Jim Michalak's Boat Designs
1024 Merrill St, Lebanon, IL 62254
A page of boat designs and essays.
(15 October 2017) We discuss water ballast. The 1 November 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.
This is a modified Ozarkian by Matt Spencer in California. It has been shortened to 16' and widened a few inches, made more curvey, with more structure added. Looks great!
1024 Merrill St,
Lebanon, IL 62254
Send $1 for info on 20 boats.
GEE....THIS ESSAY FIRST APPEARED IN EARLY 1998!
I was worried about water ballast which eventually I used in several designs. Looking back over this I think it is still valid. It was at the time a somewhat touchy subject...
I SPENT A FEW HOURS.... (days) thinking about water ballast and how it works. In general I was bothered by the comment you will often see that only the portion of the water ballast that is actally raised above the waterline by the heeling of the boat is effective in trying to right the boat.
From my study I concluded that the above statement seems true for external water ballast, but not for internal water ballast. In the study I looked at five different ballast configurations on a very simple "boat" model. Each configuration was run through the Hullform6S program and the righting moment curves from 0 to 90 degrees of heel were determined and compared.
Figure 1 shows a diagram of how to figure the righting moment of a boat at a certain angle of heel. This is a static analysis which is to say the boat is not accelerating. It can be moving, but all the forces on the boat are in balance. If the boat is being pitched and rolled about in angry seas, a much more complex analysis is required. Also, the distribution of the ballast (as opposed to simply the location of its center of gravity) becomes a factor in a dynamic analysis.
What we have here is the wind's pressure on the sail, up high, and a balancing load on the keel or fin, conspiring to tip the boat over. That "moment" or torque, is counteracted by the weight of the boat pushing down and the buoyancy of the boat pushing up. These last two forces are not in line with each other when the boat heels but are separated by a distance called the "righting arm". If the weight of the boat in is "pounds" and the length of the righting arm is "feet", the righting moment is measured in "foot pounds".
To figure the foot pounds of the righting moment, you need to know the weight of the boat and the location of the center of that weight - the "CG". Also you need to know the location of buoyancy of the heeled boat. (The buoyancy itself is the same as the weight in a static analysis.)
The CG is sort of the "average" location of all the weights. To get stated figuring a CG location, you must have a reference line and in these examples I will use the bottom of the boat as the reference. We're only going to figure the vertical (up and down) CG in these simple examples, but in a complex project you might also figure the location laterally (side to side} and longitudinal (along the length) locations.
Let's say our boat only had two elements, the hull and all its contents weighing 500 pounds with that weight centered 2' above the bottom, and let's say 250 pounds of internal ballast centered 3" (which is .25') above the bottom. So the total weight is 750 pounds.
To find the CG, you multiply each weight by its vertical location, add all those pieces, and then divide that sum by the total weight. So the example calculation would be CG = (500x2)+(250x.25) all divided by (500+250 and that equals (1000+62.5) / 750 = 1.42' above the base line.
So the effect of the 250 pounds of ballast was to lower the CG from 2' to 1.42', a difference of 7".
In this calculation the makeup of the ballast is not a factor. The only factors are the weight and location of the ballast. The only way the ballast material could be a factor is if it were of such density that it could be centered closer to the bottom. For example if the ballast were water inside a rectangular tank 6" deep, 4' wide and 2' long, it would amount to 4 cubic feet of water which is about 250 pounds and it would center at 3" above the bottom and provide the above CG location of 1.42". If we switched to lead which is 11 times denser than water, we might have a plate only .54" thick. We could mount it centered .27" (which is .0225') off the bottom instead of 3". The new CG would be ((500x2)+(250x.0225))/750=1.34'. So the CG of this lead ballasted boat is lower by less than an inch.
...FIGURING BUOYANCY LOCATION...
This is very hard to do by hand. I'm going to use Hullform6S as a tool to do this. To keep lots of variable from getting in the way, I'm going to use the above pictured "boat" as the example. It is just a box, 16' long, 4' wide, 2'deep. It weighs 500 pounds with its unballasted weight centered on the top of the box, 2' above the bottom. I'm going to ballast the box in four different ways, roll each example over at 10 degree intervals to 90 degrees, and record and plot the righting moments as predicted by Hullform6S.
So example A will be the above unballasted 500 pound box.
