Jim Michalak's Boat Designs
1024 Merrill St, Lebanon, IL 62254
A page of boat designs and essays.
(1 October 2016) This issue will take a look at the D'Arcy Bryn design. The 15 October issue will continue the topic.
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.
Bob Mueller, again, this time showing off.
1024 Merrill St,
Lebanon, IL 62254
Send $1 for info on 20 boats.
Let us go back to 2009 when the boat that became known as D'Arcy Bryn was being born. The project then was known as Billsboat. Anyway, the boat was completed this year...2016. If you think that is a long time in the works I can assure you it is normal for a new design to take that long from new paper to finished boat. Often a lot longer. Happens to other designers too. In Bill's case the years included a retirement and a new house. He is still unpacking the moving boxes. Anyway, let us start the story with a rerun of its birth...
"I GOT TO THINKING about ballast again when I got an order for a small cruiser design. This was the sort of thing that would be about the size of a Micro but without the keel and with a Toto shaped hull underwater to smooth out rough waves.
I'VE NEVER DONE ONE OF THESE...
... in the past because I found them for all practical purposes "impossible". I'd tried it several times but here was the basic problem. You start with a 15' long hull and put two or three adults on it, sitting on a watertight deck two feet above the bottom. Let's say you figure 600 pounds for the crew and another 600 pounds for the basic boat and the usual sailrig. When you try to balance the thing to selfright from a 90 degree knockdown you need another 500 or 600 pounds of ballast in the belly and your little boat is approaching a ton in loaded displacement. Now, a 15' hull will have to float quite deep to float that much weight. Micro got away with it with about 450 pounds of lead in a keel about a foot below the boat's bottom, plus a 6' beam with hard chines, a totally flat bottom. When I think back about Micro I feel Phil Bolger must have gone down the dead end road a few times like I did with trying to get the little cruiser to do the job without strange proportions or a down low keel. But in the end he made it work in about the only way it could and it is a fine boat.
But this customer wanted no deep keel and no hard chines or pram bow. He was experienced with both and is indeed a very experienced builder and boat user. What changed the equation was that the boat was essentially to be a solo cruiser.
And we could, he said, cheat a bit on self righting. In particular it did not need to be self righting with the crew on board. If it got knocked down the skipper and crew would have to go for a swim, the boat thus lightened of a burden on high would self right by itself and the crew would need to climb back in. So the crew must be prepared in that way. The skipper said that was to him preferable to hauling around a lot of ballast. I would not advise a beginner to take on such a boat but I felt this man understands the trade off. So the boat would have enough ballast to self right with no crew on board and that would be enough.
I traded a few sketches and we settled on this to start:
Note the shape and the small bottom. There is only enough floor space to sleep one, right on centerline with his pillow in the V of the bow and his feet passing through the main bulkhead and under the aft deck, Micro style. But I was careful to make the aft deck long enough for sleeping. He wanted flared sides for looks mostly I think. Flared sides make a boat harder to design and harder to build but I suppose you only do that once and then you can admire it more for a long time. In practice I think it makes a boat more comfortable and also gives it wider shoulders in a knockdown. I'm sure Bolger would say something like," If you think giving it flare to make the top wider is good, then even better yet to make it wider all the way down with plumb sides." But this one is flared.
Even at this early stage you can make a quick hack at weight and displacement. For weight I would expect something like 600 pounds of hull and rig, maybe 200 pounds of ballast, for 800 pounds on the trailer stripped. Add a motor and some gear and maybe you have an even 1000 ready to sail. Add 200 pounds for a skipper and she sails away at 1200 pounds. For a rare excursion maybe two passengers add another 400 pounds so you have a max of 1600 pounds to float.
Now the idea is to have that hull make a hole in the water (imagine a clear plastic hull pushed down into the water) such that the volume of water displaced by the hole weighs 1600 pounds. This was to be a 15' boat but it will probably have a waterline closer to 13'. then the fattest part of the hull is 5.5' wide at a point 9" above the bottom, tapering to 3' wide at the bottom. So that part of the hull has an area of .75x(5.5+3)x.5=3.2 sq ft. Now, a "prism" with that shape and length would have a volume of 3.2x13=42 cu ft. Experience has shown that a hull with ends shaped like this will have a "prismatic coefficient" of about .55, so this new boat should have a displacement volume of 42x.55=23 cu ft of water when immersed 9" deep. Water weighs about 63 pounds per cubic feet so she will displace about 23x63=1450 pounds of water and we are pretty close to our goal. Actually I do this sort of numbers juggling very early on, right at the first sketch level.
