STRONGBACK DISCUSSION etc.

[CapnK]

Senor Ebb -

Par for your course, the work is excellent and shows much thought and even more labor! The 1/2 solid dodger - great idea, will have to ponder that one. I have been planning to build (make?) a firehose-frame dodger, a la Yves Gelinas on "Jean-du-Sud" . Combining the two ideas deserves some consideration...

Anywho, I've begun (de)construction belowdecks. After dropping the mast this past Sunday, I decided to go ahead with the strongback modification while it is down, and before I put it back up.

The past couple of days I've been battling a head cold, but today I got fed up with just lying around feeling ill, and took out my aggression on the strongback and bulkhead. The strongback is now lying on the cockpit seat, and the bulkhead is cut away to within 2" of the top of the standard locker countertops. The "main cabin" is now open to the v-berth, and it really makes a difference in how the interior "feels"!!! I love it, it's like the boat is bigger already. Hopefully by the end of the week/early next, I'll begin reconstruction of the structural components I've removed. I'll be posting pics in my Gallery thread soon...

So anyway, I have a question for you concerning your strongback -

Basically - What are it's dimensions? That's to say: How many inches thick is it in its vertical and horizontal cross section? I'm trying to get an idea of what I'm seeing back on page 1 of this thread, and in the latest pics...

I've decided to go with a wide, low-profile design for the strongback, about 2" 'thick', and 8" wide (or more - I'll be removing some of the headliner to do this). Reasoning: keep the interior as visually open as possible, while making sure that my non-engineered solution is far and away stronger than what was in this boat for 40 years (which was surprisingly slapdash, once I could see it upon tearing it apart). I think it'll be all composite, glass and foam, mostly glass.

I drew a line onto the bulkhead and s/b before tearing it apart, to get an idea of how much space that was, and I think it'll be sufficient for getting the material I need in there. Yours looks to be about the same size as the strongback which was original - is this so? Thanks!

[ebb]

Hey Kurt, excellent!
Did I miss what wood you will use for the compression beam?
Depends on where the struts, the verticals are going. All the way to the cabin side (as 338) or are you keeping the forward stateroom, and will have a doorway?

Could guess that if you were going with the doorway 2 X 8 would be OK. In white oak. Wider apart and deflection could be a factor. I would go to 2 1/2" And I might think to lag in a piece of 5/16" steel on edge - cut exactly to the arc of the cabin roof - as insurance. There is a problem with this if you are laminating: your lags are going into the lams which spoils their integrity.

There is good reason why beams are usually deeper rather than flat. You know you'd have a superior beam if you put the 8" on edge. I think 338's is like 2 3/4 X 5. With smaller section plain whiteoak struts on the sides, at the ends of the beam.

Maybe you could weld up a 2 X 8 s.s. arc as a 'C' channel. That might work good. No wood, just steel. Very good, if you through bolted across the cabin top the whole length.
Don't think I'd do away with some kind of verticals going down to the berths.
Theoretically any arch can't deflect if its ends cannot move. On the Triton site at least one guy assured his cabin arc by fabricating a curved athwartship mast base - thrubolted with the beam, no doubt.


At the end of the cabinsole where the V-berth deck rises -right there at the doorway in the bulkhead - I've glued in at 2'' thick mahogany 'floor'* (crosspiece) and I will be doubling that (and maybe a bit more) thinking that I may have to put a compression pole in because the beam is not adequate. They naturally end where the original settees rise. With a pad on them and the pieces fitting well andthe remaining pieces of the old bulkhead tabbed in solid they may spread the load of the mast over the keel well enough. I've heard that poles are great to grab onto below. Imagine a well rehearsed two handed 180 swing from a crouch to a 360 landing on the Airhead...

White oak is not a wood for gluing. Tannins mean the resorcinol won't work. Can't remember about brown glue (the powdered stuff} but white oak? don't think so. There are enough complaints about Gorilla urethane glue to keep me off it. No 'soft' glue will hold stress lamination. And epoxy requires a glue line. Might talk with Smith & Co. about their epoxy AllWoodGlue.

