Design problem

Hi folks!
The last part of my exams to get my master-title in boatbuilding in germany is to design and build a small boat. I´ve drawn a 11 ft rowing dinghy, " round " stations , flat stern ( with a little help from the prolines demo version).The problem I have is that in order to calculate the weight of the boat ( I´ll build it in fibreglass) I need the surface-area of the hull. Prolines gives me this figure , but--- I need to calculate this " by hand" , so I will need a way to develope the hull surface ( "flatten" it) and then calculate the area with simpson.
Who can give me advice how to do this ???
Thank you very much indeed!

 

The trick to doing it by hand is to divide the surface into sections. The waterlines and stations make almost rectangular sections. Measure the distance between the intersections and treat them as a flat surface. Add 2-3 % and you'll a pretty accurate measurment. This is also the old method of calculating displacement. Measure the area between two stations and the waterline, multiply by the waterline spacing and repeat on and on. I hope this helps.

 

Displacement by Simpsons Rule

In order to esatablish the Displacement you must 1st prepare a Lines PLan of your boat - showing a profile, plan view and section view of the boat. ONce that is doen you can then proceed with the Disp. calculation:

1st step is to divide the waterline length of your boat into an even number of stations. 10 is the norm but any even number will do so long as there's enough to accurately represent the shape of the hull. The Distance between these stations becomes the "Common Interval" or CI. (This is normally done in the process of preparing the lines plan)

Next you must calculate the area of each of the 11 stations (0, 1, 2, ....10). There are a number of ways of doing this. I gather you don't have access to a planimeter, so, with rounded sections the easiest way is probably do the drawing on grid (or graph) paper and count the number of squares that fall within the section area. You should now have the area of each station (don't forget to multiply by 2 if you've only measured the area of one side)

Next comes the Simplsons Multiplier part. Its easiest in table form, but I don't Know how to format that on the forum so hope this is ok to understand:

Stn : Area each Stn x SM = Function
0 : x 0 x 1 = 0
1 : 0.2 x 4 = 0.8
2 : 0.3 x 2 = 0.6
3 : 0.5 x 4 = 2.0
4 : 0.75 x 2 = 1.5
5 : 1.0 x 4 = 4.0
6 : 1.0 x 2 = 2.0
7 : 0.75 x 4 = 3.0
8 : 0.5 x 2 = 1.0
9 : 0.25 x 4 = 1.0
10 : 0 x 1 = 0
-----------
15.9

The Displacement equals:

Sum of Function x CI/3 x 64 (if your working in feet) or x 1026 (if in metres)

So for my above example with say a common interval of 3 ft,

Displ. = 15.9 x 3/3 x 64
=1017.6 lbs

or if the CI = 2 metres:

15.9 x 2/3 x 1026
= 10,875.6 kg


Hope this helps......

 

Hello guest

To find the surface area of the hull by hand you need a body plan with the sections (stations) spaced in accordance with Simpson’s First or Second Rule. The larger the views then the better the accuracy of the calculation.

The most accurate method is the “secant method” but it is time consuming and there are some descriptions of it on the net. The easiest method is to measure the girth of the sections and put them through Simpson’s.

The girth is the length of the curve and say the section is a semi-circle then the girth of the section on one side (port only or starboard only) is pi*D/4. The problem here is to actually measure the distance and it is very simple.

You will need some pieces of paper that have a straight edge and a sharp pencil. The paper must be longer than the girth of the station you will measure.

Make a mark on the paper edge near the end, about 10mm or so. Put the paper on the drawing so that the mark is on the end of the curve, usually the boat centreline, and the paper is tangential to the curve. Put the pencil point a few mm away from the mark and close to the paper edge then rotate the paper until it is tangential to where you have the pencil point. Repeat this until you get to the end of the curve at the waterline or the deck or the position where you need the girth.

The amount you move the pencil depends on the amount of curvature of the section. If the curve is straight then it is checked in one move, if the curve is around the bilge you must make several moves.

Try it a few times until you become used to it, it is easier than it sounds you just need a little practice. When you are satisfied that you can do it be sure you get the same length each time.

When you have worked out the area using Simpson's Rule you must increase it slightly to account for the curvature of the hull along the length. Select a waterline that is about halfway around the sections and girth the length around the hull. That length divided by the length of the waterline meaured at the centreline gives the factor to increase the calculated area. The factor will be quite small, only a few percent.

Michael

 

use a rolling map distance meter-- there small,cheap and very accurate!

 

It depends on the scale of the drawing.

Very good on scales of 1:5 or less but useless on 1:50.

There are or were some "electronic gizmos" to do it but if you want it only once or twice why bother?

Michael

 

I want to thank all of you for your help, I finally got a result closeish to the prolines-figures, which is all I needed.

One more question, if I may:
What is an easy way to calculate the stability of a boat, and draw a graph describing it ( rightening moments)
(- Kurvenblattberechnung und Kurvenblattzeichnung)?
Thanks again !
TR

 

Just registered!
I´d really appreciate your help regarding my Questions.
Thx
TR

 

It is reassuring to get the same result by two methods. This is a very valuable lesson. You have a backup way of verifying the work.

 

TR

I do not know any site on the net that shows HOW to calculate Cross-Curves of Stability. There are a few that describe how to read them and work with them.

The simplest way to calculate the Cross Curves is by computer using one of the modern suites. The old manual methods are simple enough but they are time consuming and very prone to error. You really need an integrator and even then it will take 25 hours of laborious work but setting it up in a spreadsheet would save hours and save the hassle of double-checking. As you are designing a dinghy you probably won't need to heel the boat as far over as the usual ship of boat with a closed deck and a watertight structure on the the deck. So you'll save even more time.

You need to get a textbook on naval architecture, preferably an old one. The latest books tend to describe the integrator method and computer methods, the old ones usually describe more methods that can be done with a planimeter . I’ve checked all my own textbooks and probably Kenneth C. Barnaby’s book “Basic Naval Architecture” is the most complete.

The attached MS Word file will help , be careful it’s 3.1 MB

Michael

 

DAMN

the file is too big, I've split it into three and here's the first part

Michael

 

Do you have an ftp program? If so I would be happy to give you an ftp login on our server to upload the file to... sorry for any inconvenience, as I really appreciate all the great material and help you've provided on our forums.

 

OOOOps! I have to apologise, I misunderstood the task I was given ( I was not familiar with the terminology).
What they want is a graph describing the increase of displaced water in relation to the draft.
This is easy enough to work out, I found a description of how to do that in one of my books!
:idea:
I´ve attached a pic of the page in case you don´t get what I try to explain...

Still, thank you all for your help!
T.R.

 

Rrespected sir,
i m a final year student of engineering in raj. india. sir in the submition of my project i know the basic concept but i donot know the right way so pl'z tell me.
PRO: to design a boat by using aluminium foil which can sustained maximum weight. in the designing of the boat pl'z consider the sea-worthiness, shape . the size of the foil is 25cm.*25cm..

so pl'z give me the best solution with specific dimens.
thanking you,
u'r sincerely
anil mishraion

 

anil

I do not understand your question. Please post the question using exactly the words as they were given by the professor.

Is it a small model you must make? The foil is only 25 cm x 25 cm but that is less than the distance from my elbow to my wrist.

Karl