Jim Michalak's Boat Designs
118 E Randall, Lebanon, IL 62254
A page of boat designs and essays.
(1Nov99) This issue shows how to figure the center of gravity for your new boat design. Next issue, 15Nov99, will show how to use the Hullforms 6S program to get a stability curve for the new design.
If you are interested in free analytical boat design software go to Hullforms Download (archived copy) and download some. Next issue I'll be using the oldest version, Hullforms 6S, if you care to follow along.
BILL MOFFITT'S WOOBO
Center of Gravity
WHAT IS IT??...
Here is a figure showing AF2 with my guess at the weights of the major components and a first shot at their locations. The object is to find a "center of gravity" location which is sort of an average of all the weights and locations. It would be where all the weights might be concentrated to represent the entire thing in a simple way. For example in the longitudinal direction, the X axis on this figure, you could predict where the hull would balance level while hoisting with a single line. You could figure out where the boat should sit on its trailer and provide the proper tongue weight. You could also figure out a stability curve for the boat by knowing the cg in the Z direction, the vertical direction, and from there tell where the boat will capsize, or how much sail the boat can carry in a certain wind, or how much wind is needed to capsize the design with its given sail area.
In the figure I've shown the reference lines forming the X and Z axes that I used to make the calculations. (The Y axis would be into the page and be a lateral axis.) These reference lines can be drawn anywhere but once you start the calculations they must not be shifted.
I'm going to start by calculating the vertical cg of the empty boat. The chart below lists the individual items making up the total empty weight, plus the vertical distance from those weights to the reference line, which in this case is a horizontal line drawn through the lowest point of the hull. The third column multiplies the weight times that vertical distance.
Item Weight W(lbs) distance z(in) Wz(in-lb) Hull 460 18.0 8280 Bottom 75 2 150 Mast 40 120 4800 Sail 5 130 650 Yard 10 210 2100 Boom 10 55 550 Empty Boat Total 600 27.6 16530
A bit of explanation about the weights. The sailing rig bits are roughed in by just figuring the volume of the elements and multiplying by 30 pounds per cubic foot, an approximate density of typical wood. The sail weight is just the sail area converted to yards times maybe 5 ounces per square yard, the typical weight of sailcloth. The hull weight is figured from the weight of the plywood that goes into it, I showed how to do that in an essay about guessing weight a while back. But the element "bottom" is a twist to account for the weight of extra thick planking on the bottom. Here is how I do that. The main hull is 1/4" plywood but the bottom is two plies of 3/8" thick plywood for a total there of 3/4". I look at the hull as a tube of 1/4" plywood that is 36" deep, so its weight is centered 18" above the base line. Then the extra 1/2" thickness of the bottom is accounted for seperately as another 75 pounds about 2" above the baseline to account for some rocker in the bottom. The weights are guesses at this point.
The bottom line totals for the weight W and the moment Wz are simply the sums of those individual columns. The cg height z in the center column is what we are really looking for and is not a simple sum of the center column. To find z of the empty hull you divide the total moment 16530 inch-pounds and divide by the total weight 600 pounds and you get 27.6 inches which is sort of the average location of everything. Remember that this is a first cut approximation and don't be fooled by the decimal precision of the answer.
Actually the boat will never sail in this condition. But with the basic empty hull numbers calculated it is easy to add other items and see the effects. Let's add a 180 pound skipper sitting on a low bench with his own cg 18" above the baseline. This would be the minimum sailing condition. Here it is:
Item Weight W distance z Wz Empty boat 600 27.6 16530 Skipper 180 18 3240 New total 780 25.3 19770
So in this case we've treated the empty boat as a single entity and simply added the skipper. Since the skipper is seated on a very low bench, the effect of his weight is to lower the overall cg height a bit.
And you can go on adding and adding different combinations of people and situations. Here we'll add a crew member to the above figures. The effect is to lower the cg a little bit more since he is sitting so low.
Item Weight W distance z Wz Empty boat 600 27.6 16530 Skipper 180 18 3240 Crewman 180 18 3240 New total 960 24.0 23010
We'll figure one last combination. We will use these numbers next issue when we figure the stability of the different combinations. This last one will be 400 pounds of internal ballast centered 3" above the floor:
Item Weight W distance z Wz Empty boat 600 27.6 16530 Skipper 180 18 3240 Crewman 180 18 3240 Ballast 400 3 1200 New total 1360 17.8 24210
So the effect of the 400 pounds of internal ballast is to lower the cg over 6".
You can also figure the fore-and-aft cg the same way except instead of using the vertical dimension "z" you use the longitudinal dimension "x". The result is the longitudinal cg which effects how your boat will trim fore-and-aft. Here are numbers for the different weight conditions given above but this time the x dimension is used, x being measured from the forward perpendicular of the boat:
First the empty boat longitudinal cg:
Item Weight W(lbs) distance x(in) Wx(in-lb) Hull 460 120 55200 Bottom 75 120 9000 Mast 40 66 2640 Sail 5 130 650 Yard 10 130 1300 Boom 10 130 1300 Empty Boat Total 600 117 70090
Now with skipper:
Item Weight W distance x Wzx Empty boat 600 117 70090 Skipper 180 175 31320 New total 780 130 101410
Now with skipper and crew:
Item Weight W distance x Wx Empty boat 600 117 70090 Skipper 180 175 31320 Crewman 180 140 252000 New total 960 132 126610
Now with skipper, crewman, and 400# ballast:
Item Weight W distance x Wx Empty boat 600 117 70090 Skipper 180 175 31320 Crewman 180 140 25200 Ballast 400 100 40000 New total 1360 122 166610
Again I've made a lot of assumptions here but in general the empty boat's longitudinal cg is a little forward of center at 117" mostly because of the weight of the mast. When the skipper sits back at the tiller the cg moves aft to 130". Add a crewman sitting aft of the main bulkhead and the cg moves aft again to 132". Add 400 pounds of ballast forward of the main bulkhead and the cg moves forward to 122".
