Jim Michalak's Boat Designs
118 E Randall, Lebanon, IL 62254
A page of boat designs and essays.
(1Aug04) This issue discusses hull shapes. The 15 August issue will rerun the taped seam 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.
Larry Normandeau built this Twang in Alberta and strapped it to a float plane just to prove that anything can fly. But he won't explain what these two are inflating.
Almost everything I know about boats I learned from Phil Bolger, including hull shaping. I'm only going to discuss shapes suitable for building in plywood.POINTY BOW/PRAM BOW...
Some boats have pointy bows and and some have blunt bows, like Karl James' Jewelbox shown here:
If Jewelbox were to have a pointy bow it would need to be maybe 20% longer to do the same job. And even then it could not have that handy step in the bow that allows you to step on and off the beach easily. In smooth water as is shown in the picture the pram bow has no disadvantages and a lot of practical advantages. But the bow has to be swept up well above the water so that she is an "over the water" hull and not a "through the water" hull. But if the water gets so rough that the blunt bow won't clear the waves then she goes slam bang into it, a great geyser of water shoots aft, usually she will stop in her tracks and then start going again. My very first design, Piccup pram was like that and I could easily tell when it was time to head for calmer waters - when I got a quart of water in the face with each wave.
You can try a pointy bow as shown here with an inwork photo of AF4G:
The rough water work is improved but the bow must still be swept up above the water. If you to push the point of the bow through the water the sharp point on the bottom creates a lot of drag. If you think not then try rowing a pointy bow boat with the bow point immersed. It gets a lot harder real quick. And if the boat yaws a bit the drag goes up even more. The stern will try to pass the bow and she will try to swap ends and broach. I don't think a pram bow will do that in general as easily but it will be warning you with its slam bangs a lot sooner than will the pointy bow.
POINTY STERN/SQUARE STERN
Before I built any of my own designs I built about six Bolger designs and three of them had pointed sterns. I think there are supposed to be two advantages to a pointed stern. First is that you don't need to build a transom. But you would still have to bevel another stern post instead so you aren't getting off free in that respect, and a transom is usually a fairly easy part to build. Second is that they are supposed to be better sea boats, less affected by a following wave. That might be true for things like lifesaving boats and slow fishing boats that must soldier on in all weather.
I found pointy stern boats (usually called "double enders") have some real life problems. My first one was Bolger's 12' Teal. The pointy stern reduces the capacity quite a bit, just like the pointy bow, and the Teal was sort of limited to about 250 pounds. The pointy stern also reduces the lateral stability a lot and thus reduces the amount of sail or wind you will sail with. Lastly I found that loading a pointy stern boat to the cartop was quite a chore. You invert the hull and lift the bow up to the rack. But now the stern is balanced on one point and the boat will try to turn itself upright without fail every time! Eventually I clamped a stick across the stern top to stabilize it when inverted.
My second pointy stern boat was another small Bolger design usually called the Payson Canoe. This is a multichine boat with pointy bow and stern, totally open and a one man boat. (It became the main idea behind my own Toto design.) Still had the cartop problem although this one was light and narrow enough to keep it inverted with muscle. The freeboard was quite low, maybe 5" in the center, 10" at the bow and 7" at the stern (I'm guessing). What I found is that certain following waves, such as powerboat wakes in smooth water, would lift the boat bow and stern with the center more out of the water and lateral stability, already small in a narrow one man canoe, would get even smaller. It never rolled over but scared me a few times. And remember the stern is wide open with small freeboard. I thought it could easily swamp by the stern in certain conditions (but it never actually did). So in a case like this with a low pointy stern you had better deck it over (as I did with Toto and also gave it a small square transom to add more buoyancy and stabilize for cartopping).
