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

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.

So. The photo above and at the photo at the bottom of this post is the same model as the first photo, at the start of this thread, with roughly the equivalent of 4" rocker dished out of the middle of the side panel, made that way by grinding off the bottom of the model until it had the rocker profile I was looking for. I did this by clamping a right angle grinder upright at the edge of my work bench. And then slowly, gradually held the model over the grinder to shape the bottom profile.

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.

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.

That way all other ribs can vary as needed, as to both width and side angle. The "stem" is held together with packing tape. The transom doesn't exist. It's just packing tape.

Ok. Here's the side panel after taking the model apart. The goal was to make a boat with lots of side flare that did not have too much rocker. Where the rocker change was a bit more gradual at the upstream end and a bit more abrupt at the downstream end.

I made a straight line chine model (that did have too much rocker) and then ground off the bottom of the model with disc sander. And then took the model apart. This is the side panel. A 4" inch dish (on 30 degree sides) that bulges more toward the front end. The dish-out extends forward of the middle a bit more than it extends back.

That's what I had laid out three days ago on real plywood. Before I got cold feet and decided to make the model first. On the plywood I had a 4" inch dish. On the scale model (made from a for sale sign) I have a 4" inch dish.

The front or downstream end here is to the right. I'm really pleased my best guess three days ago was to make a 3-1/4" inch dish centered 24" inches forward of dead middle. Then I made the model, with straight line chine and too much rocker, ground off the bottom until it looked right, took the model apart and measured the side panel. And found a 4" dish centered slightly forward of the middle. Son of a bee.

One more thing worth noting:

If you widen the transom the rocker at the upstream end reduces. How useful that is remains to be seen. But it's worth knowing. The most important thing is to somehow generate a mental picture of the boat you want. And then to play with angles and widths and tricks and techniques until you get what you want. And not so much the other way around.

Finally. Note the model making technique. A plastic for sale sign is not as good as Formica counter top material. But I couldn't find any Formica without ordering a big piece. The ribs are 1/8" foam board marked to scale and cut with a drywall knife and a straight edge. Over-lapping halves can spline at the middle so the model maker can make the boat narrower or wider at any point. And to vary the side angles. Paper clips and hot melt glue hold it together. I put the temporary rib-like formers at layout lines that started in the middle, heading out left and right at 24" inch intervals.......24" inches at full scale. Smaller than that on the model.

To get an 18' foot side panel to scale down to a 24" inch plastic for sale sign I divided each real measurement by 9, because 18' feet is 216" inches. And 216/24 is 9. The inverse is handy too. 1/9 or one divided by 9 is 0.11111111111111

So for every real measurement (say 18' feet or 216" inches) if you multiply by 0.11111111 you get the scaled down distance to mark on the model.

I used an 80 grit disc sander to shape the bottom, after the model was first made.

I've now got a sharp focus image in my mind's eye about the boat I'm designing now. Three days ago it was a lot fuzzier. I'll work out final blueprint dimensions full size. But the model was useful. I'm glad I did this.

Could I have done the same thing with 3D modeling software? Probably so. But I STILL haven't learned how to use it yet. I will. It's in the to do queue now.

The photo below shows how to translate the chine shape lesson from the model to plywood.

And now the two side panels actually get dished out, as per the lessons learned from the plastic model.

Note these side panels were both cut from a single 4x18' foot panel. So the height of each side panel at the oarlock position is now roughly 20" inches. That's what's left after gouging the chine edge 4" inches at the oarlock area.

However, these panels will be used to make a temporary boat shaped form or plug, on top of which Plascore will be mounted. I'll probably make the Plascore side panels from two separate 4x18 panels, so they can be closer to 24" inches high at the oarlock area.

Scaling Boats -- usually but not necessarily from smaller to larger

Scaling boat dimensions works the same way as scaling a vector image in computer graphics. If you have a set of dimensions you like but you want to make it bigger you have to (simply) multiply every existing dimension by a constant scaling factor.

If you have a boat with a 16' foot gunwale and you want to make it bigger, so it has an 18' foot gunwale it goes like this:

18 / 16 = 1.125 so (1.125) is your scaling factor.

For every dimension in the small boat if you multiply by 1.125 you get the new dimension.

The gunwale in the original boat was 16' feet. So 16 x 1.125 = 18

If the bottom in the original boat was 48" then 48 x 1.125 = 54 (inches)

All dimensions including those related to trapezoid like rib formers get multiplied the same way and voila: you have a larger boat scaled up proportionately. It's a time consuming process.

If you have blueprints with 100 dimensions given in feet and inches you have to convert everything to inches with decimal places. And then multiply by the factor. And then convert back to feet and inches. But it works. It's time consuming but it isn't hard. Almost anybody can do it.

3/16" of an inch is 0.1875 inches (three divided by sixteen)

172 inches is 172/12 or 14.33 feet (172 divided by 12)

.33 feet is 3.96 inches (.33 times 12)

.96 inches is 15/16 of an inch (.96 x16)

It can drive you nuts. It's fussy detail work. But it isn't calculus or differential equations