Jim Michalak's Boat Designs
1024 Merrill St, Lebanon, IL 62254
A page of boat designs and essays.
(1 November 2016) This issue will continue the D'Arcy Bryn design. The 15 November issue will get to a D"Arcy sailing report.
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.
The OliveOyl prototype is about done but will go into hibernation for the Canadian winter.
1024 Merrill St,
Lebanon, IL 62254
Send $1 for info on 20 boats.
Ballast Again 3
NOW WHERE WAS I?????
...We tried to work out the ballast details for the little cruiser.
The idea was to get the cg down to 19" above the bottom, which is where the Hullform model predicted that shape hull would self right when rolled up to 90 degrees (beyond that and it will most likely lay on its side somewhat mast down as long as the spars are buoyant wood). Crashing through the numbers revealed that 380 pounds of ballast just below the bottom was needed to get that cg position with a crew of one on board, and 280 pounds needed if there were no crew on board, ie, the skipper has abandoned ship, the ship self rights when he does so and he has to regain the ship somehow. The skipper who commissioned the boat wants that second option. So there we are....almost.
As Harry, the weights guru, told me back at the missile factory, "We always knew what the thing will weigh, we just don't know where the weight is yet." In particular my guess that the bare hull structure would weigh 500 pounds and that its cg would be 20" above the base can be refined now.
At this point we now know the bare hull will need six sheets of 1/4" plywood (about 150 #), five sheets of 3/8" plywood (about 170 pounds) and three sheets of 1/2" plywood (about 150 pounds). The total ply weight is about 470 pounds and I think that should be about the weight of the hull structure so the 500 pound estimate seems secure. But a third of that weight is in the bottom and my assumption that its cg will be 20" above the base can be refined to 18". A small bit of advantage perhaps. So the resulting total cg is a bit lower than I first assumed and crunching through the ballast numbers again shows 230 pounds of lead should suffice instead of 280 pounds.
While we're on this topic I will say that the bottom is intended to be two laminations of 1/2" plywood. Any extra thickness down there is almost as good as lead as far as the total cg is concerned. I would very seriously consider making the bottom three plies thick, a full 1-1/2", and then putting the lead requirement down to 200 pounds. I think I will do that. With a bottom as bulletproof as that we can I think bolt the lead straight through the bottom, no extra structure required.
NOW, ON TO THAT LEAD....
I have cast some lead but always in smaller castings. In particular I did a long time ago a Micro ballast. That was supposed to be one 425 pound casting but Bolger's structure clearly allowed for it to be done in small sections glued and nailed to the keel structure which has plywood facings. I think my lead "bricks" were about 40 pounds each and that was easily done solo. So we can do that again with this one.
Lead weighs .4 pounds per cubic inch so 200 pounds of it would be 500 cubic inches. Lets do that in eight 25 pound "bricks". So we want 62 cubic inches per brick. So if you make a mold to cast a brick that is 1.5" thick, 3.5" wide, and 12" long, you get the 25 pound brick. Do that eight times. Each brick will be secured along the outside bottom centerline more or less as a "shoe" to total 8' long. They won't be fastened to each other since we don't need strength in the lead because we have a really thick bottom. Let's bed each brick to the bottom with latex caulk and fasten with a 1/4" stainless bolt at each end, total of 16 required. Bolt head on the inside of the hull, also sealed over with say rtv, and nut end on the outside in a recessed hole in the lead brick, also all sealed over with rtv. The idea is to allow for future removal. It should work and be done with some ease by a solo builder. Last I looked 200 pounds of scrap lead would cost about $100 but its daily value can fluctuate wildly.
NEXT TO GLANCE A RIGHTING MOMENT DIAGRAM...
Hullform will calculate righting moment diagrams in a jiffy. These are nearly impossible to do by hand in a normal lifetime so small boats always did without them until the PC was invented.
Anyway, I ran the Hullform program for this shape with a total weight of 1000 pounds (probably the minimum this boat will sail at) and the 19" cg location. Here is a graph of the results...
Hmmmmm... Interesting. This is actually a composite of two Hullform models. The first is the full hull model and the second model has a cut down stern that is supposed to simulate flooding of the cockpit deck, but not under it. I assumed the cockpit would flood at about 60 degrees so don't take that part of the curve too seriously.
