Footy design – Freeship III

If you have difficulty to see the mesh click on the icon several times that it will come and go and may be better viewed. It is important you always see the mesh and design and not to be confused. You can only modify the drawing by clicking and dragging the black dots, in doing so you will be modifying the hull.

The lower red line corresponds to the hull center line, the vertical fore and aft corresponds to bow and stern and above, the deck, then we will change the two points aft in the lower red line in the stern region:

Lowered the stern on the centerline we can return to the calculations. Here it is worth commenting on the small window that opens when you click a point on the mesh. It shows the coordinates of the point. As the two points that we have stirred are in the center line the y value of point must always be 0.0, if it are not the Freeship calculate anything. After placing the point where we want let the calculations:

and we see that the total waterline length of immersed body = 33.922 m  that will look like in our Footy 33.922 cm. This value can be increased if we change the slope of the bow. Unlike the IOM or RG 65, the Footy bow is inclined, so that the bow has to stay inside the box, take a look at the picture above. Using the inclined bow we can increase the Lwl when sailing due the crest formed in bow. But let’s not worry about that, now, let’s calculate the area needed to keep the boat with Cp = 0.58:

0.58  = 600/Asm* lwl

with lwl = 33.922 m

0.58 =  600/Asm * 33.922        soon

Asm = 600 / 33 922 * 0.58

Asm = 30.495 m2 = 30.5 m2

As the actual midship section area = 25.714 m2 = Asm,  we see that we need to increase the sectional area and increase the immersed volume.

This is possible by increasing the area of ​​the midship section and increasing the area of ​​the other sections (midship section area is the largest immersed cross section area). To do this minimizes the Profile view and maximize Bodyplan view:

As we have stirred the two mesh points in the Profile View did not see the consequence of this twitch on the cross sections, that we can see now. The latter sections were deformed,  let us move the mesh points to correct the sections. The first is what is selected in the figure below. When we select a point (by clicking on it) it is involved by a square point:

Clicking and dragging the point down:

At this point, we’ll have to increase the midship section and other sections. The Bodyplan view  has many cross sections. Let’s reduce the number of these sections. As we know that the LWL is 33.922 m, we can provide a spacing between sections: 33.922 / 10 = 3.3922 m, which will give us 11 sections within the lwl what we will facilitate the work. To do this we go to Calculations menu and clicking on Intersections and then the first icon, which are the cross sections (Stations):

Clicking the trash icon  see that behind all the sections of the window go out, then clicking the + N  appears a dialog box where you put the value above -> 3.3922 and then click OK:

We then see the new cross sections, spaced at 3.3922 now.

The new cross sections are as follows:

As standard procedure Freeship puts the midship section – section of greater cross-sectional immersed area  in the middle of total length, but we will put it in position 16.961 m which in the Bodyplan view sections is on the left side, the aft sections. To do this we will go in the menu Project -> Project Settings -> Main Dimensions and put this value in the box  –> Midship or maximal area station location by removing the check in the box –> Default at 0.5 L:

Once this is done we will increase the submerged part of the largest underwater section of the left side, our midship section:

Let’s go to the menu Calculations -> Design Hydrostatics and we see that our area came to 27.396 m2,  so, we need increase it more:

We repeat this process until we get  30.5 m2. At this point we can modify the cross section and introduce the way we want for the cross sections: circular, parabolic, square, triangular, etc. or a mixture between them. I’ll leave them nearly circular:

We see then that my final midship sectional area is 30,506 m2 with my displacement increased to 493.70. To reach 600 tonnes we go to change the other sections monitoring the displacement. An interesting observation is to see that Cp change to 0.4821, and this change is because the area of ​​the midship section increased but the displacement do not increase much, but when the displacement reach 600 he will stay with Cp = 0.58.

The modification of the areas of other sections can follow any criteria – sinusoidal or trochoidal or whatever the designer wants but now we follow the curve of areas shown in the Profile view, but we will worry that our changes not introduce peaks or troughs in the curve areas and keep continuous :

We see that this figure shows the displacement ( CB – center of buoyancy) positioned forward the section 16961 and therefore I will try to put it a little more aft increasing a little more the aft sections than forward sections.

The resulting hull is below, I triedd to do a very traditional hull, no chine, no major new features:

The calculation result was this:

I’ll put down the three views increased to serve as a reference:

The next step is to hit the part above the waterline project. We see that the maximum height in the bow, indicated in the Profile view is 1502 or 1102 is above the waterline project would be in our Footy 11 cm above. The experience from Carcara would say me that 5 cm above it would be reasonable, so we’ll have to lower the deck to 9002:

Let lowering all upper points of the mesh to look like this:

I left the height of the stern out of water with 4 cm, thus the deck have a fall of 1 cm.



About Fred Schmidt

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