If the beam appeared to be molded into the cabin, not featured, but was disappeared, nicely rounded, into the cabin painting scheme, I bet you could go deeper on the beam scantling and be positive (almost) that the mast would stay out of the accommodation. Incorporating a number of fastenings thru the lamination might insure the mast won't crack it and delaminate. Be cool if you could carve the beam out of a curved branch. Indeed, if you rounded the bottom, curved it like a branch you'd gain more strength without it being obvious.

Interesting problem.

Epoxy has to have a glue line. In curved lams prebending by steaming would allow gluing without force. If your lams are thin and easy to bend (1/8"), and if you have the time: gluing up a few at a time, letting them get hard, and continuing with a few more, etc could work. Gluing a bundle of wood together under stress is not correct anyway. The piece will want to straighten itself out (flatten) which is not what we want to happen - so why build it in?
Wide lams may be difficult to set up a jig and to clamp. But the beam is pretty small. Gluing the beam to the boat would add strength and solidity.
Screws could be used to hold lams together while gluing and backed out after set. I have a bunch of 1/4" luan 1 1/2" squares with a hole in the middle. Drive them in with a grabber to tighten up pieces, get a bulge out.

I think you have to give attention to supporting the full width of the 8" beam with 8" wide verticals. Gotta support the beam!

Go for it!

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*floors connect ribs together over the keel in traditional wood boats. Each pair of ribs, port and starboard, would have a floor, each floor fastened to the keel. Snobby using a term like that in a lit'l ole plastic Ariel. oh well...

[CapnK]

Included is a quicky graphic* I did up to illustrate my current thinking. I'm with you on the stiffener thinking: Since the vertical component of an I-beam is what makes it resistant to downward bending, I have been trying to figure the best way to incorporate that into a 2" thick structure which could resist the compression forces put on it by the mast. I've ruled out wood, because I think that in what I'm going to do it will only add extra weight, w/out adding any extra strength. I'll be relying on glass for strength. Too bad, I like the look of wood! But it will be elsewhere in the cabin, so...

In a nutshell, to build the beam: Lay up the glass onto a sheet of 3/4" thick foam (one side at a time), putting a total of 1/8" thickness glass on each side of the foam. This can be done with the foam in a horizontal position, making it easy to control wetting out, and to control alignment of the glass fibers in the cloth to achieve maximum strength in the finished beam. Once both sides are glassed and cured, carefully cut the 1" thick individual "slices" of the beam to a template taken directly from the hull, then bond the 'slices' to each other. After the slices are assembled into a unit, glass on the outer layers (top and bottom skins of beam).

When this entire beam assembly is cured, it gets bonded into/under the deck, bedded in glass and "ebb's mishmash" , drawn up into place with screws and/or bolts until cured. Use more mishmash to filet the edges of the beam to the deck for smooth transitions, and then, last, laminate on a final overskin of glass.

I plan to put 2 vertical stainless poles in place where the old doorframe verts were, they should give additional support and handholds. I 'mocked this up' yesterday to see how they would look visually, and they don't detract from the 'open' feeling the cabin has with most of the bulkhead removed (which I am loving!). At the bottom, they will tie in to reinforced floors similar to what you have done.

Whatcha think?


*(((As I was just making the graphic, the thought came to mind: "Hey, there's also the space between deck skins which could be incorporated!". Hmmm - doing so would give structure which would not protrude down into cabin space (good!), but it would involve much more work - remove outer deck skin once new beam was in place, build structure in where the current balsa core is, then replace deck skin. An idea which deserves further thought, and which someone else might be able to use at some point, so I've included it...)))

[frank durant]

note to all 'beam' builders out there....Plywood is VERY strong when on edge (vertically).4 layers of 3/8th ply would give GREAT strength as long as the bottom edge was 'trimmed' out with wood so as to keep it from deflecting for/aft under load. I know little about boatbuilding,but in home building we often use plywood as an additional laminate when building a beam to increase the compression strength. WAY stronger than conventional wood.

