DRAWING MY MODELS
In the last year I received a lot of messages asking what I’m doing when drawing a plan, what sort of criteria I take into account, which software I use, etc.
There are no mysteries and every body can do the same.
This discussion is realated to the application of theoretical and experimental data.
The regatta winner need of course a good design boat and good thumbs.
A designer can be a good developper and poor performing skipper.
It is not necessary to have both capability at once.
The Formula 1 Pilots are not the Design Engineers. A Pilot become a winner when the Racing Car is also good.
A designer shall be, in any case, a good observer of what happen on the water !
As first I would propose to buy a couple of books related to Yacht Design.
These lectures will bring the basic knowledge needed to understand how a sailing boat behaves once in water.
The first thing I do consider is the environment where the Sailing Model will be used.
The major parameter is the dominant wind and the wind force.
In a simple way I can say that low wind conditions = low Prismatic Coefficient and vice versa with high wind conditions = high prismatic coefficient : PC
Reading the Books , the names as Reynolds and Froude are often appearing in the text and in the diagrams.
To knows more about a simple click on Google will help.
For better understanding the Froude number Fn is compared with another scale that reflect the Boat relative speed Vr.
The expected maximum speed that a sail boat can achieve is function of the Length of the Water Line.
Of course the speed is direct consequence of the wind strength.
Fn = 0.40 and/or Vr = 1.25 are the maximum limits that our Monohull models can reach.
Planning requires much higher speeds and higher wind conditions. Some says that the begin of planning wil start with a Vr = 1.4 and up. Stable planning regime can be obtained with Vr=2.0 and above
Watching the diagrams published in the books, it is easy to see that the major drawback for a sailing model is the total resistance encountered in the water when sailing.
The hull form is the second thing I look for, also because of the aesthetic point of view.
Vr = 0.8 is referring to low speed due to low windy conditions and a Prismatic Coefficient of 0.55/0.57 is justified, while at Vr=1.25 is reflecting high speed and of course high wind condition. In this case a Prismatic Coefficient of 0.59/060 is supposed to offer better performances.
Sail plans are not yet part of this discussion that remain close to the Hull Design.
Of course the same boat will be facing both conditions, with high and low wind conditions, therefore is up to the designer to select the most appropriate Prismatic Coefficient that will be function of the dominants weather conditions of that area where the model will be sailing.
The hull forms are playing an important role depending on the speed that the model is expected to achieve.
Personally I do have a preference for the Symmetric type Hull
The Diagram here under are complementing the text.
The tread will continue…