Engineers? Opinions on this please...

[Cadence]

Originally Posted by Diesel Bill

Sorry, I can't tell you the loads imposed or the load carrying ability of your addition.
But as someone who has lifted many heavy loads and rigged for lifting said loads, that laminated arch looks incredibly strong. Maybe even stronger than the bulkheads it's bolted to. Others with more math skills will hopefully add to this.

Ditto on the nicely done job.

[UNCIVILIZED]

Don C L, I dare say you're getting more information that you anticipated, or perhaps even desire

[barnakiel]

Ha ha ha!

I am into a very similar project now and our boat is very much like yours.

I look at your images and i like what you did. Stuff cannot be built 'too strong'. And if it is, you will only sleep better.

We are going to leave our old beam in place and apply a twin identical beam of the same design on the opposite side of the main bulkhead. I will post pictures when done.

I do like what you built. It looks very strong. I would trust it 100%.

Cheers,
b.

[hamburking]

Hi,

I am a mechanical engineer, but don't hold me liable for my "off the cuff" opinions here.

1) Looks strong, likely stronger than necessary.

2) Its laminated the wrong way. You have vertical members through bolted with horizontal bolts. That works for you to mount like a bulkhead sandwich. But a beam needs stiffness at the top and bottom, like an I beam. Your stack relies on glue at the top and bottom for stiffness. In addition, the plywood uses thin layers of wood with alternating grain. The grain seen in the picture shows the grain running up/down. So the forces in the beam (which are greatest at the top and bottom) would induce a "rip" in the grain...its the weak direction of the grain. So you could consider your stack strength diminished by half, since wood rips much more easily in that grain direction. As another poster mentioned, it would be much stronger horizontally stacked and through bolted to the deck. A rough guess would be at least 10 times stronger.

3) You can test it. Use your scraps to mock up another similar beam...it need not be so curvy and fancy, just the same construction and rough dimensions. Then take it to your local school of engineering for a destructive test.

4) Under load, the top of the arch will be in compression. Wood is pretty good in compression. The bottom of the arch will be in tension. Wood is not so good in tension. Keep an eye on the bottom of the arch looking for openings. If you see cracks on the bottom edge, that's your clue just prior to a failure.

[Don C L]

Originally Posted by hamburking

Hi,

I am a mechanical engineer, but don't hold me liable for my "off the cuff" opinions here.

1) Looks strong, likely stronger than necessary.

2) Its laminated the wrong way. You have vertical members through bolted with horizontal bolts. That works for you to mount like a bulkhead sandwich. But a beam needs stiffness at the top and bottom, like an I beam. Your stack relies on glue at the top and bottom for stiffness. In addition, the plywood uses thin layers of wood with alternating grain. The grain seen in the picture shows the grain running up/down. So the forces in the beam (which are greatest at the top and bottom) would induce a "rip" in the grain...its the weak direction of the grain. So you could consider your stack strength diminished by half, since wood rips much more easily in that grain direction. As another poster mentioned, it would be much stronger horizontally stacked and through bolted to the deck. A rough guess would be at least 10 times stronger.

3) You can test it. Use your scraps to mock up another similar beam...it need not be so curvy and fancy, just the same construction and rough dimensions. Then take it to your local school of engineering for a destructive test.

4) Under load, the top of the arch will be in compression. Wood is pretty good in compression. The bottom of the arch will be in tension. Wood is not so good in tension. Keep an eye on the bottom of the arch looking for openings. If you see cracks on the bottom edge, that's your clue just prior to a failure.

Thank you! What do you think of the aluminum chevrons sandwiched in there providing the support? Those and the shear strength of the bolts are what I figured would be carrying the load. I see the plywood is a less than ideal choice now, especially the way I cut it.
I don't feel something is built unless it is overbuilt so now I am thinking to re-think it........

[Snowpetrel]

Don. It looks good to me, I'd go sailing, evaluate it over the season and if you are concerned at the end of the season rebuild or strenthen it then. Lifes to short to seak perfection. The thing looks substantially stronger than the mess the boat had for 50 years, and the failure mode doesn't seem sudden, as in any issues will simply show up with the end grain plywood cracking and some slight extra deflection.

Many years ago I delivered an old 28 foot wooden boat with a much smaller and slightly rotten beam like yours across the Tasman mid winter. I worried about it a fair bit, but after a few days of rough weather with pencil marks and measurements I couldn't detect any movement and stopped worrying about that and switched to worrying about the bad garboard leaks and the rusty floors under the keel tank...

You have three components to this beam. The solid wood core, the plywood sheathing and the aluminium chevrons. It looks to me like individually each would take the bending load on its own. All three dont play perfectly together, but they will help each other to some extent.

