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Wood and aluminum have similar strength to weight ratios. The greater thickness per given weight makes wood stiffer and more damage resistant in smaller scantlings. Aluminum is (with proper corrosion protection) lower maintenance and can be manufactured in longer lengths than are available in clear lumber. Both wood and aluminum can be machined with ordinary tools, but wood is easy to glue; welding aluminum is a specialized trade, the work has to be done in windless conditions so the weld can be bathed in argon gas to prevent corrosion from infecting the weld.
As drawn by James Wharram, MON TIKI has wooden masts. But for the sake of cost and (especially) speed of build, we opted for aluminum masts. A pair of aluminum tubes of the right diameter, wall thickness and length (should have) cost about $200. Add a few hundred dollars for a aircraft-grade welder to braze on the bits and pieces and we (should have) had a couple of masts for under $1000.
In fact, 33′ aluminum tubes do cost $200 each. But you have to order 12 of them because they’re manufactured in China.
And they need nine weeks lead time.
And there’s a $900 delivery fee.
That’s $3300 before we even get the welder to do his thing!
We sourced 20′ lengths of the right sort of aluminum pipe (by local, I mean manufactured in China at a standard length and stocked locally), but that didn’t come out any better. 20 footers were $500+ each, plus material and welders time to butt them together (and then throw away 15′ of left over tube!)
So back to wood.
150 board feet of clear douglas fir.
We started with about 150 board feet of clear douglas fir.
This was ripped 60mm x 25mm staves. These were scarfed to 33 foot lengths.
The edge of staves were run through the table saw twice to give them a “bird’s mouth” notch.
Now here’s the clever part.
The edge of one stave just sort of snaps into the bird’s mouth of the adjacent stave and *presto*, you have have a 33 foot, hollow octagon.
While the epoxy sets the assembly is held in place by rope (a Spanish windlass generates a lot of clamping power!)
Once the epoxy is set we attacked it with planers, then belt sanders and then finished by hand.
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Materials and man-hours together, the cost to make the wooden masts was less than the materials alone for aluminum; and a good chunk of that money when into my crew’s pockets.
Plus there’s knowing it was done right. I don’t know thing one about welding, but I know a thing or two about wood and epoxy.
Plus making a hollow wooden tube 33′ long is cool.
And making two of them is super cool!
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Very cool post, Ryan. How does being hollow affect their strength? Is it more than sufficient (which I’m assuming they are, but I’m really asking for a more detailed statement about that), and what’s the way in which you know how to analyze/estimate their strength? (Or are you actually going to build 3, then bend one until it snaps” 😉Report
I can answer this (at least qualitatively). Hollow is smart. Very smart in this application. For a given amount (and therefore, weight) of material, a hollow tube is much stronger than a solid cylinder. That’s because the average diameter is larger for a tube. And the ability of a beam to resist a bending moment (like snapping a pencil in two) is a function of… shit, I can’t remember the term right now, it’s been thirty years… center of moment?… but in this case the diameter. Think of it in terms of a lever: the longer the lever the more force you can apply. In this case the leverage is measured from the center of the beam to the center of the radial mass.
It’s the same reason that engineers building bridges use I-beams instead of solid beams with a rectangular cross-section. You can trim a way a lot of the material from the center, which doesn’t contribute a lot to the strength, while losing a significant amount of weight.Report
Moment of inertiaReport
Thank-you. It’s one of those stupid things… I could see it in my mind and I could calculate it (well, if I could remember enough calculus anyway) but for the life of me the term escaped me.Report
I had one of those last night, but I eventually got off the couch and got ready for bed.Report
Incidentally, I’ve seen a neat thing in a lot of houses these days, where instead of a simple two-by-twelve the joists are a pair of one-by-one rods with a plywood “web”–making an I-beam out of wood, in a way.
Note that if you’re worried about a column buckling in compression, then a solid beam is better–but even there you have a viable trade-off in that a large diameter better resists moment, so long as the wall isn’t so thin that the cross-section buckles.
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Hollow beams, being lighter per unit length for the moment-of-inertia, do better against dynamic loads as well. Natural frequency is roughly measured as the square root of (MOI times material elastic modulus divided by weight per unit length). From this, it’s obvious that a lighter beam has a higher frequency, which means it’s less likely to see large deflections under periodic loading (such as wind or rocking.)Report
Thanks guys.Report
David, the grains of the eight parts in your picture of the joined mast all line up in the same spiral direction except one. Is there any appreciable twisting moment on a mast?
Edit: I’m not correct. There are four pieces in one direction and four in the other. I’m still curious about twisting moments on the mast, and whether aluminum is superior because of it.Report
The scantlings (boat talk for dimension) come from the American Bureau of Shipping (ABS, http://www.eagle.org), the “bible” for USCG certificated boat. There are scantlings for aluminum, douglas fir, sitka spruce, and carbon fiber.
Interestingly, a sitka spruce mast would have been slightly larger, but about 25% lighter, actually lighter than an aluminum mast to the same engineering spec, but the lumber would have cost about twice as much.
