If you can use a calculator you can design a boat….pdf

8/11/2019 If you can use a calculator you can design a boat.pdf

1/7

Part 1

Who hasnt worked with ideas or sketches of ones own dreamboat? Shouldn't it be better, faster, cheaper and prettier than all the other boatsdesigned up 'till now? It is not difficult to design a boat if you follow some easy design steps.

Step 1) be aware of what you want

Always start by defining what you want. Most people start with some ill-defined ideas about their dreamboat. The first thing you must do is todefine exactly what you want. In order to get the ideas well described, write down a list that defines the boat. This list can be called a designbrief. It will help you through out the entire design process and it will help you keeping focus.

Here is an example of some questions you can ask yourself when you make the list: What do you intend to use the boat for? Should it beracing, cruising or fishing, or is it a combination of different purposes? Is it a boat for sail or power? How many bunks do you want? Shouldthere be a toilet and pantry? How big a boat do you what? What materials do you want to use for the boat?

Step 2) look for inspiration

All designers look for inspiration in other boats and there is nothing wrong with that. So see if there are other boats that meet the demands inyour design brief or take fragments from different designs and combine them to form a concept you like.

Be aware of the size of the boat that inspires you. It is not at all possible to just scale a boat beyond a certain limit of approximately 10-15%.If you, for instance, scale the length, beam and depth of a boat by 2 then the sail area would be scaled by a factor of 4 and the displacementof the hull will be scaled by a factor of 8. So you see the proportions of the boat no longer match.

Step 3) make some sketches

Now that you have made the design brief and found some inspiration, you should have a better idea what the boat should look like. The nextstep is to start sketching.

(click image to enlarge)

The sketching does not need to be fancy. Remember you are at the start of the design process and there are things that will have to becorrected later on. Simply make a sketch containing a waterline/chine plane, a deck plane, a profile and a midship section. Use a reasonablescale, perhaps 1:20 or 1:25, and find paper that is 11x17 or A3.

Don't make the boat too peculiar. For an amateur, the odds of designing a good boat are much better when you stick to something known.Also try not to make your boat too narrow. It is much easier to give a boat better stability or different trim if you have some displacement towork with, and the wider beam will give you that.

At this time it is a good idea to make some preliminary sketches regarding interior. Make some copies of your hull sketch and draw in yourinterior to see if the hull fits the requirements in the design brief. You may have to correct either your design brief or hull. If, for instance, youhave stated that you want to make a 14 boat with 4 bunks, pantry and toile, you will need to make some serious considerations about what ispossible.

Step 4) estimating the weight

To determine the stability of your hull, you need to know the weight of the different elements you place in the boat. There is no easy way todo this but to do it right, you have to make a complete list of all elements' weights and centres of gravity (cog). At this stage you can startwith some estimations and in part 3 of this article series there will be a deeper discussion of this issue.

In order to estimate the weight you can start by comparing your boat with other designs. From step 2 you should have some good ideas aboutwhere to start. Compare the weights of the different boats and make some simple calculations regarding your own boat. Be aware of whatweight you compare with what. Different designers have different ways to state their weight; e.g. some state it only for the bare hull andsome for the hull with all gear, crew etc.

The simple calculation may look like this:

El e m e n t W e i g h t [ k g ]

Hull with interior 1900Engine and tanks 350Gear 450

2 7 0 0

At this stage it is not necessary to take the distribution of the weight into account. That will be considered in more detail in part 3 of thisseries.
http://www.duckworksmagazine.com/04/s/designs/morten/designing/ill-1-1.gifhttp://www.duckworksmagazine.com/04/s/designs/morten/designing/Part3.cfmhttp://www.duckworksmagazine.com/04/s/designs/morten/designing/Part2.cfm


2/7

Now the question is: How does the preliminary weight match the hull sketched previously? Again it is necessary to make estimations. Whendesigning a hull there are certain coefficients that can be useful. One of them is the prismatic coefficient. This is a number that reflects therelationship between the displacement and a prism with equal area as the midship section and with a length equal the waterline length of thehull. E.g. if a hull has a waterline length of 6,0 m and the midship section has an area of 0,658 m2 the prismatic volume will be:

6 ,0 x 0 , 6 5 8 = 3 ,9 5 m 3

If the same hull has a prismatic coefficient of 0,7 it means that the hull has a displacement of:

3 , 9 5 x 0 , 7 2 = 2 ,8 4 m 3

If the hull has to float, at this waterline, in fresh water it means that the boat will have a weight of 2840 kg.

