Jim Michalak's Boat Designs
118 E Randall, Lebanon, IL 62254
A page of boat designs and essays.
(15oct06) This issue will continue the panel stiffness essays. The 1 November issue will rerun the capsize 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.
Panel Stiffness 4
A QUICK LOOK AT FIBERGLASS...I googled around a bit to find the strength/stiffness properties of epoxy and fiberglass. All sources showed big ranges so they must vary quite a bit from maker to maker and in different layups. Like wood, a fiberglass/epoxy layup has different properties in different directions depending much on the directions and densities of the fibers, etc. But in a general way I think epoxy alone has properties about like wood both in stiffness and strength. So adding a layer of epoxy to wood would not improve its strength or stiffness except perhaps to seal and heal cracks in the underlying wood.
A composite of fiberglass/epoxy is a different animal. Glass is quite strong and stiff in certain ways. One website indicated that a generic fiberglass/epoxy sheet has an E (stiffness) about twice that of wood and a strength about four times that of wood. Let's do a thought experiment. We make a test sample that has 1 sq inch cross section of wood encased in fiberglass/epoxy that also totals 1 sq. inch cross section. We put it in the testing machine and pull on it measuring force and stretch. Like this:
As the sample is pulled the wood and glass and epoxy all must stretch (strain) the same amount. But since the glass/epoxy is twice as stiff as the wood, it will take twice as much force to get to the same amount of stretch. So if the sample is loaded to 300 pounds, the wood takes 100 pounds of the total and the glass/epoxy takes 200 pounds. (You might note that this system will only work well if the stiffer material is also stronger in the same ratio. If not it can fail before the wood fails, quickly dumping all its load into the wood which might cause a failure there. By the same token in this example, the wood core should fail before the glass/epoxy and when it does it will dump its load into the glass.)
NOW BACK TO OUR MAN SITTING ON THE 2X4...
Let's say he "plates" the top and bottom of his 2x4 with a layer of glass/epoxy .06" thick. Has he stiffened the beam? Let's now look at the strains and stresses in the plated 2x4:
The strain picture is the same as always. It has to be this simple triangular shape for a beam made of an elastic material to stay in one piece. It is just telling us that the stretch and shrinking of the fibers is proportional to the distance from the middle of the beam.
But the stress picture is now a lot different. As always the outer fibers will take most of the load. But since the outer plates are now also twice as STIFF as the inner 2x4 they will take on a stress that is twice that of the attached wood that has the same stretch, as in the simple tension example. We end up again with an I beam effect.
How to calculate stiffness when we have a beam made of both apples and oranges? I think it can be done by sort of converting the glass/epoxy into "equal wood area" with the "equal" part being based on the relative stiffness of the materials. So if the glass/epoxy is twice as stiff as the wood we can draw a cross section that shows the glass/epoxy portion as twice as wide as it really is and assume it is all wood. Like this:
Now we can quickly calculate the new moment of inertia for the "plated" 2x4 and compare it to the old. It turns out the plated 2x4 has a moment of inertia of 1.50 compared to .98 for the original 2x4, an increase in bending stiffness of 50%. Not bad.
"But," says Mr. Woodlover, "you increased the depth of the beam by 1/8" when you plated it and earlier essays have shown how you get stiffness very quickly by making a beam deeper. What if the beam were simply made of all wood that is3.5" wide and 1-5/8" thick?" Good question. The moment of inertia for that beam would be 1.24. So the glass effect is really more like 20%. In fact an all wood beam that is 1.75" thick is stiffer than the glass plated 2x4. You certainly could glue a 1/4" x 3.5" stick to the top of that 2x4 much quicker and cheaper than plating top and bottom with glass/epoxy.
POP QUIZ!
In real life a fellow glasses a face of his plywood mostly to protect it from the elements. How much stiffness is also added? I hear that question all the time and if you are one who asked it you know I usually answer something like "It's not as effective as you might think." But to tell you the truth I never really ran the numbers on the problem. So lets do that.
A fellow makes a boat with a 1/4" thick plywood bottom and covers one side with fiberglass set in epoxy. How much has he increased the stiffness? A section of his sample looks like this:
. 
I'm not sure how thick the glass plating really is but I measured a bit of fiberglass which I think is 10 ounce cloth and it is .010 thick with no epoxy. I would assume the epoxy would add thickness so I am using a .020" thickness in the example. I am going to calculate the moment of inertia of a typical 1" wide piece of this. As before when we do the calculation for an all wood equal section we make the glass plate twice as wide as the wood to get this:
It is not symmetric and I had to do more involved calculations to find the neutral axis and then the moment of inertia around that axis. But the final answer is that where the 1/4" wood has a moment of inertia of .00130 per inch of width, the plated sample now has a moment of inertia of .00164, and increase of 26%. More than I would have guessed.
NEXT TIME...we capsize a boat).
Imresboat
IMRESBOAT, DOUBLE PADDLE CANOE, 15.5' X 26", 50 POUNDS EMPTY
Imresboat is a slimmed up version of Larsboat which was designed for Lars Hasselgren to replace a Folboat that had finally met its end. Lars wanted capacity for two, plus decking, as with his old Folboat. But Imre wanted something more slender and lower in the same length. The result is 26" wide instead of 30" wide. And the result of that is that Imresboat is a single seater because the capacity has been reduced with the width. Imre wanted it even narrower but I hoped to stay within the tippyness bounds of most folks. Not sure yet how narrow something like this can get before the normal guy can't sit upright in it. I do know that you apparently can get used to about anything in that respect as experienced kayakers do.
The deck opening is smaller to sit just one man but I think it is longer than the usual kayak because us older folks have trouble fitting our legs through the hole in a performance kayak. Something like putting on a pair of pants without standing up.
(Here is a photo of Paul Moffitt's Larsboat with two aboard.)
Another change I made with Imresboat is to go to thinner plywood to save weight. In this case I suggest 1/8" ply for everything except the bottom which is still 1/4". Not sure where the 1/8" plywood comes from but it used to be what they skinned doors with. Be sure to boil a sample and hope it is waterproof. Anyway, Chuck Leinweber brought a Toto to our messabout with 1/8" ply sides and 1/4" bottom and it struck me as being fine for stiffness and strength. Like Toto and Larsboat, Imresboat is taped seam construction. I'm guessing at the 50 pound weight but for example Larsboats built of 1/4" plywood have weighed between 60 and 80 pounds depending on who built them.
Taped seam construction from four sheets of 1/8" plywood and one sheet of 1/4" plywood.
Imresboat plans are $20 until one is built and tested.
Prototype News
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.
This long and lean project is a 19' version of Toon2. Shown here on its first sail in very light winds. We will wait a bit longer to get a sailing report in stronger winds.
Here is a Musicbox2 I heard about through the grapevine.
We have a Philsboat going together in California. You can see the interior room in this 15' boat:
And here is another Philsboat in northern Illinois:
And here we have all the parts of an LHF17 rowingboat. The boat is being assembled now and the builder reports it to be at the limit of the taped seam method because of the wicked twist in the garboard. That would be true with any building method but perhaps the real Herreshoff builders used steamed planks.
AN INDEX OF PAST ISSUES
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)
