The Need for Speed: how to computer-loft a hydroplane

An interesting project landed on my desk this week.  We’ll be helping to recreate a 1929 Gold Cup racing hydroplane!

The project is a collaboration with Mark Mason of the famed New England Boat and Motor, who brought the client and will rig and power the boat, and Brooklin Boat Yard, who will build the hull and deck.  Mason is renowned for his restoration of numerous vintage racing powerboats, perhaps most notably, Baby Bootlegger, designed by George Crouch.  Brooklin Boat Yard is well known for its decades of high-quality work in modern wooden yacht construction, and worked with Mason back in the 80’s in a similar collaboration.

The subject is Scotty Too, a John Hacker-designed triple-step hydroplane.  The lines were taken off the original in the time-honored way—manual measurements resulting in a table of offsets with hundreds of reference points describing the shape of the hull.  Our task is to take those offsets and create a modern three-dimensional computer model of the hull, in the process fairing out any bumps and hollows, or inaccuracies in the offset points.  Once we have created a fair hull and deck, we will work with Mason and BBY to come up with a sensible construction method, deduct for the required planking thickness, and create full-size patterns for the BBY building crew.

Scotty Too in Multisurf. The yellow ponts are offset points; the red points are magnets

I began by laboriously entering the hundred of offset points into the computer model, organizing the points for each station (there are 14, plus stem and stern dimensions) on a separate layer of the program, so I can turn it on and off to reduce confusion.  These points are not used in creation of the hull surfaces; they remain undisturbed to serve as a permanent reference to the original hull.

I then began to create a hull using the same technique we use when starting from scratch to design a new hull: I created a series of “Master Curves,” over which a hull surface is stretched, then by adjusting the master curves, I was able to fair the hull and achieve the desired shape and hydrostatic characteristics.  There are a couple of things about this project that complicate it a bit, however.

First, it’s a triple-step hydroplane!  This means that instead of a single hull surface, as you’d have with a round-bilged boat, I needed to create many mores surfaces: topsides, three bottom panels, and the vertical steps between the panels.  In addition, there’s an overhanging, Vee-shaped stern. 

Second, there are those pesky offsets to consider.  How do I get the hull surfaces close to those points, without locking myself into the unfairnesses that are a fact of life with so many points?   I created a reference system by projecting a “magnet” for each offset point.  These “magnets” are points that are constrained to reside on the hull surfaces, but are projected onto those surfaces from the offset points.  Thus, as I maneuver the master curves around, using fairing tools to achieve a smooth surface, I can see in real time just how close the surface is to each of my reference points by judging the distance between the point and its associated magnet.

Tedious, but fun!  And I was pleasantly surprised at how quickly the hull came together into a nice fair collection of surfaces.

Late this week, Mark Mason will visit to pass his blessing on the hull shape and begin the conversation about the structure.

Bob Stephens