Jim Michalak's Boat Designs
118 E Randall, Lebanon, IL 62254
A page of boat designs and essays.
(1oct07) this issue will take a look at hull wind drag. For the 15 October issue I hope to dig out the old leeboard geometry 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.
Peter Simmons' Skat at Bill Paxton's messabout last year.
Hull Wind DragBACKGROUND...
This is the sort of subject you hear comments about all the time and the results of which we have all felt first hand as we try to row in the wind or push a boat onto its trailer in a blow. I thought I would at least get the ball rolling with some basic calculations to hang some numbers on the subject. Just how much force does the wind put on your boat?
IT IS GOOD TO BE OLD...
...and have at your fingertips these old things called "books" that they used to sell to people. I always think that if the computer had been invented 3000 years ago and paper 30 years ago that paper would cause a revolution! So I have on my wall a 40 year old copy of Mark's Standard Handbook For Mechanical Engineers, about 4" thick. Imagine how many storage systems involving tapes and discs have come and gone since then. All it takes today is for Mr Gates to decide a new version of Windows is needed to make all your stored data useless. Anyway, so I've got this old book and in it are some words about "ship wind resistance" right there by the locomotive data.
...I'm going to take a look at the drag of a rowing hull, say Roar2, because I've got a lot of personal experience with it. Remember this chart we made a few month's back, a Hullform study of Roar2 that was backed up by measuring the force on the oars in my real Roar2:
As you see here by measuring the forces on the oarlock pins it was determined that the drag on the boat in calm conditions was 5 lbs at 3.25knots which is a very common "cruising" speed for a rowboat like Roar2. The test agreed fairly well with the Hullform results.
...a quick look in the old handbooks shows that aero drag is D=.0034 x S x C x V x V where D is the drag in pounds, S is the cross sectional area presented to the wind in square feet, and V is the speed of the air over the boat in knots. A fine case of apples and oranges in units as is always the case in engineering.
C is the "drag coefficient", a wonderful handful of approximation. The C of a "flat plate" is supposed to be around 1.1 to 1.3, there apparently is no total agreement. What is the C of a boat? Well, depends. But Mark's suggests a C of .9 for an unstreamlined ship. Might as well use 1 for this quick study to make figuring easy.
Now, the square footage presented to the wind will include that of your body. Are you streamlined? Maybe you can only dream of being a flat plate. Anyway, I am assigning you a C of 1 just like the ship. Let's figure the square footing of the Roar2 head on as say 1.3' x 3.5' = 4.5 sq ft. And the body sitting bolt upright at the oars might add say 1.5'(wide) by 2.5" (tall above the wale) for another 3.7 sq ft. Let's get scientific and call the total 8 sq ft frontal area.
THE EFFECT OF AIR SPEED...
...V becomes monumental as speed picks up because the speed is multiplied by itself in the equation. For first look I will use a speed of 10 knots. If you have ever rowed into a wind of 10 knots you know it is a factor in your effort. But before we run the drag number let's look at the effect on drag as speed picks up in a general way. If you double the wind speed, say 10 gusts to 20, the drag becomes four times as much. And if you triple the wind speed from say 10 to 30, drag becomes nine times as much. Somewhere in there you will be overwhelmed by wind drag as we shall see.
LET'S RUN A NUMBER...
OK, for Roar2 pushing into a ten knot wind we get D=.0034 x 8 x 1 x 10 x10 = 2.7 pounds. No big deal? It would not seem so but it is a 50% increase over the no wind load we measured and you would notice it. Remember that oars are machines that gear up speed so a 2.7# extra force at the hull would be about twice that as felt at your hands. You could deal with it but it would tire you more than usual.
Now let's say the wind picks up to 20 and you are pushing into it. Air drag goes from 2.7# to 11#, twice your original water drag. To pull at the same speed as before you would need about three times the effort as in calm conditions. What is worse is that if you slow down your stroke to ease your effort you won't accomplish much since the air drag will be little affected. Even standing still you will need to push hard to stay put, better throw out an anchor to rest. Or better to pull very hard for a short time to clear that windy spot. There is no real future in rowing against such a wind.
Now let's say the wind gusts to 30. Now air drag would be about 24 pounds. I'll bet you could not make progress into it and the load on the oarlocks will challenge some screws. I also point this out as being probably the thrust limit for a small trolling motor. If you swing downwind most likely you will only be able to "control" it from the standpoint of aiming at a safe place downwind.
BY THE WAY...
...Mark's says the head on drag is not the worst case. Expect drag to increase about 30% if you yaw 30 degrees from the wind's direction.
A LOOK AT A POWER BOAT...
Let's say you live on a small quiet lake which is zoned for trolling motors only. You have a small pontoon boat, say 7' wide with 3' freeboard plus a lot of other crap on top like a bimini and control station. Let's say it all amounts of 25 sq ft of stuff. You want to be able to take her out under control when the wind is 20 knots. How much thrust is needed just to counter the wind? Well, D = .0034 x 25 x 1 x 20 x 20 = 34#. Certainly within the range of a large trolling motor and maybe you would be OK with small dualies.