Last pointy stern boat I did was Birdwatcher, which I still have. Pointy stern was I think for looks and ease of building. But whatever ease of building was lost in the resulting complexity of the rudder system which could not use a traditional tiller because there is not enough room. So there is a linkage system that moves the tiller forward into a wider part of the boat. Also lost in the pointy stern here was a lot of floor space. Jewelbox was my attempt at a practical Birdwatcher and when I got to try Jewelbox at our Messabout a long time ago I was shocked at how much roomier it was even though it is about 5' shorter. Lastly, there is no good place to mount a motor on a pointy stern boat like Birdwatcher although we all have tried. The original idea behind Birdwatcher was that it be a homebuilt Dovekie, designed during the oil embargo for use without a motor. Yes, it will really work that way, oars and sails alone. Trouble is that you can only row it far on a calm day and any motorboat wake will stop you dead. So we all added motors or tried to.
Well, I think you can see that I note no advantage in a pointy stern boat worth chasing. I think I've only drawn one. the LFH17 rowing boat, but that was a copy of a classic.
If your boat has a squarish cross section such that the bottom is totally flat you have a flat bottomed boat. You can see many advantages right away. It usally has fewer parts so can be easier to build. In use it can have a lot more floor space. Best of all the full width of the hull is getting immersed and providing buoyancy, so a flat bottomed boat will float more weight for its size than anything. Another way to view that is the flat bottomed hull will float the same weight in less water. So if you have lots of shallow areas to boat in then the flattie is a good bet.
But there are disadvantages of course. The structure of the bottom must always be heavier than shaped bottoms mostly to keep the panels from flexing. The flexing is often more imagined than real but it is there and can scare you. Luckily with most sailing boats adding thickness to the bottom is a plus in almost every way. Not so with powerboats because weight is speed if you hope to plane the boat. So expect flatties to be a bit heavier than other shapes.
Do flat bottoms have more drag that other shapes? That is a real tough question and I'm sure the answer is "it depends". I'm fairly certain that a simple flat bottomed shape like a jonboat is the fastest power boat in smooth water although I have no proof. Trouble is that in rough water the flat bottomed can really get pounded and tossed around. In general I would say it is the least seaworthy shape. I think Chapelle used to say of old time sailing flatties that they had to be about six times as long as wide to go well. Today's flatties are usually a lot wider than that and in rough water they can get very slow and bouncy and eventually dangerous because they won't be able to carry enough speed to handle, for example to tack through the wind at a critical time. So if you want a "go anytime" rough water boat, don't consider a flat bottomed job. If you are boating in rivers or lakes that never get rough, or can choose you weather carefully, there may be lots of advantages to a flat botttomed boat.
The above photo shows the bottom of the prototype Skat, one of a few V bottomed boats I have done.
I have a feeling that for a slow speed boat such as a rowing or sailing boat, a V bottom is the fastest thing around in plywood if it is done properly. I think classic racing boats like the Lightening are of this shape. "Done properly" means to me that the bottom panels are not twisted at all and that the chines (where the bottom panels meet the sides) are not in the water in normal trim. With the chines out of the water there is no issue about drag due to turbulence there as the water tries to sort itself out as it goes around a corner. Similarly, if the boat is heading straight there will be no flow across the bottom seam. So in the V bottomed boat the water need cross no angles at all as it jouneys from stem to stern. Do I have proof?
Almost. A long time ago I made for myself a test pram called WeeVee. It was 7' long and 3.5' wide with a deep V. On a boat that small the V has to be quite deep in order to float any weight with the chines free of the water and I think this thing had about 9" of V in the center with its ends swept up really high and clear of the water. It is a fact that WeeVee would row at about 4mph with the same effort that would move a similar flat bottomed pram no more than 3mph. Frank Kahr built one and reported the same. Then he built a Vireo and finally a Robote which had the exact same cross section as WeeVee but with a pointy bow and 15' long.
Robote is fast and handy in rough water to the point that he used it to operate off surf beaches.
When you look at the above photo you can also see that the V bottom provides some depth that makes sitting more comfortable. But you can see the disadvantage to the shape. It won't beach flat (not sure how Frank got it to sit still for the photo). In order to launch from a beach you must walk it out a ways and hop it (unlike a flat bottomed boat that usually can be pushed off from a beach as you stand in it). And the V bottomed job is tippier than the flattie (although in the proper hands that means it will be more seaworthy, able to roll with punches so to speak). The tippiness can be eased a lot by making the boat wider. V bottomed boats can be a lot wider for their length than a flattie and still go very well. Skat is a copy of an old design, 12' long and 6' wide and is said to be very fast and able all things considered.
