Jim Michalak's Boat Designs
118 E Randall, Lebanon, IL 62254
A page of boat designs and essays.
(15feb03) In this issue we'll rerun an old article about estimating the weight of a new design. Next issue will rerun the old article about figuring out how large your boat needs to be to float that weight.
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...
... 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.
Annette and Elizabeth launch Dorado in New South Wales, Australia.
GUESSING AT WEIGHTHISTORY...
I worked in a missile company for about 13 years and met weight engineers all the time. Looking back at the experience I might say there were three personality types of engineers. Type A was interested in power, pay, promotion and politics and not so much in hardware. Management was mostly made of Type A's. Type B guys plugged away at their work with the idea of finishing it by normal quitting time and going home - or at least looking busy until quitting time. And Type C's were the new guys, right out of college and still full of enthusiasm and patriotism. It usually took about five years for a C to evolve into an A or B. Advance design, where the real weight guessing was done, was usually full of Type A's managing some Type C's. But when a program starting getting hot some Type B's might be brought in to do some reliable work. Then managers ran the risk of the Type B reminding everyone that the Advanced Design Group had never won a program or made any actual flying hardware in the 20 years of the group's existance. (The missiles being assembled out in the shop were designed by other companies.) And the Type B's might point out that the calculations, which had been done by the Type A's and C's, upon which the program's future was based, were hopeless.
ENTER MISSILE BALONEY...
And it came to pass that I, still a young type C at the time, was teamed up with Harry, a type B weights engineer, on an "advanced cruise missile" program. I was a strength engineer at the time but was quite curious of how the weights guys did their work. Given a half dozen concept missiles at a Monday meeting, Harry would produce detailed weights for each say by Thursday. I was wowwed!
One day Harry showed me how it was really done. He had secreted in an obscure book a graph he had made that plotted the weight vs the volume of existing missiles, the source of the info being Jane's All The World's Whatever. The data points formed a straight line on his graph. That meant that in spite of variations in size and shape, they all had the same density! So Harry's work was simply to figure the volume of the proposed missile and multiply that by that universal density and, Voila!, you have an accurate weight of the proposed missile. Doing that did not take four days, of course. It might be done in a few minutes in the privacy of the potty and the other days spent looking busy.
The reason the missile chart worked so well, Harry said, was that missiles are always jammed totally full of stuff. There was no empty volume to upset the average density calculation. The average density might change a bit with history as new technology made for denser packaging, for example when the guys in the shop found they could install oversized wire bundles into the fuselage by pounding them in with hammers.
And Harry assured me the current average density of missile was the same as that of baloney, similar to that of water. He called it "missile baloney" and you could get the correct weight of a new proposed design by imagining it to be a giant baloney sausage and that was all there was to it. So if your missile was say 20" in diameter and 20' long it would have about 43 cubic feet of missile and it would weigh about 2600 pounds fueled up and ready to go. Of couse you would not report to the Type A's that the weight was "about 2600 pounds". You would say something like 2624.6 pounds or 2587.2 pounds. If you knew the head Type A had a pet concept you would make that one be the lightest.
Harry assured me that any further work would simply be to find out how that weight was distributed among the missile's elements because the baloney had "local lumpiness". But the total could never vary far from the first guess.
I was of course sworn to secrecy about the method. But one day a young Type C engineer tried to impress the Type A's by writing up a memo of the one line missile weight graph and presenting it as his own. They fired him! No kidding.
Harry went to work on his "Grand Unified Baloney Theory Of The Universe".
ENTER BOAT BALONEY...
When I started drawing boats about 10 years ago I saw a need to estimate the weight of new designs using typical plywood construction and wondered if there might be such a thing as "boat baloney"? I came up with the following chart:
I'm afraid the method does not work so well with boats because boats aren't usually jammed full of stuff - there is lots of empty volume. And because boat hulls are built in entirely different ways by different people, even when they use the same set of plans. To illustrate that I'll recall the time I took my old Gloucester Gull dory, the Payson/Bolger boat, out rowing with Dan Knodler who also has a version of the same design. Mine was made of 1/4" plywood with taped seams and totally stripped out. It weighed 65 pounds. Dan's was built by a pro from 1/2" plywood with hardwood seats and gingerbread and weighed about 150 pounds. I think Payson said one built right to the plans as he made them weighed about 100 pounds.
