Engineers? Opinions on this please...

[Don C L]

Well, you all have no idea how much I appreciate this discussion!
First, the 1/2" bulkheads are very securely tabbed in to the hull and cabin house. The hull is very stout, not even close to any kind of oil canning. The upright supports are bolted to the bulkheads on either side. My idea was that the bulkheads would continue to distribute the load out to the hull.
The aluminum is 1/4" thick 6061, 2.5 inches wide, 3 of them separated by 1.5"
I know that galvanic issues are possible, but considering the arch is well protected and sealed I figured I had a few years before that would pop up.
I did not epoxy the whole thing together, just the three plywood arches where the two 3/4" pieces are glued and then each has the chevron let-in to it. It can be dismantled and removed without too much destruction. Remember I had to get the wires in there too before I put it all together.
I had a number of different design ideas before this one. When I finally settled on an combo wood/metal arch under the deck I originally planned SS chevrons in there but someone commented that that was WAY overkill... and it was considerably easier for me to fabricate the whole thing this way in my garage. If it is truly not up to 9K# or so, then of course I will take it apart and try again.
I was figuring the load would largely be carried by the sheer strength of the bolts. There is no discernible deflection so far, but maybe I could try measuring more carefully. I just can't figure what will bend or where it might first break. I had thought the weak point would be the top of the chevron, but in order for that to break the bulkheads would have had to have been pushed apart, no?
I agree it could be stronger and more elegant! I can take a little criticism!
The original arch was eight inches wide and had a height (I am finally getting the terms right?) of 1.5 inches and was wrapped in 3 layers of roving. I totally agree and get it that having the plywood on its side is not a strong way to build an arch. In my mind it was the aluminum bolted together with spacers that would do the job.
Is not an I-beam that is in a chevron shape stronger than a straight I-beam? Is this not a truss where the ends are securely held in place and the strength of the bulkheads and hull will prevent the ends from moving outward?

Thanks to all for all comments!

oops forgot, the span is 25"

[Don C L]

Here are a couple more shots of the original arch construction and how it is built into the cabin.
It still boggles my mind that this has to hold 9 or 10,000#.. (or more?)

[Don C L]

a few more shots..
The chevrons were cut from a single sheet.

[Snowpetrel]

Thanks for the extra photos. It would also be interesting to get an idea of deck thickness and construction, and if or how the beam is fastened or glued to the deck? Also the mast step, and a close up or information about the plies of the plywood, are they balanced, ie the same amount of material with grain each way.

If you are to keep it in place it would be stronger glued in place rather than just bolted. And a layer or two of unidirectional glass on the bottom edge would add strength far in excess of its weight.

You are right that the chevron shape helps. It is jusy going to be hard to work out by how much. Much simpler to assume it's a simply supported beam. In this case my calcs seem to suggest that on their own they are not enough, but I feel (havent done any calcs) that tied in as they are and angled they will hold the whole thing together if the plywood was to crack. This seems very unlikely, as the wood alone seems more than capable of carrying all the load. What is the exact chevron angle?

[NevisDog]

Heck that looks strong! Straight-grain timber would have been better than plywood, as Uncivilised points out, but it looks well-engineered. You haven't said what grade of plywood - I've seen some so-called 'marine-grade' ply fall apart before it even hits the water.

[NevisDog]

Originally Posted by J Clark H356

...The vertical load on the beam is the weight of the combined mast, boom and sail. The stays are to keep the mast positioned and stable. They don't add to the vertical load. ...

Uh-oh! This is totally incorrect.

Think about it for a moment - we all know that bringing the shrouds inboard (say to allow closer sheeting of a genoa) can drastically increase mast compression (= downward pressure on mast step), or that vastly over-tensioning the backstay will eventually crush the mast! I wouldn't want anyone to think the above statement is remotely related to reality. i've just come across this thread and maybe someone else has pointed this out already.

[model 10]

Originally Posted by NevisDog

Uh-oh! This is totally incorrect.
Think about it for a moment - eady.

Of course you're right but wat is the load? The whole boat? Or some factor of the righting moment"

[Don C L]

Originally Posted by Guy

Of course you're right but wat is the load? The whole boat? Or some factor of the righting moment"

Here is the article that got me worried!

http://www.fhkiel.de/fileadmin/Data/...P-Yacht_02.pdf

He had 1.4 x the displacement worth of force at 30 degrees of heel. But it could go higher.

For me that would be 11,200# but I am not pushing a racing boat hard as Jim noted. Still, one wants to be prepared.......

[J Clark H356]

The bolts don't really provide the strength for the beam, they along with the glue, make it a single unit rather than a series of independent pieces. The totality of the beam is what gives it strength.

The depth - which is the distance from top to bottom of the smallest piece will determine the load it will carry. For my earlier calculations I assumed 6 inches. I now, having looked at it with the aluminum let in that is 2.5 inches, believe it has less depth than 6 inches.

You are correct in describing it as similar to a truss. It will also carry more than calculated due to the plywood acting as a skin where the loads will be distributed along the plywood grains to the bulkhead. Can you provide the depth?