From your hull sketch you have the midship section. So the only thing to do is find the area of the midship section. Remember that the areahas to be the underwater area of the section. Multiply the area with your waterline length and you have the prismatic volume. To determinethe displacement use a prismatic coefficient between 0,54-0,75. This means that for fine ended hulls use the number 0,54.

Now see if the hull you have sketched and the weight you have estimated can be made to fit together. You are not at all done yet, but youhave managed to get started and make some preliminary sketches and calculations. In the example you can see that the weight estimated issmaller than the displacement of the hull. This is not a problem since it is normal that the weight will be higher when you get in to details withyour weight calculation.

The whole process of going through the different steps is called a design spiral and you have to go through this spiral several times beforeyour design is finished. Every time you go through the spiral you should come closer to the final result making your dreamboat better andbetter every time. Not every round in the spiral needs to cover all 4 steps, but it may be necessary for you to correct all of them.

In the next 2 articles there will be an more about line drawing and hydrostatics and also an elaboration of the weight issue.


3/7

Part 2

The Line D rawin g

(click image above to enlarge)

The most useful tool when designing boats is the line drawing. The line drawing clearly defines the hull geometry. Furthermore it is used whencalculating hydrostatic properties for the hull you are designing.


If you have followed the steps in part 1 of this article series you will already have the start of the line drawing. But there is more drawing todo before it is finished. Start with drawing your design water line (DWL). The design water line is the waterline you expect your hull will floatat. Later on it may show up that the hull does not float at exactly this waterline but dont worry about that at the moment. Next you divideyour waterline in 10 equal lengthen pieces making 11 stations. In the profile plane the stations 0 and 10 are made where the profile of the hullis cut by the DWL. Now you have a grid to start with. The next thing to do is to begin constructing the stations. The stations cut the hull inthe same way as the frames will be placed in the boat later on. But the stations have in principle nothing to do with the frames in the boat.


Now with the grid started it is time to draw some more stations in the body plane. As an example you can see above how it is done. In theprofile plane the intersections between the hull and the stations are continued into the body plane. Then the Dist 1 and Dist 2 from the half breadth plane are set out from the centreline in the body plane. In the body plane you now have 3 points that define the section. In theexample there is used a chine hull so the 3 points are sufficient for defining the section. Continue with section 1, 7 and 9. You can fill in therest later. If you are designing a traditional hull the 3 points will not be enough to define the entire section so you will need some waterlinesand buttocks as well.


You can add as many waterlines (WL) and buttocks (B) as you want. They have the purpose of helping you define the hull shape moreaccurately. When you add the waterlines and buttocks you make more intersection points so you will have more points helping you to defineyour curves. Normally waterlines and buttocks are made with an equivalent distance between them, but you can place them at any distanceyou choose.
http://www.duckworksmagazine.com/04/s/designs/morten/designing/Part1.cfmhttp://www.duckworksmagazine.com/04/s/designs/morten/designing/Part3.cfmhttp://www.duckworksmagazine.com/04/s/designs/morten/designing/Part1.cfmhttp://www.duckworksmagazine.com/04/s/designs/morten/designing/Part1.cfmhttp://www.duckworksmagazine.com/04/s/designs/morten/designing/Part3.cfmhttp://www.duckworksmagazine.com/04/s/designs/morten/designing/Part1.cfm


4/7


The rest of the line drawing is now made. Normally curves should be smooth but since the example is a chine hull you can see some bendswhere the waterline cuts the chine. Making a line drawing is hard work. It is not unusual for a designer to spend days working on one, so dontexpect to make yours in an hour or so. You must also be aware that the lines may have to be changed many times during the work. Evenlines you thought were right may later need to be altered. This is normal and there is no shortcut to that.

Drawing tools

Beside a ruler, pencil and eraser some tools for making curves are necessary when making the line drawing. If your drawing is large you canuse a thin batten held with weights, but if your drawing is smaller or there is large curvature some curved rulers are necessary. It is possibleto get a large variety of curved rulers but they are not all well suited for the purpose of drawing hull curves.