Looking at the above chart it seems that empty hull baloney has a density of about 1.5 pounds per cubic foot if lightly constructed to about 3 pounds per cubic foot if heavily constructed. Remember this is just for empty plywood hull structural weight. I would say "lightly constructed" would mean a simple undecked boat like a canoe or rowboat that borders on flimsy. "Heavily constructed" would mean a more complex decked hull with some beef to it. Actually you could make a case that heavy could be heavier than I am showing.
Let's use this issue's feature boat, the Dorado, as an example of using the chart. This boat is 5' wide, 19' long, with a cuddy cabin 3' high. It's total volume is about 250 cubic feet. If we use an average boat baloney denisity of 2.25 pounds per cubic foot as suggested by the chart we would expect the empty hull to weigh about 560 pounds. Not much to figuring that!
The above just estimates the structural weight of the hull, not the ballast, sail rig or motor, etc...
ENTER THE CREW WITH ITS JUNK...
Usually one might figure the weight of the average adult as 175 pounds. But I know some of you weigh twice that and some weigh half that. It's very important with small boats in particular to get a good estimate of the proposed crew weight and be honest with yourself about how many boating friends you have and how often they will really be boating with you.
As for personal gear, food, and water, Dave Gerr recommends about 20 pounds per person per day. Sounds reasonable. For living aboard he says 400 to 1000 pounds of junk per person.
Small outboard motors with a bit of fuel seem to weigh about 50 pounds although it can vary quite a bit. Usually an available motor is easy to weigh on a bathroom scale. (There is a commercial website at www.smalloutboards.com where the man often reports the weights of the used motors he has for sale.)
If you want ballast, the old rule for ballast is about 50% of the empty hull weight although its placement is very important. Nowadays one can check the ballast requirements a lot easier than in the olden days because computers are great for this sort of figuring.
Sail rigs don't actually weigh very much. As we saw last issue a 24' mast about the size of a Micro or Birdwatcher mast weighs about 30 pounds. And that is about the most mast a man can step by himself without special gear. The sticks used for booms are easily estimated and modern sailcloth is really quite light, a 100 square foot sail weighing maybe 4 pounds. Leeboards and centerboards can be quite large and heavy.
Add up all the bits and you will have an estimate of the total floating weight of the boat. Then you can get serious about making sure your proposed hull will float it properly by using the methods shown in the last issue.
ENTER THE PLYWOOD PANEL LAYOUT...
And it came to pass that Payson spake unto Bolger saying, "Go forth and bring unto my people drawings of plywood sheets with the boat parts laid upon that they might be knowing of how much to buy and be not wasteful."
Payson and Bolger didn't invent the plywood panel layout but they popularized it and made it a key element of instant boats. All the plywood parts of the boat are laid out in scale on standard 4' x 8' sheets of plywood on a drawing. The layout is supposed to be a guide to the economical use of the plywood. It doesn't adapt too well to the traditional cut-to-fit style of building. I see in a recent Boatbuilder magazine where Thomas Firth Jones built a Payson/Bolger catboat by traditional methods instead of the taped seam instant method shown on the drawings. He kidded Bolger and company about spending the time to lay out all the parts. But the layout is an important material and labor saver to someone building instant style.
Once you lay out all the parts you can determine how much plywood you will be using. And if you know how much plywood you are using, you know how much the pile of plywood weighs. And since an instant boat is mostly plywood, you get an excellent idea of how much the hull and plywood parts will weigh. Then you can update your idea about the density of boat baloney.
Here is the plywood panel layout for my design Mixer. All the hull parts including the leeboard, and rudder parts are there on four sheets of 1/4" plywood:
Wood density varies quite a bit. But a 1/4" sheet of plywood usually weighs about 25 pounds, a 3/8" sheet about 37 pounds, and a 1/2' sheet about 50 pounds.
