STRONGBACK DISCUSSION etc.

[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.

[bill@ariel231]

Kurt

just a voice of caution: I recommend you take a look at Brian Toss' book on rigging or a couple of web sites...

http://www.sponbergyachtdesign.com/A...ngineering.htm

or

http://www.classicmarine.co.uk/Artic...ging_loads.htm

(there are others)

from the sponberg site a rule of thumb is:

(mast_compression) = ((weight of boat) * (righting moment from CM to CB)) / (distance mast to chain plates)

rough numbers (eye-balling the boat at 30 degrees of heel)....

mast_compression = ((5200 lbs) * (1.0 to 1.5 feet)) / (4 feet) = (1300 to 1950) lbs

This number is higher than some other more detailed models. doubtlessly, Moderator Bill has some better numbers for weights & moments, but the point is there will be a large point load under the mast and you are looking at a large span.

before you commit to a radical change, you may want to build a full size model and load it with a jack or hydraulic press.

Maybe I have misread the article.. I'm just a software guy.

cheers,
bill

[CapnK]

I took a good look at an Alberg-designed Cape Dory 26 yesterday, checking out how the strongback was engineered in a very similar boat to ours, 20 some years after our boats were in production.

The CD26 has a center-pole support for the mast, with port-offset access to the v-berth. The mast support is apparently a solid wood, 2.5-3" square pole. Interestingly, the pole is offset from centerline *to starboard*, about 1.5". There is some sort of arched structure up against the overhead which the half-bulkhead attaches to (mechanically, no tabbing that I could see), but it is small, and hidden by the cabin liner. It is an upside-down triangle in shape, approx 3" wide against the overhead, and 3" to the apex of that triangle. The corners of the triangle are radiused about 1/4".

The wide piece of wood on the bulkhead against the overhead you can see in this pic is simply trim, it didn't appear to serve any structural function. Also note the seeming lack of cabin trunk side support on the port side at the bulkhead station.

From tapping against the overhead, it was evident that the deck construction in that area - forward of the bulkhead at least as far as the forward edge of the mast base - was more solid than in other areas of the overhead.

Last, and somewhat unrelated, the size of the standing rigging was obviously smaller than that which we have on our boats.

Interesting.

[Bill]

Last, and somewhat unrelated, the size of the standing rigging was obviously smaller than that which we have on our boats. Interesting.

The Ariel used the same rigging and mast (cross section) as the Triton. Sort of explains why these are such a tough little yacht.

[ebb]

Very shallow beam, (don't really see it!) - no side supports at the cabin sides.

Lots of similarity to the Ariel of 20 years earlier - looks like heavier buttocks, to keep her from squatting. But the beamless interior is very interesting. Did you knock on the sides? They'd have to be solid, wouldn't you say? Otherwise typical Alberg. I have to wonder how it is done? maybe a single post really can act like a keel stepped mast. Do the shroud-plates come thru the deck?

Numbers say only 78 of these were built.

Hard to imagine any boat near the size of an Ariel with less than 3/16" wire.
Wonder how the general construction quality compares between the two?

[ebb]

Kurt,
the end result of your mast beam project will be zero flex of any sort.

As to the deck flexing, I believe it has to be rotton core. Right? Like 338 you may have a composit whose inner layer is damn thin. In the past I questioned whether a recore could be done from the top because the inside had little or no integrity. I found a single layer each (on the inside of the composite) of cloth and mat under the mast. Easily moved with the fingers. I've just assumed that later number boats were skimped on, but it may be individual ones.

I believe now when doing a radical coresectomy that the foredeck area should be carefully supported from inside with boards and battens or even an inflated bag or two. Cutting the deck out in small sections at a time so as not to loose the camber of the deck.

No bulkhead or knee was built-in to the underside of the deck/cabin roof in 338. Seems to be the way it was done back then. I would make sure they are well tabbed in this time so that the deck becomes part of the system, not a symptom of something wrong. How this can best be done is obviously open for discussion. The forward knees for the lowers ought to be able to support the deck. Wide tabbing and healthy core would take care of any flex in this area. No?
{The widest part of the foredeck and the rounded nose of the cabin is unsupported from inside - except by its monocque shape and the sides of the hull. Flex in this area would be strange - unless it was contributed by weakened deck.
The mast is supported by the whole top of the cabin and indeed by the whole top of the molded deck. Imco it's possible that some of the downward force of the mast could be translated into that cabin-nose/wide foredeck area. Loss of camber would be an indicator. Long straight-edge would show it.}

I managed to get glue under the liner port and starboard sides and carjack the sags and openings 338 had there up flat to the roof - I felt very uncomfortable messing with the liner so I opted for monolithic knees off the hull. I may go back and tab them in to the top. (But I might also remove them because my chainplates are going on the hull.) The liner as you know does not extend all the way out to the hull under the side decks so there is actually some prime attachment for part of the top of the aft knees (aft lowers) and the main bulkhead (or what remains of it under the decks.) Would not tab onto the liner. The knees tabbed in solid must contribute a good deal of stiff to the hull and deck in way of the shrouds what ever method of chainplate attachment.

IMHO, NOTHING should be able to move in the shroud/mast/deck/cabin/beam area. That is, nothing of the boat itself (the plastic) should move at all. The rig depends on it. The shrouds in big water can't suddenly get loose because of 'oilcanning' of hull or deck and not support the mast, both sides. Sudden loose shrouds could be a cause of the mast breaking in a knock down, doncherthink?

It does seem far fetched: but if the weight of the boat is supported by the mast (ala Sponberg) then two tons of concentrated force could do some strange distorting of plastic laminate! Would have to see it to believe it - like in frame by frame time lapse. Monocque structures can be crushed, I'd think they can be twisted on an axis, and relative flat thin planes can be oil-canned. Generally '60s plastic lams were substantial and made from excellent ingredients. That's what we gather, anyway. But composits with cores must have been fairly new and simple back then - now we know that the opposing skins must be equal in strength/thickness. David Pascoe, our favorite online marine surveyor talks about this on
http://www.yachtsurvey.com/HiTech.htm

I feel that the mast bulkhead along the hull from the deck down to the keel has to be very strong WITH WIDE TABBING. - and ideally the bulkhead should be connected across the keel, so that mast forces are shared on both sides of the boat. As above, so below. The circle must remain unbroken...in the bote, load, in the bote ....

[CapnK]

Bill - Thx for the suggestion, I have Toss' tome on the subject, will have to reread the pertinent chapters. The websites you posted are most informative, and give much food for thought. Once I decide on *something*, I will indeed make a test piece first, and do what I can to see that it will be up to the job!

Ebb - I don't see any obvious signs of core rot in the area, but do plan on making danged sure of it before wrapping it up. Eventually I'll be going with external plates, and the areas around the bulkhead and chainplate attachment points will recieve much strengthening and stiffening in the process. Like you, I don't want it to move at all, oilcanning is for powerboat engine rooms only.

The circle shall remain unbroken.