if you can use a calculator you can.pdf

7/30/2019 If you can use a calculator you can.pdf

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If you can use a calculator you can

design a boat

1Who hasnt worked with ideas or sketches of ones own dreamboat? Shouldn't it be

better, faster, cheaper and prettier than all the other boats designed up 'till now? It is notdifficult 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 ideasabout their dreamboat. The first thing you must do is to define exactly what you want.In order to get the ideas well described, write down a list that defines the boat. This listcan be called a design brief. It will help you through out the entire design process and itwill 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 be racing, cruising or fishing, or is it acombination of different purposes? Is it a boat for sail or power? How many bunks doyou want? Should there be a toilet and pantry? How big a boat do you what? Whatmaterials 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. Sosee if there are other boats that meet the demands in your design brief or take fragmentsfrom 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 aboat beyond a certain limit of approximately 10-15%. If you, for instance, scale thelength, beam and depth of a boat by 2 then the sail area would be scaled by a factor of 4

and the displacement of the hull will be scaled by a factor of 8. So you see theproportions 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 abetter idea what the boat should look like. The next step is to start sketching.


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The sketching does not need to be fancy. Remember you are at the start of the designprocess and there are things that will have to be corrected later on. Simply make asketch containing a waterline/chine plane, a deck plane, a profile and a midship section.Use a reasonable scale, 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 aremuch better when you stick to something known. Also try not to make your boat toonarrow. It is much easier to give a boat better stability or different trim if you have somedisplacement to work 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 your interior to see if the hull fits therequirements in the design brief. You may have to correct either your design brief orhull. If, for instance, you have 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 to do this but to do it right, youhave to make a complete list of all elements' weights and centres of gravity (cog). Atthis stage you can start with some estimations and in part 3 of this article series therewill 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 about where to start. Compare theweights of the different boats and make some simple calculations regarding your own

boat. Be aware of what weight you compare with what. Different designers havedifferent ways to state their weight; e.g. some state it only for the bare hull and some forthe hull with all gear, crew etc.
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The simple calculation may look like this:

Element Weight [kg]

Hull with interior 1900

Engine and tanks 350

Gear 450

2700

At this stage it is not necessary to take the distribution of the weight into account. Thatwill be considered in more detail in part 3 of this series.

Now the question is: How does the preliminary weight match the hull sketchedpreviously? Again it is necessary to make estimations. When designing a hull there arecertain coefficients that can be useful. One of them is the prismatic coefficient. This is anumber that reflects the relationship between the displacement and a prism with equalarea as the midship section and with a length equal the waterline length of the hull. E.g.if a hull has a waterline length of 6,0 m and the midship section has an area of 0,658 m2the prismatic volume will be:

6,0 x 0,658 = 3,95 m3

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

displacement of:

3,95 x 0,72 = 2,84 m3

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

From your hull sketch you have the midship section. So the only thing to do is find thearea of the midship section. Remember that the area has to be the underwater area of thesection. Multiply the area with your waterline length and you have the prismaticvolume. To determine the 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 madeto fit together. You are not at all done yet, but you have managed to get started andmake some preliminary sketches and calculations. In the example you can see that theweight estimated is smaller than the displacement of the hull. This is not a problemsince it is normal that the weight will be higher when you get in to details with yourweight calculation.

The whole process of going through the different steps is called a design spiral and youhave to go through this spiral several times before your design is finished. Every timeyou go through the spiral you should come closer to the final result making your


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dreamboat better and better every time. Not every round in the spiral needs to cover all4 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 alsoan elaboration of the weight issue.


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2The Line Drawing

The most useful tool when designing boats is the line drawing. The line drawing clearlydefines the hull geometry. Furthermore it is used when calculating hydrostatic

properties for the hull you are designing.

If you have followed the steps inpart 1of this article series you will already have thestart of the line drawing. But there is more drawing to do before it is finished. Start withdrawing your design water line (DWL). The design water line is the waterline youexpect your hull will float at. Later on it may show up that the hull does not float atexactly this waterline but dont worry about that at the moment. Next you divide your

waterline in 10 equal lengthen pieces making 11 stations. In the profile plane thestations 0 and 10 are made where the profile of the hull is cut by the DWL. Now you

have a grid to start with. The next thing to do is to begin constructing the stations. The
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stations cut the hull in the 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 anexample you can see above how it is done. In the profile plane the intersections betweenthe hull and the stations are continued into the body plane. Then the Dist 1 and Dist2 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 the example there is used achine hull so the 3 points are sufficient for defining the section. Continue with section 1,7 and 9. You can fill in the rest later. If you are designing a traditional hull the 3 pointswill not be enough to define the entire section so you will need some waterlines and

buttocks as well.

You can add as many waterlines (WL) and buttocks (B) as you want. They have thepurpose of helping you define the hull shape more accurately. When you add thewaterlines and buttocks you make more intersection points so you will have more pointshelping you to define your curves. Normally waterlines and buttocks are made with anequivalent distance between them, but you can place them at any distance you choose.


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The rest of the line drawing is now made. Normally curves should be smooth but sincethe example is a chine hull you can see some bends where the waterline cuts the chine.

Making a line drawing is hard work. It is not unusual for a designer to spend daysworking on one, so dont expect 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. Even linesyou thought were right may later need to be altered. This is normal and there is noshortcut to that.

Drawing tools

Beside a ruler, pencil and eraser some tools for making curves are necessary whenmaking the line drawing. If your drawing is large you can use a thin batten held withweights, but if your drawing is smaller or there is large curvature some curved rulers are

necessary. It is possible to get a large variety of curved rulers but they are not all wellsuited for the purpose of drawing hull curves.


