Designing a jetboat with DelftShip

[Lurvio]

This guide has been in my head for some time now and I finally got around to actually making it. DelftShip (formerly FreeShip) is a free software for hull design and basic hydrodynamics.

The software.


On Home-leaf, are most of the controls needed for basic design,
 - Project section has Project settings where you can edit the data given when starting a new model.
 - Edit section has reversing button that saves you many times when you screw something up.
 - Layer sections has buttons to add and delete layers, a selector to which layer the new planes (e.g. extruding) are added and the layer properties window (more on that later).
 - Point section has useful tools like Align points (becomes familiar with this guide) and some intersection tools that are helpful in the later stages of design.
 - Edge section has the extruding tools, Split edge to add a point to an edge, Crease tool etc.
 - Face section has tools to creating faces, mostly used to fill holes
 - Hull display has the options to what is shown on the model (control net, intersections, hull data, etc.)
 - View section has the perspective setting and view modes (editing in wireframe only)

On Tools-leaf are additional tools, Move tool is most useful in basic design.

At the bottom there is Incr. distance with a value behind it. By clickin on top of it a dialog is opened, I changed the value to 0.010 (1 cm). This value defines how much a point is moved when the arrow buttons are used on the dialog that opens when a point is selected.

Mouse controls
 - left button down selects an area to zoom to
 - right button down moves the model in the window
 - scroll works as a zoom
 - ctrl (on keyboard) and left button down, from left to right selects points and edges that are fully inside the selection, from right to left selects points and edges that are at least partially in the selection

Keyboard controls
 - ctrl + a zooms to extends
 - ctrl + z reverses the screw ups.
 - esc deselects everything
 - ctrl down points can be selected and deselected one by one with the mouse. Multiple (like the whole chine) edges can be selected.

With these we are ready to start a model.

[Lurvio]

The hull created in this tutorial is basically a bigger version of the boat I am building so the shape is what pleases my eye. This tutorial should give you readers enough skill to create any hull shape that happens to work for your needs.

Starting a new model shows this dialog.


I only edited the length and beam as the rest are not important. The result is something like this.


Now select these three points and create a new face.


Then we select and remove everything else.


And we have a starting point for a transom, the first point is moved to origo (zero on all axis) and will be the centerline at the bottom of the transom.


Next point is the outer edge of delta at transom. Delta will be 30 cm wide at transom.


The last point is a bit trickier as it defines the deadrise of the hull we're creating. 65 cm width and 16 cm height means a deadrise of 14 degrees.


I split the top edge and moved the new point to centerline and upwards.


Now we start with the bottom. Select an edge to extrude. I have a hull length 3,6 meters so it is easy to devide by six. The delta is half the length so three sections, 5 cm closer to centerline with every section.




Now we create the delta (new plane). The points need to be selected in order for this to work. A new layer can and should be created before taking this step. In fact all the plates of the hull should be on separate layers.


Here with two points (edge split) and edges added. The control net works best when every point has four edges connected to it (three on open edges and centerline).

[Lurvio]

Then the rest of the bottom is extruded.


At this point we have a flat plate so the bow needs to be shaped. The Move tool (Tools-leaf) is best for this so the bottom shape stays constant. Here the front is moved up 45 cm.


Getting the shape aft of the bow to look nice takes a few tries/moves. The tip of the delta is a problem as there is an extra edge connected to that point and it causes a sharp bend to the keel line in the drawing. This bend (transition from flat (delta) area to the twisted bow area) should be kept small  to ensure the plates fit together. There are tools to check for this, but more on them later on.


Next step is to select all the edges between delta/centerline and chine and split them to get extra points. I added new edges (if the edges were selected in order it is easy add the longitudal edge as the points are selected automatically after their creation) but this is not mandatory as these points are only used for reference.


Now we start the actual design process by shaping the chine. I started by moving the transom-chine corner 5 cm inwards.


To shape the chine I use the top-view and move the points only on Y-axis (transverse) until I have the shape I want. This is going to be a blunt nose design so the bow end is not sharp.


Moving the points causes the bottom shape to change so that has to be fixed. The Align points tools works wonders here, but the points need to be selected in right order for this to work. In the image the left point (by the delta) is selected first, then the right (chine) point and last is the center point.


Aligning can be done on all axis at once or only on one and as the point is already positioned on longitudal and tranverse, only the height needs to be fixed.


The rest of the chine points need to be aligned one by one.


Here is the finished bottom plate

[Lurvio]

Next comes the side plate. Select the chine edge and extrude upwards and out. I extruded 40 cm up and 7 cm out.


Then we repeat the process just like the bottom. Add the extra points (and edges) and in side view shape the lower sheer line.


