Jim Michalak's Boat Designs
118 E Randall, Lebanon, IL 62254
A page of boat designs and essays.
(1October 2015) This issue will look at center of gravity calculations. The 15 October issue will continue the topic (and most likely be out a few days early as I am going to the Port Aransas show about that time).
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.
Yes, it is my old Roar2, well worn and well used, back in the water after the summer flood that had this lake level ten feet deeper than shown. A perfect day for a row.
118 E Randall,
Lebanon, IL 62254
Send $1 for info on 20 boats.
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
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:
And the first D'arcy Bryn is to the point the builder can sit and relax in it and imagine boating. You can follow the builder's progress at http://moffitt1.wordpress.com/ ....
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.) Double layer bottom on and glassed, hull returned to upright. He is decking it now...
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
15oct14, SailOK2014, Jonsboat
1nov14, Chine Runners, Piccup Pram
15nov14, Lugsail Rigging, Caprice
1dec14, Sail Area Math, Ladybug
15dec14, Poly Laminates, Sportdory
1jan15, Sharpie Spritsail, OliveOyl
15jan15, Knockdown Recovery, Dockbox
1feb15, Mike Monies, Laguna
15feb15, Cartopping, IMB
1mar15, WeeVee Lessons, Vole
15mar15, Bulkhead Bevels, Frolic2
1apr15, Capsize Lessons, Riverrunner
15apr15, Hollow Spars, Slam Dink
1may15, Boat Costs, Blobster
15may15, Small Boat Rudders, Roar2
1jun15, Emergency Flotation, RB42
15jun15, Thailand Mixer Cruise, Mixer
1jul15, Rend Lake 2015, Musicbox3
15jul15, Box Boat Stability, Mikesboat
1aug15, Taped Joints, Cormorant
15aug15, Plywood Butt Joints, Paulsboat
1sep15, Navigator Cabins, Vireo
15sep15, Boxboat Stability 2, 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