Creating a Deck

Main Deck 01.3dm (4.2 MB)
In the attached, I have a main deck outline and centerline. I would like to get a surface that goes follows the outline and goes through the centerline.

I have tried networksrf but get nothing.

I’ve tried sweep2 but don’t get a smooth matchup.

What would be the best way to create this.

The deck is supposed to have a parabolic cross section. You should get that with a 3 point spline.

I guess I’d Sweep1 that section curve in both directions along the centerline, then trim with the edge curves, but the CL is a mess at the ends, I’d make a new curve.


The centerline in your file corresponds to straight cross sections.

Are the parabolic cross sections supposed to define the height of the centerline? Do the parabolic cross sections scale with width or are they the same shape curve which is trimmed to width?

They scale. They should be 3 point parabolas. Edge, Center, Edge. This is a place where close enough for government work is fine.


The shortest curve shown in the image would be the parabola.

Use Sweep2 with one pair of edge curves as the rails and the center curve as the section for one half of the deck. Then use Sweep2 with the other pair of edge curves as the rails and the edge of the previously created half deck as the section for the other half. This ignores the centerline curve which as Pascal said is a mess towards either end.

Most boat and ship decks have circular, not parabolic, cross sections with constant radius. I have a simple and reliable method to create such decks. All that is needed is radius of the cross section and the edge curve.

Definitely a parabola

Agree it looks like a parabola. Usually the same curve is used without scaling for each deck beam. The mast deck beam curve was created, then each deck beam was made as a portion of that curve.

Do you want my method for creating a deck surface using a single, unscaled master curve?

I agree with David on a single master camber board being made. Your scan looks like a lot of trouble to reproduce in your model. In the shipyard, the loftsman would take those offsets and lay them out to the scales noted and make a wooden camberboard that would sit on top of the sheerstrake or gunnel. It gets centered and then moved from station to station or bulkhead and the centerline that results sort of gets taken account for in that manner.
I’d like to see David’s method but I usually find the widest point of the beam (snap to quad) and then draw a line from sheer to sheer there. If this were 50’ beam and the drawings asked for 5" rise in the middle I would draw a vertical line from the midpoint of my line 10" high. Then I create a arc (start,end,midpoint) to the end of the 10" line and that gets me very close to a 5" midpoint. Sometimes it takes a bit of tweaking. Then I emulate the loftsmans technique with that arc.

Here is what that looks like using your scan in Rhino. The yellow highlighted line matches the tick marks very closely and when I scale it vertically to 5" (red line) I can measure a radius, 3485’-8 3/4". This is why they work in that manner because I doubt the loftsman had a set of trammel points that long!

Good catch. Expanding the height of the the circular arc along the long dimension results in a more accurate curve for picking off intermediate points.

David, I didn’t do any point picking to the scan. I was just using it to demonstrate how I use an arc for the deck camber and how close it is to the age old lofting technique. Here is what I’ve alway seen in the old textbooks.

If you really want to get old school!

Using a circular arc and then expanding it in one direction will result in an elliptical, not circular curve.

I know David. The way I described at first does not involve scaling which would do just as you say. I was just trying to illustrate the archaic drafting method that Jim is trying to follow. I guess I should have shown my way and not add the complication of the ‘point picking’.

 I had never seen the batten trick in the video, pretty neat. It is supposed to create a true arc I think. I found another example that claims so. Sam Rabl knew every trick in the book, hell he wrote the book on lofting! 

I can't image why one would want to use a parabola for a deck camber and it takes just a few seconds to do one at each station in Rhino. How do you go about it?

Here is a nice little tool online.

My method is described in a new thread Deck Method

Thanks David

Jim and David,

 That divided circle deck crown illustration I posted yesterday had me wondering so I used that method in Rhino to see if it did in fact create a true arc. Close but not quite. 

Jim, Davids method of making a constant camber deck that he posted in the new thread is brilliant! You should go through the process it was very enlightening. I do decks all the time and thought I had it figured out but always knew that my crown up towards the bow was getting a bit wonky so I compared with Davids method and ran some Rhino analysis processes on both and can see the error of my ways. 

Thanks again David, I owe you a beer sometime. You have become one of the type of contributors to this forum that have made it such a great resource.  

@jodyc111 Thanks for the very kind words. I should mention that my method works for deck master sections of any shape, not just circular arcs.