The fairing process is the same except it's done on (transverse) Y-axis.


Just a quick peek of what we got so far in Shade-mode


Next we extrude the deck corner plate. I use the same distance on both Y- and Z-axis, the former being a negative value. Here the reference points are removed. If the extra longitudal edge was added it is now easy to select that and hit Edge collapse. Usually all the control points get removed automatically with the edge.


I edit the height of the corner plate in perspective mode by simply moving the points on both axis the same amount so the angle stays the same.

Here is one way of making the corner plate to the transom, by extruding inboard some amount (20 cm on my case) and the selecting the new points and changing their Y-axis value to zero.


Here is the corner plate finished and the transom ready (add plane to fill the holes. The corner plate at transom should be on same layer with the transom plate to save on welding there.


Then we extrude the deck plate. This is simple as most of the upper sheer (except one section aft is left of) is extruded inboard by some amount (40 cm).


And the new points selected and moved to zero on Y-axis.


After filling the hole aft we have our deck plate.


Add a new plane to the nose. I split the vertical centerline twice and moved those points to same heights with the points on chine and lower sheer. Two edges are added as folding lines (they need to be creased).


Then the forward edge of the deck is moved aft to form the last corner (or chamfer). The keel line front point is moved aft and down to finish the nose plate. To be totally accurate (with the bottom shape) a Plane intersection can be made a few cm (6cm on this model) behind the front of the hull and make the bottom part of the nose to that point, but eyeballing is usually accurate enough. Here is a wireframe of the hull with stations (in Intersections-window) added every 60 cm.


Here is the hull in Developability check view. The plates are all green in the middle so everything is fine there. Red corners mean they need to be welded, green corners can be folded.


If you flip the hull and look at the bottom, there is probably a lot of red in the bottom plate at the forward end of the delta. That is caused by the fourth edge connected to that control point as explained earlier. The other views can be used to check the model, especially the Environmental mapping mode with Metal selected as background (click right mouse button - Mode - Environment map). If there are no twists in the reflection the model should be fine to continue.

[Lurvio]

So, how do we get the model to become a reality? Go to Tools-leaf and hit Unfold (was plate developments in earlier versions). The result is probably something like this.


There are a couple of plates that look fine but most are twisted out of whack. That is because a wrong layer was selected for some of the steps earlier.  The layer window looks like this.


I am missing two layers altogether and none are named, so I fixed that.


::edit::
heemi.v8 deserves a thank you for pointing out this feature of the program.

So to get all the faces on the model changed to the right layer, hit interior edges button on Hull display tools. Here I have coloured all the layers different for better visualization.


Now we can select one or more of the faces. The stubs that appear on all the intersections of the interior edges show which side of the face the plate thickness is meant to be on. It is safer to have all the normals (as they are called in the program) pointing outward. 


When the faces are selected, you go to the layer selector (the drop down menu) and select the layer you want the faces to be moved to.


After all the faces are in the right layers the result should look like this.


Finally we have usable coordinates that can be used to cut our aluminium. The negative latter values can be used as positive ones if they are all negative, the plate will show upside down.


If you have the plate at hand you can use the coordinates and measure them straight to the plate and draw cutting lines between them. Or write the coordinates on paper and draw them on a 2D cad software (I have TurboCad) and send the files to be cut on a machine. The old FreeShip versions had the ability export the cut files straight to .dxf files, but it is since reserved for the commercial version of the software.

[Lurvio]

There are a couple of ways to create bulkheads. Easiest way is to open the Intersections-window (Tools-leaf) and Stations. Remove all the stations from the list and then create a new one to where the bulkhead is wanted.


If the new station does not show up, hit Stations button on Hull display. With the station visible got to File-leaf - Export - DXF Polylines


In the dialog select the file location and set the export unit to millimeters. 


The file should look like this. Check the scale. If the export unit was meters the drawing will be one thousandth scale. Mirror the line to get the full bulkhead.


Another way is to use the Plane intersection tool.


New edges are created into the control net.


Now the new points can be selected and a new face created. This can only be used on fairly straight hull sections, not on the forward half of the hull. The green line needs to be creased.


Reflection shows a straight hull, good so far.


Stringers and other inner structures can also be made by extruding. Start on the centerline of bottom/delta and extrude downwards, then adjust all the points to the same height. Then extrude outboard to where the stringer is needed (20 cm off centerline), then up to the top of the stringer (15 cm above delta) and finally ourboard (5 cm) to create the top of the stringer.


Then use the Intersect layers-tool to cut away the part under the bottom. This tool is a bit fiddly to work with so be patient.


Two stringers ready.


And a view of the bottom with the stringers.

[Lurvio]

So, we have a hull drawn up. Does it float? Does it float the right side up? How much load can it hold?

