Notes on Boat Design Software
by Gavin Atkin gmatkin@clara.net 

I havenít said much to anyone  about the software tools I used to develop the Bluestone and the Castles in the Air, and I feel I can usefully put this right.

My current favourites are Hulls, the free version of TurboCAD 2D (though Iím now migrating to the paid-for version 6.5), StarOffice (sunís freeware equivalent of Microsoftís Office), and a variety of freebie graphics packages.

Hull modelling

Thereís no doubt about it Ė Iíve been a fan of Gregg Carlsonís program Hulls for ages, and now that itís capable of modeling hulls with up to 10 strakes, I think itís making great strides. Iím looking forward to the day it does ballast calculations along with all the rest.

To me, Hulls doesnít just  work, it works well, and itís free. Nevertheless, it has certain limitations, and so here are some of my ways of working around them.

  • While the two-dimensional dxf files Hulls produces seem to be accepted universally, the three-dimensional CAD files it outputs are acceptable to a disappointingly small number  of CAD packages .

The answer to this is to use the offset table; print it out from Hulls, and then manually enter the data as co-ordinates.  Itís fiddly, but it gets you there.

  • Hulls doesnít (yet) allow  the designer to dial in ballast data to reveal on screen how a hullís hydrostatics will change.

You have to do this with a pencil  on the back of an envelope. Itís all a matter of moments, which, in the case of boats, work like horizontal levers: figure out the total displacement, work out the centre of gravity (COG) of the hull from the on-screen model (the coordinates in the bottom right-hand corner come in handy for this) and then do the arithmetic using the COG and the weight of the ballast you think you wish to add. Work out the moments using the horizontal distance between the centre of bouyancy (COB) and the total COG.

  • The lines in the drawings Hulls produces are segmented and are not smooth curves they may seem to be Ė look closely, and you will find that that elegant sheerline you modeled is actually made up  of 12 short straight lines Ė three between each station.

A good CAD package such as the free version of TurboCAD can help to fix this. First explode the drawing, and then extend each end of all the short lines to ensure that each of them intersects. Then use your CAD packageís curve drawing tool set to snap to intersections and construct a curve based on the intersections and Ė et voila! Ė an elegant smooth curve.

  • When the dxf output of Hulls is inputted into a 2D CAD package (or one that projects ), the chines (or strakes) are actually doubled up. What youíre seeing is the port and starboard strake together. Selecting and deleting  one copy of each segment will leave you with a drawing you may find easier to work with.

There are two things I canít help you with: while one can print out a set of offsets, I havenít yet found a way to output them  to a text file, and itís important to always save your files before heeling your model too far (the crash that may result could otherwise lead to the loss of several minutesí work),

CAD

Despite the adverts and lack of measurement tools, I canít live without TurboCAD 2D (like Hulls itís free and obtainable from ĎTurbo Cadí). I suppose Iím biased because I now know my way round its buttons and menus and donít want to learn anything new. Nevertheless, I do believe that itís the most user friendly  software of its kind available.


Gavin paddling one of his one sheeters

One point though; I have found that the dxf files that the paid-for 6.5 version produces are not universally compatible, while TurboCAD 2Dís dxf output is. So, even if you have 6.5 or 7, it may well be worth keeping a copy of 2D on your machine so that you can do a little converting work when you need to.

Graphic output

And then thereís the question of how to make CAD drawings into gifs. This can be a fiddly and irritating task. The first job is to find a way to turn the CAD drawing into a bmp bitmap image Ė but having done so youíre likely to find that certain details , particularly text or dimensions, are too small to see and have to be either removed or re-sized.

My approach  is to use Star Office (free software from the Sun Computer website). I open a new Star Office drawing, and import the dxf file, which appears small on the screen. Then I play with the page format and size and position menu items until Iíve got a biggish image  (as big as A4 or larger), and then I hit the export button, and choose the bmp option. Youíll have to fiddle with this until youíve got what you want by trial and error.

Once youíve got this part right, youíre on your own with your favourite paint and photo editing software. I find that blurring lines, then fiddling with the contrast and colour tones often produces nice images. Colouring using the fill function works well too, so long as you donít make the colours too strong.

I hope this helps!

Gavin Atkin, London, March, 2001

 

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