Engineers? Opinions on this please...

[Don C L]

I have a deck-stepped mast on a Columbia 29. The support needed replacement after 50 years. I am no engineer but I THOUGHT I had over-built this support arch sufficiently, but now I am not so sure after reviewing a new article about mast compression forces. So I am wondering if any of you folks out there might be able to help with their unvarnished opinions. I am thinking now I may need to rebuild it, but I sure would rather go sailing! The boat's displacement is 8000#. The arch is bolted to the bulkheads, a 2' span. Uprights support each end of the arch. The arch is made of 6 layers of 3/4" plywood, a 1.5 inch wood spacer, with 3 chevrons of 6061 aluminum, 1/4" thick and 2.5" wide let-in to each 1.5" if that makes sense. The photos should make it clear. The whole arch is then bolted together with 3/8" bolts and the uprights supporting the ends are bolted through the bulkheads as well. I am pretty confident about the bulkhead support and connection, but I am not now so confident about the strength of the arch itself. It certainly MUST be stronger than before though. Supposedly now, according to a new article I saw, there may be more compression force on the mast, over 9000#, than I had figured originally. So anyone with a little engineering expertise who might give me a thumbs up or down here? Thanks!
The original arch was built of laminated plywood wrapped in glass. I removed the section that had damaged wood and built the new arch to go up into that void, allowing the top of the aluminum chevrons to fit up there. I was trying to minimize the loss of headroom passing through to the v-berth.
BTW I have installed it but I did not finish it off yet. The plan is to cover the arch bolts with a finishing trim and arches, touch up interior, so it still looks a little rough. I have been sailing with it and I have pushed it a bit, and it hasn't shown any problems at all so far.

[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.

[Cheechako]

Looks overbuilt to me. The weak point may be the bulkheads.
I are an engineer but lost my train. In scientific/engineering terms that is "hell for stout" or "built like a brick S*%thouse!

[markwesti]

Looks good Don , Westsail does a similar fix for early W32's . The only difference is they through bolted it with carriage bolts through the cabin top and the beam is not as substantial as yours . But there is a compression post .

[UNCIVILIZED]

First, a disclaimer. When faced with a problem, I tend to start with the worst case scenario, & then go from there. So some of the below may be a bit overblown. The mindset’s an old habit trained into me by Uncle Sam.

I don't recall the math that goes into figuring something like this. However I can categorically tell you that it's quite common for the mast steps on many boats to see loads that are greater than the weight of the boat. Sometimes several times the weight of the boat. It depends upon the rig design, & boat design.

On some boats the loads are utterly huge. Such as my old 2-tonner, which had cap shrouds, headstay, backstay, & lowers made of -17 rod, 2 sets of intermediates per side that were -12 rod, a baby stay that was 10mm wire, & runners & checks, both made of 5mm Amsteel blue. So that even if the rigging wire was tensioned to but 20% of it’s breaking strength, the load imposed by them was about 37,000lb, AKA twice what the boat weighed. And this was at rest, with no sail loads on things, nor jumping off of waves, etc.
And yes, some of the above math is a bit oversimplified. But it gets the point across I think.

As to your boat. I get the impression that there's more lumber up there now than when you removed the old structure. Which if that's the case it's probably strong enough. However, know that plywood is much, much weaker in this application (or any application where the force vectors are predictable) than is straight grain, plain lumber. Especially given the knots & voids found in plywood, even the good grades. Since at best a bit more than half of the wood is oriented in the best direction to deal with the loads.

Think about how in most boats where you see clear finished ceiling beams, if they're laminated, all of the laminations are horizontally oriented. This is to maximize the properties of the wood in dealing with loads placed on top of the deck. And in vertical beams & on the insides of the cabin house, all of the laminations are vertical, for the same reasons.

Also, I'm uncertain as to what purpose the inletted aluminum pieces serve, other than as a clamping surface for the bolts which hold the layers together. Since when you start to mix materials like that, unless you look up their mechanical characteristics (numbers) & match them with one another, one will be significantly stronger & stiffer than the other. And it'll be the one to carry the primary bulk of the load... until it doesn't, & then the load transfers over to the less stiff material. Unless, that is, you build things in a manner akin to an I-beam, with the stronger, stiffer material forming the beam's flanges, & the flimsier material acts as the sheer web. As in fiberglass sandwich construction.

In the 1979 Fastnet race, many of the rudders which failed were a combination of carbon fiber & aluminum. Especially in the rudder stocks. Yep, bad choice. But not so much at the time due to the galvanic interaction of the two. It was that there wasn't sufficient carbon fiber in the stocks to carry the full rudder loads. Though it did try to, carbon fiber being much stiffer than aluminum. But after a bit of time, & many flex cycles, the carbon reached it’s yield point & failed. And the aluminum which was left to carry the loads was insufficiently strong to do so. Thus the rudders/rudder stocks failed.
I’m kind of doubting that this will happen with this arch, but it’s something worth knowing.

