Science Buddies: "Ask an Expert"

I am currently researching for how the length of a bilge keel on a boat can affect the boats roll,I have been having trouble finding websites with this information. Do you have any good starting points on where i can find that information?

Your question has to do with the field of naval architecture. I have no specific knowledge of that field, but do have an extensive background in general fluid mechanics.

I know enough to be able to tell you that how a ship rolls is controlled by many more variables than just the length of the keel. One must take into account the depth of the keel, the shape of the hull, the shape of the prow, the weight distribution above and below the water line, the speed of the vessel, and many more variables.

I discovered that the field of naval architecture is complex enough that there is little specific information on the internet. But you can do a search for the term "naval architecture" and discover there are numerous books on the topic.

Good luck in your research.

Whats the relationship from the length of a bilge keel to a boats roll?

Hi,

Please post all your questions on one project as replies to your original post. This allows the experts to quickly understand the history and context of your query and simplifies things for everyone.

I assume you are using this Science Buddies project:

https://www.sciencebuddies.org/science- ... p038.shtml

Have you looked in the books listed in the bibliography for the answer to your question? If not, that would be the obvious first step. In your shoes I think I would try to answer the question experimentally by making and measuring the roll periods of different depth keels, and plotting the results.

As Mr. Neu said the theory looks complex to understand. If you want to try, here are some resources that you can start your research from, though these sites may not answer the question directly:

http://books.google.com/books?id=6C0CAA ... ory&f=true

http://www.martv.com/TECHPAPERS/All%20P ... ethods.pdf

http://www.iwwwfb.org/Abstracts/iwwwfb1 ... b17_41.pdf

http://e-book.lib.sjtu.edu.cn/isope2002 ... 19p114.pdf

http://www.ultramarine.com/hdesk/docume ... hmitke.pdf

This thesis might be worth browsing:

http://vbn.aau.dk/files/107857/fulltext

Here is an article explicitly considering ship roll frequency:

http://www.hindawi.com/journals/mpe/2010/934714.html

If you want to drastically simplify the solution to your question, you can imagine that your ship will bilge keel is a simple pendulum for purposes of rolling from side to side in quiet water. The theory of a simple pendulum is described here:

http://en.wikipedia.org/wiki/Pendulum

To apply this model to your ship the pendulum length is the vertical distance from the roll axis of rotation of your ship to the center of mass point of the ship. Here is an explanation of center of mass:

http://en.wikipedia.org/wiki/Center_of_mass

It should be clear from this that as a practical matter it is not going to be easy to calculate or report a simple answer to the question of what the roll frequency is going to be for various shapes of bilge keels, if for no other reason than it is hard to predict the mass and center of mass for various model shapes. Understanding the behaviour of simple pendula as a function of the pendulum length should help you predict the qualitative behavior of the roll frequencies as a function of bilge keel depths however.

I hope this long winded answer helps. Good luck with the project!

Best regards,

Barrett L Tomlinson

I think this is a much easier and simplified question which is better long or short bilge keels? or which is used more? Thank you very much for your help.