The most complete set of ship curves is the Copenhagen ship curves. The ship curves are produced by Linex ( www.linex.dk ) . The set of curves are outstanding but unfortunately it will also be quite expensive to buy them all. I can recommend that if this is the solution you wantyou can buy curves A1, A2, A11/15 and A11/29. They will fit most needs.

If you want to spend less on curves you can use 3 curves from the 103DT Burmester Set. The curves can be used for making hull curves but

they are not outstanding. I have often seen the curve set in bookstores for $15-20.CAD

In this article series cad for hull design is not described but there are many cad applications available for hull design. They range in price fromfree to many thousand dollars. So all the sketching and drawing on paper can be avoided and instead you can do all the work on yourcomputer. It shall be said at once that using cad for hull design is great, it saves a lot of the hard work for you get results quickly. But there
http://www.linex.dk/http://www.linex.dk/http://www.linex.dk/


5/7

are problems when designing with cad programs. First of all, are you sure that the hull you designed with the cad program is the hull youwanted in the first place? Many times, if you are not careful the hull you get is the hull the cad program or the limits in the cad programdesigns for you. So you sort of loose control over your hull design process. Anyway it can never give you any problems knowing the old fashionway of doing the design process. It might even give you an advantage when using a design software.


6/7

Part 3

Hydrostatic

There are many hydrostatic properties that can be calculated for a hull. Within the limits of this article series only the displacement and thelongitudinal centre of buoyancy (LCB) is calculated.

From the line drawing it is possible to calculate every station's underwater area. In the example a chine hull is used, so calculating the areasshould not be a problem. If you are designing a traditional hull it can be more difficult calculating the area under a curve. One possibility couldbe to draw the stations on graph paper and then count the squares under the curve. This will give a reasonably accurate result. Remember

that your line drawing is only made with a half station, so the area has to be multiplied by two in order to get the entire area of a station.

Station Area [m 2 ]

0 0

1 0,1584

2 0,3474

3 0,4936

4 0,5952

5 0,6578

6 0,6880

7 0,6920

8 0,6750

9 0,6418

10 0,5972

When you are through calculating the areas you should have a table like the one above. Next step is to use Simpsons rule for approximatingdefinite integrals. It sounds complicated but it is quite easy to use. Every station area is first multiplied with Simpsons factor (SF). Then thesum of all the products are divided by 3 and multiplied with the longitudinal distance between the stations.

Station Area [m 2 ] SF Res.

0 0 1 01 0,1584 4 0,63362 0,3474 2 0,69483 0,4936 4 1,97444 0,5952 2 1,19045 0,6578 4 2,63126 0,6880 2 1,37607 0,6920 4 2,76808 0,6750 2 1,35009 0,6418 4 2,5672

10 0,5972 1 0,597215,7828

In the example the longitudinal station distance is: s = 0,648 m.

The displacement for the example hull becomes: 0,648 x 15,7828 / 3 = 3,409 m 3

If the boat has to float in freshwater the weight at the construction waterline will be 3409 kg. But since saltwater has a density of approximately 1025 kg/m3 the boat will have a weight of 3,409 x 1,025 = 3495 kg when floating at the construction waterline in saltwater.Thus saltwater has a greater buoyancy than freshwater due to the higher density.

To determine the longitudinal centre of buoyancy (LCB) it is necessary to make some further calculations with the station areas. The nexttechnique introduced is the moment calculation. This technique can be used in various situations. First it is necessary to define a fixed stationand in this example the fixed station will be station 10. Then every station contributes to the moment calculation by their area multiplied withthe longitudinal distance from station 10.

Station Area [m 2 ] Arm [m] Moment

0 0 6,480 0

1 0,1584 5,832 0,924

2 0,3474 5,184 1,801

3 0,4936 4,536 2,239

4 0,5952 3,888 2,314
http://www.duckworksmagazine.com/04/s/designs/morten/designing/Part2.cfmhttp://www.duckworksmagazine.com/04/s/designs/morten/designing/Part1.cfm


7/7

If you can use a calculator you can design a boat….pdf

Documents

Transcript of If you can use a calculator you can design a boat….pdf