# Mik the stick is back (Waterline position)

Discussion in 'Boat Design' started by Mik the stick, Sep 4, 2014.

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### Mik the stickSenior Member

I have recovered my equilibrium enough to enjoy boat design again.
All you naval architects out there draw your boats and make your calculations and produce boats. I am not a NA but am managing to teach myself with the help of those who have answered my questions. Time to give something back perhaps.:idea: Naval architects publish drawings stating draft and length etc. I don't know how a designer knows what the draft is going to be unless he knows where the waterline is. Fancy design software may give him the answer but I'm sure he could calculate it some how, again I don't know how easy that might be. But I have a simple rather accurate method I would like to share.

A Tug has a LWL of 74ft and displacement of 91.36 tons.What is its draft (Tc)

Taylors formula for wetted surface area is 15.51 x (91.36 x 74)^0.5 = 1275 square feet.
Denny's formula gives almost the same result if you knew what Tc is. the published Tc is actually 5.79 but you don't know that yet.

Denny's formula 1.7 x 74 x draft+(91.36 x 35/draft) = 1281 when draft = 5.79ft. if you start with a draft of 3 and work up using a calculator The draft will work out at 5.6 feet. Pretty close don't you think.

If for some reason the Tug was overloaded to say 101 tons Taylor gives WSA at 1341 and Denny's formula gives new draft at 5.9 feet. The boat would be 3.6" deeper in the water.

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### TANSLSenior Member

Precisely tugs are a type of ship whose forms have changed much over time. Although I have no hard data, I have doubts that the formulas that were valid for the old hulls continue to have some value for the current ones.

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### gonzoSenior Member

You can decide in advance what the draft will be and the location of the waterline. To accomplish that it is necessary to go through the long tedious process of calculating the weights of everything, and inspecting the build to make sure they comply with the design.

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### Mik the stickSenior Member

the dimensions of the tug I used are from a text book using the Hassler Taylor and Denny formula to show how close these formulas estimate the WSA. The result is about a 2-3% spread.

Because I'm not a NA I can't always do things as a NA would. What I'm trying to do at the moment is.

1/ Decide L and B
2/ Decide D/L Ratio which give a weight to work with.
3/ I can use the formula posted to estimate where the waterline will be.
4/ I can then calculate the hull and deck surface areas and use scantling rules to calculate weight.
5/ Add weight of equipment, engines, fuel and furnishings.

I have not yet completed my calculations But I think it will work. I am aware that if a boat has to be say 20,000lbs for any particular reason then something will have to give. the boat will be smaller than I wanted and or have a smaller engine with less fuel and furnishings.

If I was designing a shoal craft for example it could be perhaps 20ft-60ft but the one unchanged design parameter might be a Maximum Draft 0f 2.25ft.

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### MikeJohnsSenior Member

I'm curious, why a tug?

A tug is a tractor in the maritime world. It's hullform is a compromise based on the requirement float the propulsive machinery. A massive power plant and a large prop.

It's a poor hullform for efficient propulsion at reasonale speeds, most tugboat type lookalikes have a sensible power boat hullform below the waterline.

The only reason to design a tug complete is if you are building a tug.

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### Mik the stickSenior Member

The text book I have has a drawing of a tug in the back. It seems to be the authors preferred vessel to teach the subject

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### rwatsonSenior Member

Good for you - broadening your mind. I wish you all the luck with it.

Its probably a good policy - use a difficult design type like a Tug to ensure a thorough understanding of the principles.

Just out of curiosity, have you had a chance to try out a cheap program like Deltship or Freeship for example, and see how much easier it is to make a machines mind work hard, rather than wear your own out ? All these problems are largely automated now - though you will need to know what the underlying logic is, as Tansl can confirm.

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### TANSLSenior Member

I also agree that the greater the challenges, the better. However, an overly ambitious challenge can wallow in discouragement.
A modern tug is one of the most difficult hulls to be designed.
Otherwise rwatson, totally agree with your opinion, "though you will need to know what is the underlying logic."

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### NavalSArtichokeSenior Member

While I am an advocate of bettering oneself, I also think there is a right way and a wrong way to go about this process.

Developing arcane 'rules of thumb' like is presented in the OP is, IMO, an illustration of the wrong way to go about this. Trying to equate the wetted surface of a vessel, which due to the complexities of hull shape is only an approximation, to the draft overlooks more fundamental approaches, which are taught to student naval architects.

There are several good books which introduce one to the principles of naval architecture in a clear and concise fashion, without developing all sorts of ad hoc rules based on shaky foundations. I mention two titles here:

Brian Baxter, 'Teach Yourself Naval Architecture'
a classic text, which in its original form could fit into a shirt pocket, yet provided a clear description of the key topics in NA and provided a set of problems for the student to work out.

and

'Ship Stability for Masters and Mates', by Capt. D.R. Derrett; revised by Dr. CB Barrass
gives a thorough grounding into the basic physics behind ship stability to the officers in charge of running an actual vessel.

If one is truly serious about becoming knowledgeable about a subject, it should be studied carefully and diligently and systematically, not in a hit or miss fashion.

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### TANSLSenior Member

Formerly, when starting a new project, the designer had to use a range of aspect ratios, properties of the hull, etc. to obtain a first approximation of the forms of his vessel. If the first approach was detected incorrectly had lost hundreds of hours of work, drawing and calculations. That's why they were so important relations between dimensions. For this reason hull´s shapes chaged very little in centuries. Currently the designer can afford to do something wrong because, quite easily and quickly, he can change it.
But as rwatson says "you will need to know what is the underlying logic."

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### Mik the stickSenior Member

rwatson I have a program (delft ship I think) how ever I like to use a pocket calculator and formula.
NavalSartichoke I thought what I suggested was reasonably accurate or I would not have presented it. I will try to buy Teach yourself naval Architectute. I don't know much about NA, though I think I know more than you think I know. My fundamental problem is even with a good textbook I have to teach myself.

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### TANSLSenior Member

Mik, each manages its resources as he wants but let me tell you you're in the stone age. With these tools you can only do a project in your life, no time for more.

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### Mik the stickSenior Member

Tansl As I'm not a real NA that doesn't matter learning is much of the enjoyment. My original post was to present a method of where to draw the waterline after I have added all structure weights crew etc. I was not trying to teach granny how to suck eggs.

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### TANSLSenior Member

I apologize if my words have bothered you. No was my intention. I would like to help you, if that´s okay with you.
The first phase of a project is to define the main dimensions of the ship. I agree that this can be done with a hand calculator as long as the formulas you're using are valid for the type of boat you want to project. From there, all other calculations, which of course can be done with a hand calculator, are so cumbersome that can cost you an eternity. That's all I wanted to point out.
I think you need some kind of software to carry out your project.
Btw, my respects to Grandma.

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