Jim Michalak's Boat Designs
1024 Merrill St, Lebanon, IL 62254
A page of boat designs and essays.
(1 October 2020) We calculate displacement volume. The 15 October issue will discuss choosing a design.
NO MORE ORDERS UNTIL NOVEMBER, MAYBE...
...an update from my blueprint paper supplier, Freedom Paper in Houston, is that they expect to be ready to ship in 4 to 5 weeks. I have used Freedom Paper for about ten years now with no troubles at all, always very quick shipping. Not sure why they got caught short this time but I don't want to give up on them.
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.
FIGURING DISPLACEMENT
THE VERY BASICS...
One of the real basic errors in boat design is to not match the capacity of the hull to the weight it needs to carry. It's actually pretty easily done. Let's take an example: a fellow wants a 12' skiff to handle two adults. He figures the empty boat will weigh 120 pounds and the adults together at 350 pounds. So he has to float a total weight of 470 pounds.
One thing is for sure: no matter what the shape of the hull, it will sink into the water until it pushes aside, or displaces, 470 pounds of water. (They way I heard it was that ancient Greek mathematician Archimedes figured out the basics while sitting in the bath tub.) Fresh water weighs about 62 pounds per cubic foot and salt water usually about 65 pounds per cubic foot. So the volume of fresh water displaced by the 470 pound boat is 470/62 = 7.6 cubic feet. That was easy!
Next the problem becomes one of shaping the underwater part of the hull such that it displaces 7.6 cubic feet with good flow lines.
METHOD 1: THE PRISMATIC COEFFICIENT...
We talked about the prismatic coefficent in the last issue. Remember that we construct a prism with the same length as the waterline length and the same cross section as that of the boat's maximum beam. The hull will fit neatly into the prism. Divide the volume of the underwater hull by the volume of that prism and you have the prismatic coefficient.
What is interesting to me is that the prismatic coefficient doesn't vary much from one displacement hull to another, as far as normal small boats are concerned. Jewelbox, with a flat bottom and squared off ends, has a prismatic coefficient of about .60
And Toto, with very pointy ends and multichine cross section, has a prismatic coefficient of about .50
So I figure if you were to assume a halfway normal displacement hull has a prismatic coefficient of .55 then you would always be within 10% which is actually pretty good.
So without drawing a line I might say that the 470 pound weight will need a "prism" with a volume of 7.6/.55 = 13.8 cubic feet. The prism could have any combination of cross section and length that will have that 13.8 cubic feet total volume.
Remember that the prism has the same length as the hull's waterline length and cross section as the boat's maximum cross section. Remember we want a 12' boat, but the waterline will be shorter if we want some rake to the ends (for looks) and maybe enough rocker to make sure the stem and stern don't drag the water (for low drag). We might guess that the waterline length will really be about 10'. Then the maximum cross section would have to have 13.8/10 = 1.38 square feet of area below the waterline. If we wanted a flat bottom 3' wide (wide enough to have sailing stability and narrow enough for reasonable rowing) we would have a draft of approximately 1.38/3 = .46' or 5.5".
If we wanted a different cross section than the simple box shape I'm assuming here, we would have to tinker a bit. Draw up a cross section with the 1.38 square foot cross section and see if we like it. Actually in this case I'm going to flare the sides out to 4' at the top of the wale at the cross section. But that won't affect the area under the waterline much. We might sketch out something like this:
METHOD 2: THE CURVE OF AREAS...
This method of the "curve of areas" is an accurate and flexible way of determining the volume of almost any oddly shaped thing. It's actually pretty easy to do but first you need a drawing of the proposed boat (unlike the prismatic coefficient which just needs a general idea of the boat).
Let's take our proposed boat and draw a line on it that represents 5.5" draft, our guess at what is needed to float 470 pounds. Every now and then along the length of the hull we take cross sectional cuts of the hull and measure the area at each cut below the 5.5" waterline. The cuts can be taken anywhere but there must be enough to get a good definition fo the hull. Actually for a normal boat about five cuts will do. The more you take the more accurate your work but five will get you really close. Our example boat might be like this:
The dimensions of the underwater sections are measured right off the paper drawings and the area of each is calculated like this:
Next we graph those underwater areas spacing them the same distance apart as they are on the real hull to get the "curve of area" like this:
The area enclosed under the curve of areas is a volume and is indeed the volume of the underwater hull - it's what we are looking for. In the old days you would measure the areas with a planimeter, a clever and expensive gadget. But you can do well by just breaking down the curve into triangles and trapezoids, figuring the area in each trapezoid, and adding them all up. In the example, second trapezoid from the left is figured by 1/2(175+209)X12 = 2304. Adding the sections together gives us a total of 12500 at 5.5" draft. That equates to 450 pounds of water. So the first guess was off by 20 pounds or about 4%. By the way, the drawing showed a waterline length of 9'5" instead of the first guess 10'.
As mentioned before this method can be used to figure the volume of almost anything. Let's say we wanted to find the weight of a mast. Say the mast is 24' long, 3.5" in diameter at the base and 1.5" in diameter at the top, with a curved taper between. The figuring of the volume might look like this:
1920 cubic inches equals 1.1 cubic feet. Wood usually weighs 25 to 35 pounds per cubic foot. At 30 pounds per cubic foot the mast would weigh 33 pounds.
