RG 65 Design with Freeship – 3

If you find interesting to have your drawing colors as my drawing, then go to menu File – -> Preferences and set the color as mine, to change the color click on the current color:

My colours

Let’s take a look at Menu -> Calculations and see what are the values ​​that the boat has for the main quantities that we need to control in the project.

Clicking in menu → Calculations opens a dialog box with :

Intersections – later we will use this feature

Hydrostatics – Click on it, clicking this sub menu you can access a box with the leak points.

Leak points are grid points that indicate where the mesh has no continuity like at the top of the hull at the height of the deck and in the transom. Check the light green mesh points, these are the leak points. These leak points are normal and not have to worry about them, however if by chance there is a leak point on the centerline of the boat the program will not do the calculations and you have to fix it by putting the value of y equal to zero. When you click a green dot (light or dark) of the mesh a box appears giving the point value of x, y, z . If any point on the centerline of the mesh is not at y = 0 is a leak point and you have to change the value of y in the box to zero.

As we do not touch at any point of the design click a point on the center line and check the box with x, y, and z and see that it has zero value for y. When we click on it  change to red

The axes have the following direction:

x -> source (x = 0) in the stern and the positive direction is to bow.

y -> source (y = 0) on the center line, positive direction to the left (Portboard)

z -> source (z = 0) at baseline (Base line) positive direction up

You really do not have to worry about the box of leak points, click OK. If the page show the calculations with all data is because everything is OK, if you see a dialog box saying it was not possible to calculate, verify which point of the grid center line is not with  y = 0, correct and return the menu page of the calculations, will appear the calculations (unless you have more points with y non zero, but there’s just see what is not, and put y = 0).

This window gives the values ​​of the main hull dimensions and magnitudes of hydrostatics on the draft of the project we set at the opening of the project, that first dialog box.

Let’s see some highlights of this window:

Water density – 1.025 t / m³ density This is for salt water and we need exchange it for fresh water that is 1.000 t / m³. We design the boat for fresh water because the condition is worst. Let’s just make this change:

Go to Project menu and then click on Project Settings and click on the tab Hydrostatics and change Water Density of 1.025 to 1.000. Click also in the box: Enable automoving model to baseline

Back to the Menu Calculations to open it again with the calculations:

. Beam over all – in spite of setting a maximum breadth of 1.3 m, mesh is formed with 1.292 m, it is inherent in the creation of the mesh, the process that generates the mesh. Not much difference to what we thought, remembering that our RG 65 has a beam that is 10 times smaller than 0.1292, a difference of only 8 / 10 millimeter of beam we want.

Displaced Volume – 0.859m ³ – Volume of water displaced by the hull, is equal to the volume of the submerged hull. The weight of water displaced equals the weight of the boat (Archimedes’ Principle)

Displacement – 0.859 tonnes (in fresh water the volume is equal to the Displaced Volume because the density of freshwater is 1.0 t / m³). This greatness is one of the most important because it shows us that the our boat to be at the design draft 0.4 m should be weighting 0.859 t.

Here I must pause to explain the following:

Did the number that represents the weight in tonnes, in this scale 10:1 we are using, is the same number as our boat would have on RG 65 in kg, so if this boat in the window has 0.859 tonnes displacement, our RG 65, had a displacement of 0.859 kg if built in scale 1:10.

If by chance the GR 65 that we are trying to build is to have1 kg we need move the mesh, changing the shape of the hull to increase the volume submerged to the displacement to be 1.0 t. This is the advantage of letting fires the icon that shows some of hydrostatic quantities on the screen, let’s stirring in hull form and instantly see that the displacement value was.

If by chance I want my RG 65 with a weight of 700 g = 0.700 kg, the boat of the screen should have 0.700 tons of displacement and I have to decrease the volume of the submerged hull by modifying the shape of the hull through the grid, moving in grid points.

Prismatic coefficient – 0.5287 This value is good for light wind where the boat does not reach the maximum speed he can achieve, personally I not use prismatic coefficient down like this, I use 0.55 or up. Take a look on the page Naval Architecture in the Blog Menu to see the text about prismatic coefficient. The prismatic coefficient has a relation on the wave resistance which together with the frictional resistance are the most important resistances.

Wetted Surface – 5.153 m² (our RG 65 would have a wet surface of 0.05153 m² – dividing the value of the screen for 100 because it is area). This value, with the data I have is reasonable to good, here, would be interesting if someone was in possession of data from RG 65 wet surface (area of ​​the hull in contact with water) could add something.

Longitudinal Center of Buoyancy (CB) – 3.144 m Here we have an indication that the center of gravity of the volume, local application of Buoyancy, which is the resultant of the forces of water pressure that pushes the hull up, is at 3.144 m from astern. This data is one of the most essential since it indicate that we need have the resulting centers of gravity of the weights, that make up the total weight of the boat, to be exactly at 3.144m to the stern, for the boat to be exactly on the design waterline.

Longitudinal Center of Buoyancy (CB – Center of Buoyancy in %) = – 3.496 % (negative value because it is aft midship section ), is the percentage of the distance from the Center of Buoyancy to midship section to the length of the waterline project (Total Length of submerged body), is an important aspect to the resistance of the hull in the water. I think a value between -1% to -2% would be better. To change this current value would have to put more volume to forward, shifting the Longitudinal Center of Buoyancy to forward.

Waterplane Center of Flotation (CF is the center of gravity of the waterline area) = 3.018 m, personally I like to keep close to the CB, little aft, near the middle of the boat

Well, seen these figures now I need:

1 – If I want to do a RG with 900 g, so my project will need have a Displacement = 0.900 t, and I will have to increase my volume by increasing the stations area in the immersed part.

Here lies a good conversation – of course, the lighter the boat less the resistance, however a very light boat means a bulb with less weight and thus less righting moment to put the boat at an angle of heel to have a good efficiency of the sail.

Here is the X of the RG 65 equation.

What is the weight of the bulb to be chosen? What is the length of the keel?

Imagining that all boats have an equal weight of hull (smallest possible) what would be the ideal total weight of the boat? In fact, towards the end when the project gets closer is that I will choose the final weight of the boat, for now we leave it with 0.900 kg or 900 g or 0.900 t for our design.

2 – To increase the prismatic coefficient to 0.55 or more we need decreasing the immersed area of the station with the bigger  immersed area and increasing the area of ​​the other sections immersed areas and I can also utilize this to increase the volume to 0.900 m³.

3 – When I’m make this, increasing the volume, I would prefer to increase the volume forward of the midship section, in the fore part of the boat, so that my LCB and LCF to come more closer to the midship section.

4 – In the same time I will stay alert to changes in the values ​​of the wet surface for not increase and even decrease it, if possible.

 We need change the position of the stem and the aft part of the hull also.

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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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