[CapnK]

Frank -

I assume that is so because the wood fibers run in different directions in a ply? (as opposed to the same direction in a normal piece of wood.)

I'd considered using plywood in this manner, but thought I might get a stiffer vertical with the glass 'ribs' like in the graphic.

So - I could do the beam then with 14-16 'slices' of 1/2' ply, bonded together with a couple layers of 6 oz cloth between each, make it 1 3/4" thick in the vertical dimension, and cap it with 1/4" of glass...? All this would get the same final treatment - completely encapsulated and bonded to the underside of the deck, as noted above.

It might be quicker to do that way. One thing I liked about all-composite, though, was no worries about water penetration. Could take care of that with much care in sealing things up I guess...

[ebb]

Indeed. I do like the multiple I-Beam composite, great graphic. It's an engineering challenge! EG how thick do you make the webs? The more you think about it the more feasible it becomes.

The verticle supports inside, two placed about where the original doorway is, should do it. Could also buildup wide pads on the beam to spread the 'point load' of the supports.

Mike Goodwin introduced us to maranti "Aquaply" and "Hydroply" which are superior in every way to American fir - that stuff would make a good arch - ala Frank's post. Thanks, buddy! 3/8" of maranti equals 3/4" american fir. I would not use FinnPly. Would caution that cutting the 2" wide curved pieces out and gluing those together side to side to get your width is not such a good idea. There still will be flex don't you think? Could you put it on the floor and jump on it??? Even with the extra wide width I think there is not enuf 'meat' in 2 inches. Would like to debate this. I'd be happier with 2 1/2". Plus the two columns inside. As you know it's imco. Frank, though, of course meant bending 1/4" over a form and gluing up a 2, er, 2 1/2" stack! Only with meranti.

A composit structure composed of two skins separated by foam, endgrain balsa, honeycomb -essentially a nonstructural filler - will be stiffer than a solid lay up. Also lighter of course. But we can't forget that the load on the beam is concentrated in one place. Spreading out that point load is very important. Correct?

What we learned about an unrestored Ariel with a flattened cabin-crown under the mast is that the original Pearson structure (beam, strut braces and bulkhead) while funky was still working and in good shape. (On 338 anyway.) The structure had probably 'settled' or shrunk over the decades allowing the rigging to pull down on the mast. That is, it was more of a result of aging materials rather than the weight of the mast, or rot. After the two screws that hold the round wood maststep to the beam inside were removed, the composite cabin top popped back up to it's original crown!! Or pretty close to it - how about that?? That's the challenge confronting the renovator. We need to make some structure that will not move over time. This time. Or one that can be adjusted up when needed. I'll take the immovable, with some redundancy built in. And everything GLUED IN. We live in the age of incredible glue.

If one had the time, testing various structural ideas would be revealing. We'd need the various arcs and a carjack and some sort of meter. But doing this in the imagination is a good alternative. One can 'see' the various ideas and how they might be expected to react to a carjack (or an unmetered jumping attack by a twohundredfifty pounder wearing Redwings!) trying to straighten them out.

I'm oldfashioned and think solid heavy oak will do. But a well thought out composit of carbon fiber and the best epoxy would be much lighter and more in keeping with the technologic advances of this era......... Would be a curious jig to come up with! Vacuum bagging, anyone?

The load on the compression beam is constant - and no doubt there'll be times when sailing that the load exceeds the norm, would be constant AND 'pumping.'

Gotta go to work


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(Supposed to be working...) How about bent square (rectangular) steel tube? Easy to get the arc done. Get the best curve, weld four of them togther side to side. Fill the odd angles and spaces up top with epoxy mix using the beam with a film release. Take it down. Then glue it back up with rubber. No?

Might get a clear span out of that one. And however the supports go, they also could be the same tube all welded together. There's that old problem of what the finish will be: chromium oxide? Wait a dang minit... Use the best quatity iron (the other steel) tube and get it galvanized. No stinkun chromium oxide and no sticky epoxy!!!

[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.......