Footy design – Freeship II

We have a reference for Footy weights:

As there is no limit to sail area the project is still more freely to the total weight to be chosen and you can play with the bulb weight and sail area.

The value of 1000 cm2 can be increased if the wind is light.

Well, as we have to define the dimensions of length, maximum beam and draft in order to use the Freeship, let’s start with:

Total length = 380 mm

Maximum breadth = 100 mm

Draft = 40 mm

Entering Freeship in the menu File , click New

the window is filled with primary data from Freeship, let’s change the values ​​for our boat, but if we use the actual values ​​can not take advantage of the calculations performed by the software because the accuracy of the numbers used by Freeship. So, multiply by 100 the linear dimensions from our boat. Thus, the measures to put in the dialog window will be:

length = 380 mm => 0.38 m x 100= 38 m

beam = 100 mm => 0.1 m  x 100 = 10 m

draft = 40 mm => 0.04 m x 100 = 4 m

To pass the values ​​calculated for the the actual size to our Footy divide  the lengths by 100 the areas by  10,000 and volumes by 1,000,000

Moreover we will change:

no. points in the longitudinal direction -> from 6 to 7

no. points in vertical direction -> from 5 to 6

these points are the points that we need modify their positions to change the shape of the hull.

Switching we have a 7 x 6 = 42 points to modify the shape of the hull.

Clicking OK to pass the screen:

 

this is the default Freeship boat for  measures we provide.

The Freeship standard medium for water is salt water  but the critical condition  is fresh water, then we must make this change.

For this, we clicked on Project and Project settings and get:

Here we can fill in the blanks, but let’s click on Hydrostatics and get:

Let’s change the density of 1.025 (salt water) to 1,000 (fresh water):

Click OK and the calculations will be made to fresh water.

We need evaluate its characteristics in order to have the notion of the changes we have to do, so go to the menu Calculations and clicking in Design Hydrostatics and we get:

The quantities we need to start monitoring is:

Displacement = weight of water displaced by the immersed volume, which is equal to the weight of the boat (Archimedes’ Principle)

Prismatic coefficient

Wetted surface

Longitudinal center of buoyancy

Waterplane center of flotation

For those not really familiar with these magnitudes I suggest read this page:

https://iomdesign.wordpress.com/naval-architecture/

Go to the values:

Displacement = 418.43 tonnes

Prismatic coefficient = 0.5287

Wetted Surface Area = 274.5 m2

Longitudinal center of buoyancy = 18,384 m

Waterplane center of flotation = 17,642 m

These values ​​are for a boat 38 m lenght, to go to our Footy would be:

Displacement = 418.43 tonnes = 418340 kg => 418340 kg/1.000.000 = 0.418340 kg = 418.43 g

How we work in kg and Freeship in tonnes simply divide the calculated weight  418.34 tonnes per thousand  => 0.41834 kg or read the Freeship value  418.34 tonnes in grams => 418 , 34 g to pass the values ​​found for our Footy.

Prismatic coefficient = 0.5287

Wetted Surface Area = 274.5 m2 => 274.5/10000 = 0.02745 m2 (wetted immersed area of ​​our Footy)

Longitudinal center of buoyancy = 18,384 m => 18.384/100 = 0.1834 m = 18.384 cm (EC of our Footy)

Waterplane center of floatation = 17,642 m => 17.642/100 = 0.17642 m = 17, 642 cm (CF of our Footy)

We see then that we can read the linear dimensions (length)  provided by the software in cm directly, to take back to our footy.

Well let’s analysis the values. The Freeship informs us that the displacement is 418.43 g but is not sufficient because we want  600 g. We must then increase the volume until it has  600 tonnes.

The prismatic c0eficiente Cp is 0.5287. This coefficient should be between 0.52 and 0.60. At low speeds the hull should have little Cp and go up according to the speed increase. How we can have only one Cp for each displacement seems to me that in the case of Footy, which has no limitation on sail area, it is better to have a high Cp, say 0.58 and have a sail with more than 1000 cm2 for light winds for the Footy get easier at full speed.

The formula for Cp is: Cp = Volume / Area of midship section * Length of waterline waterline (LWL).

To  increase Cp we need to increase the volume or decrease the midship section area. As we have to increase the volume of 418.43 m3 to 600 m3 let’s see what is the midship section area necessary to obtain the Cp of 0.58:

From Freeship: Total length of submerged body = 30,776 m. Here we see a big difference between the total length L0a = 38  m and Lwl and we’ll see why in the drawing. The reason is the very hight stern of the Freeship standard boat. We may have the stern closer to the water surface  to increase the lwl to obtain a higher speed potential. Our formula is:

0.58 = 600/Asm* Lwl

So, let’s do the first work in the drawing,  that is lower the stern. Once we have the new Lwl we can use the formula.

Freeship window is composed of four windows, three of them represent the views necessary for defining the shape of the hull: The Profile View that are longitudinal sections, the Bodyplan View are the cross sections and Plan View that are horizontal sections, and the fourth window is a real 3D view of our boat.

To move the stern we will maximize the window  Profile view that looks like this:

Note the mesh lines drawn in blue outlined in red and with black points, these 42 black points are those who speak up above. This mesh involves the hull and to change the shape of the hull we have stirred these 42 points. This mesh can be removed from the screen going on the menu Visibility  and clicking Control net, if we do we will keep the screen like this:

We can access this command looking for the icon in the menu bar and clicking on it:

is the first icon.

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About Fred Schmidt

Engenheiro Naval interessado em projeto de veleiros radio-controlados. Naval Architect interested in RC sailboats design
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