Planimeter Question

[outofthenorm]

I'm thinking about buying a Planimeter from ebay for occasional use as I play with designs. I don't know anything about them, so can anyone advise me on what to look for. What's a good brand, and what features do I need for hull measurement?

Thanks in advance - Norm

[bainbridgeisland]

K&E is most common. Most offices I have worked for have at least one. I use a Dietzgen D-1802 I bought in 1973 that I like very much.

General Info:
There are two main components, an anchor pole arm and a measuring arm. Most anchor pole arms are of fixed length; sometimes they are adjustable for different scales. A calibrating tool is also provided with many planimeters. All the tools I have used measure in inches and use a wheel and veneer measuring scale. Most planimeters use a pointer to follow the lines but I like the optical indicator on my Dietzgen.

There are two ways to use a planimeter. The most common is with the anchor pole set outside the area to be measured. Place the pointer of the measuring arm about the middle of the area to be measured. Set the anchor pole arm at about 90-degrees. One end of the anchor pole arm fits into the recess on the measuring arm. The other end has a pin that anchors it to the paper. Place a tic mark on the edge of the surface to be measured and use the tiny handle on the measuring arm to slide the pointer to the tic mark. Write down the numbers off the scale. Smoothly follow the perimeter of the surface clockwise with the pointer and stop at the tic mark. Read the numbers off the scale and subtract the previous reading. This is the area of the surface. All the planimeters I have used actually measure in inches, tenths of an inch and hundreds of an inch. So a reading of 572 would be 5.72 square inches.

Calibration and scale: If your instrument measures in inches, is fixed length anchor pole arm and perfectly calibrated, you can adjust to allow for various scales by inverting the scale and squaring it. For example, 3/4" =1'-0" becomes 4^2 / 3^2 = 1.7777... (5.72 * 1.77778 = 10.169 ft^2) I usually do not count on perfect calibration. You can easily check by drawing a circle, calculating the area of it and then measuring with the planimeter and comparing answers.

The other way of using a planimeter is with the pole anchor inside the area to be measured. I haven't needed to do that in 20-years and have forgotten how.

To increase accuracy due to user error, go around the perimeter multiple times and divide the answer by the multiple.

For years, most of my work is on the computer yet I use my planimeter regularly. The most common use is during preliminary design. My first sketches are always in pencil and I commonly use the planimeter to calculate volume, center of buoyancy and prismatic coefficient. Such a sketch, combined with preliminary loading, structure, weight study and powering constitute the first revolution of the 'design spiral'. After this step, pretty much everything is on the computer. Working this way saves tons of time compared to folks I work with who only use a computer. I get the proportions right with a pencil and then refine the design with the computer.

Hope this helped.

David Mancebo

[paladin]

Norm...I will be home on the 20th (they have me locked up again) and I can copy my K & E manuel for you if you like and send it by e-mail. Send a private message if this will help.

[Ian McColgin]

Of more diverse use and greater accuracy is a good analytic triple arm beam balance, if you can get one or use one at your local high school or college. Some modern digital grocers' scales are also that good but beware: Digital to four decimals does not mean accurate even to one.

The other thing you need is K&E or other maker of superbly consistant graph paper.

Weigh a few samples of graph paper, sans margins, to determine that it is reasonably consistant and to learn the scale.

Wear latex gloves so the moisture from your hands does not affect the measure. Use a very hard fine point pencil to draw the hull sections. Cut exactly to the inside of your drawing, thus leaving out the weight of the graphite. Weigh the section and calculate the area from that.

I developed this method in college analytic chemistry because I was terrible at even finding much less solving integrations and I was even worse at getting up at ohfrickingdarkthirty to go to the computer building, punch a buch of cards, and generally mess with the many ton clattering monster.

Anyway, think of all the other things acqu isition of an accurate scale makes possible . . .

[outofthenorm]

Thanks everyone. I knew there would be some expertise around here.

David - You said "hope this helped" Did it ever! It sounds easy enough, so long as one has a steady hand and a little patience for the math. This work is just for my own enjoyment, so I work exclusively on a drawing board. I'm used to thinking in inches and eighths, and software like FreeShip and other drafting programs expect me to be digital, so I haven't translated anything on to the computer yet.

Paladin - Thanks, I've sent a PM

Ian - Very clever idea. If I'm understanding this right, I would take the weight of say 16 square inches of some 8x8 to the inch paper, then divide by (16x64=896) to calibrate a weight per square. Then I weigh the cut out section, and simple multiplication of weight x 1/64th gives me total area in inches. Cool.

It's akin to the old T Harrison Butler notion of balancing a cut out of the immersed portion of a section on the edge of a razor to help locate the heeled center of buoancy.

Okay, I'm off to scout ebay for a compensating polar planimeter AND a high quality digital scale. Nothing better than an excuse to buy a cool tool.

- Norm

[paladin]

Ian...sorta laying here thinking about your methodology.......for instance.....I have some 7mm mylar grid......and since gold is measured to the pennyweight...I got those scales.....mylar is heavier than the paper, but doesn't absorb moisture so......I should get a more consistent reading if I do my initial drawings on paper, then transfer to the mylar....and it should be more accurate...or.......