# How do Naval architects calculate tonnage

Discussion in 'Boat Design' started by Mik the stick, Dec 28, 2012.

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### Mik the stickSenior Member

I have looked at many specs for powerboats like the Diesel Duck. This has dimentions 36.66 x 12.75 x 4.75 made in 1/4" steel 32600lbs. A naval architect must be able to estimate this weight. Just how is it done.

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Firstly you need to understand the terminology.

To explain briefly, "tonnage" refers to a volume, a volume that can be filled with "stuff", which means how much earning capacity a boat has. Every boat commercial that is, is assigned a Gross Tonnage (GT) and an Net Tonnage (NT). This determines how much port fees they pay, owing to their carrying capacity.

How much a boat weights, is referred as its "displacement". The units may well be in tonnes, but it is always referred to as the displacement.

A successful design relies heavily upon an accurate weight estimate. How its done..is by designing it...not guessing. Unless you know the person and can ask them directly!

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### rick grayJunior Member

the calculations are done for hull and framing by weight of steel plate per square foot. 1/4' plate weighs about 10# per square foot steel sections are listed weight per foot. a steel sales book gives weights.

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### MikeJohnsSenior Member

It is part of the design process. Called a weights and moments study.

But if you want the displacement of an existing vessel and the designer is not contactable then you need to either find some lines drawings or take lines off the hull, and measure drafts and calculate immersed volume, or maybe just find a reasonably accurate travel lift weight from the last haulout.

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What is published is not necessarily accurate, the first DD in steel was built in China by SeaHorse and floated something like 6" deep, it was overweight by around 10,000 pounds.....1/4" plate in a 36' pleasure boat (with framing) is crazy overbuilt....George even specified 1/4" plate on the deck!

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### JSLSenior Member

A 36' boat with 1/4" plate. On a 'normal application' pleasure boat this is heavy. But that's okay if you like running into things. I have been reviewing a 52' steel trawler (not my design thank goodness) that has a 3/8" bottom and will when finished displace about 115,000 lbs. (51.34 L tons). Even icebergs feel threatened.
However, if the boat is for 'rough' work, it has to be tough. A 26' sorting-tug can have a 5/16" bottom & 3/8" plate sides. But, it also has a 300 hp engine on a 42" nozzel propeller so it can pull several thousand pounds.

Regarding the weight estimate, it is a very detailed lift-off of ALL weights (and their centroids) on the boat. You must even allow for over-rolling of plate and paint. It is probably the most boring but critical part of the design process. Once completed, the numbers are run with the hydrostatics to determine where and how the boat will float.

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### IkeSenior Member

Lets get back to the basic question. As was said the designer does a weight and moment study. I'll just talk about the weight part of it. The designer starts with a rough idea of what he wants. This is expressed in a drawing and on that drawing is a waterline. At the beginning it is an educated guess. But when the designer calculates the immersed volume (the part underwater) and how much it displaces in weight, that weight should approximate the weight of the boat. But what is the weight of the boat? The designer then estimates the weight of every part of the boat. As some said we know the weight of 1/4' steel plate. If we multiply that by the square footage of steel plate we have the total weight of the plate. This is done for everything in the boat.

My first job in naval architecture was at a shipyard doing weight studies of ships. Everything in the ship had to be listed in a very thick book (fortunately now we have computers to do this) and all the weights added up. I mean everything, materials, nuts bolts, equipment, liquids (fuel, water, oil, sewage) The total should come out somewhere close to the desired displacement. If it is greater, then the boat will sit lower and have a greater displacement. If it is less then it will sit higher and displace less.

This affects it's performance and stability so these studies are very important. It is a long and tedious process, but vital to the performance and safety of the vessel.

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### rick grayJunior Member

Tell me how!! sounds good to me. i am trying to design and build, modify as i go along, but my concern is with the balance of the amphibian. there is of course larger areas of sections aft, but engine weight is just at or aft of midship. so i don,t know how it will sit at rest in water,and assume it will get sufficient plane angle. i know 35 cu ft of void will support 1 ton in salt or 36 cu ft in fresh water,to get displacement of weight of craft.so only way i know is to balance the craft on a cross beam untill i find a balance . center of gravity ,metacenter? it won.t be top heavy (I am sure?) Primative and scary!

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### IkeSenior Member

How much time have you got? I would go over my bandwidth data limit. I suggest you go to your library and find a book on naval architecture. Try Principles Of Yacht Design by Lars Larsson and Rolf Eliasson. You may be able to find a used copy on Amazon. The last chapter shows a sample weight calculation.

see another thread on this http://www.boatdesign.net/forums/boat-design/calculating-weight-displacement-40592.html

If you do a search on BoatDesign.net you find other threads on this.

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### jehardimanSenior Member

Mik and Rick;

If you really want to know the ins and outs of weight and moment calcuations, get a copy of the Weight Engineers Handbook from the Society of Allied Weight Engineers. More than enough data to make sure you get it right. FWIW, paint, welds, and bolts not accounted for in the plating and foundation plans can be ~4-7% of the deadweight of a ship. And don't forget the fluids in the pipes either.

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