Engineers? Opinions on this please...

[Snowpetrel]

Yeah. Looks pretty solid to me, much more than I have seen on similar boats. The plywood has much lower UTS and sheer strenths than solid wood, but the advantages of this composite solution is that any failure should be noticable first as cracks in the ply, being as it is likely stiffer than the aluminium.

So any failure with this (and I think its highly unlikely given the boat and the rig type) will be instantly noticable and gradual as the aluminium takes the load and yields slightly while the plywood cracks. So a gracefull failure mode. If you were still worried it could be considerably strenthened with some unidirectional glass (or enough carbon) under the beam. If you used carbon you would need to do some calcs because it will take most of the load. Uni glass is a bit more of a team player, but nowhere near as strong or stiff.

[UNCIVILIZED]

Yes, as mentioned by Jim, my mentality is that of "overkill never fails". And I thought about mentioning the bit about a beam's stiffness going up with the cube of it's thickness, but my earlier post was already on par with Godzilla's rant through downtown Tokyo.

The more monocoque you can make the structure the better. Even if that means wrapping much of the area in clear packing tape, & then bedding the load bearing bits in an epoxy/milled fiber bog, so that there are zero gaps in between anything. Then unbolt it, pull the tape, & reassemble, for a smooth, tight fit. Though it may need a tiny bit of coaxing to come apart even when you use tape, & watch out for mechanical interlocks.

You can also measure the deflection in the boat before & after tuning the rig (no load vs. loaded), as well as under sail vs. in a static condition. And even with & sans spar. Typically you do this in the fore & aft plane, & it's a good tool. Plus it's fun to freak out owners occassionally, showing them that we bent the boat... only to have things go back to normal once we ease off the hydraulic backstay, & she's back in the slip.
Growing up. Where's the fun in that


PS: You going to get a nice pretty piece of Koa to slice into veneers, & cover up the bolt heads, & laminate lines. Shouldn't be too expensive

[estarzinger]

It looks strong

My only substantive comment . . Aluminum does not do well sandwiched in wet wood - turns to mush - so seal it up real well. I personally would rather have used a different material than the aluminum, like say g10 sheet . . . But I suspect the way you have done it will be fine for a long while.

[lancelot9898]

It looks pretty well done however, I would run it by a structural engineer to see what is said about any stress risers introduced by the bolts being so close together. If the original structure had this design then nothing to worrying about.

[scallowayuk]

Originally Posted by 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.

it looks strong but unciviliseds comment about using ply for laminated beams is absolutely correct , a weak material as only half the laminates add strength ,,the ones running longditudinally, and even then it must be proper marine ply as cheaper ply uses inferior timber with knots on inner layers .

[jimbunyard]

I'd have used solid, close grained spruce, fir, or yellow pine instead of plywood, full profile aluminum plates instead of chevrons, and 8 bolts instead of 12 in the arch, but that's just me.

I'd guess your design is at least twice as strong as original, the addition of the pillars on the aft side of the bulkhead, which I assume terminate on top of stringers, will add stiffness not existing in the original structure.

I would assume this is a good thing, but it could induce distortions in other less well-reinforced areas, something to keep an eye out for but not to worry excessively about.

I agree with Jim, set a benchmark before tensioning the rig and see how much it changes as you tension it. I'm betting on no, or almost no deflection of the arch itself, though there might be a little compression as all the elements seat together...

[UNCIVILIZED]

I think that the number used for plywood as compared to standard wood is 22%. It's a LOT weaker. And it's not too late to add some G-10, or FRP plate. That or some full sized slabs of aluminum. Full sized meaning as big as the beam itself.


I'm surprised no one's commented on the Koa yet ???

[Thuid]

The only thing I can think of when looking at the pictures was: The plywood arch was laminated multiple wide layers laying flat. You have now changed the orientation of the fibers by standing ~half of them on end. The fiberglass/plywood layers originally formed an arch with vertical strength but also side compression strength (e.g. egg shell, bridges). It is now orientated differently and broken into sections. How was the ends of the original arch braced sideways? You may have load support but have definitely changed the side compression forces if they are a factor to be considered in your construction.

Hope this makes sense, a bit tired now and not really sure if I am.

[UNCIVILIZED]

Originally Posted by Thuid

The only thing I can think of when looking at the pictures was: The plywood arch was laminated multiple wide layers laying flat. You have now changed the orientation of the fibers by standing ~half of them on end. The fiberglass/plywood layers originally formed an arch with vertical strength but also side compression strength (e.g. egg shell, bridges). It is now orientated differently and broken into sections. How was the ends of the original arch braced sideways? You may have load support but have definitely changed the side compression forces if they are a factor to be considered in your construction.