Example B is shown below. It's the same as A exept it has a 250 pound lead fin with the weight of the fin centered 2' below the bottom. So the CG of this combination is .66' above the boat's bottom. But, the 250 pounds of lead displaces some water, right? Its volume amounts to .36 cubic feet of lead displacing the same amount of water which is 22 pounds of water trying to float the lead back up. So the total ballast effect of the lead while it is under water is actually about 228 pounds. If the boat heels to the point where the lead is totally out of the water, it has 250 pounds of ballast effect.
Example C has the same 250 pound fin configuration as example B except a water filled tank, 2' long, 6" wide, and 4' deep, is used instead oflead. That amounts to 250 pounds of ballast water centered 2' below the hull so the CG of the total boat/ballast combination is the same .66'above the bottom. Again, the 250 pounds of external ballast displaces some water. The water ballast displaces its own weight in water! So the ballast weight is exactly balanced by the buoyancy of the displaced water. So this underwater tank has no ballast effect as long as it is under water. When the boat heels enough to raise it out of the water, it becomes effective.
Example D is really the same as C except the ballast tank has been moved inside the hull. So now it is 6" deep, 4' wide and 2' long. The ballast weight is centered 3" above the bottom of the hull and the overall CG is at 1.42' above the bottom. We don't have a separate external tank displacing water. But compared to example A the hull sinks a bit deeper to float that extra weight so the statics of the Hullforms analysis is a bit different.
Example E has a 250 pound V shaped ballast tank on its bottom. Whether it might be called internal ballast or external ballast is in the eye of the beholder. I included it because I thought it might represent some water ballasted trailer sailers you can buy.
The results of the Hullform6S study are shown below:
Curve A is, I think, pretty typical of a light flat boat with no ballast. The maximum righting moment of about 450 foot pounds is reached quite quickly although I think a real boat would have the peak at about 20 degrees. The boat looks to capsize at about 45 degrees heel. My experiences with Jinni, about this size and weight, were similar. It capsized twice in the time I had it. Both time it went over well before it shipped any water over the rail.
Curve B shows the how effective metal outside ballast can be. Not only is the maximum righting moment about twice that of example A, it still has substantial righting ability at 90 degrees of roll. It will tend to roll upright at any angle of heel up to about 110 degrees.
Curve C, external water ballasted fin, is an interesting one. Until the fin starts to exit the water as the boat rolls, it has no effect. When fully out of the water (about 80 degrees of roll) it is as effective as metal. In between its righting moment goes to about zero. If rolled to about 50 degrees it would stay there until acted upon by an outside force such as a wave or maybe the crew moving about. If rolled to a greater heel angle it will try to return to 50 degrees. If rolled to a lesser degree it will continue to roll fully upright.
Curve D, internal ballast, has about 50% greater maximum righting power than no ballast or the external water fin. It should easily outsail a water fin boat up until it capsizes at about 65 degrees of heel. At that point the water fin boat gets back on its feet while the internal ballast boat flops over.
Curve E, V ballast tank, cuts across about everything as a compromise. It doesn't have the initial stability of the internal ballast boat, but it has positive stability at high roll angles. I've heard water ballasted production boats behave this way.
I can't see any obvious winner here. All have advantages and disadvantages. However, if you had a particular type of boating in mind, the chart may help you make a choice. For blue water sailors, the metal fin seems the way to go. Lots of righting ability. For inshore sailors where a rare capsize won't mean death, perhaps the internal water ballast, with its simple trailering abilities due to light unballasted weight and very low draft, will be to your advantage. The V tanked boat might be best for someone who cares a bit more about ultimate stability. A combination of all of the above might be in order for some folks.
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:
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
1nov16, D'Arcy Ballast 3, Piccup Pram
1dec16, Sail Area Math, Ladybug
15dec16, D'Arcy Thoughts, Sportdory
1jan17, AF3 Capsize, Normsboat
15jan17, The Weather, Robote
1feb17, Aspect Ratio, Jewelbox Jr
15feb17, Aspect Ratio 2, IMB
1mar17, Normsboat Capsize, AF4Breve
15mar17, Underwater Board Shape, Harmonica
1apr17, Capsize Lesson, RiverRunner
15apr17, Measuring Leeway, Mayfly16
1may17, Scarfing Lumber, Blobster
15may17, Rigging Lugsails, QT Skiff
1jun17, Rowing1, Mayfly14
15jun17, Rend Lake 2017, Mixer
1jul17, Rowing2, Viola14
15jul17, Rowing3, Vamp
1aug17, RowingSetup, Oracle
15aug17, Taped Seams, Cormorant
1sep17, OliveOly Capsize Test, OliveOly
15sep17, Plywood Butt Joints, Philsboat
1oct17, Sailing OliveOyl, 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