Jim Michalak's Boat Designs
118 E Randall, Lebanon, IL 62254
A page of boat designs and essays.
(1Apr10)This issue will look at ideas to strengthen masts. The 15 April issue will rerun the sharpie sail rig 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
He is on a never ending search for ice cream and internet access. When he finds both at the same time he posts photos like:
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!
This is Shannon Boyd in Texas with his first project, a beautiful Toto.
Beefing A Mast
I GOT AN INTERESTING QUESTION....
... the other day. He asked," Can I strengthen a hollow mast with a steel bar in the center hole?" "Too tough a question to answer," says I. But maybe not.
As you probably know, to me the best way easily to beef up anything like a mast is to simply make the wooden mast larger in cross section. The reason is that the mast will pick up strength to the third power of the diameter or the width so, for example, increasing a round mast from 3" to 3.5" should increase strength by almost 60 percent! And stiffness will almost double!
But maybe that is not the problem often. Maybe the boat is complete and maybe the skipper is getting worried about his hollow mast strength in rough conditions. Changing the mast diameter will be a big job, a new mast and new mast partner at least. What about putting a steel bar in that hollow mast? At least in the base, run higher than the mast partner which I think is where it is most likely to break.
WHY BREAK AT THE MAST PARTNER????
This is for free standing unstayed mast. That is to say there are no guy wires supporting the mast. Let's take as an example a lug sail which has just one line, the halyard, attaching the whole rig to the top of the mast, more or less. The sail usually has a boom too but that is down low near deck level and in fact may not be attached at all to the mast. Anyway, the halyard pulls down and to the side. Usually the down pull can be a lot more than the side pull and that would depend on the geometry at the instant and how tight you string the rig. Usually the tighter the better since tight normally means less sail twist and more efficiency to windward. But it is the side pull that moves the boat.
Might as well say right now that the down pull ends up as compression in the mast and it can be considerable to the point of destruction if it is overdone. I know my old Hobie had this one sort of figured out because its rope halyard had an ending of steel cable with a pressed fitting say about 6" from the end where it joined to the sail. So the correct routine was to haul up the sail with the rope halyard and worry that fitting into a slot on the top of the mast which in effect meant the halyard from then on carried no force, instead the sail's tip was in effect secured to the masthead. So no halyard compression in the mast, although it seemed to me that the Hobie 16's mast was a section of bridge girder or something. Stout. Closer to home, my Birdwatcher has something similar in that Bolger designed the original version with no halyard at all! The sail is tied to the mast head permanently. Reefing is done with the ancient sharpie method of brailing the sail to the mast. Not too elegant perhaps in use but the masthead on the 24' Birdwatcher is tiny, maybe 1" square.
And on stayed masts the compression is added to by the forces developed in the stays. It can become enormous to the point that I think stayed masts usually fail by buckling in compression. It happens in an instant. Everything falls down around you with nothing remaining above the deck.
Anyway, with the side pull at the top of the mast the mast sort of acts like a big lever. The bending forces in the mast are zilch at the tip and then gradually build as you work to the mast partner. At the mast partner that side force meets its first reaction and that puts and end to the buildup of bending moment. So I would usually expect the location of the mast partner to be the location of the maximum bending in the mast. Add to that some compression due to the halyard tension and then the direct compression of partner against the mast and you have a somewhat complex picture. That's where it is most likely to break. I've had masts break elsewhere but only due to construction sins that I took a chance with.
NOW, LET US HANG SOME NUMBERS FOR EXAMPLE.....
Let's say we have 50 pounds of sail side force at the mast tip being dumped there by the halyard which gets it from the yard which is tied to the sail, etc. Let's say we have 10' of mast from tip to mast partner. Then we would have 10 x 50 = 500 ft pounds of bending at the mast partner. Let's call that 500x12=6000 inch pounds.
Let's say we have a 3" round wooden mast to start with. Recall that the maximum bending stress in a round section will be Mc/I where M is the bending moment (in inch pounds), c is the distance from the center of the section to the outer fiber (1.5" in this example), and I is the "moment of inertia" of the round section. A peak at the handbook says for a round section the I = .78 x r^4 or .78 x 1.5x1.5x1.5x1.5 = 3.9 inches to the fourth. With a predicted bending moment of 6000 inch pounds it works out as a maximum stress of s=6000 x 1.5 / 3.9 =2300 psi. Good wood is stronger than that, maybe up to 10,000 psi if you are rich enough for perfect Sitka spruce. But Lowes pine maybe, maybe not. No knots or cracks near the mast partner please.
NOW, LET US ADD A METAL DOUBLER...