Hope this makes sense, a bit tired now and not really sure if I am.

So are you saying that if you were to look at the boat from the side, he's rotated the orientation of the plywood 90 deg? If so, that's Bad Juju defined, as the planes of the plywood which are oriented vertically are providing almost zero strength. Particularly when you consider that you can cleave (split) veneer in the direction of it's grain with a butter knife, by hand, literally. And plywood's pretty brittle to begin with anyway. Ergo my 22% number. Which may or may not be correct.

[Thuid]

Yup. Bad juju for sure. But also what was the arch butted into on it's ends? It is braced to support from below to take vertical load but be aware there may be side bracing coming into play especially in arch construction. Not saying it needs it but from the looks of the original construction it was an engineered arch and not a straight forward beam. A simple exercise is to bend a playing card into an arch and then lay it flat on a table, press down and it collapses. Now brace the sides of the card as well when it is on the table and press down. It's a lot hard to flatten.

(Sorry for all the edits, brain is really tired right now.)

[UNCIVILIZED]

Here's some food for thought:
Beam of Destiny and Jock Straps » Olson 30 Class Association
http://www.cruisersforum.com/forums/...ml#post2356033

[foufou]

To check what deflection you vet while sailing you could put a temporary compression post under the arch with a gap that has crushable foam inserted. A tight fit. Then go sailing a d check how much the foam has changed.

[Evan_Gatehouse]

Modulus of Elasticity of plywood in bending - around 10,000-12,000 MPa.

Modulus of Elasticity of Aluminum 5086 - 71,000 MPa

So the aluminum is 5x STIFFER than the plywood. The plywood is mostly along for the ride and won't take a lot of stress until the aluminum has taken most of it. It is not completely useless, but it's less useful than you think.
Good way to visualize this is imagine a rubber band and a steel cable both attached to a hook Now add a big weight to the hook. The steel cable is very stiff and doesn't stretch much and takes the weight. The rubber band just sits there because it doesn't take any load until it starts to stretch a lot. This is an extreme example but it helps to get the point across.

You've also got the aluminum not the same depth as the wood so that complicates matters more. The bending stress in a horizontal beam is highest at the top and bottom sides of the beam. There is a lot less in the middle - this is why I beams have less material in the middle and big flanges top and bottom.

Yes mast compression loads can be higher than boat displacement. On the old IACC monohulls the total compression loads were around 100 Tons. On a boat that weighed about 20-25 T.

Getting back to the arch. Yes, it's a moment connection if the beam is all bogged in there with thickened epoxy. That will reduce the deflection but raise the stress actually. Reducing the height of the beam (from 8" to 6") is a really bad idea in terms of strength too as another person pointed out. At this point I would sail it carefully and re-evaluate after a season of sailing. It might be strong enough. But maybe leave it uncovered without the trim just in case you need to redo it. Measure from the center of the arch to floor and make a record of it. Measure again while sailing and at the dock after a hard day's sailing. You're looking for permanent deflection. A bit of flex while hard on the wind isn't the end of the world - but look up at your mast and see if it is deflecting to leeward excessively. If the mast support flexes too much, your rig tension will decrease.

[sailorchic34]

To my old eye's it looks a tad weak. Mind you when I last had static's and strength of materials, I was still using a slide rule.

The aluminum is thin and has little real strength. It's not thick enough nor deep enough in depth to provide real strength. A kip or two on a good day. but not near enough. Same goes for the Beam Thickness, or width buys a little but depth of web buys far more strength then width when dealing with compression loads.

The other issue is aluminum does not have a yield point. Which means as some point in the future the aluminum would crack and break without warning. I would NOT use it in this location.

If it was my boat I would replace the aluminum with 1/4" thick x 6" or 8" deep stainless or even better epoxied black steel. I would not make the arch less thick then the original. But that's just me.

I would use 1x8 oak, steam it to bend and then lamidate 3 pieces with
the steel on each side.. That then would be nicely over built. 2-1/4" (three 1x8) of oak alone would have a strength of 3,000 psi. Add the steel and it's adding another 6k-10k psi which is probably enough. when divided x 2

[Evan_Gatehouse]

Of course aluminum has a yield point - it's a ductile material just like steel!

If you use a 1/4" x 6" steel beam, deflection will be very low (0.006" inches says my beam calculator), but bending stress is around 24,000 psi. Steel yields around 36,000 psi so you won't bend it but that is assuming a 10,000 lb load so your safety factor is pretty low because the rigging load could well be 15,000 lbs thus your safety factor goes to 1.0 and the beam is right at the edge of bending. If you go to a 8" deep piece of steel the stress will be a lot lower (2.3x lower)

As I pointed out above you can't just add the strength of individual components like oak and steel together.