# Keel Bulb Design Software

Discussion in 'Sailboats' started by Adam16, Nov 6, 2020.

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1. Joined: Nov 2020
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Hi

I saw that there was a software section but thought this might be more appropriate to my requirement. I am looking for a some sort of design software that can determine the weight in lead for a particular bulb shape. I have a project on that requires a bulb of around 75kg but I am not sure how to go about designing a bulb and calculating the amount of lead required for the particular shape. Hopefully there might be some software that allows you to draw up or import a drawing that can calculate what weight it would be.
TIA

2. Joined: Jan 2006
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Excuse the Imperial measure, if it is helpful 2.4 cubic inches of lead weighs one pound. 75 kilograms comes to about 165 pounds. So.... 2.4 times 165 suggests that 402 cubic inches of lead will do the job. (6587 cubic cm.......I think) The shape of the bulb is a whole other matter.

If the bulb is in the shape of a streamline then the starting wild guesses is that the area of the largest section times the length amount to about two thirds the volume that you need.

You can do the calculations manually by using Simpsons rule. Divide the length into an even number of spaces. That means that you will have an odd number of sections to fiddle with. Find the area of each of the stations, If it is a round shape the square of the radius times pi is the area of the section. Make a chart of the sections such as 1, 2, 3, etc. Place the area of the sections in the chart. Now multiply that area, of each of the sections as follows. . The first multiplier will be One the second will be four, the next will be two. continue to use the two and four multipliers until the next to last one which will be four, the last one will be one.. Your multipliers will look something like this..............1,4,2,4,2,4,2,4,2,4,2,4,1 Take the sum of all the multiplications. Multiply the sum by the distance between the sections. Divide that product by the constant three. The equivalent will tell you how many cubic inches or cubic centimeters of volume that you have measured.

Forgive my clumsy explanation please. Doing the arithmetic is not nearly as complicated as it sounds. Easy with a hand held calculator. I think it is fun, but then some think that I am a nutcase. If you have access to a program such as Solidworks, or similar, you can do the math in a few seconds. Otherwise do it the hard, fun way, like the old time NAs had to do.

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### jehardimanSenior Member

Not so much wild guesses if you use Pappus's Second Theorem. Draw the half shape for the bulb you want. Find the area (A) and the centroid (c) from the axis of revolution. Volume = 2*pi*Ac. For "most" streamline shapes A= 0.707 rl, where r is the radius and l is the length and c = .42 r. So Volume = 0.59*pi*r^2*l...give or take for the first turn around the design spiral. The largest section is pi*r^2 and the largest section*length is pi*r^2*l; then messabouts's "wild guess" is 0.66*pi*r^2*l.....near enough.

Edit corrected.... no "2*" in the calculation of messabout's volume...

Last edited: Nov 6, 2020
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### jehardimanSenior Member

Wait... did I just fark that up?
....yes, but no...
Largest section is pi*r^2...but messabout (and my math) is correct... Volume = 0.66*pi*r^2*l

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### jehardimanSenior Member

Ok people...let this be a lesson...don't drink and text...

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### Earl BoebertSenior Member

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7. Joined: Nov 2020
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Hi Due to illness I have not got back to this since I first posted, I would like to thank all for your posts on this matter. Now to start working through the solutions, Oo

Thank you guys

8. Joined: Nov 2020
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Thank you

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10. Joined: Nov 2020
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Thank You

11. Joined: Mar 2013
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As messabout said, your bulb needs a volume of 6.6 liters. If you want it the really old fashioned way without math, make a wire frame of the shape you want (steel wire), cover in cling wrap, fill with water. Modify until it holds 6.6l.
Otherwise, any software that can calculate the internal volume of the desired shape.

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### Ad HocNaval Architect

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### HowlandwoodworksMember

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### The QSenior Member

That article along with this.. Keel and Rudder Design Methodology – Vacanti Yacht Design LLC (vacantisw.com) scan down and click on the pdf.. Were what I used to design my keel. Both the Star and my boat have limited draft, so taking advice from the articles, the latest Mader 8300 shape gives more useful aero foil on the keel and less "torpedo".

Is my keel successful? I have no idea, first launch next spring covid and life permitting..

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