Well, there is nothing kinky about it. I don't see any sudden swings that you would see with a light flat bottomed boat that will capsize all of a sudden. This one settles in at about 700 ft-lbs. By the time it starts to fall off at around 60 degrees the cockpit would be flooding and I would hope the skipper is getting scared enough by then to release the sheets if he can get to them. Well, so far so good.
NOW FOR A LOOK AT THE SAIL AREA...
The above curve is for what is probably the lightest sailing weight for this boat, that is with a single skipper and almost no gear. If she weighs more you can multiply those righting moments by the ratio of the higher weight, so if she weighs 1500 pounds then the max righting moment would be about 1000 ft-lbs. But don't forget eventually at about 90 degrees the righting moment goes to zero and any ratio times zero is still zero.
The righting moment is resisting the overturning moment produced by the sail's sideways force and by the force on the leeboard that is opposing the sail's sideways force. When a boat is close hauled almost all of the sail's force is really to the side and I will assume that for now to make like simpler here. The center of the sail area on this boat is about 12' above the center of the leeboard area so to make a 700 ft-lb overturning moment the sail's force would be just 58 pounds. Now the question is "How much wind will produce 58 pounds on the 113 sq ft main sail?"
OK, in a general way the force of the air on a sail or wing or whatever is F= .0034 x S x C x V x V where S is the "area" in square feet, C is a coefficient related to foil shape etc that we need to look up from test data, and V is the wind speed in knots. So this mainsail has an area of 113 sq ft, and the C is usually about 1.5 when the wind is fully applied on a typical good sail. It works out that V would be equal to 10 knots. We would expect the boat to be overpowered if the wind exceeded 10 knots. That is not very windy but lug sails usually reef well so the skipper will need to keep an eye out. On the other hand in order to sail well in light winds you need to have a lot of square feet up there. As an aside I should say that you find as you get elderly that a small sail area to handle higher winds and a good motor to handle lower winds is a very good combination.
WELL, WE'RE ALMOST DONE....
One last quicky check we can make now is to get a wag at the stress in the mast. Most masts really are sized by how much they bend more than their strength but once we have gone this far the strength check is pretty simple.
The weak point for the mast is bound to be right where the mast meets the mast partner because that is where the bending is the highest. I'm talking about free standing masts like this one, masts that are supported by guy wires are a whole different animal.
If you know your boat is going to capsize if the force on its sail exceeds 60 pounds then you can start with that number I think as a check. After all, if the sail force exceeds that then she goes over and you have an entirely different problem. (Once I mentioned to Phil Bolger that the boat is a lot more stable in pitch so a boat getting blown downwind could have a much higher mast load than one that is close hauled. Phil thought about that for a few seconds and simply responded," Yes, that makes sense. But sailboat masts always fail when close hauled." I've always accepted that and only make the close hauled check.)
Anyway, this boat has its sail area centered about 7' above the mast partner. So the actual bending moment of the mast at the critical point is about 60 x 7 = 420 ft-lbs at the capsize point (we need to convert that to 5000 inch pounds to be consistant in future calculations). The elastic bending stress in the mast is then easily calculated. The proposed mast there would be 3" square. The maximum fiber stress in that square section would be stress=6xM/(txtxt) where M is the bending moment and t is the mast thickness. It works out to be 6 x 5000/(3x3x3) =1100 psi. Perfect Sitka Spruce might handle up to 10000 psi but probably not pine from Lowes. So the stress sounds low but there are many warnings. For one the righting moment was taken at the lightest boat weight with no one hiking out. And this is a static analysis. A dynamic analysis with hull rolling etc might run the numbers way up, but I'm too stupid to do a dynamic analysis. Like the great engineers of olden days I'm satisfied with static numbers and a big margin of safety.
PICCUP PRAM, SAIL/ROW PRAM, 11' X 4.5', 90 POUNDS EMPTY
Piccup Pram was the first boat of my design to get built, back in 1990, I think. I still have the prototype and use it regularly. I designed it to be the best sail/row boat I could put in the back of my short bed pick up truck. But I found it to be a good cartopper, too. It has capacity and abilities I had previously thought impossible in a 90 pound cartopper. The photo above shows the original 55 square foot sail on Pensacola bay a long time ago. Piccup is a taped seam multichine hull which can take a fair amount of rough water.
Piccup continues to be one of my most popular designs and I get nice photos from builders. Here is one of Richard Donovan hoping for more wind up in Massachusetts.
