Jim Michalak's Boat Designs
118 E Randall, Lebanon, IL 62254
A page of boat designs and essays.
(1Sep05) This issue will detail some structure in Bobsboat. The 15 September issue will take a look at the Russelville, Arkansas messabout.
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.
Going for a ride in Brian Walker's prototype Veep14.
Designing Boats 9SOME STRUCTURE...
In the past few Bobsboat episodes we have figured out the shapes of all the panels and then all the required bevels. We are ready to draw real structural details now.
One guiding rule to keep in mind is that structural elements in a boat are almost always defined by stiffness and not strength. Second guiding rule is that if you double the thickness of a structural element you will usually increase it bending stiffness by a factor of 8! So half inch plywood is eight times as stiff as quarter inch, and three eights is 3.4 times as stiff as 1/4", etc.
Starting from the top of the boat down, here is how I figure it:
Funny thing about decks is that they can be quite heavy. Two of my first boats, the Bolger Jinni and Birdwatcher, had decks from 1/4" plywood. I suppose the largest unsupported areas were about 18" square. I am not heavy compard to a lot of you, about 170 pounds. When I step on those decks they creak but I don't think they ever cracked. Bolger was big into 1/4" ply back then and he is not a heavy man. Most folks would not be happy about walking on a deck of 1/4" plywood even if it doesn't crack. So a deck that will really get walked on should be 3/8" thick minium. But a deck that usually won't get walked on can be 1/4" I think and that might include decks for canoes and rowing boats and smaller sailboats where you can't step away from the cockpit anyway. Bobsboat is pretty large and at times folks will step on its decks just to get on and off the boat from a beach so I would use 3/8" ply on its decks.
It is worth noting that I would never use 1/2" since weight up high in any boat will be a problem.
In my opinion the sides of almost any boat can be quite light. The 24' Birdwatcher has 1/4" ply sides and there is no problem. So I specify 1/4" ply on almost anything I draw. Sometimes on the larger boats I might draw 3/8" ply mostly because such a boat will have perhaps no 1/4" anywhere else. Someone who is watching his weight could go with 1/4" will no exceptions I can think of, even with a 30'+ Cormorant. I think the only argument against the lighter ply might be its impact resistance but it is seldom the sides of a boat that take a beating. On the little canoes and such I will specify 1/4" only because it is easy to get - lighter 3/16" or even 1/8" will suffice if you want to spend the extra for less, so to speak. In a boat you need to carry by hand a long way that can make the difference between a boat you like to use and one you hate to use.
One my boats you won't have separate bilge panels unless it is a taped seam multichine. Flexing does not seem to be a problem because these panels are seldom very wide. But some judgement is in order with these because they can take a beating at times. Even on the little canoes I would use at least 1/4" here - you might try 1/8" to save a bit of weight but there is a possiblity of getting a rock through it in certain conditions. So as a general guide I would use 1/4" ply for bilge panels for boats under say 14' long, maybe 3/8" ply from there to say 20' long, and maybe 1/2" ply for boats even larger. For Bobsboat I will specify 3/8" ply.
Bottom panels of plywood boats are really challenging.
I had several Bolger boats when I started. I built my Teal with a 3/8" bottom as specified on the plans - quite solid for such a small boat.
My old Bolger Jinni had an open center cockpit 4' wide and 6.5' long with a bottom of 1/4" plywood with three small stiffeners on the outside bottom. In rough water I looked to flex an inch although I suppose it was actually a lot less. There seems to have been no harm to it but it was scary at times to watch. And any gear you had sitting on that bottom would hop clear with each wave. It never failed structurally.
Next was a Bolger Gull dory built entirely with 1/4" ply and taped seams. Over 15' long and only 65 pounds! We took it everywhere. Loading and unloading from a cartop was done in a minute or so. But I've seen the very same design done with thicker ply which weighed 150 pounds and was no joy to cartop. My Gull was stiff enough. The 1/4" bottom had no stiffeners at all but did have a thin layer of fiberglass cloth on the outside. That gave abrasion resistance but added no stiffness. From that experience you will see that I never specify more that 1/4" for the bottom of canoes and rowing boats. It is strong enough and stiff enough and anything extra you add will only detract from the experience.
Last came the Birdwatcher. Bottom is two layers of 1/2" ply laminated together to form a 1" thick plate. There are areas 5' wide and 6' long that are unsupported. Still it is solid as a rock and remember that a 1" thick bottom will be 64 times as stiff as the same floor in 1/4". In the bottom the weight on a sailing boat is a plus for the most part. It is ballast. But I will suggest that if ballast weight is all you need then a hundred pounds of steel is a lot easier and cheaper to put in place than a hundred pounds of plywood. I can tell you that gluing on the second lamination on a 24' boat takes a lot of planning and a lot of glue and fasteners to secure while the glue cures.
Power boats have a much more difficult bottom situation because performance is for the most part just a result of high power/weight ratio. Remember the old rule that it takes a horse to plane 50 pounds of weight. A sheet of 1/2" plywood weighs about 50 pounds. So you can't just pile it on like you can with a sail boat that will be trailered (any cartopped boat has to be light to be successful). My AF4 supplied me with guidance on this subject but not because I wanted it to. The original drawings had a 3/8" bottom with three longitudinal stiffener/skids on the outside. When I built my prototype I bought some 3/8" ply but like most plywood nowadays it was undersized and is really 5/16". It has been nothing but trouble both from the standpoint of flexing and leaking (through knots and such) but has never failed. As with the old Jinni it can be frightening to watch in rough water and guaranteed to make you slow down. When Max built his AF4 he knew of my troubles and made his with a 5/8" bottom and a single 2x4 stiffener down the bottom center. His is like iron although his boat is about 150 pounds total heavier, not all of it in the bottom. So another 3 hp needed but maybe it was worth it. The current AF4 drawings show a 1/2" bottom.
