Jim Michalak's Boat Designs
1024 Merrill St, Lebanon, IL 62254
A page of boat designs and essays.
(15 October 2016) This issue will continue the D'Arcy Bryn design. The 1 November issue will continue the topic (eventually we will get to a 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.
Frank Coletta's wonderful custom Shanteuse in Port Aransas, Tx. Hmmmm...wonder what the neighbors think??
1024 Merrill St,
Lebanon, IL 62254
Send $1 for info on 20 boats.
NOW, WHERE WAS I.....
...To recap we were plugging away at a tough assignment of a small but heavy cruiser. In the first stage it looked like this:
There was an agreement between myself and the skipper that it would be a one man boat on a cruise. He did not want to haul a lot of ballast around and was willing to accept a boat that would self right on its own in a knockdown as long as the skipper hopped overboard as it went to 90 degrees, thus relieving the hull of some weight up high. Then the boat would self right and the skipper has to climb back on somehow. If we needed the ship to self right with the skipper in place a few hundred more pounds of ballast would be required to balance his weight and the skipper said he would rather swim than haul around lead.
The design progressed to where I had drawn the panel expansions and made a card model:
And made a Hullform model to get a good hack at where the center of gravity (cg) had to be for the boat to self right from 90 degrees.
The Hullform model said the boat will self right from 90 degrees of roll if the cg is at 19.0", or below, measured from the bottom of the hull. So imagine the hull balanced on an edge like this:
Now it becomes a question of doing a weight and balance estimate for the design and find a ballast weight and location that will give us the cg at 19" from the bottom.
To progress you need to have a drawing of the boat with a sail rig up and really an estimate of everything that will be in the boat. At this time I had a sail rig which used the same sails the skipper had for his present boat:
Now to paste in the weight guesses. Like this:
There can be a lot of gosh and golly hand waving, on top of pure ignorance, in a job like this. The sail rig isn't too bad in that you can take the yard, for example, say it is 2" round and 11' long and that makes it 414 cubic inches in volume. Typical wood might weigh .017 pounds per cubic inch (say 25 or 30 pounds for a cubic foot with a lot of variation) so you guess the yard will go about .414x.017=7 pounds or so. I've guessed 10 here being rational and conservative in engineering nature anyway and note that there are some ropes and pullies up there too and that I like round numbers. But you see how it is pieced together a step at a time. When I first worked at the missile factory I wondered how the weights engineers did their work and the answer is slowly and meticulously....calculating shapes and volumes and keeping ponderous track of rivets and rivet holes. But they did something I am copying here in estimations in the first stages. As Harry once said, "We always know how much it weighs. We just don't know where the weight is. Yet!".
An example of that is in the gross hull estimate of 500 pounds. Well, it will be more than 300 and less than 800 if it goes by the plans. But as the design progresses we can refine that weight and its location on the cg diagram and refine our ballast estimates with it. At this stage it is a guess.
...using the above scale model we measure upward from the bottom, which is here our baseline and datum line, and make a chart of it all with all the elements, with weight and height above the baseline, and find the cg location which might be thought of as the "average" location of all the weights together. Here is my chart:
There you have it. The "h" is the distance from the datum to the item. I've also figured the "L" which would be the distance from the bow tip to the item so I can get the horizontal location of the cg also and maybe can trim the boat level by also getting the ballast located best fore and aft. Anyway, the bare boat (with a 50# motor) looks here to weigh 609 pounds and the cg is 28.9' above the datum, well above the 19" desire. You can look at the chart and see that the hull itself and the main mast are the main contributers to the high up cg. No surprise there.
I've also shown in a separate calculation the cg location with a 200 pound skipper on the aft deck. Turns out I've placed him pretty much on the average for the boat so the vertical cg hasn't moved much but his weight aft moved the logitudinal cg aft.
NOW THAT NASTY BALLAST...
The plan is to bolt the ballast in lead bars to the bottom of the hull so that its average location will be at -2". The bottom of the calculation page shows the algebra. To get the cg down to 19" takes 276 pounds of ballast at -2".
Now before we fly off the handle about how I thought it would be less, I say it still will, in the end, be less than 276 pounds of lead. Here is why. As the structure is better defined the hull weight, given at first guess as 500 pounds 20" up, will be broken down into smaller pieces more accurately located. In particular this boat is to have a very thick plywood bottom, at least an inch thick. So I expect maybe we will have a hundred pounds of plywood there which will come right off the lead weight. I hope.
Even without that idea we are still in business I think. Total boat weight ready to go will be about 900 pounds on the trailer. And with the skipper and his lunch we might be 1200 pounds sitting in the water.
And let's look at the lead requirements if the skipper wants to stay on board for his knockdown. Now we need another 100 pounds plus to balance his weight as he sits on the aft deck. If he stands up all bets are off. Figuring ballast to counteract live folks on deck is just a dice throw. No telling what they will do at the time and their weight in a small boat is major portion of the total.
