Jim Michalak's Boat Designs
118 E Randall, Lebanon, IL 62254
A page of boat designs and essays.
(1Mar10)This issue will compare wooden and metal spars. The 15 March issue will rerun the lugsail rigging essay.
THE BOOK IS OUT!
BOATBUILDING FOR BEGINNERS (AND BEYOND)is out now, written by me and edited by Garth Battista of Breakaway Books. You might find it at your bookstore. If not check it out at the....
ON LINE CATALOG OF MY PLANS...
...which can now be found at Duckworks Magazine. You order with a shopping cart set up and pay with credit cards or by Paypal. Then Duckworks sends me an email about the order and then I send the plans right from me to you.
MUST SEE WEBSITE!!!!
...Our old buddy Garth Battista of Breakaway Books has escaped! He towed his Cormorant and his family, between East Coast blizzards, from New York to Miami and is now acruisin' with them all in the Bahamas, etc.. He keeps an updated log at
Garth is like most of us, an inland waters guy who went out and built a boat for himself. Seems like just yesterday he sent photos of his first Toto. THREE CHEERS AND GOOD LUCK!
Mick Cross plys the canals of Jolly Olde England in his Harmonica. Actually he has two of them.
Wood Vs Aluminum Spars
BACK TO THE DAYS OF YESTERYEAR....
...long long ago I had a third interest in a Hobie 16, my first boat. They were all the rage then. A very clever design. But I quickly found it ain't no one man boat, at least not for the young man I was then. I quickly moved on to a Snark Mach II which is very much a poor man's Sunfish. Truely a one man boat, slack bilges and about 3' wide, a shaped styrofoam block covered with a thin plastic shell like a big streamlined beer cooler. I suppose it weighed 80 pounds. It has 84 sq ft of nylon sail, more than the much heavier Sunfish, on aluminum spars. It was very low in the water and a very wet experience and I recall well the big waves and Carlyle Lake washing right over it at times. But I must say at times it would really fly. One day I sailed from the south ramp up and around the northern islands and back to the south ramp, 18 miles total, in three hours. It was reaching up and back in a perfect wind all on plane I guess. That sail, still the fastest I ever done certainly in a monohull, was as I recall quite effortless.
Later that year on Labor Day I got caught in a squall with it. Capsized and blown away from shore on a lake three miles wide, I hopped a watery ride with a Hobie crew that had also capsized so at least I wasn't alone. My Snark drifted away from us. The Hobie crew had a knowledgable skipper and the first order of action was to swim out to the mast tip and lash a spare life jacket there lest the boat turtle if the mast filled with water. Eventually we righted the Hobie right about the time the rescue arrived. The rescue boat had already rounded up my Snark and were relieved to find its skipper (me) was still alive. Two of the Hobie crew thought the experience was so cool that they did not accept a tow home. Instead they kept on sailing. As we who were rescued were brought back to the sail harbor we saw another rescue being towed in, a Y Flyer totally awash and floating almost totally below the surface to the point that I thought they were towing in a white sail, not a hull. The Flyer's rig was totally destroyed, nothing left above the deck.
I'm going to add a little more to this story. I recall when the storm was really bad that there seemed to be no "surface" to the water, just different densities of H2O from solid to mist in a gradual conversion about two feet thick. The next day my arms and legs were bruised all over. In general if I had not been rescued I think my life jacket would have sustained me until I fetched up on the other side of the wide lake. Without a life jacket there would have been no surviving it, at least not for me. So in a way it was a good learning experience I suppose.
MORE LESSONS LEARNED...
The Snark was rearranged by the storm. The rudder and daggerboard were totally gone, the plastic rudder cheek ripped right off the boat. The aluminum mast was bent a bit. The aluminum yard had lost the extension that plugs into its end which adds about two feet to the length. I called around for repair parts the next day and got the answer that the sailing season was over and the dealership closed. So I fixed it all myself. A new wooden daggerboard and rudder and new aluminum rudder cheek and yard extension. Hell, if I can fix this boat I can make my own from scratch, right? So it was a life changing storm in a way although I'm sure making my own boat was inevitable.
ON TO THE BOLGER TEAL...
By this time I had learned about Phil Bolger and had a copy of The Folding Schooner and made a Teal from the plans in that book. Like many of you I went to tinker with the details, something Phil never seemed to mind. As I recall the Teal hull was per plans but I decided that low tech wooden mast needed updating. Wicks Aircraft Supply is close to here and I knew from fixing the Snark that they had all sorts of aluminum tubing in stock. The thing is that you can't really replace a tapered wooden mast straight off with aluminum tubing because you can't get tapered tubing. Even flagpoles, which appear to be tapered, are I think rolled from a uniform tubing such that they are thicker at the narrow tip than at the wide base. So the taper is in appearance only, the weight of the tubing is the same per foot along the entire length. The weak point of the mast is at the base. That is where a wooden mast usually breaks and that is where the Snark mast got bent (just a little bit, I still used it). Besides, Wicks didn't stock aluminum tubing in long lengths, maybe10' max. So I dreamed a dream to telescope a smaller tip tube into the top of the larger base tube. Hey!, while I was at it I dreamed a dream to make it telescope with a halyard so I could reef the sail by shortening the mast!