There is quite a lot of sheer at the point where the beams join the bulkhead. It looks like this is only being taken by the ply and the aluminium with the aluminium being the dominant part, but again this looks fine to me. From an engineering perspective it isnt the most elegant solution, but it looks very very strong.

[hamburking]

Originally Posted by Don C L

Thank you! What do you think of the aluminum chevrons sandwiched in there providing the support? Those and the shear strength of the bolts are what I figured would be carrying the load. I see the plywood is a less than ideal choice now, especially the way I cut it.
I don't feel something is built unless it is overbuilt so now I am thinking to re-think it........

Think of an I beam, which is really what you want in there. You have done a great job of the skinny middle bit, which takes little load. And a ...sorry... really lousy job of the wide ends which take all the load. The aluminum inserts are also oriented to be part of the skinny, non-loaded section.

If I were surveying your boat, or hired to evaluate your design ...sorry... I would have to recommend tearing it out and building it right. Sure, it looks good, but has only a small fraction of the strength it would have if the sheets were horizontal, not vertical. Obviously you put a lot of time and effort into this, and most of the comments say it looks so good. But its not. Its weak. I suggest you speak with someone locally to get a second opinion. Someone who works with beams, like a house builder...they use manufactured beams all the time.

[captlloyd]

Originally Posted by UNCIVILIZED

Don C L, I dare say you're getting more information that you anticipated, or perhaps even desire

Yes, this thread brings to mind the movie "
Lord Jim" with Peter O'Toole. His imagination ran wild, imagining his ship had sprung a huge leak and he abandoned a perfectly floating ship. I would guess whatever is supporting that arch would fail before the arch. I do not recommend hooking a garden hose up to a fire hydrant.

[estarzinger]

just fyi . . . . stuff like this does exist . . . . https://www.mcmaster.com/#fiberglass-i-beams/=180074f

[hamburking]

Of course...the standard test for the compression post....tighten the rig and see how many threads are left on the turnbuckles. If the post (or arch, or deck) is weak, you will run out of threads before you have the required tension in your rig. Whenever looking at boats (to buy) I always inspect the turnbuckles to check for a bad compression post/base/arch/whatever. If the turnbuckles are done up as far as possible, then there is a problem.

[wyb2]

Originally Posted by hamburking

...
1) Looks strong, likely stronger than necessary...

Originally Posted by hamburking

...would have to recommend tearing it out and building it right...Its weak...

I'm confused

[hamburking]

Originally Posted by wyb2

I'm confused

It looks strong. But that does not mean it IS strong. Or rather, it IS strong, but not in the direction of the forces which will be applied by the mast when in compression by the rig.

[Snowpetrel]

Originally Posted by hamburking

It looks strong. But that does not mean it IS strong. Or rather, it IS strong, but not in the direction of the forces which will be applied by the mast when in compression by the rig.

The plywood is not optimal, and its orientation is not ideal either, but about half the grain is running the correct direction and the grain running the wrong way still helps tie the fibres together and strengthens the beam against longitudinal splits. On balance I would have thought the plywood portions have about half the strength of solid wood used this way. Laminating plywood horizontally into a curve would have been slightly stronger but not by much, since the curve is slight and half the grain still runs the wrong way.

And speaking of the curve, even though small it turns it into a partly arched structure so a small portion of the tension in the bottom is reduced as long as the ends are well secured and buttressed by the bulkheads.

Hopefully the whole thing is glued together rather than just bolted. This will help spread the load across all the parts.

Hopefully the top of the beam is well bonded to the cabintop, moving the neutral axis up due to the fibreglass cabin, increasing the effective beam depth, therefore considerably reducing the max fibre stress. This will be hard to model accurately, due to the different materials. But will help a lot.

It would be fun to play with some real numbers!

[wyb2]

Originally Posted by hamburking

It looks strong. But that does not mean it IS strong. Or rather, it IS strong, but not in the direction of the forces which will be applied by the mast when in compression by the rig.

got it.

Throwing this out to all: does anyone think the new setup is weaker than the original?

[J Clark H356]

It's late and I will try to calculate it tomorrow and let you know how much load it will safely carry. I've not calculated mast load before, so I'll have to think about the loads for that. The more bolts the better as it must unify all the materials to give you an effective beam. This is similar to a laminated glued beam and the bolts are having the effect of the glue. You must calculate the section modulus of the various components which is fairly easy once the sizes and type of materials are known. I need to know exactly how far the "span" is. Span is the dimension between supports on each end. This is no more than a simple beam calculation and I have been doing these for years. I'll review your materials again tomorrow and if I completely understand the assembly sizes and thicknesses, I can do the calculation for the load it will carry with a margin of safety.