Here is the hull with a waterline at 20 cm above delta.


Here is the layer window. There are some settings that affect the data the program can give out. A selection on Hydrostatics means that the layer is used in the calculations. Layers that have inner structures (stringers, bulkheads etc.) have to be ticked off. Developable option can be used when using Unfold-tool. It is easier to pick the coordinates with only one plate unfolded at a time. Then comes the important part, density and thickness. Density is easy, aluminium weights about 2,7 kg/dm³. The plate thickness is something that depends on a lot of things. There is a lot of experience (trial and error) based guestimates around on what thickness works for each part of the hull. In the image are my guestimates for the hull.


Project settings. Changing the Draught value can be used to find how much load the hull can hold at a specified immersion.


Project settings. These values/selections should give pretty accurate data. Water density is changed to fresh water.


Here is the Hydrostatics report. There are a few values of interest. Displacement is probably the most important, Longitudal center of buoyancy is the other. LCB alone does not tell that much but when compared to Longitudal center of gravity (at bottom of the page), we can start estimating how the hull would float. In this model the LCB value is 1,188 and LCG is 1,460, i.e. the boat is nose heavy. This is just an example and doesn't have much value when thinking about the finished boat. To get a usable LCG value, at least most of the components (engine, pump, tanks, floors etc.) have to be placed in the model in correct places and weights. For example if you know the engine weight and center of mass you can add a sphere to the right location and then play with thickness and density values to get the weight right (no need to actually draw an engine like I did on my project).

[Lurvio]

There is a lot of small tricks that can help in the actual building of the hull.

Aft end of the bottom plates (including delta) can be extented 10-20 mm. This will help with the welding of the transom to the bottom plates.

I had to take 4mm off the bottom of the bulkhead on my small jetboat. This was because I had an 8 mm delta and 4 mm bottom plates and I welded them with the outside being flush.


More will be added, when they come up. If you have tips that you've used, feel free to post (or PM) them and I'll add them here.

[Lurvio]

For future updade

[Moab]

Layers are confusing the hell out of me!!!  I have been reading alot and messing around with the program alot but I am having no luck understanding how to make a new layer.  Can I assign one to a certain plane?  How do I see the different layers when I want to? 

After extruding the bottom out to where the delta pad ends you say "Now we create the delta (new plane). The points need to be selected in order for this to work. A new layer can and should be created before taking this step. In fact all the plates of the hull should be on separate layers."  But I dont see how to create a new layer.  Also what order do the points need to be selected in?

Just to be clear here are my questions:

Can I assign a layer to a certain plane? 

How do I see the different layers when I want to? 

I dont see how to create a new layer.

Also what order do the points need to be selected in?

Thanks for this great tutorial.  It is a whole different way of creating the hull than the other tutorial I read and seems much simpler and more applicable to our style boats.

Matt.

[Lurvio]

MoabTuco

There are a couple of confusing mistakes on my part yet to be fixed. I have mixed plane and face in a couple of spots. In the quote you posted the text should be: Now we create the delta (new face). The points need to be selected in a clockwise or counterclockwise order around the perimeter where the new face is to be created. As for the layers, first you create a layer, it's as simple as clicking New layer-button. The Delete empty layers-button will become active as the layer is empty at this point.

In short, when a new face is created (created between selected points or extruded), it will be appointed to the layer selected in the dropdown menu.

You can edit the layers settings in the Layer dialog.


Hope this helps and if not, ask away.

cheers
Lurvio

[Moab]

So, can I move a face from one layer to another?

Or does the face need to be created while the layer that I want it in is in the drop down menu?

I would like to go back through my old model, that I didn't create any layers on, and assign the faces to different layers.

Thanks for all the help, Matt.

[Lurvio]

::edit::
After a tip from heemi.v8 the guide was edited for a lot easier method of changing faces to different layers.

To my knowledge the only reliable way to change layer is to remove the face from the original layer and create again to the new layer. It is extra work, but easy if you have the points. If you have two plates that are on the same layer, but are not connected to each other, you don't necesarily need to separate them, you just cannot set different properties to them. 


cheers
Lurvio

[heemi.v8]

Hi Lurvio,
Thanks for the message. I have seen this topic and i like it thanks for putting it up. I actually learned a couple things from you I have a way to change layers easily. rather than try to explain i am working on making a video or two to show how its done.
Also they way you extrude the hull from the transom forward is quite smart would save a lot of time from the way i do it. What if you want to make a variable degree hull? this method wouldnt work right? i have a video that i may make about that too.
Jamie

[heemi.v8]

here is the video. its still uploading but should be live soon. hope the quality is ok cos i didnt check it right through.

http://youtu.be/9wR2sXQFgmg