# waterline length

Discussion in 'Boat Design' started by genius1981, Jul 30, 2013.

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### genius1981New Member

Hello everybody

I am writing from spain and a I want to know please how can I calculate stimated waterline length in a new construction boat to use in UNE-EN-ISO 12215 to calculate scantling. I have boat planes with the measures but i need this data. Thanks all

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### Mike GrahamJunior Member

I'm not entirely sure where the difficulty is. The design waterline should be marked on the plans and a more-accurate estimate is probably given by the hydrostatics analysis. Do you have a hullform with no design waterline/draft indicated? Designing a hull without defining this is not reasonable. Where did you get the plans?

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### daiquiriEngineering and Design

In general, without knowing the details of your boat and of the plans you have:
If the known measures include the draft T, then draw a horizontal line at the distance T from the deepest point of the keel. There you have the waterline, so now you can measure the waterline length.

Cheers

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### genius1981New Member

I know what you say, but I have the boat with total lenght and beam and general measures but if I dont know displacement, draft or other I cant know the value. Sometimes I read in other proyects that they take stadistics with other similar boats and then when have stimeted value I can calculate displacement or if I have stimeted displadement I have stimeted WL length. But how I know total weight if I cant calculate scantling and all structure elements. Sorry for my english. Thanks

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### Mike GrahamJunior Member

You can't choose a hullform if you don't have any approximation of the displacement. What if she sinks?

It's a basic fact of engineering that you can't design anything without making assumptions about the answer before you start, getting feedback as you refine the design. Design is a fundamentally iterative process. In the context of naval architecture, we call this the "design spiral".

Bottom line, you need to make some assumption about displacement early on in the design. Whoever designed the hull had some sort of draft in mind in designing it, so if you know what that is, you might use that as your target. Otherwise, you might be able to derive one by starting with what looks about right to you on the plans or imagining the structural sizes that seem about right to you as an initial weight. You might also consider starting with the displacement for some boat similar to the one you intend.

Are you hoping actually to design and build a sailboat?

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### TANSLSenior Member

In large vessels is normal to take what is called "scantling draft" which is an estimate of the full load draft, adding for example 7%, since at that time no one knows the total weight of the boat .
Once you know the scantlings, as indicated in the spiral design, you can better estimate weight/depth and recalculate the scantlings (although usually not necessary). The scantlings excess is included in the "designerÂ´s safety factor".

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### daiquiriEngineering and Design

Genius,
evidently I have misunderstood the situation here. Thought that you have boat plans and data, where for some strange reason the designer didn't indicate the design waterline (and hence Lwl).
Now it looks more like you'd like to design a boat but don't know where to start... Is it so?

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