Ok, maybe the skipper is getting a little worried. He has a bunch of fat people on board for the first time and has stability he has never imagined before. The wind is picking up and he is in a position to "blow the sticks off" as they used to say. When he gets home he slides an aluminum tube over say the bottom 4' of the mast such that it extends from the step to a foot or two above the mast partner. It is a sliding fit, not glued or fastened so that the two pieces are locked together. But the fit is good enough that the wood and tube must bend together. They will share the load.
IT BECOMES A QUESTION OF STIFFNESS...
...and stiffness in bending is determined by our old friends E and I. Recall that E is the "modulus of elasticity" of the material. Think of it as a spring rate. To measure E you might take a 10" bar of the material with say a 1" cross section, put it in a universal testing machine (like the one Vincent Price used to commit suicide in "The Fly"), pull on the ends of the bar while you measure the stretch of the bar. Almost all structural materials are "elastic" in that they stretch and rebound like a rubber band as long as you don't overdo it. So if you put a 10" aluminum bar as described into the machine and stretched it with 1000 pounds it would stretch a thousandth of an inch and it would have an E of 1000x10/.001=10 million. And you would have the right answer! All aluminums have an E of 10 million more or less as long as you don't stretch them beyond their "elastic limit" (at which point they don't rebound but instead stay stretched for ever).
So now you put a stick of wood in the machine and repeat the test. My handbook you would expect it to stretch about about .007 thousandths. Because the handbook says the E for wood is about 1.5 million. But wood is really fuzzy stuff and it will vary a lot more than aluminum. But in general wood is say 1/7th as stiff as aluminum in such a test.
Next, we will calculate the EI 's of the two pieces. We already know the wood has an I of 3.9 and an E of 1.5 million, so EI is 5.8 million. If you knew the real applied load you could calculate the actual bend of the mast but if your mast is tapered then the I is changing constantly and it ain't easy and is best left to the student.
We didn't say how thick the aluminum tube was but let's say it is .09" thick and 3" in diameter. Another quick peek at the old handbook and we find I for a tube is 3.14 x t x r^3 so for this tube is would be 3.14 x .09 x 1.5x1.5x1.5= .95 inches to the fourth. So EI for the aluminum tube here would be .95 x 10 million = 9.5 million.
The tube is about twice as stiff as the wood. If they bend to the same curve, and they must if they fit together, then the tube will absorb about twice as much bending as the wood. So if the bending was 6000 in pounds, then I would expect the wood part to take about 2000 inch pounds of that and the aluminum tube about 4000 inch pounds. So the stress in the wood is greatly reduced.
LET US NOT CELEBRATE YET....
You have to check the stress in the tube next. Back to Mc/I. For the tube we already know the numbers. Stress here is = 4000x1.5/.95 = 6300 psi. OK, celebrate. Almost any structural aluminum can handle that.
ONE MORE WORRY...
What happens where the tube ends leaving the wood to soldier on alone? Same old problem except that the bending moment higher up the mast will be less. For example if the tube extends 2' above the partner, then the bending moment there now has an 8' arm instead of a 10', so the bending there is 8 x 50 x 12 = 4800 inch pounds instead of 6000. But it is still a worry and these transitions are always trouble makers. There was a saying back at the missile factory that "structures always fail at the joints." Yes.
NOW ON TO PLAN B....
...Let's say the above mast was made with a square cross section, say 3" square with a 1" square hole down the middle. Can we reinforce it with a 1" square steel bar placed in the hollow center.
Same procedure as above. First the I of the new mast. Another quick peek at the handbook and we find the I of a square is (a^4)/12. So for this square mast we start by taking I = 3x3x3x3/12 =6.8 inches fourth. But the center 1" is hollow so we must subtract that where Icenter = 1x1x1x1/12 = .08 inches fourth. Total is 6.8-.08=6.7 inches fourth and note that almost no I was lost due to the hollowing of the mast (and not much weight was saved for that matter either). So EI for the wood mast at the partner would be 6.7 x 1.5 million =10 million. Note that the square wood section is much stiffer than the round of equal width.
Now for the EI of the steel bar placed in the hollow center. We already know the I is a tiny .08 inches fourth. But E for steel is about 30 million, 20 times stiffer than wood. So the steel filler has EI of 2.4 million.
The steel EI is only about one fourth that of the basic wooden mast so here I would expect the wood section to take say 4800 inch pounds of the total of 6000 inch pounds which the steel's share is about 1200 inch pounds. Nothing like the benefit of the outer aluminum tube and that is to be expected because in bending it is the outer fibers of the thing that are doing all the work. Lastly a quick check of the bending stress in the steel would be something like stress = 1200 x .5 / .08 =7500 psi and there is no problem. The wood will fail long before the steel. But I suppose you could see this system as giving the wood a 20 percent reduction in stress, or the mast is 20 percent stronger.