Let's say you have a Shanteuse box shanty, 6' wide and 4' deep and you want to cruise at 20knots. How much air drag there?. D = .0034 x 24 x 1 x 20 x 20 = 23 pounds, the same as for the pontoon boat above since they have about the same frontal area. But here you want to cruise instead of survive as with the electric pontoon. But the tests we ran a few years ago where we measured prop thrust showed that even a 15 hp gas engine can produce over 100 pounds of thrust. I would say that in this case the air drag is still a lot smaller than the water drag.
Let's say you have a Birdwatcher, 5' wide and 3.5' deep. What is the drag in 10 knots wind? So D = .0034 x 17.5 x 1 x 10 x 10 = 6 pounds. (I'm sure designer Bolger would argue his Birdwatcher has a drag coefficient less than 1 and I would agree. Streamlining can make a big difference, for example the forms of the old dirigibles seems to be the lowest drag with a C of about .04). You could handle it. I did for a long time. Birdwatcher was usable for short times in a 10 knot wind. You could row at about 2.5mph for long times if the water was calm, no motor boat wakes allowed.
...We'll look again at leeboard size and geometry.
LARSBOAT, DOUBLE PADDLE CANOE, 15.5' X 30", 65 POUNDS EMPTY
Larsboat was built by Lars Hasselgren to replace a Folboat that had finally met its end. Lars wanted capacity for two, plus decking, as with his old boat.
I took Toto and lengthened it with a 30" plug in the middle to gain capacity. But lengthening a hull with a straight plug like this usually improves a boat in almost every way and Larsboat should be faster than Toto in good conditions. In this case the plug meant I didn't have to refigure the shape of the twisted bow panels as I would if I'd lengthened Toto with an overall stretch. (I can figure twisted panels pretty reliably now, but not back when Toto and Larsboat were drawn.)
The decking was quite simple because even the original Toto could take a forward deck of flat sheets with a center peak. I should add that I feel the decking is very optional. This prototype weighs 61 pounds and deleting the deck might cut another 10 pounds or so. The undecked boat also would have a better cartopping shape. I'd keep the stern chamber. It will ease your mind about taking a big wave over the stern.
This would be a preferred project for someonw who intends to do a lot of cruising and camping. In the Toto camping I've done the sleeping room has been OK, but the storage is limited. Larsboat would be better both because of increased capacity and because there is dry storage under the bow deck.
The basic hull is taped seam construction needing four sheets of 1/4" plywood for the decked version and three sheets for the undecked version. No jigs or lofting required. Plans are two blueprints with keyed instructions for $20.
The photo above is of Bob Smithson's Larsboat. He customized the decking a bit. I think he also built the boat of 1/8" ply to save weight. I've forgotten what his boat weighed but he did say it was sufficiently rigid for him.
Bob Hoyle built this one without a deck down in Florida:
Paul Moffitt built this one. You can see this is a much better two person boat than the shorter Toto:
And remember Garth Battista's vertical Larsboat?
And the old outboard motor guru Max Wawrzniak often goes for a paddle in his Larsboat:
I also drew up a trimaran sailing rig for Larsboat called "Trilars" which is sold separately for $10.
It is pretty much a clamp on rig except for the small mast step and partner. Down in Texas Charles Nichols built a model of it:
Then he built what they call down there a "Charlars" shown here with me behind the wheel at the Conroe, Texas messabout in 2002:
It's different from the Trilars. The main hull is per the Larsboat blueprint but built without the deck. I thought the rudder and leeboard were just like the Trilars print. The sail is a large balanced lug instead of a sharpie sprit. The floats are triangular in cross section instead of flat bottomed as on the Trilars, and I'm pretty sure the Charlars is wider than the Trilars which I drew narrow enough to trailer without disassembly. There wasn't much wind that day but I got a little sailing in with the Charlars and it seemed fine with the potential of being wicked fast. My only thought was that it was fast and stable enough (it was stable as a dock) that in rough water it might spear a wave and ought to have the fuller deck, something I think the paddling Larsboat can get by without in most cases.
And finally Jeff Blunk built a Trilars in Colorado. Haven't gotten a sailing report yet:
And Errol Smith down in Australia made a Trilars and has sailed it a lot. But it is not quite a Trilars and in particular has a centerboard instead of the blueprint leeboard (probably makes it a one man boat)..
Larsboat plans are $20 and the add on trilars plans are still $10 for now.
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 I heard about through the grapevine.
This is Ted Arkey's Jukebox2 down in Sydney. Has a temporary rig and he is working on the blueprint rig.
And the Vole in New York is Garth Batista's of www.breakawaybooks.com, printer of my book and Max's book and many other fine sports books. Boat is done, shown here off Cape Cod with mothership Cormorant in background, Garth's girls are one year older. Beautiful job!
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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