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The most complete set of ship curves is the Copenhagen ship curves. The ship curvesare produced by Linex (www.linex.dk). The set of curves are outstanding butunfortunately it will also be quite expensive to buy them all. I can recommend that ifthis is the solution you want you 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 cadapplications available for hull design. They range in price from free to many thousanddollars. So all the sketching and drawing on paper can be avoided and instead you cando all the work on your computer. It shall be said at once that using cad for hull designis great, it saves a lot of the hard work for you get results quickly. But there are

problems when designing with cad programs. First of all, are you sure that the hull youdesigned with the cad program is the hull you wanted in the first place? Many times, ifyou are not careful the hull you get is the hull the cad program or the limits in the cad

program designs for you. So you sort of loose control over your hull design process.Anyway it can never give you any problems knowing the old fashion way of doing thedesign process. It might even give you an advantage when using a design software.
http://www.linex.dk/http://www.linex.dk/http://www.linex.dk/http://www.linex.dk/


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3Hydrostatic

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

From the line drawing it is possible to calculate every station's underwater area. In theexample a chine hull is used, so calculating the areas should not be a problem. If you aredesigning a traditional hull it can be more difficult calculating the area under a curve.One possibility could be 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 linedrawing is only made with a half station, so the area has to be multiplied by two in orderto get the entire area of a station.

Station Area [m2]

0 0

1 0,1584

2 0,3474

3 0,49364 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 approximating definite integrals. It sounds

complicated but it is quite easy to use. Every station area is first multiplied withSimpsons factor (SF). Then the sum of all the products are divided by 3 and multipliedwith the longitudinal distance between the stations.


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Station Area [m ] SF Res.

0 0 1 0

1 0,1584 4 0,6336

2 0,3474 2 0,6948

3 0,4936 4 1,97444 0,5952 2 1,1904

5 0,6578 4 2,6312

6 0,6880 2 1,3760

7 0,6920 4 2,7680

8 0,6750 2 1,3500

9 0,6418 4 2,5672

10 0,5972 1 0,5972

15,7828

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 m3

If the boat has to float in freshwater the weight at the construction waterline will be3409 kg. But since saltwater has a density of approximately 1025 kg/m3 the boat willhave a weight of 3,409 x 1,025 = 3495 kg when floating at the construction waterline insaltwater. Thus saltwater has a greater buoyancy than freshwater due to the higherdensity. To determine the longitudinal centre of buoyancy (LCB) it is necessary to makesome further calculations with the station areas. The next technique introduced is the

moment calculation. This technique can be used in various situations. First it isnecessary to define a fixed station and in this example the fixed station will be station10. Then every station contributes to the moment calculation by their area multipliedwith the longitudinal distance from station 10.

Station Area [m ] 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

5 0,6578 3,240 2,131

6 0,6880 2,592 1,783

7 0,6920 1,944 1,345

8 0,6750 1,296 0,875

9 0,6418 0,648 0,416

10 0,5972 0 0

5,5464 13,828


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The longitudinal distance becomes: 13,828 / 5,5464 = 2,493 m

This means that LCB is laying 2,493 m forward of station 10 i.e. between station 5 and6. The moment calculation is made from station 10 but the moment calculation can bemade from any station desired. Just remember from witch station the calculation is

made when evaluating the result.

Weight calculation

The weight calculation is carried out in order to calculate the longitudinal centre ofgravity (LCG) for the boat. In principles all elements, even every screw or nail, should

be counted in the calculation. But of course it would be a large job to do that, so inpractice all major parts should be counted in and then a certain amount added for thesmall things left out or forgotten.

It is normal to divide the weight into different main areas. You must determine which

areas are suitable for your hull, but a minimum must be; hull structure, superstructure,interior, installations, ballast, rig and sail (if a sailboat) and payload.

In order to make the rest of the weight calculation you must make some seriousconsiderations regarding the construction, appearance and layout of your boat. Youhave to decide how you would construct the hull, what the cockpit and roof should looklike, how the interior should be and what engine installation you will have. When thesethings are decided and drawn you can make a trustworthy weight calculation.

For every area considered, not only must the weight be known, but also the centre ofgravity must be known. With many things the centre of gravity is not known preciselyso an estimated centre must be used. The centre of gravity for every component is usedin a moment calculation similar to the one made in the hydrostatic part. Later on, whenthe calculation is finished the resulting centre of gravity for the boat is found. Belowyou can se an example of the weight calculation for the hull structure.

Hull Structure

ItemWeight

[kg]LCG[m]

Moment

Scantlings plywood 431 2,910 1254,21

Scantlings epoxy and glass 312 2,910 907,92

Transom plywood 38 0 0

Transom epoxy and glass 23 0 0

Frames 172 2,490 428,28

976 2590,41

LCGhull = 2590,41 / 976 = 2,654 m forward of station 10

Now it is only a question of finishing the rest of the main areas in the weightcalculation. When that is done you can make a new weight and moment calculation thatgives the total weight and LCG.


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Item Weight [kg] LCG [m] Moment

Hull structure 976 2,654 2590,30

Superstructure 586 2,937 1721,08

Interior 637 3,236 2061,33

Installations 779 1,345 1047,76

Payload 590 2,493 1470,87

3568 8891,34

LCG = 8891,34 / 3568 = 2,492 m forward of station 10

As you can see, the weight is a bit less than the displacement calculated. The differenceis thus so small that in practice it is nothing compared to the uncertainty in thecalculations. Furthermore it can be seen that the LCG is laying a bit aft of the LCB. Thismeans that the boat will have a small trim, but again it will have no influence in

practice.

It is not certain that your first set of hydrostatic and weight calculations will come outlike the example above and therefore it may be necessary to go back through the designspiral, each time correcting and refining the drawings and calculations.

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