# I need some help from boat makers.

Discussion in 'Boat Design' started by BoatBoyBuilder, Apr 5, 2012.

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### BoatBoyBuilderNew Member

I am currently in the designing phase of my boat building project.

http://www.imgur.com/R8dGm.jpg here are the designs.

Specs: 19ft long
7ft wide
8ft tall at it's tallest point
weight: 3300lbs.
I need to know if it will float. I have done the math it it just confused me, I consulted a physics major and I think they got side tracked saying I would need some 1500 cubic meters to do this. Based on the design, and the basic specs, would this boat work?

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### PARYacht Designer/Builder

Not for nothing BoatBoy, but if you have to ask this question, you shouldn't even be attempting to design a boat.

There are dozens of designs for this type of thing, most quite cheap and the math is already done for you. Do yourself a big favor and avail yourself a set of real plans and stop guessing about the important stuff, like if it'll float or not. Try Glen-L.com

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### rwatsonSenior Member

BBB

The problem is that you are living in an uncivilised part of the world where they don't use metric measurement, so you would normally need to do two years of uni before you can figure out if a boat will float. But, if you want to join the rest of the world, you can work it out like this :-

19ft is 5.8 metres
7ft is 2.1 metres

lets say the sides are (you havn't said) .5 metres high.

then the total inside volume of the hull (how much water you could store inside the boat if you filled it up) would be :

5.8 x 2.1 x .5 = 6.1 cubic metres

There are 1000 litres ( 1 litre = about 1 quart) per cubic metre, so your boat could hold 6,100 litres of water, if you filled your boat with water.

The best bit, is that each litre of water = 1 kilogram. (1 kilogram = 2.2 pounds). So your boat can hold about 6000 kilograms of water. This is called hull displacement. It means also, that I would have to have 6000 kilos of steel or cement or other heavy stuff to push your empty boat under the water. That's a lot - isnt it.

From there you can figure that, if your boat weighed 400 kilos ( around 800 pounds) with all the wood and paint, and you had 4 people on board weighing 130 kilos, that would total around 1000 kilos.

So if your boat take's 6000 kilos to sink it, but the total weight it has to hold up is only 1000 kilos - then it would float.

Now - if you read all the way to the bottom here - and you think it makes sense ( or it might if you think about it a bit) , then you have a chance to actually make you boat idea work.

You might also like to Google the man Archimedes, and learn about his discovery about floating things. (archimedes principle)

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### hoytedowCarbon Based Life Form

BBB, rwatson is funny, huh, but he is right about the flotation.

Make your first boat about half that size and a fourth the expense to learn with. I concur with PAR to build a kit boat first. Once you have done that, many of your questions will have been answered for you.

See http://en.wikipedia.org/wiki/Buoyancy

"Stability
A floating object is stable if it tends to restore itself to an equilibrium position after a small displacement. For example, floating objects will generally have vertical stability, as if the object is pushed down slightly, this will create a greater buoyancy force, which, unbalanced by the weight force, will push the object back up.

Rotational stability is of great importance to floating vessels. Given a small angular displacement, the vessel may return to its original position (stable), move away from its original position (unstable), or remain where it is (neutral).

Rotational stability depends on the relative lines of action of forces on an object. The upward buoyancy force on an object acts through the center of buoyancy, being the centroid of the displaced volume of fluid. The weight force on the object acts through its center of gravity. A buoyant object will be stable if the center of gravity is beneath the center of buoyancy because any angular displacement will then produce a 'righting moment'."

http://www.fourmilab.ch/hackdiet/www/subsection1_4_2_0_7.html

1 Gallon of fresh water weighs 8.34 Pounds.
1 Cubic foot of fresh water weighs 62.42796 Pounds.

Weigh the boat materials, cargo, machinery, equipment and persons on board in pounds. Divide that sum by 62.43 and it will tell you how many cubic feet of water will be displaced.

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### gonzoSenior Member

Based on the drawing, it will probably capsize. Also, the propeller won't be getting any clean water produce little or no thrust. Scantlings are one of the most important part of the design too.

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### PetrosSenior Member

plans are a good idea, but if you are doing this for the fun of designing your own boat, there are a few suggestions I would make:

Find a hull shape for an existing design (see links below) that is simple to build and about what you want and copy the shape, but simplify the design and lay out to make it easy to build. that way you will have a proven shape at least.

I would build a large model of it first to see if behaves the way you expect, about 8 or 10 ft long will give you something big enough to ride in and if it works out you will have yourself a low cost dingy.

You can simplify your design by squaring off the front and just sweeping the bottom up to meet the deck at the front. like a barge or a scow. Easier to build, more room inside, more displacement and more stablity.

You might look at these old designs of free plans for ideas:

http://www.svensons.com/boat/

http://www.polysail.com/oldboats.htm

Good luck. Have fun and Post pictures of what you end up building.

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### IkeSenior Member

I vote with PAR but, I would also suggest you go to the library and check out a few books on building simple books.

Such as:
Instant Boats By Harold Payson. http://www.instantboats.com/
or;
Boats with an Open Mind by Phil Bolger. http://www.ace.net.au/schooner/sites2.htm

Also a book on stitch and glue boatbuilding.
Building Instructions Booklet For Stitch & Glue Construction http://store.devlinboat.com/booksandvideos.aspx

These will give you some ideas and also a better idea of how boat plans are drawn, and how boats are constructed.

In simple words learn something about boatbuilding first before you try design your own. No one here is saying don't. What we are saying is boat building is not as simple as it appears. Do a little study before you launch into it. (sorry about the pun) Having to redo and redo and redo things while building can get expensive. Learn from others mistakes. Try this http://newboatbuilders.com/pages/fl12.html

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RWatson; your remark about living in the uncivilised part of the world is sad but true. Why even the pommies...er I mean brits have forsaken the notion of measureing their body weight in stone. Actually the brits are the ones who layed that silly Imperial measure on us in the first place. Now they have essentially abandoned it in favor of sensible metric measure. They left the yanks to suffer and failed to offer any sort of apology.

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### rwatsonSenior Member

Yes - its a cruel joke allright. I understand that some American Scientific types use Metric, but not your Rocket Scientists.

eg.
"CNN NASA lost a 125 million Mars orbiter because a Lockheed Martin engineering team used English units of measurement while the agencys team used the more conventional metric system for a key spacecraft operation"
http://articles.cnn.com/1999-09-30/...iter-spacecraft-team-metric-system?_s=PM:TECH

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### hoytedowCarbon Based Life Form

Had NASA stuck to English measurements instead of trying to be like France we probably would have been successful.

11. ### CatBuilderPrevious Member

Wow, I never saw that before. MKS units are the standard units used at NASA, ESA, and by all physicists, including those at Martin, Martin Marrietta, Lockheed Martin, or whatever the company is now called. I used to work at NASA, working along side Martin employees on several of our projects. The main problem here would be outsourcing.

It is best to have one organization in charge of a technical project. Martin was way, way out of line not using MKS units. Hopefully, someone was disciplined.

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