# Basic principles of boat designing



## firecat1981 (Nov 27, 2007)

You can download some free plans and then build them to scale models.
http://www.bateau.com/freeplans.php#.UucEIBAo7IU

This is how I calculated rough displacement and draft for my builds. It was pretty accurate. Hope it helps.


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## topnative2 (Feb 22, 2009)

I believe, u.s. coast guard has formulas on their site


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## Brett (Jul 16, 2008)

> easy way to calculate displacement of a balsa wood model


The operative word there is "displacement".
Meaning: how much water is moved out of the way?
Easy way to calculate is the fish tank method.
Fill tank to the top with water, set model in tank causing water to overflow.
Remove model and measure the drop in water level.
The drop in water level x the length x the width of the tank is the displacement.

Very old school technique from the half model days.


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## firecat1981 (Nov 27, 2007)

Brett, instead of spilling water everywhere couldn't you just fill it part way and measure how much the water rises? :-?


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## Brett (Jul 16, 2008)

The idea is to emphasize "moving water out of the way".  

Besides, more fun making a mess than being neat and tidy.  ;D


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## WhiteDog70810 (May 6, 2008)

> Done any one know any decent web info/links on boat designing site that would be relevant to a 5th grade science project.  My head is in the clouds reading all the math I've found.  He narrowed it down to how do different shapes help boats do different things.  I'm looking for a easy way to calculate displacement of a balsa wood model as well.
> Thanks


The math is easy if you build a floating box, but it gets complex once you add curves in any dimension to the equation. I long ago forgot the math that taught me how to calculate the surface area of a curved surface and the volume of the space it defines.

I am partial to the "build it and see what happens" technique. If it is your first design, pick a fairly simple hull form from a proven style and backwards engineer it using models until it looks right to you and sits right in the water. Your weight/length ratio will vary a significant amount from your model to the final product, so calculating displacement from a model is difficult for an amateur builder. It also doesn't matter since the final product will probably be different than your projections. The smartest guidance I can provide is to buy a good set of plans from a real boat designer. I am not smart enough to follow my own advice so why should you? ;D

Nate


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## firecat1981 (Nov 27, 2007)

It's a science project, it would be very expensive to buy a bunch of plans just to study the hydrodynamics of them. 

How about get a single electric model boat motor and use it to test out a few models build from door skins and wood glue? 

Or better yet set up a fish tank or tub with a powerhead at one side to create strong flow, a few drops of food coloring will show how the water moves around the boat. Kind of like a wind tunnel.


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## DuckNut (Apr 3, 2009)

There is an entire forum related to boats, hulls and design.

http://boatdesign.net/

I am sure there is the answers you need over there as most of the builds are designed for production.  If you can't find it - ask and they will answer.

edit: don't forget it is a 5th grade project and some of the calculations are too advanced.


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## david_kohler (Jun 29, 2009)

Thanks a bunch! I've figured out a way not to do calculations and just do results from tests to three different shape hulls designed for different purposes. Like how much the water line changes with 5 grams place on each hull. I liken it to boy scouts pinewood derby scale for a 5th grader. It got too complicated quickly.


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## WhiteDog70810 (May 6, 2008)

> It's a science project, it would be very expensive to buy a bunch of plans just to study the hydrodynamics of them.


Mea culpa. I read the original post too fast. I thought he was looking for a "5th grade science fair project" explanation for an AP/college prep math problem (I'd like to hear that explanation also). I failed to catch that it was literally for a 5th grade science project. 

:-[

Shallow Breathe,

Attempt to the greatest extent possible to help him keep the length, width and weight of each model standard across the models to minimize variability. You'll probably have to add weight to two of the hulls to accomplish this. You want the only variable to be hull form if you are looking at draft with added weight. If your son presents a clear hypothesis and can explain the reason to minimize variability and the difference between accuracy and precision in addition to his actual results by the end of this, he will really impress the judges and have a much better grasp of the scientific process.

Have fun!

Nate


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## david_kohler (Jun 29, 2009)

> > It's a science project, it would be very expensive to buy a bunch of plans just to study the hydrodynamics of them.
> 
> 
> Mea culpa.  I read the original post too fast.  I thought he was looking for a "5th grade science fair project" explanation for an AP/college prep math problem (I'd like to hear that explanation also).  I failed to catch that it was literally for a 5th grade science project.
> ...


 ;D   You just helped me figure out that part down as I was thinking about this at work today! I really do have a lot of weights for pinewood derby cars as my son was in cubscouts that will work perfect.  You know kids love to do things about what they like, and lucky for me he loves going with his dad on the boat. I'm sure from the stories I heard on here, it won't be long till time with girls beats time with Dad on the boat.


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## oysterbreath (Jan 13, 2009)

What you are looking for is *Archimedes' principle. *Google it. It is formulated in such a way that you can figure out how much water (at a given density) can be displaced by an object with a known density and volume to reach a state of equilibrium. That's not the best description.....but you get the point. The other related law is *bernoulli's principle*. These two form the foundation of basic fluid dynamics...I think. lol


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