The ABS standard does not contemplate construction technique; only material, wall thickness, and overall diameter. There’s no torsion on the mast. The sail rotates around the mast in accordance with the relative angle of the wind.
A hollow wooden mast is about 40% the weight of a solid mast of the same diameter, but with very little lost of strength for the lost material weight. Traditionally wooden masts have been solid because that’s the way tree grow, and weight is not at a premium on bigger boats.
But it’s hard to find trees that make good masts where we live, plus catamaran are more sensitive to weight than monohulls, plus the bird’s mouth technique, while looking a little intimidating is actually remarkably easy. So a hollow wooden mast makes sense for all kinds of reasons!Report
It’s a bit surprising that they don’t include a knockdown for a mast that’s built-up rather than monolithic. Were you required to perform any quality testing of the adhesive bonds? (i.e. witness samples in a cup to make sure it cured, lap-shear samples to verify strength)Report
Personally, I think you’re better off with wood.
In a fire (see Jaybird’s post above), metal loses all of its strength at once, but wood retains its strength.
Metal will turn plastic at its yield strength, and you might as well have a rubber band at that point.
I’m not familiar with the various grades and alloys of aluminum, but I know the stuff turns to ash under brazing temps.
I’ve worked a few ethanol plants up at the top of the tanks, and there was on-site tigging going on there.
Wind isn’t so much of an issue as preparing for it.
But from a safety vantage, I think you’re better off with wood.
And it’s some pretty cool work too.Report
While all that is true, I think if you have a fire on your boat hot enough to make a metal mast go plastic, then losing the mast is the least of your worries.Report
I think the yield strength of aluminum gives around 540 or so. It’s fairly low.
Worst-case scenario is a fuel fire.
It’s feasible.
But yes, there are certainly other notable concerns in such a case.Report
Yield strength is low, but heat conductivity is high. Alot would have to go terribly wrong to concentrate that type of localized heating.Report
A plastic gas tank and two full scuba tanks would do the trick. It’d be a rough day at the office .Report
Awesome stuff, David. You must be very proud – those glue ups look amazing.Report
To talk about the strength of wood is a bit complicated, because it’s anisotropic and non-homogenous. In that sense, it’s more like a carbon fiber composite than it is a bulk material like aluminum.
In some modes of stress, for some woods, I’ve seen data that suggest wood is actually better even by volume than at least some common alloys of aluminum. There are good reasons why fiber composites are useful materials, and that’s just as true of wood as it is of high tech carbon fiber/epoxy. I’m a long way from an expert on the stresses subjected to boat masts, but given the USCG’s acceptance of carbon fiber and the basic geometry of the situation, I think that design does a decent job of playing to wood’s strengths.
Wood is also, for its strength, probably better in dynamic load environments than metal. Less brittle. Although it’s correspondingly less effective for long-term static loads, I don’t think that’s likely to be an issue here.
Contra Will H, I’m not sold on wood’s superior thermal performance. It’s true that wood loses strength gradually, as opposed to metal. But you start to see reductions in performance at relatively low temperatures (although still well above anything you’re likely to see in a normal boating environment). Similarly, I’d be more concerned about environmental degradation in a wooden mast than in an aluminum mast (I’d also be concerned about possible degradation of epoxy, both in this design and in carbon fiber composites).Report
To be honest I’d rather have wood than carbon fiber, as far as masts are concerned. A tree is designed by billions of years of evolution to be a really tall relatively straight thing that primarily experiences bending loads. Carbon fiber structures are very heavily dependent on process control–you don’t just slap some fiber on and hit it with a hair dryer–and there are many brilliant ideas for the use of CF composite that foundered on the reality of process development.Report
You might think differently if you sat through a few hurricanes.
The scrub pines tend to shear off branches. Sometimes they just shear in two.
Oaks will bowl right over after the water loosens up the dirt around them. They come up roots and all, leaving a big hole.
I’ve never seen a carbon mast do either of those things.Report
Actually, so would I, unless I’m doing a racing boat or something where every pound counts.
Here’s a secret about carbon fiber: in anything except pure axial tension, it’s terrible. Not only is the strength terrible off-axis or in shear or compression (that’s what the epoxy matrix is compensating for) they will fracture and irreversibly lose strength if they undergo compressive strain. There are ways to protect against that, with careful design and, as you say, processing. But still, unless you really need the performance carbon fiber offers, I’d pass.
Also, unlike aluminum, I’m not sure epoxy/carbon fiber is going to give you an advantage in environmental degradation and maintenance over wood. UV exposure and wetting (especially together) certainly can degrade epoxy.Report
So maybe the best way to go if you wanted to get hi-tech would be a pre-stressed composite of a CF rod inside an aluminum tube. Expensive as hell, probably, but it would be very strong and stiff. That’s how we build bridges.
