Jim Michalak's Boat Designs
118 E Randall, Lebanon, IL 62254
A page of boat designs and essays.
(1apr00) This issue goves some thoughts about the shaping of hulls. Next issue, 15apr00, will present some ideas about chosing a design.
Tim Webber is kicking off the messabout season with his TEXAS MESSABOUT at Lake Conroe near Houston on April 15. For more details click here.
A WORD ABOUT COLD WATER...
Now is a good time to run the annual warning about boating in cold weather: Every Friday the St. Louis paper gives the fishing report and I read it because it gives the water temperatures of the local waters. Right now they are about 50 degrees F. Now, here is some valuable info that came with the life jacket I bought last year:
32.5.....................................under 15 min.................................15 to 45 min
32.5 to 40...........................15 to 30 min...................................30 to 90 min
40 to 50..............................30 to 60 min...................................1 to 3 hrs
50 to 60..............................1 to 2 hrs........................................1 to 6 hrs
60 to 70...............................2 to 7 hrs.......................................2 to 40 hrs
70 to 80..............................2 to 12 hrs......................................3 hrs to indefinite
If you wonder how the data was gathered click here, scroll down about 60 percent, and read. Clearly anyone falling into our local waters currently needs saving right away. There may a good chance that you won't save yourself unless you are very close to shore or can get right back into your boat. I've also heard the cold water can be completely incapacitating to the point where you MUST wear a life jacket to even float. It's quite sobering when you think about what can happen.
Frank San Miguel & Co. in AF3
This is a pretty interesting subject which creates all sorts of arguments. One often sees the equation for "hull speed" or a chart of speed vs. power and everything is tied to waterline length or power/weight ratio. You are left with the idea that the shape of the hull makes little difference. But it does. The usual charts are meant for average hulls, as a rule. By my own experience, I can really tell the difference in rowing a boat. Two boats may be the same size and weight, and yet one can be 50% faster than the other because the hull is shaped for better flow or perhaps has less surface area. Here is an example: the flat bottomed and hard chined Moby Dink will row at no more than 3 mph while WeeVee with its deep V bottom will clip along at 4 mph even though they are the same length and weight and made from the same pile of stuff. (But WeeVee is a pretty extreme boat, not suitable for the family rowing.)
Is there any guidance for shaping a hull for efficiency, especially for the plywood hull?
Phil Bolger has hinted in his books of his theories for shaping hulls. About ten years ago he sent me a copy of his shaping theory in an article which he said had been turned down by the magazines. I'll regugitate what I can remember about it. His idea helps you visualize the water flow.
(Right here I should say that in my experience at the missile industry the field of fluid flow has its share of gosh and golly. There are theories and formulae to fit any situation. Once we were putting eight small brackets about the size of your thumb on a missile body so that it might fit a new launcher. The aero engineers cried real tears and presented charts on parasitic drag, etc., etc., telling all that the range would be ruined. The program had no choice but add the brackets and fly some real missiles. They went farther than those with no brackets! "Of course they did!" replied the aero guys. "Those brackets acted like vortex generators, energizing the boundary layer and reestablishing low drag laminar flow." They had a chart for each occasion.)
AFLOAT ON SEAS OF PEAS...
Phil's theory starts with the idea that the boat is crashing through a sea of frozen peas. As it moves along it pushes each pea out of the way, the push being somehow proportional to the speed of the push and also the angle that the hull meets the peas. The push is perpendicular to the surface of the hull. The sum of all those little pushes is the drag caused by the form of the boat. (Drag caused by surface area friction with the water is another story.) Sort of like this:
Right away you can see that a long finely shape boat should have less form drag because the angle of that the hull makes with the peas is more shallow than a short blunt boat. No surprize there. And a narrow boat will push the peas with less force than wide boat.
So, ignoring the surface area part of the problem, making a boat longer and narrower should make it faster.
But most boats are limited in some way as far as length goes and with a given weight, the beam and depth follow quickly. Then the problem becomes, "What can I do to shape the hull lines for efficiency with a given weight and waterline length?"
I'm reminded of Howard Chapelle's saying, "Water doesn't like to be surprized." So when I lay down a line on a paper I try to keep it as smooth as possible. I've been told Joe Dobler's more advanced designs were all done with sections of circular arcs. I don't do that but try to use a normal untapered spline which deflects through just three points. The resulting curve is not a circular arc since its ends will have no curvature, but it is still very smooth and has the advantage that real full sized wood will want to take the same curve. I'm not sure how Bolger does it.
Here is a picture of a typical flat iron skiff:
It's the sort of boat that we've looked at all our lives. It has a pointy bow with the stem fairly deep in the water. Bolger would say something like this: The bow sides curve a lot more than the bottom. Therefore the pressure on the sides is greater than the pressure on the bottom. So the water on the sides will try to move from the higher pressure on the sides to the lower pressure on the bottom, eddying around the sharp chine as it goes. The eddies are energy robbers that slow the boat.
