Before we start to modify the shape of the hull will look at the running resistance of the hull Freeship launched.
We go to the Calculations menu and clicking on Resistance and then in Delft Yacht Series or Holtrop-88 for Sea Ships and we have to fill in various data and we now need to talk about speed.
The fastest speed that a displacement boat (because the force that pushes up is arising mainly from water displacement made by the hull) can achieve is given by:
Vmax = 1.34 * √ (length waterline in m / 0.3048) (knot)
The hull on the screen has a length of waterline 5.256 m :
Vmax = 1.34 * √ (5.256 / 0.3048) = 5.56 knot
Well, as we have set the maximum speed to put in place in the dialog box – end speed – we will establish 6.0 and establish 10 calculations of resistance in the range 0.0 – 6.0, then Step = 0.6.
In the next part of the box – Hull – We mark the option – Extract data from current hull and put 0.4 in Hull Draft option instead of 0.000 now, as we just do nothing in keel and rudder let’s stay as is, and hit the calculator at the top of the dialog box:
We see that the graphic appears the running resistance of the boat and if we clicked on Results we have:
In the figures above we have the input data
Here the numerical results of resistance
By the results we see for speeds above 4.8 the recommendation of a Prismatic Coefficient Cp = 0.55 or more are confirmed but the calculations indicate an ideal position for the Center of Buoyancy – CE – of – 3.2%, so the different I guess, but as the project grows we will see how this percentage will evolve.
Well, let’s work in the hull shape, so, close the window and maximize the Profile View:
To set the zoom window, to get the whole picture in the window, click the left mouse button and drag it up or down.
As we have seen, the longer the waterline length is, the greater is the boat speed, by this we need increase the waterline length as much as possible.
We will do this by placing the stem in the vertical and lowering the stern of the boat.
The stern I do not down totally, let it be slightly above the waterline so that when the astern boat wave forms the stern not to bury causing vortices that increase resistance.
To modify the stem, just click on each grid point on the stem and put x = 6.5 and give a return, see:
And see, as the displacement has already increased – displ = 0.87 t .
Come to the stern. Click at the lowest point of the grid in the stern and drag down to where I want to:
Immediately I see that my displacemente go to 0.89 t.
This figure also shows that to move the mouse with the points we lost a little accuracy in position, this point moved with the mouse should be with x = 0 because the point is at stern, yet, but it is at x = – 0.0071 or aft of the aft limit, in this case is just going into the box and put x = 0.