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# "The devil is in the details."

Updated: Apr 28, 2020

Or how to make hull lines fairing and keep everyone happy.

When you do hull lines smoothing you have to think how to not destroy designer's ideas and at the same time not to make shipyard disappointed of your hull surface. Here I would like to show some topical details which appear again and again.

The smoothing of the ship surface has never been a simple task. The simplest part of it, approaching the original surface provided by the customer. Next comes the fun part. Approximate surface repeats all the same defects that were in the original surface. And, the main task of smoothing comes down to how to remove the defects of the original surface and not to deviate too far from it. However, many customers have their own ideas about the shape of the surface, tradition, and, finally, just delusions. Modeling the original surface designer usually does not care much about the details. In the process of smoothing, just the details are very important. The shape of the flat side and flat bottom lines, the angle of connection of the stem to the base plane, the angles of entry of the frames into the base plane, all this is very important for the correct construction of smooth surfaces. In this case, it often becomes necessary to substantiate one or another decision for the customer.

Quite often, the initial surface model consisting of a large number of joined Bezier surfaces. Visually, this surface is quite well smoothed. Therefore, any sufficiently large deviation of the resulting surface will raise natural questions from the customer.

I noticed that most problems arise if the boundary lines of Bezier surfaces do not have a smooth connection. So, for example, the boundary lines of four surfaces that come into one point around this point must belong to the same plane. This is the simplest condition for smooth conjugation of four surfaces in the vicinity of a corner point. If the corresponding boundary line of the surface is smoothly connected with, there are no problems.

In this case, we have two vectors that form the plane. If some of the lines do not have smooth conjugations, this means that all the tangent vectors of these lines near the corner point must belong to the same plane to satisfy the smoothness condition.

As a rule, the designer does not think about this at the initial stages of designing the ship surface. At this moment he is most interested in the shape of the hull as a whole. The problems are very clearly visible if you build a large number of sections in the area of ​​such a point. Sections in this area are not smooth, and often have knuckles. Since in the final smoothing of the surface we usually use one patch of ​​the surface on the fore ship, we cannot, and should not, repeat the mistakes of the initial design stage. Sometimes it is difficult to explain to the customer the legality of such a decision.

Consider some of these cases.

1. The shape of the line of flat deadrise.

The line of the flat deadrise has a knuckle at the transition from the inclined plane of the bottom to the horizontal keel. In the view from the top, such a line is depicted by one smooth curve. In the view from the front, this curve has a knuckle at the transition point to the horizontal keel. When modelling the ship surface with Bezier patches, we should have a join of two surface patches at the knuckle point. At the same time, to avoid knuckle on the surface around this point, the angle of entry of the frame to the knuckle point must be strictly defined. At the initial design stage, as a rule, no one thinks about it. The line of the flat deadrise bottom itself in the top view must also have knuckle.

There are three ways to solve this problem:

- Make a knuckle point on the boundary line of our surface area. At the same time, we can fulfill the conditions for a smooth curve in the top view. The disadvantage of this approach is that a knuckle point appears on the border of our surface. This suggests that the surface at the knuckle point is not smooth enough. If the customer insists on such a curve, you can go for it.

- Another solution to this problem is that we extend the inclined part of the flat bottom line below the base plate and cut the surface along the base plane. In this case, the horizontal line of the keel will be the intersection line. With this approach at the point of knuckle will be knuckle.

- In my opinion, the correct approach is to set the three control points of our curve strictly on the line of the horizontal keel. In this case, the shape of the curve will be somewhat different, but this will simplify the work on modelling the surface in this area and will give the desired smoothness of the surface.

2. The angle of entry of the frames at the point of attachment of the stem to the base plane.

The angle of entry of the frames near the point of attachment of the stem to the base plane cannot be arbitrary. The vector of the tangent of the structure should belong to the plane formed by the vectors of the line of the stem and the line of the flat bottom. Considering that this point is the corner point of our new surface, this is very important. No matter how you try to change the shape of the angle of entry of this frame by changing the position of the control points of the surface, you have little to succeed. Try changing the entry angles of the stem or

flat bottom lines, and you will immediately see the difference. All of the above applies to the initial surface model. Often the designer does not think about these aspects. Therefore, very often in such areas of the initial surface has knuckles on the waterlines. Such regions are usually checked first when the study of the initial surface.

3. Lines of the flat side, passing into the line of the deck or horizontal knuckle.

Often the flat sideline has a knuckle at the junction point of the deck. Due to certain restrictions on the number of boundary lines of our surface, we are forced to make a knuckle point at the surface at this point. This is nothing more than a tribute to tradition. Of course, the flat sideline is very important for determining the flat part of the hull surface, but there is no need to create knuckle there. This is just a tribute to tradition. There are several solutions to this problem:

- Creating knuckle on the surface boundaries. This solution is not very good since it leads to a local non-smoothness of our surface. Because the lines of the flat side and the lines of the deck form a plane in this area, this smoothness is almost not noticeable.

- Round transition line. This is most preferable because it does not cause problems with the surface. At the same time, the frame sections look very natural. For me, this is the easiest and preferred way.

- Another way of having the right to exist is to bypass our surface above the deck line and trim it's by deck plane. The surface in this case also looks natural, but this option is more difficult if you plan to build surfaces above the deck line.

Here I showed only some aspects that need to be paid attention to when smoothing the ship surface. In summary, you can only advise treating the original surface model with care, but at the same time, do not repeat the errors of the original model