A LAST THOUGHT...
...I haven't gotten into the idea of adding fiberglass or carbon cloth reinforcement to the outer surface of the mast. For that to have any meaning the exact layup of the fibers has to be known and even then I'm not sure if I'm smart enough to figure it. Best left to the student. If the fibers are stiff and mostly run the length of the mast then they will be very effective in taking the bending. If they are very stiff and thin, say a thin coating of carbon fibers running the length of the mast, they might be so effective at taking the bending that they will take essentially all of it right up until they snap, at which point you are back again to just a wooden mast. Nothing simple about composite stress analysis I think.
FROLIC2, CUDDY MULTI SKIFF, 20' X 5', 400 POUNDS EMPTY
The photos show the prototype Frolic2 built by Larry Martin of Coos Bay, Oregon. Larry built the boat quite quickly this past winter including sewing the sail to the instructions given in the plans. He reported sailing it for the first time on a ripping day with an occasional 2' wave. I always advise testing a new boat in mild weather, especially a new design, but Larry got away with it. Looking at his photos, the neat work, and simple efficient rigging suggests to me that Larry has been sailing small boats a long time.
Frolic2 has a small cabin, probably only for one to sleep in because the multichines that make the boat good in rough water also rob you of floor space. To say it another way, the nice big floor space of a flat bottomed sharpie is what pounds in rough water and makes you uncomfortable. But Frolic2 has a 6' long cockpit so someone could sleep there too. There is bench seating. The cabin top has a slot top down the center and you can stroll right through the cabin standing upright in good weather and out the front bulkhead to the beach. The mast is offset to one side so you will need not have to step around it. Phil Bolger showed us how to do this about 15 years ago and it works. But Larry went conventional with his boat, mounting the mast on centerline and decking in the front of the cabin. On a slot top cabin you use a simple snap on tarp to cover the slot in rain or cold or bugs.
Frolic2 was designed for rough water, long and lean, especially in the bow, and with multiple chines. She's really a takeoff of my Toto canoe in shape. Larry omitted the motor well you see in the lines, and the oarlocks too (The wind must blow just right all the time in Oregon?) but I intended this to be a multi skiff sort of boat with rowing and motoring abilities. You can't row a boat of this size in any wind or waves but in a calm you can travel far if you have patience. I didn't fool around with a gadget motor mount - I put the motor well right in the middle and offset the rudder instead of the other way around. This worked out very well on the high powered Petesboat. We'll see how it goes on a narrow boat because the second prototype is getting the blueprint well as you see in the photo below of the Colorado Frolic2 still being built. You need little power on a boat like this, 2 or 3 hp is more than enough.
The lug rig is for quick easy stowing, rowing, and towing. (The blueprint sail is actually the same size and shape as that of a Bolger Windsprint, a boat which weighs maybe a third as much as Frolic2 and is much narrower. But I think the Windsprint might be over sparred for its size and weight.) Larry reports the rig is about right for the boat, sailing fine with a reef in and three adults on a windy day. The lug sail can be closer winded reefed than when full, perhaps because the sail is then shorter and the yard better controlled (less sail twist). For that matter a sharpie sprit sail the same size as the lug might be smarter in rough water conditions if you can live with the long mast. Switching rigs won't be hard. The mast can be relocated almost anywhere in the slot top without altering the hull to any degree. You just need extra partner and step fittings.
Update, 2006. Jeff Blunk's Colorado Frolic2 eventually found its way to Illinois and then to our Rend Lake Messabout in the hands of Richard Harris. We had a chance to try it out and I was quite pleased. It was fast and powerful. At one point with three men on board, with Max at the tiller, I went forward to tweak the sail which took a couple of minutes with me staring up at the sail. That done I looked back and saw that we were really rolling along.
And Gary Blankenship's Frolic2 completed the Everglades 300 mile challange with him reporting sailing for hours at 7 knots or more. Last count he completed three of the challenges with a 4th overall in 2007. Here is Gary's Frolic2:
Construction is with taped seams from eleven sheets of 1/4" plywood and two sheets of 1/2" plywood.
Plans for Frolic2 are $35.
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 old outboard book and many other fine sports books. Beautiful job! 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. But he has used it extensively on his Bahamas trip towed behind his Cormorant. Sort of like having a compact car towed behind an RV.
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:
A Twister goes together in good shape. He moved the cabin bulkhead aft for more cabin, less cockpit:
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Plyboats Demo Download (archived copy)
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Brian builds Roar2 (archived copy)
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Herb builds RB42 (archived copy)
Barry Builds Toto
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