Montana Riverboats Test Domain Roadkill

Scale Model Making

Scale Model Making

Are models worth much? Yes. At least if you are trying to design a completely new hull shape. You bet.

I used to make dory models with formica, which bends well without distorting. But the lumber yards don't have formica scraps any more. You have to order a 12' foot piece to get any at all. 1/6th scale is handy, where an 18' foot boat is a 36" inch model.

Here I used a plastic "For Sale" sign. But they only come 24" inches wide so I had to use an odd scale, to make 18' feet become a 24" inch model.

If you are painstakingly fastidious with measurements and cutting a good looking model can get you fairly close to full size working dimensions. But they'll still be too far off to actually build with. I usually start with a plastic model and then jump to a full size model, made with plywood side panels and adjustable rib like formers. Then I fiddle with widths and side angles until it looks right. And then glass it up.

3D software like FreeShip may replace the plastic model step. I'll find time for that soon. This model tells me 30 degrees side flare with a straight line chine makes waaay too much rocker. 

If you warp the side panels a bit by having wide flare in the middle and less side flare at the ends, and if you dish out the chine some so the chine edge is a curve instead of a striaght line, and (if you possibly) widen the transom a bit too, you can reduce rocker as much as you want. Dishing out the chine a lot does (unfortunately) mean you have to make a separate side panel for each side, at least if you want more than 20" inches of side panel height at the oar locks. Keeping width in the middle for as far out as possible, in both directions, and then bending in sharply as close to the ends as possible also helps reduce rocker. 

The final gnat's ass dimensions still have to come from adjusting and experimenting with a full size model. I like making these small scale models as a first step. It's fun. But I still end up disappointed about how much they tell me. A full size (adjustable everything) model, on the other hand, tells you exactly what you want to know.

This model was made with a plastic sign, marking pen, calculator, foam board ribs and a hot melt glue gun. And a some packing tape at the ends. 


Next step. Ok.

I took the model above, which has lots of side flare and a straight line chine, and therefore too much rocker, out to the shop. There I slowly ground off the bottom of the model until I got a rocker profile I liked. The next step after this one is to take the model apart and examine the shape of the resulting side panel, and its chine edge. And then scale that up to real dimensions. And then and only then cut the real side panels. 


The next step after grinding it to shape is to take the model apart and lay the side panel flat. And then to examine the shape of the resulting, post-grinding chine edge. The above model does now flatten out the middle, leaving all the rocker out at the ends, with a slightly more gradual rocker profile at the upstream end. This is a scale model of a 17' foot boat (18' foot side panels) whose bottom widths are close to symmetrical either side of the middle......just as wide 4' feet back from the middle as 4' feet to the front of the middle. But the rocker profile is doctored. Significantly.


In the side panel piece above, after taking the model apart temporarily there should be layout pencil lines. Measure down from the gunwale edge to the curved chine at each layout location to determine an approximate measurement at each layout location. Write those measurements down.  In a later step, when you are working with a full size model you can use those measurements to set up the proper chine dish, on the full size panel.

Now put the model together again.

The photo below shows a view of the model from above, so you could see the rib-like formers. I used a fixed width and fixed side angle rib in the middle rib. That isn't necessary. But I did it that way. All other forming ribs are foam core trapezoid shapes that spline in the middle, and are held together with paper clamps. 

Play withs widths and side flare as needed to get an approximate shape, as you like it. To change side flare you will need to make new 1/8" foam board forming ribs.  But that isn't hard. Or expensive.

So now, after repeated adjustments and building and re-building the model you can now measure everything up as accurately as you can, and then multiply each measurement by 6 to get the full size dimensions.  Now you can make a full size model with plywood, 1x6 scraps or chip board for the ribs and perhaps AC constrution plywood for the side panels.  Or even with more expensive Meranti Hydrotech, which would be the final finished side panels.

The dimentions you got by scaling up from the model will only be approximate.  You will attach the newly made full size panels to adjustable ribs, as described in the New Hull Designs section.

proximate.  You will attach the newly made full size panels to adjustable ribs, as described in the New Hull Designs section.