Then paint it up to look like wood 😉Report
Yeah, carbon fiber metal matrix composites can be pretty awesome. But if you thought carbon fiber/epoxy was expensive and challenging to process…Report
Nice project, and have enjoyed reading your progress. A couple notes on the wood. Wood fibers can lose considerable compression strength when wet. Do you have any treatment for the interior surface of the hollow?
What is the plan for securing the masts to the rest of the structure? Typically metal brackets create a problem with the fibers being crushed if ever the moisture swelled the cell structure. Some of the ancient lash boats are very interesting. Mostly a lost art.Report
Now THAT is cool looking. I’ve already got the wheels turning in my head about doing something similar around the metal support beams when I finish my basement.Report
That’s the prettiest bit of workmanship I’m likely to see in a month. Along with Citizen, I’m wondering how this mast will be seated.Report
What sort of treatment do you plan for the masts? As a carpenter, I cringe at the idea of putting bare fir, even cvg fir in a wet environment under critical stress. Will it be epoxied? Soaked in Penetrol? Fiberglassed? The hollow design will be handy, since you can just pour something down the middle while rotating it, and get much better penetration than you could with a solid beam.Report
Fiberglassing might be beneficial because it could keep the seams in compression without creating stress risers or crushing like banding might.
One of my worries with a hollow wooden sailing mast would be how it reacts to cannon shot. Losing a mast in the middle of a broadside engagement is always bad.Report
I would be concerned about matching the expansion rates under saline conditions.
Red lead is the best thing for wood that I know in salt water.Report
Here’s a question for the wood experts: I’m building raised beds. Originally I was going to make them out of untreated redwood, but I’ve come into some kiln-dried, heat-treated, white pine that has a nice price point (free).
Since the wood is free, I’m willing to give it a go even if it rots out in a couple of years, (since I was figuring redwood would only last about 5 as it was), but if there’s a cheap, non-toxic sealant that will add appreciable life to the wood, I’d use that. It’s the cheap + non-toxic part that is a pain in the butt.Report
Land-Ark is an awesome, non-toxic, all-natural wood preservative, made from concentrated wood resins. Unfortunately, it’s not cheap, but it might be cheaper than buying all new redwood, depending on where you live.Report
Non-toxic sealers have been out there for a long time now. I’ve used this stuff for fence posts.Report
I gotta say: the thought of using kiln dried white pine for raised beds hurts me, bro.Report
Dude, eco-friendly people reuse whenever possible. The wood is ex-shipping pallet. If I didn’t use it for this, it’d go in a chipper and become a Pres-To-Log or something.Report
You’ve got a treasure there Patrick. Old pallets are the hottest thing in home decoration.
http://www.dumpaday.com/genius-ideas-2/35-amazing-uses-for-old-pallets/Report
Some of those are really cool; a few make me worry about splinters.
A few I want to do myself now.Report
You have to be careful with them, because a bunch of the older pallets are treated with all sorts of nasty crap.
The KD-HT designation is post-2002, the process was developed in Canada it looks like to get certification for EU environmental standards. The wood is dried and then heat-treated.
It kills the bugs and prevents shipping pallets from carrying on feisty passengers across national lines and whatnot. Much better for the environment than piles of pallets treated with this stuff.
The junk pile for Caltech shipping is a couple hundred yards south of my window. Yesterday someone left three pallets for some enormous piece of equipment that were made of 4 10′ 2″x8″s with a 11/32 plywood top surface and 2×4 supports. I took two.
So instead of having a 12′ long, 5’5″ wide, 8″ deep raised bed made out of redwood, I’ll have a 10′ long, 5’5″ wide, 16″ deep raised bed made out of white pine, with some leftover bits to make a scaffold that I can use to hang bird netting/shade cloth.Report
Yeah, I was thinking of the planter projects and then saw this article; I’m thinking using a pallet to hold planting pots might be safer than just filling the thing with wood.Report
Yeah, you’re being a hell of a lot more eco-friendly this way than using redwood.Report
Not an expert but will offer some ideas for inspiration.
Best if you could put the timbers on gravel or something that drains.
Old sealant formulas prescribed one part linseed oil, to 2 parts gum turpentine.
Ive heated wax and applied like paint for small projects. If it soaks into the surface and end grains it can have some longevity.
Whitewash if pride isn’t a problem.
If the environment is uncommonly dry, I have used vegetable cooking oil (the cheap stuff) as after it oxidizes it becomes nearly varnish.Report
I have a 15′ x 15′ plot that used to be turf, back in the corner of the property. I took the turf out, re-graded it (half-assed, by hand), put down weed blocker and then covered the whole thing to a depth of about 3″ with sand.
The raised bed will go on top of the sand, centered. The sand doesn’t grow weeds, so the path around the bed will be low maintenance, and it drains pretty well, so that should keep the bed from getting soggy.Report
I would go with Kilz and an acetone wash.
Get the stuff in a bucket and mix it acetone-heavy about 2:1 or 3:1.
The acetone will make it soak down into the wood.
Plus, you cover more area with less primer.Report