"But," you say, "there is more wood in the boat than just the plywood. There is the framing wood too." That is true. But then again not all the plywood gets used. Here is a general rule that I use with plywood boats with nail and glue joints that require stick framing and conventional chine logs - allow about 30 pounds per sheet of 1/4" plywood, 45 pounds per sheet of 3/8' plywood, 60 pounds per sheet of 1/2" plywood, etc. Taped seamed boats are lighter than framed boats and can indeed be estimated fairly well using just the weights of the plywood sheets. I think about the only warning I might give when using this method of estimating the weight of a new design is that often the ply panel layout also contains temporary forms that won't be permanently in the hull. Those areas should be excluded from the total. So when I look at the plywood panel layout of Mixer shown above, I think instantly, "No more than 100 pounds in four sheets of 1/4" plywood and taped seams. In fact with the sail rig and hatches stripped off for cartopping it might be around 80 pounds." I don't remember if David Boston who built the first one weighed her.
By the way encapsuating a boat like Mixer will increase its weight. The numbers I mention above would include minimal paint but not epoxy coating. So you might add maybe 10 pounds per gallon used.
Now let's use this issue's featured boat Dorado as an example of getting a close weight estimate of hull weight by using the ply panel layout. When I lay out all of Dorado's parts I find it needs about eight sheets of 3/8" plywood and two sheets of 1/2" plywood. So a guess at the hull weight, more refined than the baloney weight, would be eight times 37 pounds for the 3/8" sheets and two time 50 for the 1/2" sheets. That totals 400 pounds. When I wrote up the catalog blurb below for Dorado I said 500 pounds thinking of her framing, a glass coating and some motorboat parts that the sailers won't have. But I could be wrong. The original 560 pounds estimate using the baloney theory seems as good as anything.
... Now that we have guessed the weight of the boat, let's figure how big it has to be to float that weight.
DORADO, POWER CUDDY SKIFF, 18' X 5.5', 650 POUNDS EMPTY
Dorado is a Southern Hemisphere constellation and also this boat designed for Ashley Cook in New South Wales. The idea behind it was for a rough water power boat with a sleepable cuddy cabin. We started with my Frolic2 sailboat (which itself evolved from the Toto canoe) and straightened out the stern lines to adapt it for planing power. After mocking it up Ashley said he and family needed another 3" of headroom so Dorado B was born and built. The plans show it both ways. I should add that even though Dorado started out as a modified sailing hull, it probably would be a terrible sailer just as Frolic2 would be a terrible power boat on plane. The two worlds just don't mix and the idea of "just hoisting a simple sail" on Dorado is bound to be a sure loser both because it will sail poorly and because you will lose the simplicity of a straight power boat. Not that the idea isn't worth more study. It would take a special person to do all the tweaking involved with making the two-way boat work well.
The shape works! Ashley has been running DoradoB into the rough stuff. In the above photo he is off the east coast of Australia with nothing but ocean between him and New Zealand. He says the big curling wave to his side is a permanent fixture and fatal to boaters trying it. He is using a new 30 hp two stroke motor which still is in its break in period. The Coast Guard would say that is the max for this hull. With the 30 Ashley reports cruising at about 20 mph with two adults on board. As an aside I should mention that in boats like this one needs to pay attention to the prop specs to prevent the motor from going over redline at full throttle. If the usual prop is for a heavier and slower boat then I would expect the motor to rev too high at full throttle in a lighter faster boat. The idea is to select a prop with a pitch that will put the tach on the red line at full throttle.
The hull is made with taped seams needing two sheets of 1/2" plywood and eight sheets of 3/8" plywood, a total of about 400 pounds of wood. I would expect the stripped hull to weigh about 500 pounds with framing and fiberglass. I would add at least 150 pounds to that for a motor and its gear which is why I estimate the empty weight to be about 650 pounds. Add two adults and fuel and now you are around 1100 pounds total. The old rule for motors on planing powerboats is a horse for each 50 pounds so if you havea 25 horse motor you will be able to cruise this boat at 2/3 throttle. I would say 15 hp is the minimum you might use and expect to plane, even then you must stay very light and run near full throttle.
Plans for Dorado are $40 when ordered directly from me. You can also get them through www.duckworksmagazine.com.
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.
Here are the prototypes abuilding that I know of:
The Australian Twang builder is done and is testing, searching for the ideal seating position.
The Oracle builder in St. Louis is done, waiting for lake ice to melt.
Here is a Skat 12' catboat started in Alabama.
A Piragua18 is being completed in Georgia:
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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