By the way, WeeVee was so short and deep that it would run aground well short of any beach and would balance there on its V and you would be afraid to move in any way thus throwing it this way or that like a carnival ride. You might say, "Why not lop off the bottom of the V and give it a flat spot?"
If you chop the bottom of the V to give it a flat spot you will get a multichine boat. Here is Sandra Leinweber painting the bottom of Ladybug which is a multichine:
In the case of Ladybug the bottom is flattened in its center from stem to stern. Bolger did a lot of these about 20 years ago including his Gypsy and Bobcat and Cartopper. My first one was my Piccup pram although I quickly followed that with the original Roar rowboat. After that came Woobo which was essentially a 15 pointy bow Piccup Pram. All were fast and able all things considered. Much better than a flat bottomed boat and far as handling and speed in rough water. Essentially these might be thought of as flat bottomed boats with the sharp chine corners angled off. So I guess that sharp chine angle really can cause problems in certain conditions.
Here is that same Ladybug sailing in Texas very happily with three grown men on board:
So I've drawn a lot of multichines over the years. Do they have any short comings? Yes. The multichine shape by nature has very little lateral resistance. They often would just as soon go sideways as forward. One of the first I tried was a Bolger Gypsy, this one totally bald on the bottom and me rowing it with all the sailing boards up. As I rowed into a calm cove I dug in one oar to turn it and it spun a couple of 360's without ever changing its direction of translation! It was not noticable with sailing boats like Piccup where the leeboard will drag a bit in the water to keep it straight. But the problem was quite noticable with my original Roar rowboat which would blow around like a bubble in high winds. So now I show all of these with skid/stiffeners/skegs on the bottom to give at least a bit of lateral resistance. But not for all of them. For my Roar and Toto I found a different solution.
Toto was from the very beginning an attempt to improve the Payson canoe. It has the same multichine cross section and length as the Payson canoe but as noted earier I squared off the stern to make it easier to cartop and decked the stern for security. There was one other idea I wanted to try. All plumb bow boats, like Payson canoe or the Ladybug shown above, can become really cranky and slow if the lower tip of that plumb bow is immersed in solid water. You note with Ladybug that she trims right with the bow tip clear. With the smaller canoe you quickly become limited in weight capacity because you can't keep the bow tip clear of the water, for example if you try to paddle with a passenger. It is MUCH harder to paddle that way. So when I drew Toto I gave it a bow that you see a lot on advanced multiplank dories. (I sure copy a lot!) Turns out it was pretty easy to do and build although I hate to say how long it took me to be able to figure the plank shapes on the drawing board. Unlike Skat and Ladybug, etc., where the planks curve with no twist, the Toto has a twist to the bilge planks in the bow region which allows the shape you see. No getting around it. Here is a photo of Barry Johnson's Toto under construction:
Toto was right on from the start. I still have it and use it all the time, unchanged since it was first built. The bow indeed can accept an overload without getting cranky at all. She was fast and excellent in rough water. After I had used her a year I chopped the bottom bow off my original Roar rowboat and converted it to the Toto bow and I think was able to make that way a good comparison of the two bow shapes. Essentially I think the Toto bow is clearly a "through the water" bow while the original plumb bow multichine (like Ladybug's) is clearly an "over the water" bow. What I noticed first was that in light chop the "tap tap tap" sound of the old bow was replaced by a gurgle sound as the new bow cleaved the water. Also I saw that the new bow "grabbed" the water better and gave the boat a lot more lateral resistance and she held on now in a cross wind. As with the old dories, the Toto bow will take on the rough stuff as seen here when Ashley Cook rides his Dorado through some Australian chop:
The Toto bow has the edge everywhere except that I think it is a bit slower in smooth water and it uses up a bit of interior volume, or shall we say the plumb bow boat can be shorter and have the same capacity.
...We'll tape some seams.