AT THIS POINT...
I do a "lines" drawing and I take "offsets" a regular points, enough so that I can define the shape in numbers. I use those numbers to make a model with Hullforms, a free hydrostatics program you can download with the link shown in the links section of this website. Here is the Hullform model:
Hullform is a really smart program. I have no idea how it works but it works very quickly on any computer and I've never caught it trying to tell me a lie. So it tells me that this shape will displace 700 pounds at 5.7" draft, 1000 pounds at 7", 1200 pounds at 8", and 1400 pounds at 9". So my quicky guess was only off by 50 pounds. So the single skipper scene will draw 8" and the party boat scene just over 9". In real life add about 3" to those numbers since we are going to have a center skid with some lead hanging on it.
I am reminded now about why I always gave up on projects like this. If you wanted full knockdown recovery with a crew of say three you might need another 300 pounds of ballast, or more, and the weight just gets out of hand for such a short boat.
NOW BACK TO HULLFORM...
Hullforms will do a lot more than just give you displacement and different drafts. You can flip your model on its side. If you know the weight and cg location it will tell you if your boat will right from different angles of roll. A calculation like this is very very tedious to do by hand. So much that it essentially was never done much by hand and it took the computer to help us out.
When I first started doing these it was clear that the Hullform model has to be modified to give meaningful results since the cockpit will flood long before the hull is rolled sideways and then you have a different shape displacing water. Instead of high sides aft you will have just the low raised deck to float her there. So I modify the model to look like this:
With the newer versions of Hullform you can get a curve printed out of righting moment along a range of roll angles. But you have to tell it the weight and cg information and you have to figure that yourself and it ain't always easy. At this stage what you can do is put in our guess at the weight and run the program at 90 degrees of roll using various cg locations and find the cg location that gives a zero righting moment. If we can keep the cg below that point she will self right. Doing that, the winning cg location for this hull is ...(drum roll please)...19.0" above the hull bottom.
SO, HOW MUCH DO YOU WEIGH, ANYWAY????
....Well, I'm not going to get into that just now. I've decided to conclude this essay next issue in order to give a blowbyblow account of a weight and cg guestimation. So you've got two weeks to shape up because I will have to ask. But in the meantime...
HOW ABOUT A CARD MODEL...
I felt that the design had jelled to the point where it would work for the right skipper and I was going to refuse any more changes. Might as well press on with more details. At this point, with the offsets well defined, (you can actually use Hullform to fine tune your offsets), I dragged out my flat panels spreadsheet that we talked about a long time ago. To review, you imagine each plywood panel in the hull to be made of small triangles. Each offset point defines the common points of joining triangles. Then the spreadsheet uses the xyz of the offsets to determine the true length of each of side of each triangle, thus the true shape of each triangle, then it hooks them all together and spreads them flat to give, you hope, the true flat shape of the original panel so you can lay it out on plywood. I've always called this the Rable Triangle method although I would think that Sam Rable, who published the idea in the 1950's (special thanks to old buddy Dave Carnell for never throwing anything away!) used the method in big shipyards long earlier than that. They probably drew the triangles full sized on steel plates. It won't work with scale hand drafting because little drafting errors throw the final shape way way off. But even my Apple 2 had no trouble doing the math in a flash, hiding the fact that it took me weeks if not months if not years to finally fall into the needed equations.
Anyway, I figured the shapes of all the panels in 1/8 scale, made card copies of the panels and taped them together. An important step because if the model don't fit then the real boat don't fit. OK, here are three pics with the decks still off. First bow shot:
Stern view (there is a "temporary form" in the middle of the cabin and the bulkheads don't have openings under the rear deck yet):
Upside down (note how the fine bow and multichines really eats into the living floor space):
And then with the decks on, from the front:
And the stern:
Sort of looks like a boat.....
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:
Larsboat plans 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.
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
15oct15, Hullforms Model, Jonsboat
1nov15, Port Aransas2015, Piccup Pram
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
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