You can also use the area of curves to find the CG of the object, too. For real simplicity I've seen designers in the aircraft industry draw the curve on cardboard, cut it out and balance the thing on a knife edge. The item's CG will lie on that balance line.
THE COMPUTER METHOD...
Finally you can use your computer to figure it all out. I'm sure all of you know that getting the computer to spit out the right answer might take a lot longer than doing it by hand. The program I used to get the drawings above was Hullform6s which can be downloaded from the links shown at the end of this page. What were the answers that Hullform6s predicts for the 12' skiff with 470 pounds displacement? 5.5 " draft, waterline length of 9'4", and prismatic coefficient of .55! So the first guesses were quite good. But the area of curves method was also excellent, within 4%. It showed the same waterline as Hullforms. Perhaps with more data points to flesh out the curve the hand method would get the same answer as hullforms but I wouldn't bother. You will never guess the weight of the finished boat within 4%.
Larsboat
LARSBOAT, DOUBLE PADDLE CANOE, 15.5' X 30", 65 POUNDS EMPTY
Larsboat was built by Lars Hasselgren to replace a Folboat that had finally met its end. Lars wanted capacity for two, plus decking, as with his old boat.
I took Toto and lengthened it with a 30" plug in the middle to gain capacity. But lengthening a hull with a straight plug like this usually improves a boat in almost every way and Larsboat should be faster than Toto in good conditions. In this case the plug meant I didn't have to refigure the shape of the twisted bow panels as I would if I'd lengthened Toto with an overall stretch. (I can figure twisted panels pretty reliably now, but not back when Toto and Larsboat were drawn.)
The decking was quite simple because even the original Toto could take a forward deck of flat sheets with a center peak. I should add that I feel the decking is very optional. This prototype weighs 61 pounds and deleting the deck might cut another 10 pounds or so. The undecked boat also would have a better cartopping shape. I'd keep the stern chamber. It will ease your mind about taking a big wave over the stern.
This would be a preferred project for someonw who intends to do a lot of cruising and camping. In the Toto camping I've done the sleeping room has been OK, but the storage is limited. Larsboat would be better both because of increased capacity and because there is dry storage under the bow deck.
The basic hull is taped seam construction needing four sheets of 1/4" plywood for the decked version and three sheets for the undecked version. No jigs or lofting required. Plans are two blueprints with keyed instructions for $20.
The photo above is of Bob Smithson's Larsboat. He customized the decking a bit. I think he also built the boat of 1/8" ply to save weight. I've forgotten what his boat weighed but he did say it was sufficiently rigid for him.
Bob Hoyle built this one without a deck down in Florida:
Paul Moffitt built this one. You can see this is a much better two person boat than the shorter Toto:
And remember Garth Battista's vertical Larsboat?
And the old outboard motor guru Max Wawrzniak often goes for a paddle in his Larsboat:
Larsboat plans are $20.
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.
We have a Picara finished by Ken Giles, past Mayfly16 master, and into its trials. The hull was built by Vincent Lavender in Massachusetts. There have been other Picaras finished in the past but I never got a sailing report for them...
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 Deansbox seen in Texas:
Another prototype Twister is well along:
A brave soul has started a Robbsboat. He has a builder's blog at http://tomsrobbsboat.blogspot.com. (OOPS! He found a mistake in the side bevels of bulkhead5, says 20 degrees but should be 10 degrees.) This boat has been sailed and is being tested. He has found the sail area a bit much for his area and is putting in serious reef points.
AN INDEX OF PAST ISSUES
THE WAY BACK ISSUES RETURN!
MANY THANKS TO CANADIAN READER GAETAN JETTE WHO NOT ONLY SAVED THEM FROM THE 1997 BEGINNING BUT ALSO PUT TOGETHER AN EXCELLENT INDEX PAGE TO SORT THEM OUT....
THE WAY BACK ISSUES
15oct19, Herb's OliveOyl 2, Jonsboat
1nov19, Herb's OliveOyl 3, Shanteuse
15nov19, Herb's OliveOyl 4, Piccup
1dec19, Taped Seams, Ladybug
15dec19, Plywood Butt Joints, Sportdory
1jan20, Sail Area Math, Normsboat
15jan20, Trailering, Robote
1feb20, Bulkhead Bevels, Toto
15feb20, Cartopping, IMB
1mar20, Small Boat Rudders, AF4Breve
15mar20, Rudder Sink Weights, Scram Pram
1apr20, Two Totos, River Runner
15apr20, Water Ballast, Mayfly16
1may20, Water Ballast Details, Blobster
15may20, Mast Tabernacles, Laguna
1jun20, Underwater Boards, QT Skiff
15jun20, Capsize Lessons, Mixer
1jul20, Scarfing Lumber, Vireo14
15jul20, Lugsail Rigging, Vamp
1aug20, Prop Slip, Oracle
15aug20, Sharpie Sail Rigging, Cormorant
Table of Contents