What is really interesting about Bolger's theory is that he ties together the shapes of the different hull panels, the sides, bottom and bilge panels. By his idea, those panels should all have the save curvature, if possible, for the best flow lines. The same curvatures would mean the panels have the same pressures and the eddying would be minimized. So the Bolger boat would have perhaps the hull narrowed and especially have the bow rocker increased a lot. Usually a firm following of the Bolger rule would result in a flat bowed scow. Bolger's version of the same boat might look like this:
Starting to look familiar? Here is the Bolger Micro.
This particular Mico belongs to John McDaniel, the photo taken at last year's Midwest Messabout. It's one of the finest examples you will find, probably the best built boat I've ever seen. You can see the Bolger theory at work here. The bottom is highly rockered, the bow sweeping very high. The bow is also squared off and would sweep even higher if it were not squared off. But even here the bow is not swept up to the degree of matching the bottom and side curves. Phil also believes that the plumb sides give superior motion over flared sides. All these elements combine to give what we all call the Bolger Box.
About now the historians are saying," Hey, he's reinvented the garvey!" They are nearly correct although the Bolger hull would be a bit more slender in the bow. If you look at figure 12 in Chapelle's great book American Small Sailing Craft, you will see that the "Old form of rowing punt, from Chapman, still to be found in America" follows the Bolger idea almost exactly.
Bolger has always been quick to point out that his highly rockered hulls were faster and noisier than the traditional flatties. I would add that they probably handle better too. I don't know of any real life test that has been done to check the theory, for example using two hard chine boats together that were identical except for the hull shaping. It would be interesting.
"But," you ask,"What about waterline length? The big rocker in the bottom of the Bolger box gives away a lot of waterline length and that length determines speed." Good question and I don't know a full answer. One comment would be that the waterline length sometimes determines the top speed of a displacement hull and that your boat usually isn't going that fast. Still that doesn't explain the speed that some of these boats attain. I was on John's Micro in a high wind and we got 8 knots on his knotstick with the stern wave trying to climb on board. The Micro probably has a waterline length of about 12' and would have a top speed of about 4.5 knots using the traditional formula. And I don't remember feeling it was "planing" either.
By the way, you can tell at a glance at the hull lines whether the sides and bottom have equal curves. Look at the lines below. If in the end view the chine line bisects the side/bottom angle then the two panels have equal curvature.
The lines above are for Piccup Squared. But I've used the theory quite a bit including Moby, Pencilbox, Cubit and Jewelbox, all shaped exactly to the theory. They are all scow bowed. Karl James' Jewelbox is shown below, it's roomy simple scow hull very apparent and you can see the chines sweeping well above the waterline fore and aft. These boats all sail quite well, with the warning that any wide flat bow will bother you in rough water. It's possible that the pointy boats are better then.
VARIATIONS ON THE IDEA...
Nobody likes the looks of these scow boats. Bolger would say something like, "No one wants to face the fact that the performance they like is the result of the looks they don't like.
The scow bow is the first thing to go. Phil usually leaves a small flat bow transom, but not as wide as it would have if the sides and bottom had the exact same curve. Often the small bow transom gives a little extra room exactly where it is most needed and as with Micro gives a place for boarding steps. Still the bottom is swept high above the waterline at stem and stern.
I haven't used the little bow transom. I've either gone full scow bow or full pointy bow. But the idea behind the Bolger theory is kept in mind and the bow is swept up above the water as you see here in the pointy bow AF2.
I haven't copied Phil's plumb sides, my boats always have a little bit of flare to the sides. I don't totally follow the idea that plumb sides make for a better boat. Phil would say that plumb sides give the widest possible bottom, but I think the last thing you want is a really wide flat bottom. At least not if you want good overall performance in a variety of conditions. Somewhere else Phil wrote that the general rule for good performance in sharpie hulls has the bottom six times as long as the beam. By modern standards that would be a very long narrow hull.
But it doesn't take much flare to ease the looks of a sharpie. Five degrees is enough, that's about what the AF2 has and it looks pretty traditional to me. Also I've always felt that a boat with flare is a little more comfortable and that the extra deck width, as with AF2, provides a bit more righting moment in case of a knockdown. But I suppose the flare is still mostly for looks. If you use a lot of flare, as with a dory, you will get the situation of a very narrow bottom which won't sail well.
Let's say you do flare the sides to extreme and add a panel on top of the flared panel to get a "multichine" hull as with Piccup, shown here:
Piccup was designed before I knew of the Bolger theory. Still it is a very fast and able boat. I'm thinking when the water crosses a 30 degree angle bilge as with Piccup, the drag effect is nothing compared to crossing a 90 degree angle bilge. In fact a boat like this is very close to a round bottomed hull and has little resistance to side motion.What about that sharp upper chine? It's totally out of the water during normal sailing.
