tutorial02.pdf

DELFTship tutorial – Leak points

DELFTship

DELFTship tutorial – Leak points

Description

This tutorial shows some of the most common causes of leak points and how to repair them. Leak points

keep the program from calculating hydrostatics as soon as they become submerged. As the hull definition is

also used for creating tanks, leak points in the hull model will also show up in the tanks. Tanks containing

leak points are not allowed.

In short every edge should be connected to two faces. If that is not the case then the edge is considered to

define the boundary of the surface. Examples are the sheerline when no deck has been fitted, or a transom

that not has been closed yet. All points on the boundary of the surface are considered leak. Think of the

sheerline becoming submerged, water would rush over the edge. Exception to this rule are edges on the

centerplane of the ship. They should always be connected to 1 face only since the hull is symmetrical and

DELFTship automatically includes the virtual other half of the model. By including the virtual starboard side of

the model edges on the centerplane are in fact also connected to two faces, one physical face and one

virtual face.

Now what is a leak point?

➔ A leak point is a point not located on the centerplane and connected to an edge with only one

face attached.

To solve the problem the boundary edge has to be identified and connected to a neighboring edge if

possible. Remember that leak points are not always a bad thing. Leak points located on the sheerline of a

small dinghy for example correspond to the reality. In this particular case the leak points can become a

problem though if you need to calculate intact stability at large angles of heel. The solution would be to add a

(temprorary) deck to make the model watertight.

For a more elaborate description on leak points the reader is also referred to the DELFTship manual.

DELFTship tutorial – Leak points

DELFTship

1. Identifying leak points.

Description

This step shows how to identify

leak points in the first place

since that is the first step on

the road to a solution.

Turn on the controlnet of your model

by selecting Display => Controlnet

from the mainmenu if the control points

are not visible.

Leak points can be recognized by the

color that is used to draw them on the

screen. The light green points are leak

points. Sometimes locked points

(drawn in grey) can be leak also!

In version 3.2 and later of the

program it has become easier

to identify leak points since it

has the possibility to suppress

the drawing of normal points.

Select Display => Leak points only

from the mainmenu to hide the normal

points from your control net. Only the

leak points are drawn now, so every

point that you see is a leak point,

regardless of its color.

The image to the right shows that the

model contains leak points on the

sheerline and at the stern where the

transom would be fitted.

2. Leak points on the centerplane

Description

Often leak points can be found

on the centerplane. In almost

any case this is caused by

incorrect placing of the Y-

coordinate of that particular

point.

Click on the leak point. The coordinate

window appears displaying the X, Y

and Z coordinate of the point. Make

sure that the value for the Y coordinate

is exactly 0.000

DELFTship tutorial – Leak points

DELFTship

3. Removing double edges

Description

Another common error is the

positioning of two different

points on the same location in

space. If that is the case one of

the points must be removed

and its geometry connected to

the other point

The model to the right shows part of

the bow of a kayak. The model has

been created to check the volume of a

buoyancy tank. The leak points are

caused by the fact that the bulkhead is

not properly connected to hull.

One of the first things you should do

after having selecting a leak point is to

move it (for example by using the

arrow buttons on the coordinate

window or the cursor keys) to check if

a crack appears in the geometry. This

is shown on the right side of the

illustration. You can see that the

bulkhead is not properly connected to

the hull. There are two edges

connected to the yellow point that are

double.

One of the edges causing the

problem is removed.

Select one of the double edges and

delete it. The connected face will be

deleted also resulting in a hole in the

model.

The geometry is fixed by

adding a new face in place of

the old one that properly

connects all the edges again.

Select Display => Leak points only

from the mainmenu to turn on the

display of all the control points again.

Select the four points with your mouse

in counter clockwise order. Not that the

bottom left selected point is drawn

differently than the top left selected

point! This indicates that there are two

points on exactly the same location.

The selected point is partially obscured

by the other point that is drawn on top

of it.

Add a new face to the four points by

selecting Edit => Face => New from

the mainmenu.

The presence of the double point is

confirmed by the fact that after having

DELFTship tutorial – Leak points

DELFTship

added the new face still one leak point

remains.

This final step shows a special

trick to avoid loosing part of

your geometry when deleting

items.

To fix the last leak point we could

delete the lower panel of the hull. But

by doing so we would lose the

foremost control point of the bottom.

Here's a better solution:

Select the two aft most points of the

lower hull panel and increase their y-

coordinate. Now select the four control

points forming the new lower hull panel

and add a new face.

Now we have two lower bottom

panels in our hull. The old one

is removed and the knuckle

lines are restored.

Select the aft edge aft the old lower

bottom panel and delete it. The lower

bottom panel connected to this edge

will be deleted by the program also.

All that's left to do is to restore the

knuckle lines. The resulting model is

free of leak points.

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