Richard's Piccup has the larger 70 square foot sail that prefer myself. It's the same as the original but is 2' taller. This balanced lug sail sets on a 12' mast and rolls up easily for storage on its 9' yard and boom. The idea was to be able to store the rig easily in the boat during rowing and it works. There is a pivoting leeboard and kickup rudder on the boat and they can be left in place raised while rowing. Converting to full sail takes a couple of minutes as you step the short mast, clip on the halyard and tack lines, hoist the sail, lower the boards, and off you go. And the balanced lug sail reefs very well although reefing any small boat is best done on shore.
Here is a Piccup by Vince Mansolillo in Rhode Island, a nice father/son project. Piccup will be large enough to hold both of them. You can see the large open frameless cockpit, large enough for sleeping. And you see the buoyancy/storage boxes on the end.
But Piccup will take two adults as seen in the photo of Jim Hudson's boat. Jim's boat has a polytarp sail as does my own Piccup.
These boats have proven to be good for sail rig tinkerers (be sure to read and apply the Sail Area Math essay before starting). Here I am in Piccup with a polytarp sharpie sprit sail. The rig is different from the originals but the hull here is totally unchanged (except for paint) from the original shown on the beach at Pensacola.
I think my own Piccup has had about six rigs of different sorts and was always the test bed for the polytarp sail experiments. But, hey!, that's nothing compared to the tinkering the late and great Reed Smith did with his out in California. Here is his Piccup rigged as a sharpie sprit yawl!
Here is Rob Rhode-Szudy's yawl rig Piccup that was featured in his essays about building Piccup that you can access through the old issue links.
Here is another by Doug Bell:
This one is by Jim Islip:
And this one by Ty Homer:
Piccup Pram uses taped seam construction from five sheets of 1/4" plywood.
Plans for Piccup are still $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.
We have a Picara finished by Ken Giles, past Mayfly16 master, and into its trials. The hull was built by Vincent Lavender in Massachusetts. There have been other Picaras finished in the past but I never got a sailing report for them...
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:
D'arcy Bryn is done and sailing here on its first voyage, on the Texas 200. I never suggest you should test a new boat on a trip like this but it worked this time. I will print a full story in a future issue.
The first Jukebox3 is on the (cold) water. The mast is a bit too short - always make your mast too long. A bit more testing will be nice...
A brave soul has started a Robbsboat. He has a builder's blog at http://tomsrobbsboat.blogspot.com. (OOPS! He found a mistake in the side bevels of bulkhead5, says 20 degrees but should be 10 degrees.) This boat has been sailed and is being tested. He has found the sail area a bit much for his area and is putting in serious reef points.
AN INDEX OF PAST ISSUES
THE WAY BACK ISSUES RETURN!
MANY THANKS TO CANADIAN READER GAETAN JETTE WHO NOT ONLY SAVED THEM FROM THE 1997 BEGINNING BUT ALSO PUT TOGETHER AN EXCELLENT INDEX PAGE TO SORT THEM OUT....
THE WAY BACK ISSUES
15nov15, Hullforms Results, Caprice
1dec15, Sail Area Math, Ladybug
15dec15, Sailing For Nonsailors 1, Roar2
1jan16, Sailing For Nonsailors 2, OliveOyl
15jan16, Sailing For Nonsailors 3, Robote
1feb16, Sharpie Sprit Rigging, Laguna
15feb16, Trailering Plywood Boats, IMB
1mar16, Hollow Spars, Slam Dink
15mar16, Bulkhead Bevels, Frolic2
1apr16, Capsize Lessons, RiverRunner
15apr16, Wood Vs Aluminum Spars, Mayfly16
1may16, Scarfing Wood, Blobster
15may16, Prismatic Coefficient, Roar2
1jun16, Figuring Displacement, Mayfly14
15jun16, Rend Lake 2016, Mixer
1jul16, Ballast Calculations 1, Dorado
15jul16, Ballast Calculations 2, Robbsboat
1aug16, Ballast Calculations 3, AF4
15aug16, Taped Seams, Cormorant
1sep16, Butt Joints, Vireo
15sep16, Old Outboards, Philsboat
1oct16, D'Arcy Ballast, Larsboat
15oct16, D'Arcy Ballast 2, Jonsboat
Mother of All Boat Links
The Boatbuilding Community
Kilburn's Power Skiff
Bruce Builds Roar
Rich builds AF2
JB Builds AF4
JB Builds Sportdory
Puddle Duck Website
Brian builds Roar2
Herb builds AF3
Herb builds RB42
Barry Builds Toto
Table of Contents