The situation with the lateral cg is a bit different for two reasons. One is that most boats are symmetric so the empty boat cg will be on centerline, or have y=0 if you use the centerline as a datum. With AF2 some of the sail rig is not on centerline but I'm going to ignore that for now. Then the only items that would enter into the lateral cg would be the crew assuming they were not sitting on centerline. The effect on heeling can be substantial. The second reason is that I don't know of any free design programs that figure the effect of a cg off the centerline. But in a future issue I'll show you how to figure the effect on heeling moment with hand calculations.
So we'll assume this AF2 is symmetric and that means we can skip the first "empty boat" table.
Now with skipper:
Item Weight W distance y Wy Empty boat 600 0 0 Skipper 180 18 3240 New total 780 4.2 3240
Now with skipper and crew:
Item Weight W distance y Wy Empty boat 600 0 0 Skipper 180 18 3240 Crewman 180 21 3780 New total 960 7.3 7020
Now with skipper, crewman, and 400# ballast:
Item Weight W distance y Wy Empty boat 600 0 0 Skipper 180 18 3240 Crewman 180 21 3780 Ballast 400 0 0 New total 1360 5.2 7020
Let's put our cg guesses into one last table showing the four weight conditions we looked at. We'll use this stuff later (trust me).
Condition weight cgx cgy cgz Empty 600 117 0 27.6 With skipper 780 130 4.2 25.3 With two men 960 132 7.3 24.0 With two men and ballast 1360 122 5.2 17.8
We'll use Hullforms to make a computer model of this AF2.
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WOOBO, ROW/SAIL SKIFF, 15' X 4', 150 POUNDS EMPTY
Woobo was designed for the "perfect skiff" competition that Wooden Boat magazine had about ten years ago (hence the name "Woobo"). I didn't win anything and the experience led me to never enter another contest. Although the contest was well conceived I had the impression the final judging was done by no one who had read the contest objectives. The usual result is that the winning boat is the slickest looking boat but not the best functionally. The winning design in this case was quite good but I'd still bet that Woobo is superior functionally in every way. With the demise of the great paper magazine Small Boat Journal about ten years ago, Wooden Boat is about the only way left for a fellow to buy a magazine off the shelf at a book store and get introduced to homebuilt boats. But small simple plywood boats are becoming rare in Wooden Boat and I'm not sure how relevant the magazine is to us anymore.
The boat shown in all the photos was built by Bill Moffitt. The prototype Woobo was built by Dick Scobbie in Illinois and I had a chance to see it get built and used. Dick's boating experience at that time was having made a stripper canoe out of Popular Mechanics magazine. This was his first plywood boat and he had no sailing experience. He built the boat, sewed up his own sails per the instructions that come with the plans, and taught himself to sail. You can do that too. In the Midwest that is the usual procedure because there is no tradition of this sort of boat. There are sailing clubs but they are strictly for racing.
Woobo has the same multichine cross section as my Piccup Pram but, being four feet longer, has still more volume, comfort, seaworthiness, and speed. She takes six sheets of 1/4" plywood compared to Piccup's five and that is a fair indication of the extra work and cost involved. If you're gonna trailer your boat anyway, Woobo would be more boat for the buck. But you can't cartop a boat this heavy.
Woobo's sail rig is a 74 square foot balanced lug set on 11' sticks with and a 12' mast. She has a pivoting leeboard and rudder. She also has a small self draining well in her stern for a tiny outboard or for wet and muddy things like boots and anchors.
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Here is another photo of Bill Moffitt's boat while sailing on the Gulf. Bill made his sail very recently using the patterns presented on this site a few issues back. He used a white polytarp kit from Polysails. I wanted to show this photo because the two large shaping darts are very clear in this backlit photo. This was Bill's first experience with sailmaking and both he and I would say it was quite a success. Looking at it there appears to be a bit of a wobble in the lower dart but the rest of the sail shapes perfectly to my eye. Also there is almost no twist to the sail overall, a great advantage in sailing to windward. I think proper placement ot the halyard on the yard is critical here and Bill has it right (I'd expect more twist in higher winds). This sail was taped together but Bill said the tapes were starting to lift in some places after a couple of weeks and he was going to sew it all up. Dave Grey who sells the Polysail kits points out that the adhesive tapes should be sewn on to be permanent, although the adhesive by itself will hold for a while. I think that is the future of polysails, myself - assembly with adhesive tapes which are then sewn down.
Woobo has taped seam construction needing no lofting or jigs. 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.
Here are the prototypes abuilding that I know of:
Jonsboat: I suspect a few of these have been built but I've never gotten a report. Here are photos of one being built by Chuck Leinweber of Duckworks Magazine
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Mayfly12: A Mayfly12 is going together up in Minnesota. The decks are on and he's into the sailing bits. By the way, the sailing bits on almost any sailboat large or small consume about half of the effort in labor and materials. Just when you thought you were about finished!
AF4B: A builder in Virginia is building AF4Breve, a 15.5' version of the 18' AF4. I tried to talk him into building the 18' version but he had two very good reasons to go shorter - a short trailer on hand and insufficient building space for the larger boat. The AF4B is essentially a "scrunched" version of AF4 but comes from a whole new set of drawings. The prototype is in the final building stages where the sanding and filling and painting seem endless.
AF2: An AF2 has been started in Oklahoma that should be close to the plans. Bottom (double layer of plywood) is on and will be finished to final paint before the boat is flipped upright for the the first and only time.
BACK ISSUES LISTED BY DATE
Herb builds AF3 (archived copy)
Hullforms Download (archived copy)