JONSBOAT, POWER SKIFF, 16' X 5', 200 POUNDS EMPTY
Jonsboat is just a jonboat. But where I live that says a lot because most of the boats around here are jonboats and for a good reason. These things will float on dew if the motor is up. This one shows 640 pounds displacement with only 3" of draft. That should float the hull and a small motor and two men. The shape of the hull encourages fast speeds in smooth water and I'd say this one will plane with 10 hp at that weight, although "planing" is often in the eye of the beholder. But a 15 or 20 horse motor is often just a little heavier and just a little more expensive and the extra power can be useful if you start to carry extra passengers (recalling that to plane a boat you usually need 1 hp for each 50 pounds of weight.) The prototype was built by Greg Rinaca of Coldspring, Texas and his boat is shown above at a Lake Conroe messabout. But here is another one finished about the same time by Chuck Leinweber of Harper, Texas:
In the photo of Chuck's boat you can see the wide open center that I prefer in my own personal boats. To keep the wide open boat structurally stiff I boxed in the bow, used a wide wale, and braced the aft corners.
I usually study the shapes of commercial welded aluminum jonboats. It's surprising to see the little touches the builders have worked into such a simple idea. I guess they make these things by the thousands and it is worth while to study the details. Anyway, Jonsboat is a plywood copy of a livery boat I saw turned upside down for the winter. What struck me about that hull was that its bottom was constant width from stem to stern even though the sides had flare and curvature. When I got home I figured out they did it and copied it. I don't know if it gives a superior shape in any way but the bottom of this boat is planked with two constant width sheets of plywood.
Greg Rinaca put a new 18 hp Nissan two cycle engine on his boat, Here is a photo of it:
The installation presented a few interesting thoughts. First I've been telling everyone to stick with 10 hp although it's well known that I'm a big chicken about these things. Greg reported no problems and a top speed of 26 mph. I think the Coast Guard would limit a hull like this to about 25 hp, the main factors being the length, width, flat bottom, and steering location. Second, if you look closely at the transom of Greg's boat you will see that he has built up the transom in the motor mount area about 2". When I designed Jonsboat I really didn't know much about motors except that there were short and long shaft motors. I thought the short ones needed 15" of transom depth and didn't really know about the long shafts. Jonsboat has a natural depth of about 17" so I left the transom on the drawing at 17" and did some hand waving in the drawing notes about scooping out or building up the transom to match the requirements of your motor.
I think the upshot of it all is that short shaft motors need 15" from the top of the mount to the bottom of the hull and long shaft motors need 20". There was a lot of discussion about where the "cavitation" plate, which is the small flat plate right above the propellor, should fall with respect to the hull. I asked some expert mechanics at a local boat dealer and they all swore on a stack of tech manuals that a high powered boat will not steer safely if the cavitation plate is below the bottom of the hull, the correct location being about 1/2" to 1" above the bottom. But Greg had the Nissan manual and it said the correct position is about 1" BELOW the bottom. Kilburn Adams has a new Yamaha and its manual says the same thing. So I guess small motors are different from big ones in that respect.
But it seems to be not all that critical, at least for the small motors. Greg ran his Jonsboat with the 18 hp Nissan with the original 17" transom for a while and measured the top speed as 26 mph. Then he raised the transom over 2" and got the same top speed!
I've gotten several Jonsboat photos although I imagine I have misplaced several. Here is one by Jim Hauer:
And another by Barry Johnson:
And another by Don Graham:
There is nothing to building Jonsboat. There five sheets of plywood and I'm suggesting 1/2" for the bottom and 1/4" for everything else. It's all stuck together with glue and nails using no lofting or jigs. I always suggest glassing the chines for abrasion resistance but I've never glassed more than that on my own boats and haven't regretted it. The cost, mess, and added labor of glassing the hull that is out of the water is enormous. My pocketbook and patience won't stand it. Glassing the chines and bottom is a bit different because it won't show and fussy finishing is not required.
Plans for Jonsboat are $25.
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.
Out West the Picara gets its fiberglass coat:
The Deep South Skat is done! Hopefully a full story in a few weeks:
Another Picara, this one with a 1' stretch in the middle, going together in Arkansas. Sailmaking done and its on to glassing the hull.
The AF4G is upright on its trailer now:
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