEM strongback. Although it is *not* rotted, it *has* cracked (see pics, cracks are evident). Without diagonals, it was under more stress, I'm sure. Additionally, it is a solid 1/2" less in the vertical dimension than what ebb measured on his strongback (3.75" vs ebb's 4.25"), and only 1 3/4" wide (1" less than ebb's!) - so it is a significantly smaller beam. Yet she still sailed on for 40 years almost...

One thing I noticed, and mentioned before IIRC - the flexation (is that a word? it is now... ) of the side decks, forward of the bulkhead, which resulted in some cracking of the inner deck skin there. I see no evidence of this cracking on the after side of the bulkhead. It is because of this evidence of force transmittal that I decided to go with the wider beam. I think it was point loading which was responsible for the cracking - the deck was trying to bend around the bulkhead. (!!!) So, to clarify what I think may be a misconception because I didn't clearly state it before: The width of my upcoming beam is only in the smaller part an attempt to make up for its reduced thickness; primarily, in my addled mind, the increased width is to spread loads across the structure while stiffening it to alleviate/cancel any torsional loads. Do dat make sense?

By way of explanation: I know that the beam will flex downward, even if only incrementally, when the boat is under strain of full sail and pounding into seas. Said bending will transfer the load out to the cabin trunk sides. Visualization: hold your hands, halfway cupped, fingertips touching at the top of a small arc, to simulate the arch shape of the inside of the cabin trunk. The mast sits on your fingertips. Imagine it pushing down, simulate it with your hands, you can see that it transfers that force out to the sides. End result: the cracking I see. A wide beam will spread that load across more surface area, which translates into more strength, and less of a load at any one point. Right? It will serve in a similar manner to do what the diagonals Katie never had would have done, had they been there.

*That's* why I want the width. Spreading the love...

I'm not counting on the entirety of the strongback to support the mast, just a small part of it. I'll be supporting the mast in the vertical with 2 metal poles I will put roughly in place of the former doorway frame. The poles will contact the strongback beam and cabin sole on wide bases. Up top, under the mast, center-to-center of these bases will only be about 16", so the span which needs to be strong enough to resist the crushing force of the mast will be unsupported for less than 14".

I am still designing it in my mind, haven't yet settled on that which feels "right". I've been thinking 2" thick on the beam because I think that done properly it will be strong enough, as well as a being visually nice. I haven't ruled out a thicker spot in the beam under this area, tapered into the rest of the beam for visual flow and load-spreading. I suppose I also could go with a metal plate in this span area, and go even thinner on the beam, since it will be poles doing most of the work.

Last, I'd assemble the beam of vertically-aligned plywood pieces with resin *and* glass cloth between the layers of ply, like in my "boxes" drawing, not just resin or glue. By using plywood instead of foam for the core material, I think it should add strength/stiffness.