I also now think I was incorrect in saying the mast load was only the direct weight. Tensioning the mast down with the stays is an excellent point and I think does increase the vertical load. How much I do not know.

Wood is not like steel which can absorb much more,than it's elastic deformation load. Wood quickly just breaks once overloaded. Take a wooden pencil and bend it and it snaps, a piece of similar metal will bend, but not break. The bending past it's point where it will recover is absorbing much greater stress. This is not present in wood as shortly past it's elastic limit, it snaps, releasing all the energy or stress.

[Snowpetrel]

Ha, principles of YD shows even more than this, for the 8 tonne 40 footer yacht the design load is 17 tonnes, but this is a pretty high performance boat. You would have no where near the forestay and backstay tension, and the old girl would start bending and grao2ning at the seams if you wound her up that tight.

If you carefully monitor the deflections long term you will see if there is any permanent deformation showing overload past "yeild" or the recovery limit of the wood.

[Don C L]

Originally Posted by J Clark H356

The bolts don't really provide the strength for the beam, they along with the glue, make it a single unit rather than a series of independent pieces. The totality of the beam is what gives it strength.

The depth - which is the distance from top to bottom of the smallest piece will determine the load it will carry. For my earlier calculations I assumed 6 inches. I now, having looked at it with the aluminum let in that is 2.5 inches, believe it has less depth than 6 inches.

You are correct in describing it as similar to a truss. It will also carry more than calculated due to the plywood acting as a skin where the loads will be distributed along the plywood grains to the bulkhead. Can you provide the depth?

As near as I can tell the measurement you are referring to would be 3.25 inches. However, isn't that only true if it is a straight horizontal beam?

[Snowpetrel]

Originally Posted by Don C L

Here is the article that got me worried!

http://www.fhkiel.de/fileadmin/Data/...P-Yacht_02.pdf

He had 1.4 x the displacement worth of force at 30 degrees of heel. But it could go higher.

For me that would be 11,200# but I am not pushing a racing boat hard as Jim noted. Still, one wants to be prepared.......

DYNA is a very different kettle of fish to your lovely little classic yacht. Here is an except from the report on the dyna's rig

The pretensioning of the “DYNA“ rig is performed with a mast jack. It was not possible to achieve the necessary pretensions computed by AES using turnbuckles only"

By the way Chris Mitchell from AES is one of the top rigging engineers with some very interesting stuff http://www.aes.net.nz

Also being a fractional rig with a narrow staying base the rig loadings go through the roof to get both a narrow enough sheeting angle and decent forestay tension from the swept back spreaders. This article really shouldn't concern you with your simple and relatively low stress rig.

[Don C L]

Originally Posted by Snowpetrel

Ha, principles of YD shows even more than this, for the 8 tonne 40 footer yacht the design load is 17 tonnes, but this is a pretty high performance boat. You would have no where near the forestay and backstay tension, and the old girl would start bending and grao2ning at the seams if you wound her up that tight.

If you carefully monitor the deflections long term you will see if there is any permanent deformation showing overload past "yeild" or the recovery limit of the wood.

Now is that a linear function, comparing sizes of boats and loading? By that I mean that since an 8 tonne 40 footer could expect 17 tonnes would it then follow that a 4 tonne 29 footer could expect 8.5 tonnes of force? Yes I won't have her wound up that tight, but that is 18,740 pounds. That's practically a Westsail 32 perched on my cabin house! Even half that is more than I had originally thought...
I think I am starting to feel some of my sailing summer slip away... and seeing the tools coming out again...

[Don C L]

Originally Posted by Snowpetrel

DYNA is a very different kettle of fish to your lovely little classic yacht. Here is an except from the report on the dyna's rig



By the way Chris Mitchell from AES is one of the top rigging engineers with some very interesting stuff AES - aes home page

Also being a fractional rig with a narrow staying base the rig loadings go through the roof to get both a narrow enough sheeting angle and decent forestay tension from the swept back spreaders. This article really shouldn't concern you with your simple and relatively low stress rig.

Well that is a bit of a relief!

But in looking at what I have put together, where would you imagine it would fail?

edit: I don't know the angle, I'll have to check but it looks like 160 degrees or so.

[Snowpetrel]

The righting moment of these more modern hulls is far more than your low freeboard wineglass hull. The rig loads track righting moment not displacement.

Some of these modern designs might have twice the RM/tonne of Displacement than your old girl, and much narrower staying angles. These both generate much higher loads. Throughout the rig and hull.

I am figuring the first sign of failure would be permanent deformation other than the whole structure bedding in. Then the outer piles of the plywood would crack and the aluminium would start to yeild slightly as it takes over from the plywood. The rigging would go slightly slack relieving a lot of the loads and preventing further failure unless its all cranked back up again. Id think it would take a very high one off peak load to do this, not normal sailing loads, eg a sudden high speed grounding, or capsize, or falling off a huge wave when punching very hard to windward i gale conditions like this one.

https://youtu.be/EnpX9uYDJSQ