One of the problems to design a Footy is choose a total length because, as by rule, it has to fit inside a parallelepiped 301.5 x 153 x 301.5 mm in length, width and height respectively.
The diagonal of the box is 457 mm and the horizontal surface diagonal is 341 mm.
The difficulty is that we need accommodate the keel and rudder in the box.
We know that the longer the waterline length greater the speed of the boat. As a curiosity we check what is the maximum speed possible for two Footy , one with length 305 mm and one with 457 mm
Vmax = ((0.305 / 0.3048) ^ 1 / 2) * 1.34 = 1, 34 knot
Vmax = ((0.457 / 0.3048) ^ 1 / 2) * 1.34 = 1.64 knot
about 22% higher.
However the geometry of a footy with a length of waterline 0.457 m or 457 mm would be interesting, I would say almost impossible and I do not know if it would be very effective because it would have a bow and a stern extremely thin and the bow inclined to aft.
Another problem would be the space and position available for the rudder, see the figure above, to the length of the Pool Shark increase, the rudder would have reduced its height and its width increased or have to move forward, reducing its efficiency.
The keel would have the same problem, would be lower and wider, signifying lost of efficiency, while here, we had an aggravating factor, the bulb would be closer to the hull and the boat would lose the ability to carry a good sail area.
The height of the keel including the bulb from the bottom of the boat, in the Pool Shark, is 17 cm and the rudder has height 11 cm, the width of the keel is 4.2 cm and the width of the rudder is 3.1 cm.
Measurements of Pool Shark, AMYA champion 2010:
Loa = 385 mm (total length)
Lwl = 350 mm (length waterline)
Beam = 115 mm (maximum)
Displacement = 600 g
Height of the bow above the waterline = 60 mm
The Pool Shark 3D has great maneuverability, is very stable and can withstand quite a strong wind.
When I started the Carcara, my first Footy design, I had take the data from internet and displacement is a key project data, I would say is the most important data, I work with a displacement of 400 g. When I received the Pool Shark 3D saw that the weight was with 600g.
Being the size of Carcara practically equal to the Pool Shark 3D I saw that I would have to increase the displacement and the mold was finish.
The solution was to increase the depth of the Carcara about 1 cm and I increased the freeboard by 1 cm (height of the hull above the waterline). For the stern would not get immersed I moved the CG (Center Gravity) of the boat forward.
The Carcara has measures:
Loa = 38o mm
Lwl = 360 mm
Beam = 90 mm
Displacement = 600 g
Height of the bow above the waterline = 40 mm
The Carcara have a performance similar to the Pool Shark 3D and in lighter winds I noticed a better performance of Carcara, principally in windward.
I am thinking that in Footy class the expertise of each sailor (since the boat is balanced) will determine the result of the race, regardless of the boat, because the littles differences in velocity between boats.
From my point of view, at least for now, the important thing is the choice of displacement and sail area capacity, this, according to the weight of the bulb.
As in RG 65 the weight of the bulb is critical, the Pool Shark 3D has a 300 g bulb.
The experience with the Pool Shark 3D and Carcara say me that the Footy weight must be around 600 g with a maximum sail area around 1000 cm2, and in lighter air a few more.
The Carcara weights spreadsheet :
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)
Longitudinal center of buoyancy
Waterplane center of flotation
For those not really familiar with these magnitudes I suggest read this page:
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.
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:
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 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:
Depending on the route of the race a boat can spend more time inclined than vertical, so personally, I study very carefully the inclined boat. Unfortunately I do not know a free software that will do it. Draw the inclined waterlines and redo the calculations for this situation is extremely important.
Today let’s try see the inclined waterlines using existing resources in Freeship.
Let’s go to menu Transform –> Mirror:
Let’s rotate 35 degree. Menu : Transform –> Rotate
See the various windows and principally the Plan View, but we can see better on Perspective View. Maximize de Perspective View. Click the mouse right button and choose Mode –> Shade, use the bottom and left window rules to rotate the 3D drawing:
See the waterlines ends, they are inclined in relation to the old centerline and has a curvature between them. I think that this tends to put the boat crossed over the direction of the course. Normally I work to eliminate this distortion to the maximum I can.
An other interesting point is the chine in the fore centerline formed by the two sides of the boat.
Let’s go work on waterlines. Minimize Perspective View and maximize Plan View.
Trigger the Control net:
Let’s minimize the Plan View and maximize the Perspective View:
1 – Go to eliminate the rotation ==> Menu Transform –> Rotate –> Longitudinal Axis –> (Type) – 35 –> Click OK and we have a rotation for the other side.
2 – Before we adjust the mirrored I will go adjust the points changed in the Plan View in the transverse section:
Minimize the Plan View, maximize the Bodyplan View:
Let’s adjust this, clicking in the two centerlines points in the left side and in the right side we can see that they are 0.6657 away from centerline. Remove the control net. Going to the Menu ==> Transform –> Move –> OK –> Transverse Axis –> type 0.6657 –> OK :
(NOTE: I don’t know why this occurs – the transverse translation)
I was able to make new waterlines like explained before.
Select Control Net and select the left point:
The next step is adjust the points, but care, adjust only the points in the aft sections positioned on the left side, the other side, right side, we will use the points window to take them equal to the left side.
Now we will go select the points changed in the left side take the measures from the point window and transfer to symmetric points at right:
X = 0.00; Y = 0.8136; Z=4.2792
Choose the symmetrical:
Change the values on window – Change the signal for Y:
Is more easy unrotate and work with the boat up:
Improved, but is far from being acceptable.
We need improve the sections forward to have a better entrance, the side aft need be improved to have better inclined hull symmetry.
But this is another phase. Let’s see the calculations.
When the hull heel, the draft normally diminish. I tried to see some times the calculation and diminishing the draft in the menu Project –> Project settings I arrived to a 2.9 m inclined draft that give us the 600 tonnes needed, the result is:
Total length of submerged body – 29.729 m 34.688 m
Prismatic coefficient – 0.5864 m 0.5769 m
Wetted Surface Area – 484.79 m2 351.27 m2
Longitudinal Center of buoyancy – 16.818 m 16.868 m
Waterplane Center of flotation – 16.809 m 16.408 m
The inclined length is very diminished, this is caused by the new draft that is more lower.
The prismatic coefficient is a little more hight what is good because if the hull is heeled is because the wind is more strong and the velocity potential is more hight when a more hight Cp is adequate.
The wetted surface area value is some strange, I do not know if it is because we have mirrored the boat, in this case the right value is the half. Is my first time using the software in inclined situation. I will check this latter.
The difference between the two Longitudinal Center of Buoyancy is about 0.050 to aft that will be in our Footy 0.05 cm, that is a very little difference. This signifies that the hull when inclined do not sink the bow or the stern. If LCB goes to forward was better and we would have a stern sink.
Waterplane Center of flotation changed 0.4 m that in our Footy signifies 0.4 cm, without problem.
The boat design is a spiral that when we pass to this point we need restart again, optimizing the results obtained. The post objective is popularize the boat design technical aspect involving the Footy. I hope that peoples like this opportunity not so common and start to look our boats with more technical eyes . I end here, because now it’s just repeating the same steps, which I will do to finish the project. When finished I’ll put the result here.