Calculating weight and displacement?

Discussion in 'Boat Design' started by JordieS, Nov 15, 2011.

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JordieSJunior Member

Hi all,

Just wondering how you estimate (or calculate) boat weight and displacement to determine the waterline.

Thanks

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Firstly you have a GA..a general arrangement and then a specification.

The GA and spec defines what the boat shall be required to achieve and do. From this you make a list of every item on board, add it up..and hey presto, you full load displacement.

Once you have this you check the weight against the hydrostatics for the hull you have selected, or designed, for the design. From that you obtain the waterline.

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JordieSJunior Member

Ok I'm really wondering how you estimate the hull and deck material weight, be it wood or fibreglass etc

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What does the specification say...?

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JordieSJunior Member

Perhaps a better question would be, how do you determine the amount (or thickness) of the fibreglass required for your hull.

If I had a design how would I decide how many fibreglass layers to use?

Thanks

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FMSSenior Member

How many fiberglass layers and what weight and fiber orientation is a large part of the design.

To design, you have to calculate the loads, strength and stiffness required at all locations for a given use - will the boat be a 60 mph boat slamming through surf or will it be a lightweight paddleboat used in calm water. A reasonable safety factor needs to be provided. But you don't want to overbuild with too much material in the wrong locations which will never be the weak links. That results in a heavy, inefficient boat, costly in material waste and in performance.

If you are conceptualizing, you can look to similar boats for rough weight estimates to get a rough idea, but at some point it all needs to be "designed" or calculated.

http://www.boatdesign.net/forums/materials/scantling-question-beginner-14826.html is on topic

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JordieSJunior Member

Are there any books you recommend that would deal with things like determining the thickness, boat strength etc. Does Dave Gerr's Elements of boat strength provide useful information such as calculating what amount of material required etc?

Thanks

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PARYacht Designer/Builder

Jordie, no offense buddy, but you are no where close to the level of understanding necessary to even remotely consider designing hull shell scantlings. Do yourself a favor and read the "usual suspects" in yacht design texts, which will greatly improve your understanding of how things work and are done. In doing so, extremely basic questions such as this, will be answered for you and you can move onto more specific questions, that don't require an encyclopedic answer here.

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JordieSJunior Member

I understand I only draw boats on Rhino for fun at the moment. I love boats. PAR since you are a proper designer and builder are there any books you can recommend? I will probably start of with buying the most recommended one before others.

People always mention Dave Gerr's elements of boat strength, would this be a good starting book?

Thanks

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LandlubberSenior Member

People always mention Dave Gerr's elements of boat strength, would this be a good starting book?............of course....it is one of the best modern books available.

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PARYacht Designer/Builder

Elements of Boat Strength assumes you already have a fair grasp of what's going on (calculating displacement, volume distribution to achieve design goals, etc.). Yes, it's a good book to have, but you need to know how to apply the information first.

"Principles of Yacht Design" would be the first book you need to wade through, then you can move onto more specialized topics, such as sail or power considerations. The book store here has the major titles you need. Use the search tool for "yacht design" "boat construction", etc.

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gonzoSenior Member

Elements of Boat Strength only addresses scantlings and not design per se.

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JordieSJunior Member

Is Principles of Yacht Design by Lars Larsson?

I'm mainly interested in powerboats so does this book cover powerboats? Amazon.com says it follows the design of a 40 ft sailboat?

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JRDSenior Member

It covers all the basic hydrostatics, drawing methods with explanations about scantlings design to ISO rules. There is one chapter on planing power boats, powering and prop design. Im not sure what books are available on power boats, but this one gives the theory, formulae and shows workings for each calculation. I think if you can fully comprehend the information in this book, you will be a short step from progressing to power boats... with a few more books and lots more questions here! Good luck

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IkeSenior Member

Jordie,
Just to give you an idea of how complex this can be;

My first job in Naval Architecture was calculating weights of materials on ships, where their center of gravity was, transversely, vertically and longitudinally, and the moments this creates.

Yes, they were ships not small craft, but the principle is the same. First you had to know what the material was, how much it weighed per cubic foot, then determine how many cubic feet and where it was positioned.

This is called weight and moment studies and is used to determine weight of the entire vessel, displacement, where the waterline will be (and does it match the designed waterline). It is used to determine trim angles, heel angles if any (boats are supposed to float upright but often don't) stability and other factors.

You have to do this for everything on the boat. Leave anything out and it is not accurate. Even then you can end up with boats not floating on their DWL or having a strange list. see http://www.sponbergyachtdesign.com/Forensic.htm#Cheshire

PS: This was before we had computers to do a lot of the drudge work for us. It all had to be done by hand using blue prints, engineer scales, slide rules (the hand calculator had just come on the market and cost about \$600. in 1971) or mechanical calculators. It was very tedious. But it is extremely important to know how to do this.

Last edited: Nov 17, 2011
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