So you pull on the halyard rope, it runs over a pulley through a slot in the top of the base section then back down to the bottom of the tip section, and the tip section telescopes out of the base like magic. Did it work? Yes it did. I still have it out in the shop even though the Teal hull was sold off long ago (with a wooden mast). I fear Bolger never bought into it since he loved simplicity. After I sold the Teal I used the magic mast on my Bolger Jinni until I got the real Jinni wooden mast done. But I never used another aluminum spar for several reasons, maybe none of them are good. One thing was the aluminum spars were surprisingly noisy and cold. Not a very good reason I know. But let's look at some number details and I'll admit I am doing this for the first time ever and know not how it is going to turn out.
Let's dream up a 16' long typical mast. Let's say the wood version is 3" square at the base tapering to 1.5" at the tip. That is about .65 cubic feet of wood. Typical wood weighs about 30 pounds per cubic foot so you are looking at about 19 pounds of wood.
Now let's figure an aluminum tube doing the same job. Let's say it is 3" round with a wall thickness of say .060" (I'm guessing but you don't want really thin and really thick is overkill (we will check that soon)). That stuff weighs .7 pounds per foot for a total of 11 pounds. So it is lighter than the wood stick even though it isn't tapered.
A quick peak at Wicks Aircraft Supply website shows 3" x .065 wall tubing 6061 aluminum (good stuff) is over $11 a foot. So you are approaching $200 in mast material and that might not scare some folks.
Here we'll do a quick check of the bending strength of the mast at the base (where the maximum bending loads are in a free standing mast).
Getting out the old handbook (Jeez, I used to do this sort of thing for a good living) I see good wood might have a breaking strength of say 5000 psi although it varies an awful lot. Now, the bending stress of an elastic beam is supposed to be stress=Mc/I where M is the bending moment (we're going to figure it), c is the distance from the "neutral axis" to the "extreme fiber" which in this case will be the outer edge of the wood which is 1.5" from the center of the mast, and I is the "moment of inertia" of the section. The I for a square like this mast is 1/12 width to the fourth power, or 6.76 inches to the fourth for this 3" square mast. Rearranging to solve for the M when the stress at the extreme fiber is 5000 psi gives us a bending moment at failure of M=5000x6.75/1.5 = 22500 inch pounds.
Now for the dream aluminum mast. Same system but here the max allowable stress for 6061-t6 aluminum is 35000 psi (keep an eye on this if you buy aluminum since untreated aluminum might be no stronger than good wood). The moment of inertia of the tube section is I = Pi x Rcubed x thickness which for our dream mast would be 3.14 x .06 x 1.5^3 = .64 inches to the fourth. So the bending moment in this tube when the stress equals the 35000 psi max stress is M = 35000 x .64/1.5 = 15000 inch pounds. That is quite a bit less than the wooden mast but this aluminum mast is also quite a bit lighter. So one might increase the wall, if it could be purchased, to .06 x 22500/15000 = .09 inches to get equal strength, and then the weight would go up about the same ratio to about 16 pounds. Then these two are getting pretty equal in strength and weight. Interesting. Now the aluminum guy will say he can get stronger aluminum and then the wood guy will say he can get stronger wood and I will leave the room as they argue.
Here I would expect the aluminum mast to dominate but we can check that quickly to a certain degree. The stiffness of a beam like a mast is proportional to the its EI. I is the moment of inertia that we already calculated. E is the "modulus of elasticity" of the given material. To measure the E you take a stick of the material, put it in a big testing machine (like the one used to kill Vincent Price in the original "The Fly") and pull the stick apart until failure all the time measuring how much it stretches. For most of the experiment the stretch measured will be proportional to the load applied. So maybe 1000 pounds applied stretches it .001 inch, then 2000 pounds gives .002 stretch, and so forth. Eventually most materials get "soft" just before they break and the stretch gets out of hand, a very useful trait in some structures where you want a bit of warning before they collapse. E is a big number. For example if the above loads were applied to a 10" long stick with 1 sq in of cross section and it stretched .001" it would have an E of 1000/(.001x10) = .1 million psi. The value of E for wood varies a lot from stick to stick but my handbook thinks it is about 2 million for southern pine or fir. The E for aluminim is 10 million and it is interesting that the E for metals does not change from temper to temper as long as the metal has not been stretched beyond its elastic limit (after which it goes softer).
So the EI for the aluminum dream mast with a revised .09" wall would be 10E6 x .96 = 9.6E6.
