Another beautiful AF4! This one by Kevin Widany.
Contents:
Contact info:
Jim Michalak
118 E Randall,
Lebanon, IL 62254Send $1 for info on 20 boats.
Jim Michalak's Boat Designs
118 E Randall, Lebanon, IL 62254
A page of boat designs and essays.
(15December 2012) This issue will delve into hull shaping. The 1 January issue will remind us about bevels, again.
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.
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Another beautiful AF4! This one by Kevin Widany. |
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HULL SHAPING
HISTORY...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 twenty 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.
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?"
CURVING PANELS...
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. 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.
Contents
SPORTDORY
LIGHT ROWBOAT, 15' X 4', 70 POUNDS EMPTY
SPORTDORY
Sportdory is an attempt to improve upon the Bolger/Payson dory I built about 15 years ago. This boat is slightly smaller than my old dory. In particular the bow is lower in hopes of cutting windage. the stern is mostly similar. The center cross section is about identical. This boat has slightly more rocker than the original Bolger dory.
The hull is quite simple and light, taped seam from three sheets of 1/4" plywood, totally open with no frames. The wales are doubled 3/4" x 1-1/2" pieces to avoid the wale flexing my first boat had. I've added an aft brace to stiffen it up and give the passenger a back rest.
Mine once covered 16 statue miles in four hours. In rough water you will feel the waves are about to come on board but they won't. But if you try to stand up in one it will throw you out with no prayer of reentry.
The prototype was built by John Bell of Kennesaw, Georgia. Here is a photo of John's Sportdory under construction. You can see the sides and bottom, precut to shapes shown on the plans, wrapped around temporary forms and "stitched" together with nylon wire ties in this case. I'm quite certain that with this design one must leave the forms in place until all the structural elements like the wales and cross bracing have been permanently installed. If they are removed before then, the assembly will change shape and you won't get the same boat. In particular I think the nose will droop to no one's benefit.
One might wonder about a comparison of Sportdory, Roar2 and QT. They are all about the same size and weight, a size and weight I've found ideal for the normal guy. They are small enough to be manhandled solo yet large enough to float two adults if needed. They are all light and well shaped for solo cartopping. Roar2 is probably the most involved to build and the best all around of the three. Sportdory is simpler and lighter, at least as fast and as seaworthy, but most likely will feel a little more tippy and less secure. You shouldn't really try standing up in either of these two. QT will be the least able of the three as far as speed and seaworthiness but may be the easiest and cheapest of the three and is stable enough to stand up in. So take your pick.
Sportdory 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.
I think David Hahn's Out West Picara is the winner of the Picara race. Shown here on its first sail except there was no wind. Hopefully more later. (Not sure if a polytarp sail is suitable for a boat this heavy.
Here is a Musicbox2 out West.
This is Ted Arkey's Jukebox2 down in Sydney. Shown with the "ketchooner" rig, featuring his own polytarp sails, that is shown on the plans. Should have a sailing report soon.
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:
And the first D'arcy Bryn is taped and bottom painted. You can follow the builder's progress at http://moffitt1.wordpress.com/ ....
AN INDEX OF PAST ISSUES
A NOTE ABOUT THE OLD WAY BACK ISSUES (BACK TO 1997!). SOMEONE MORE CAREFUL THAN I HAS SAVED THEM. TRY CLICKING ON...
which should give you a saving of the original Chuck Leinweber archives from 1997 through 2004. They seem to be about 90 percent complete.
Hullforms Download (archived copy)
Brian builds Roar2 (archived copy)
Herb builds AF3 (archived copy)
Herb builds RB42 (archived copy)