In the same way it appears that you would need another 100+ pounds of lead to counteract the weight of crew in general sitting on the aft deck. If you want self righting with say three 200 pound men you will need over 300 pounds of extra lead down low, the total adding about 1000 pounds to the floating weight of the boat. Now we are over 1600 pounds which was the original plan. The boat will float that weight. The original goal at that weight was for "reasonably good flow lines" which to me means the transom and chines are just starting to touch the waterline. She will float a lot deeper than that, just slower. Total weight sitting on the trailer would then be about 1000 pounds with about 40 percent of that in lead down low and if you read the old time books you might think I have reinvented the old "40 percent" rule.
HERE IS SOMETHING REALLY IMPORTANT...
...if you read the old books you might also find that "self righting" means a lot of different things. To Howard Chapelle it meant for a sharpie to recover from a 45 degree heel. I am pretty sure that is not good enough. For an ocean sailor I think it means recovery from a 140 degree roll or even a full 180 degree roll (one reason I'm not an ocean sailor). I've chosen 90 degrees. I have found that OK for the usual small boat sailing. A boat with wooden spars will not in my experience turn turtle even if it goes past horizontal because the wooden spars have emergency flotation way out from the hull. But the boat may lay stable on its side, especially if the sails absorb water. Not that I'm trying to scare anyone. But you need to know.
Believe it or not I still have words to write about this. I think next time I will do the more refined hull cg effort and show my idea of the ballast installation.
JONSBOAT, POWER SKIFF, 16' X 5', 200 POUNDS EMPTY
Jonsboat is just a jonboat. But where I live that says a lot because most of the boats around here are jonboats and for a good reason. These things will float on dew if the motor is up. This one shows 640 pounds displacement with only 3" of draft. That should float the hull and a small motor and two men. The shape of the hull encourages fast speeds in smooth water and I'd say this one will plane with 10 hp at that weight, although "planing" is often in the eye of the beholder. I'd use a 9.9 hp motor on one of these myself to allow use on the many beautiful small lakes we have here that are wisely limited to 10 hp. The prototype was built by Greg Rinaca of Coldspring, Texas and his boat is shown above when first launched with a trolling motor. But here is another one finished about the same time by Chuck Leinweber of Harper, Texas:
In the photo of Chuck's boat you can see the wide open center that I prefer in my own personal boats. To keep the wide open boat structurally stiff I boxed in the bow, used a wide wale, and braced the aft corners.
I usually study the shapes of commercial welded aluminum jonboats. It's surprising to see the little touches the builders have worked into such a simple idea. I guess they make these things by the thousands and it is worth while to study the details. Anyway, Jonsboat is a plywood copy of a livery boat I saw turned upside down for the winter. What struck me about that hull was that its bottom was constant width from stem to stern even though the sides had flare and curvature. When I got home I figured out they did it and copied it. I don't know if it gives a superior shape in any way but the bottom of this boat is planked with two constant width sheets of plywood.
Greg Rinaca put a new 18 hp Nissan two cycle engine on his boat, Here is a photo of it:
The installation presented a few interesting thoughts. First I've been telling everyone to stick with 10 hp although it's well known that I'm a big chicken about these things. Greg reported no problems and a top speed of 26 mph. I think the Coast Guard would limit a hull like this to about 25 hp, the main factors being the length, width, flat bottom, and steering location. Second, if you look closely at the transom of Greg's boat you will see that he has built up the transom in the motor mount area about 2". When I designed Jonsboat I really didn't know much about motors except that there were short and long shaft motors. I thought the short ones needed 15" of transom depth and didn't really know about the long shafts. Jonsboat has a natural depth of about 17" so I left the transom on the drawing at 17" and did some hand waving in the drawing notes about scooping out or building up the transom to match the requirements of your motor.
I think the upshot of it all is that short shaft motors need 15" from the top of the mount to the bottom of the hull and long shaft motors need 20". There was a lot of discussion about where the "cavitation" plate, which is the small flat plate right above the propellor, should fall with respect to the hull. I asked some expert mechanics at a local boat dealer and they all swore on a stack of tech manuals that a high powered boat will not steer safely if the cavitation plate is below the bottom of the hull, the correct location being about 1/2" to 1" above the bottom. But Greg had the Nissan manual and it said the correct position is about 1" BELOW the bottom. Kilburn Adams has a new Yamaha and its manual says the same thing. So I guess small motors are different from big ones in that respect.
But it seems to be not all that critical, at least for the small motors. Greg ran his Jonsboat with the 18 hp Nissan with the original 17" transom for a while and measured the top speed as 26 mph. Then he raised the transom over 2" and got the same top speed!
There is nothing to building Jonsboat. There five sheets of plywood and I'm suggesting 1/2" for the bottom and 1/4" for everything else. It's all stuck together with glue and nails using no lofting or jigs. I always suggest glassing the chines for abrasion resistance but I've never glassed more than that on my own boats and haven't regretted it. The cost, mess, and added labor of glassing the hull that is out of the water is enormous. My pocketbook and patience won't stand it. Glassing the chines and bottom is a bit different because it won't show and fussy finishing is not required.
Plans for Jonsboat are $25.
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
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
1oct16, D'Arcy Ballast, Larsboat
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