RG 65 Design with Freeship – 7

Let’s see the  menu:    Mode -> Gaussian Curvature.

Right click in the Perspective View window  and select Mode and then  Gaussian Curvature

The perspective is red with some blue spots and some green parts.  These blue stains indicate locations of the hull where there are bumps, we say that these stains are not fairing and is then necessary to take them away  moving the mesh points near it so it gets all red, see below figures:

Let’s move the point indicated by the arrow and selected red in the picture above, eventually we also have to move  the points near

When we have not more blue stain, the job is done. See the other bumps:

Finally, the entire hull fairing:

The choice of displacement

The lower the boat total weight  the lower hull resistance.  Let’s imagine that everyone can make the lightest possible hull, weighting, say 350 g, I do not know what the real value.

Let’s assume that this boat has a bulb with 500g and is positioned at 0.30 m below the waterline.

We can separate the righting moment of the hull into two components, the moment due to buoyancy, moved from its position when the boat is in heel and the moment caused by the bulb when it is withdrawn from the vertical of the boat because the heel.

The moment made ​​by buoyancy should not vary greatly from one boat to another because the short length of RG 65.  Suppose that all boats have more or less the same value for this moment.

The bulb moment will be M = Weight of bulb * Q * senα

where:

Q = bulb CG  distance to design waterline center line

α = angle of heel

So our keel bulb generate a moment M = 0.5 *  0. 30 *  senα = 0.15 * senα  (kg * m)

Well, what would be the distance for a bulb with 300 g did the same moment?

0.50 m as M = 0.3 * 0.5 * senα = 0.15 * senα  (kg * m)

And our boat would shift to a much smaller displacement, from 850 g to 650 g with much less resistance.

Obviously this can be an exercise with weights and measures exaggerated, but the operation is this.

What about the keel?

The keel must have a certain area to generate a lateral force capable of counterbalancing the power side of the sail made by the wind. We then need keep the area of ​​the keel.  If our keel was 7 cm for width and 0.30 m long (area 2.1 cm * m) the new keel can be 4 cm wide and 0.50 m long (area = 2.0 cm * m). This new keel may have even greater efficiency than before due to the higher aspect ratio.  But everything has a limit.

Thus the design of the RG 65 must take into account this aspect, and then must be studied for several displacements because each competition depend on the statistics of wind speed and so we can use a particular bulb and keel, so, a given displacement.

Thus an RG 65 must have (more or less, I have not deepen into the calculations  in this math, but it is just a warning for this detail) about 3 bulbs and each bulb about 3 fins.

Obvious that for light winds the light bulb and shorter keel and for very strong winds the heaviest possible bulb and the longest possible keel, are the two extremes.

Due to know the  area of ​​the sails, we can do some calculations and determine which bulb and keel for each wind.

Obvious that nature is not mathematics, but who knows the region knows for sure what the wind speed is more constant.

This is the solution for x GR 65:

1 – A hull optimized for various displacements

2 – A suitable family of keels and bulbs.

So that our procedure these days, we worked only one displacement, in fact has to be done all over again for about four displacements mainly: 0.7 kg, 0.8 kg, 0.9 kg and 1.0 kg and 1.1 kg may . In reality we have to do several projects with various options and choose the best, which has less resistance for the various displacements.

To study the moments of  keels and bulbs family that is appropriate to use in RG 65.

Let’s take a look at what can be done about a family of keels and bulbs:

We see that up to 0.15 kg * m we can use the 300 g (0.3 kg) bulb with fin length 0.30 m to 0.5 m, from there you can use the 400 g (0.4 kg) bulb  with keel length of 0.5 m and 0.4 m after we can use 500 g bulb with fins from 0.45 m to 0.50 m prioritizing the bulb of lesser weight.

The problem, at least for me is knowing the data of the moments in sail as a function of wind direction to choose the best bulb and how far the keel can be increased without introducing problems such as low efficiency or other problem. Anyway this worksheet clearly shows the problem (and solution).

One doubt: will be the 0.5 m keel  feasible, 0.5 m is efficient for RG 65?

If is feasible, who uses 4oo g bulb  with keel less than 0.4 m is penalizing in 100 g the total weight of the boat and who uses bulb keels 500 g in less than 0.4 m is also penalizing in 100 g.

Also obvious is that research on the dimensions of the sail is extremely necessary, it can greatly help in the solution.

Other restriction is that we’re just optimizing the boat for  stern wind because we do not have a software for optimize the boat in heel.

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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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