The EI for the wooden mast would be 2E6 x 6.7 = 13E6. I'm surprised! The wooden mast would be stiffer than the aluminum except for one very important thing. The wooden mast is tapered towards the top and that greatly reduces its stiffness there since, as you recall, the I is a function of the width to the fourth power. So if it tapers from 3" to 1.5" at the top, the top would be just 1/16th as stiff as the bottom. The calculation of the deflection of a tapered beam is not at all simple and here I will leave it to the student.
WHAT'S IT ALL MEAN...
I guess it is a toss up. That is why you see masts both ways.
Remember that Hobie skipper? First thing he did in a capsize was to swim out to the masthead and lash a lifejacket there to keep it from sinking and turtle the boat. (Hey! I've never seen a real turtle swim upside down!) An aluminum mast must be sealed from water to prevent that and so it must be if you use a metal tubing mast. Don't take a chance.
BLOBSTER, SAILBOAT, 16' X 6.5', 750 POUNDS EMPTY
Blobster has a lot of features I like in a boat. Lots of volume for its size, sort of like Micro or Scram Pram. The multichine shape is almost exactly like Scram's but this one does not have a Birdwatcher cabin. It has the more traditional cabin with a raised watertight deck behind. Also it has one feature I would love to have in my personal boats - a step-through bow so that when you beach you can go forward through the cabin and out the front without going into the water or climbing over the bow. The cabin also has a slot top roof.
This shape of boat with multichines has proven good in rough water and with fair speed in spite of its blobular proportions. Blobster has about 600 pounds of water ballast in its belly and should be OK to 90 degrees heel although such depends mostly on weight distribution of the crew, something the designer has little control over. On the other hand, if the crew jumps overboard the boat will be almost assured of righting without their help. Then the problem becomes reboarding. Be prepared!
Sail rig is a large but simple 139 square foot balanced lug on an 18' mast. Mast is stepped off center to allow you to walk upright down the slot top and out the front. Should be rigged in an instant with no one going on deck ever. All very low tech built with common materials but effective.
Blobster uses taped seam construction. Five sheets of 1/4" plywood, eleven sheets of 3/8" plywood and one sheet of 1/2" plywood.
Prototype plans for Blobster are $30.
Some of you may know that in addition to the one buck catalog which now contains 20 "done" boats, I offer another catalog of 20 unbuilt prototypes. The buck catalog has on its last page a list and brief description of the boats currently in the Catalog of Prototypes. That catalog also contains some articles that I wrote for Messing About In Boats and Boatbuilder magazines. The Catalog of Prototypes costs $3. The both together amount to 50 pages for $4, an offer you may have seen in Woodenboat ads. Payment must be in US funds. The banks here won't accept anything else. (I've got a little stash of foreign currency that I can admire but not spend.) I'm way too small for credit cards.
I think David Hahn's Out West Picara is the winner of the Picara race. Shown here on its first sail except there was no wind. Hopefully more later. (Not sure if a polytarp sail is suitable for a boat this heavy.
Here is a Musicbox2 I heard about through the grapevine.
This is Ted Arkey's Jukebox2 down in Sydney. Shown with the "ketchooner" rig, featuring his own polytarp sails, that is shown on the plans. Should have a sailing report soon.
And the Vole in New York is Garth Battista's of www.breakawaybooks.com, printer of my book and Max's book and many other fine sports books. Boat is done, shown here off Cape Cod with mothership Cormorant in background, Garth's girls are one year older. Beautiful job! I think Garth is using a small lug rig for sail, not the sharpie sprit sail shown on the plans, so I will continue to carry the design as a prototype boat.
And a new Down Under Blobster is off cruising under outboard power as it waits for its sailrig.
A view of the Caroline prototype showing a lot of the inside, crew on fore deck. Beautiful color:
And here is another making I think its maider voyage in the Texas 200. (I'm told the Chinese rig will be replaced by the blueprint rig.)
I gotta tell you that on the Caroline bilge panels I made an error in layout and they are about 1" too narrow in places on the prototype plans. I have them corrected but it always pays, even with a proven design, to cut those oversized and check for fit before final cutting.
And a Deansbox seen in Texas:
And in Texas Gordo Barcom has completed the first Laguna and I hope to give a full report soon. Here he blasts along on his first flight:
A Twister goes together in good shape:
AN INDEX OF PAST ISSUES
BACK ISSUES LISTED BY DATE
Mother of All Boat Links
The Boatbuilding Community
Kilburn's Power Skiff
Bruce Builds Roar
Rich builds AF2
JB Builds AF4
JB Builds Sportdory
Hullforms Download (archived copy)
Plyboats Demo Download (archived copy)
Brokeboats (archived copy)
Brian builds Roar2 (archived copy)
Herb builds AF3 (archived copy)
Herb builds RB42 (archived copy)
Barry Builds Toto
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