RG 65 Design with Freeship – 5

Let’s take a look at the resistance calculations to see the progress of our new boat:

By increasing the length of the waterline we modified the relation  Lwl / BWL and Cp :

(LWL = length of the waterline project and BWL = Beam on the waterline project, Cp – prismatic coefficient)

 As we increase the length of the waterline and the volume immersed not increased in the same proportion the prismatic coefficient decreased. 

 Cp = Immersed Volume / ( Max Immersed Sectional Area * Design Waterline Length )

 Also with the increase of Lwl the ratio Lwl / BWL soared. 

 These two effects prevent Freeship formulas for calculating the resistance by Delft because with these new values our boat is outside the dimensions of the formulas are valid for use.

 For this reason the Freeship shall calculate the resistance by Holtrop.

 Formulas are the problem, you can not compare now the calculated values ​​of resistance with that we calculated before because the calculation processes are different.

 A real boat from 6.5 m in length have beam in average 2.8 m, a relation Lwl / BWL = 6.5/2.8 = 2.3. Ours has to Lwl / BWL value 6.5/1.093 = 5.94.

Our boat have sometimes differents proportions compared with real boats.

 We can use the Delft calculations if our bean would be at least 1.3 m for BWL and Cp at least 0.52. Our Cp is 0.474.

Then we have no possibility to know if we are changing for the worse the resistance of our project each time we modified it.

  And no point in making the project within the parameters for Delft because we do not know the resistance of the boats that lie outside the geometric relations valid to use the formula, which may have a lower resistance.

Hence the need for the designer to know the resistance of RG 65 boats out there, but we do not know.

  And then the project becomes a lottery.

 We will then begin modifying the shape of our boat to make it stay the way we wanted him to stay, the way we think a boat RG 65 and later take a look at resistance

  We will now hit the contour of the hull, put a freeboard compatible with our RG 65, changing the number of stations rising to 11 which is what we normally use to make a RG 65, put waterlines in multiple 0.05 m in order to have data calculation for choosing the final weight of our boat.

 To change the number of stations and water lines and change the spacing between them will use the menu Calculations  -> Intersections

Let’s click on the Stations icon – the first icon, and pops up the dialog box:

Click the trash and the list of stations disappear, then clicking on the + N and a dialog box appears asking you to put the spacing you want for new stations, we put 0.65 to stay with spacings between 10 stations:

We click OK and the new window showing the position of the 10 stations :

Let’s change the waterlines, click on them, the third icon:

We click on the trash and then N + and appears in the dialog box to put the spacing you want, where 0.05 m:

We click OK, and the position of our new waterlines appears.

Maximize the Profile View and adjust the design within the window to see the heights of the water lines and keep them as reference :

Let us now set our edge of the hull. I will choose for our RG 65 3.5 cm freeboard aft and fore 5 cm which corresponds to 0.35 and 0.50 m in our design.

For this I will click on the grid top spots and bring down the line of the hull to stay where I want.

You can see the dialog box coordinates of the point I moved, he is at Z = 0.8935, as I want a freeboard of 0.50 and have a draft 0.40 the Z should be with 0.90, but there is only going to hit 0.90. 

You see clearly that the grid lines passed over the points below, so I’m going down all the points below before continuing:

Now I can lower all the other grid top points:

Let’s minimize the Profile View and see how is the appearance of our boat:

Continues !!!!!!!!!!!!

My Brazilian blog is one day in advance:



About Fred Schmidt

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