Remember Piccup Squared?
It was designed exactly to the Bolger flow theory with side and bottom panels bent to the same curve. It has the same length, weight, sail rig and overall configuration as Piccup except that it is a hard chined flat bottomed scow where Piccup has the multichines. I always felt that Piccup Squared was as fast as Piccup in smooth water but that Piccup was superior otherwise, especially regarding handling in rough water. There is no doubt that 11 foot flat bottomed scows are not the best in rough water.
So Piccup Squared was a real attempt to compare the two types of hull shapes but I was never able to sail the two boats side by side and that might be the only way to test the effect.
As for a planing power boat, the shaping gets simplified quite a bit. When planing the bow is well out of the water and could be any shape. In smooth water the best planing surface aft is bound to be very close to a flat plate although some would have a slight reverse hook in the aft bottom to trim the boat. I would think that in smooth water the old simple jonboat shape is the best. Not so in rough water. There I think the V is the best. It's easy to draw a V bottomed boat where the water flows smoothly from stem to stern crossing no seams at all. We've found that displacement boats made this way can be quite fast. They say that deep V power boats need a lot more power than the equal jonboat althought the ride will be a lot smoother.
I like to combine the shapes a bit with a deep V entry to cut the water and have them blend into a multichine shape aft with a small flat center plank in the manner of the old Swampscott dories. I've never felt the Bolger ski nose boats were the right way to go except in the case of very short wide hulls. I was always taught to abhor those inward 90 degree angles between bottom and the sides of the ski nose. And if you fair those angles with a plate you end up with a multichine hull.
The grand idea is to try to visualize how the water is reacting to the hull plates as it flows along and to smooth its path.
I'll give some thoughts about chosing a boat.
FAMILY SKIFF,SAILBOAT, 15' X 5.5', 350 POUNDS EMPTY
The Family Skiff is large enough to swallow up three men or maybe a family with two kids. She has two benches that are 7' long and there should be plenty of room for all. I would say her fully loaded maximum weight might be 900 pounds and her empty weight about 350 pounds, leaving about 550 pounds for captain and crew and gear.
At the same time the Family Skiff can easily be handled solo, although with just the weight of the skipper she will not be a stable as when heavily loaded. The boat also has two large chambers for buoyancy/storage and I can see her used as a solo beach cruiser because the benches and the floor space are large enough for a sleep spot. This shape of hull has proven to be both fast and able in rough water, a lot better than a similar flattie. I've made her deep, too, with lots of freeboard.
The balanced lug rig sets on short spars and sails very well reefed, in fact can be set up with jiffy reefing. The spars are all short and easily made and stowed, the mast being but 14.5' long and setting 97 square feet of sail. In addition there are oar ports for those with lots of time and little money and a motor well for those with lots of money and little time. Two horsepower is all that a boat like this can absorb without going crazy. The motor well is actually an open self draining well that uses the full width and depth of the stern. It will come in handy for storing the wet and muddy things you don't want inside the boat, like anchors and boots. I've suggested in the plans that the rudder can be offset to one side a bit to give more room for the motor. both Petesboat and Frolic2 have used an offset rudder with no harm other than perhaps needing a curved tiller to make it more comfortable. In the case of Petesboat the 60hp motor was on centerline and the rudder way over yonder to one side with linkage to a centerline tiller. Pete said the large rudder offset had no effect on steering.
Family Skiff uses taped seam construction needing two sheets of 1/4" plywood, five sheets of 3/8" plywood, and two sheets of 1/2" plywood.
Prototype plans for Family Skiff are $20.
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:
Jonsboat: Greg Rinaca near Houston sent this photo of his completed Jonsboat, land bound until he rounds up a motor.
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We also have the start of a Jon Jr, a 12' x 3' personal sized jonboat, down in Texas. Here is the latest photo with the bottom going on.
Mayfly12: A Mayfly12 is going together up in Minnesota. The decks are on and he's into the sailing bits. By the way, the sailing bits on almost any sailboat large or small consume about half of the effort in labor and materials. Just when you thought you were about finished! Here is a construction photo from last summer.
Robote: Robote is supposed to be a fast somewhat extreme rowing boat based on my old WeeVee design, thus it has a deep V bottom but is stretched out to 14' long with a very pointy bow. It was drawn as a custom job but if it works out I'll put it in the catalog. The builder has the boat taped together and hopes for trials by April.
AN INDEX OF PAST ISSUES
BACK ISSUES LISTED BY DATE
Herb builds AF3 (archived copy)
Hullforms Download (archived copy)
Plyboats Demo Download (archived copy)
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