Jim Michalak's Boat Designs
118 E Randall, Lebanon, IL 62254
A page of boat designs and essays.
(28FEB98) This issue hopes to show the basics and effects of water ballast on static stability. Next issue, about 15 March, I'll continue the subject with a look at putting water ballast on a real boat.
MORE NET TREASURE...
If you click on Blue Peter Marine.you will take a trip to Australia. This particular page has a free"Hullform6S" program you can download. This is a "thinking" program that seems very good at calculating displacement and stability and lots of other things that can be very difficult to do by hand. It is not really a CAD program to allow you to easily draw a picture of a boat. The best approach seems to be to design the boat first, and reconstruct it in the Hullforms program by typing hull offsets in the proper place. The program runs very nicely in DOS even on old slow machines.
KARL JAMES' JEWELBOX
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.
JEWELBOX, BEACHBOAT, 19' X 6', 800 POUNDS EMPTY
Someday I may get to put my full catalog on the net. For now I'll put one design in each issue.
Here is a boat that is supposed to be self righting to a great degree without ballast! Jewelbox has the combination proven by Bolger's Birdwatcher of high sides with a thick bottom and low down crew weight to act as ballast. Karl James told me his boat has righted from having its windows totally submerged. Thanks to Tim Webber for the scans. The Texas grapevine reports the original boat has transferred hands as Karl has gone on to designing his own. No surprize there since Karl had told me of adventures with 5 different boats he had owned. His Jewelbox has been all over the country, including the Mexican trip shown in the photo above, and a lake/canal journey across the Florida peninsula. Here's the write up from my catalog....
The Jewelbox prototype was built by Karl James of Winnsboro, Texas. Here's Karl raising the sail. You can see how safe the operation is. It's all done from inside the boat including stepping the mast.
In her first season this boat sailed in Mexico, the Apostle Isles of Lake Superior, Montana's Fort Peck Reservoir, and Yosemite Lake. Karl also brought it to our Midwest Messabout and I had a chance to sail her. We had almost no wind that day and as we ghosted along I asked him if the sail area was about right. He replied that it was right for a Texas wind, hinting that everything was bigger and better in Texas.
I watched Karl launch and rig Jewelbox solo in 15 minutes without getting his feet wet. Stepping the mast is a 30 second operation and it brings up one of the advantages of a lug rig. Jewelbox sets almost 160 square feet of sail on a 20 foot mast you can step solo without strain. Not only is the lug mast shorter than most other types, it can be lighter because mast deflection doesn't harm the set of the sail as it does in other types. So rigging Jewelbox is about like rigging a Sunfish. But this boat will sleep three adults inside the self-righting Birdwatcher cabin with lots of dry storage under the fore and aft decks. Karl trailers his with rudder and 6 hp motor in place ready to go. There's a self draining well in the bow to hold anchors,, muddy shoes, etc., with a step through bow transom.
I should mention that Karl made some beautiful segmented hard covers for the walkway slot. But later he went to a simple snap on fabric job. He greatly prefers the fabric job for camping. The hard covers are used for trailering and storage.
Jewelbox uses self erecting construction throughout. She needs 2 sheets of 3/16" Plexiglass, 7 sheets of 1/4" plywood, and 9 sheets of 1/2" plywood. It's all simple glue and nail construction with no jigs or lofting. Plans are $35.
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. (If you order a catalog from an internet page you might state that in your letter so I can get an idea of how effective this medium is.) Payment must be in US funds. The banks here won't accept anything else. (I've got a little stash of foreign currancy that I can admire but not spend.) I'm way too small for credit cards.
Anyway..... Anytime a design from the Catalog of Prototypes starts getting built I pull it and replace it with another prototype. So that boat goes into limbo until the builder finishes and sends a test report and a photo. There are three boats in that catagory right now.
Scram Pram, a 16' multichine Birdwatcher type, has been completed and had its first sail near Savannah. I'm hoping for a good photo one of these days. The builder said it's hard to get good photos of your own boat and he's quite right about that. The surest way is to get a good beach shot of it.
And Skat, a 12' cat boat daysailer, is being built near Phoenix. Latest word from the West Coast Spy is that the boat has been glassed on the outside and ready to flip and finish.
The Kansas Boat Psychologist keeps plugging away at his Fusebox. The front porch is done. The hull has been flipped and the bottom installation started. I'm leaving Fusebox in the Prototypes Catalog for now because it has appeared on at least three web pages. It looks as though the name "Fusebox" may give way soon to the name "Harmonica" at the prototype builder's request.
Here on the AF4 home front, the boat is essentially done.
The painting, which is never really done on my boats, went OK. The schedule was for two coats of primer and two coats of top paint. But I found the plywood had such a good surface that one coat of primer seemed sufficient. Besides, one coat over the whole hull pretty well used up my gallon of primer. (Anybody know why oil based primer is sold to prep wood for latex paint???) Below the wale, the hull is painted dark Hunter green. Three coats needed to get sufficient density here but I've still got half a gallon left. Above the wale and interior are painted flat buff (Dairy is what they called the color.) Only one coat seemed needed to totally cover the primer and I still have 3/4 of a gallon left. I'm concerned that the flat color will hold dirt too well. Our shores here are very muddy and keeping a boat clean is very difficult. In that same line, I've varnished the floors with three coats, two quarts being needed. (Couldn't find gallon cans of oil varnish.) Varnished floors seem to hold up much better and show dirt less than painted floors.
I mounted hardware and windows. Not much hardware compared to a sailboat.
Started prepping my old trailer for the new boat according to the stuff I wrote a few issues ago. Will make a point of weighing the trailer before and after loading the boat, probably the first time anyone in history has ever done such a thing.
This boat will have to live outside. So I'll need to make a custom cover for it. I've done this before using polytarp, trimmed and sewn to shape with a drawstring and tiedowns around the perimeter. I may dart this one to make it form fitting. After that I'll need to dream up a tarp cover for the cabin walkway. The bimini will be a summer project.
Right now I'm 110 hours and $420 into the project. . With luck, AF4 will be launched in the first week of March. Actually we've had very good weather lately for testing a new boat like this. But usually in the Spring it gets so windy here that one must be quite careful. Testing new boats on windy days is probably the biggest downfall for the inexperienced. You can get in big trouble and lots of folks do. You just HAVE to wait for a day with light winds.
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