Bobsboat will get a 1/2" thick bottom. That will be stiff enough for a slower hull. No need to do the extra work of laminating a bottom either.
Bulkheads can be quite thin for the most part. One of their functions is to keep the boat in shape but that is easily done since decked boats in particular form a rigid box. They also stiffen other panels such as the bottom in particular and prevent flexing. Again they don't need to be especially heavy to do that. Bulkheads also introduce loads into the hull shell and the obvious thought here is the bulkhead you hang your mast on in a sailboat. In general I think they can be pretty light if the loads are introduced into the edges of the bulkhead and not into the center. So 1/4" plywood is almost always enough.
I think there is a rule of thumb in larger powerboats that the transom should be 1-1/2" thick plywood. I don't design large powerboats and usually use 1/2" plywood with a major framing member, like a 2x8, across the top that the motor will clamp to. The idea is that the solid framing carries the load and will be secured with glue, bolts and screws to the side of the boat. If there is a problem with this it will be with twisting under the power and weight of the motor. Traditionally this was dealt with with a "knee" that ran like a triangle from the motor area to the keel of the boat. My own favorite solution is to put a slop well right in front of the transom that boxes it all in. Not only does it reduce twisting but it stiffens the entire stern of the boat in every way. And if you are sloppy like me the "slop" qualities, where all the slop drains out through the stern and not into the bottom of the boat, are well appreciated.
...We'll go for some boat rides at the messabout in Russelville, Arkansas.
AF4GRANDE, POWER SKIFF, 21.5' X 6.5', 700 POUNDS EMPTY
Af4 Grande is a 20% scaleup of the original AF4. Scaling a design can be interesting. If the design is scaled up by a factor of1.2 as it was here then the areas of all the panels are increased by a factor of 1.2 squared (1.2 times 1.2) which is 1.44. With a plywood boat, if you keep the panel thicknesses the same as the original, you would expect the hull weight to also increase by that amount. In thecase of AF4G that is about what I did. I used the same skin thicknesses that Max Wawrzyniak used on his AF4, 5/8" bottom and 3/8" elsewhere. He said the weight of the bare hull was about 425 pounds. So I would expect the 20% scaleup to weigh about 1.44 x 425 = 612 pounds. In this caseit isn't quite so simple and I'm guessing the bare AF4G hull would weigh about 700 pounds since the larger boat has built in seats and another bulkhead and some thicker framing. By the way, I would expect the cost of the project both in labor and materials to increase in the same proportion as weight.
With that weight in mind we might add 120 pounds for a motor larger than AF4's 10 or 15 hp and a bit of fuel. So the minimum weight now is about 820 pounds. Let's add another 80 pounds of misc. gear and we're up to 900 pounds without crew. Add maybe 500 pounds for a small family and then we have an all up weight of 1400 pounds. There seems to be a general rule of thumb for powerboats that it takes a horse for every 50 pounds of weight to plane and then you need 28 hp for this boat. The Coast Guard gives advice about powering boats and this one would max at 35 hp based on length, width, and the fact that it has a flat bottom which can be prone to handling problems if overpowered. On the other hand, this boat used solo and stripped might weigh in at 1000 pounds total and do quite well with 20 hp. In my experience with AF4 these hulls will plane with less than a horse for each 50 pounds. AF4 planes with about 7 hp at about 600 pounds total, a horse for 85 pounds. But if you try to plane it with 7 hp you have to run full throttle all the time. Better to run a 12 hp motor at 60% throttle. Using the same number for the 1400 pound AF4G the 28 hp motor might plane the loaded boat at 60% throttle. All this is preliminary guesswork.
Here is another interesting thing about the straight 20% scaleup. All of the volumes increase by a factor of 1.2 cubed (1.2 x 1.2 x 1.2) which is 1.73. So the interior volume of the AF4G is 73% greater than that of the AF4. Not only that but the volume of water the boat displaces can be in the same proportion which means it can be heavier by 73% and look the same in the water. So scaling up a boat gives big increases in room and capacity. On the other hand, scaling a boat down can result in a shocking decrease in capacity. For example if you were to scale down a 15' rowing dory by 20% to 12.5' you might expect the capacity to go from say 400 pounds to 230 pounds (400/1.73=230) and it quickly becomes barely a one person boat!
Eric Shadow built the prototype in New York and I think it is into its second season. It is a large and straight forward job. You can see in this photo how little water the boat needs even though it is over 20' long - maybe 4" will float it.
Construction is simple nail and glue like the original AF4. AF4 Grande needs nine sheets of 3/8" plywood and five sheets of 5/8" plywood. I don't think these boats need epoxy coatings if they are stored under cover. The chine corners need to be armored with fiberglass set in epoxy and inner seams given a fillet of thickened epoxy to keep water from creeping into the seams.
AF4 Grande plans are $40.
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.
The out West Picara has its roof and some major sail rig bits done:
The down South Picara is more or less complete now. Should have an updated photo soon.
This long and lean project is a 19' version of Toon2. Next issue I will have a photo of the painted hull awaiting its sailrig.
This is a slightly modified Veep14. I'm told the sheer is raised a bit and the stern given a bit of flare but the bottom is per the plans.Writeup next issue:
A Vector builder is keeping a website of the project at http://firstname.lastname@example.org/ and here is a photo of his boat on its first sail, just before the storm hit. I also have photos of a Vector completed by Pete Mohylsky in Florida. Hopefully a report soon:
Here is a Musicbox2 I heard about through the grapevine.
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
Table of Contents