One concern though is the gaps between the top of the beam & the coach house roof. Ditto the ones between the posts on either side of the doorway, & the underside of the beam. Since unless all of those parts fit together well (tightly, with no gaps), then the loads will seek out & concentrate themselves at the weakest points. Probably the (relatively) thin skinned bulkhead at one of the joints between the arch, & the vertical posts. But without seeing how everything fits together live, it’s really tough to say.


Bottom line, the biggest concern would be the gaps between each of the multiple structural pieces. Eliminate them, & then things will perform much more like a 1-piece monocoque structure, including the bulkhead that it's all attached to/surrounding/sandwiching.

[rognvald]

Don,
Although I am not a structural engineer, I can assure you . . . your fabricated arch would outlast the Pillars Of Hercules. I agree with Cheech.
Good luck and safe sailing. P.S. Nice workmanship!

[Don C L]

Originally Posted by UNCIVILIZED

One concern though is the gaps between the top of the beam & the coach house roof. Ditto the ones between the posts on either side of the doorway, & the underside of the beam. Since unless all of those parts fit together well (tightly, with no gaps), then the loads will seek out & concentrate themselves at the weakest points. Probably the (relatively) thin skinned bulkhead at one of the joints between the arch, & the vertical posts. But without seeing how everything fits together live, it’s really tough to say.

Thank you!
Interesting stuff! You probably saw that article I cited in another thread.

Yes, there is no gap between the roof and the top of the arch, it is all filled and faired with Marine Tex, so that is tight. Between the uprights and arch there is still a bit of gap in a couple spots, and I have actually dismantled those in order to fill those gaps too.
It was my thought that the aluminum chevrons would be the real backbones of strength in the load bearing ability of the arch. But your point about the two having different responses to the stress is a good one.
BTW the arch is 6" wide which is also the size of the mast base directly above. The original arch was 8".
Reassuringly for me was that I put a car jack under the arch and loosened the bolts in prep for some kind of improvements, and it was clear there had been no movement since first installed.

[UNCIVILIZED]

Sorry, I don't know what other article & thread you're referring to. As to this new arch, are you saying that it's significantly smaller than the old one? And BTW, what was the reason(s) that caused you to think it needed rebuilding?

[jonnypaget]

I think you are fishing for compliments [emoji12]

[Don C L]

Originally Posted by UNCIVILIZED

Sorry, I don't know what other article & thread you're referring to. As to this new arch, are you saying that it's significantly smaller than the old one? And BTW, what was the reason(s) that caused you to think it needed rebuilding?

Ha! I forgot, it was your thread! here it is, post 25

http://www.cruisersforum.com/forums/...-184737-2.html

The new arch is narrower but thicker. Here are a couple shots of the arch before and then showing the damaged mid section. And I was sailing with it that way for quite a while! The PO had bolted in those SS straps and I guess they held it from collapsing but the arch was not rigid and water had gotten in there and the flexing had weakened it. Still, pretty amazing for over 50 years. I don't know how many years those straps had been on there.

[Don C L]

Originally Posted by jonnypaget

I think you are fishing for compliments [emoji12]

Ha! Thanks for the compliment, but I am not. I just have no way (now) of testing to know if it will hold 9 or 10K (or more?) pounds! Doubt is a horrible thing.
And I am jonesing to get sailing this summer!

[wyb2]

Did a real rough calculation using these equations:
Beam Bending Equations Calculator Supported on Both Ends Single Load at Center | Engineers Edge

and this to check my work:
Beam Calculator

assuming:

1. 8000 lb load
2. Only the aluminum (because I have no idea how to guess at the modulus of plywood) (conservative)
3. Simple beam instead of arch (conservative)
4. Simply supported (not the case - conservative)

Spits out max deflection of about 0.25 inches and max stress about 60,000 psi. 1/4" doesn't sound bad, but 60,000 psi is high, 6061-T6 has a yield strength around 40,000.

But, my assumptions were very conservative. Say the wood takes half the load - that's 1/8" deflection and 30,000 psi. Say the arch with fixed supports doubles the strength - that's 1/16" deflection and 15,000 psi.

I'd say you are definitely in the ballpark. It's clearly stronger than what was there before, those SS strips bolted to the bottom weren't doing jack.

[wyb2]

Originally Posted by Don C L

...The new arch is narrower but thicker...

One note after re-reading - in a simple rectangular beam increasing the width increases it's strength linearly, make it 2x as wide and it will be 2x as strong.

Increasing it's height (what you are calling thicker) increases it's strength to 3rd power - 2x as tall will be 8x as strong.

I = base * height^3 over 12

[Jim Cate]

Don. you could try the simple experiment of measuring the deflection whilst you load up the rig during the tuning process. If the deflection is insignificant with all the standing rigging loaded up to static tuning, my gut feeling is that all will be well. That feeling is also supported by the fact that with the rotten old beam nothing bad happened.

I suspect that the extreme figures that Unciv has bandied about don't represent the situation in an older, low tech boat such as yours. It's a far cry from the race boats that he is so familiar with, and I bet the loads are far smaller.

Jim

[atoll]

looks massively strong........don't forget to add extra ballast to the keel to compensate for the extra weight above