Sailing dinghy plans, paper to digital—Newbie needs help


I’m new to SU and have started to learn it through one of the subscription tutorials. What I’d like to achieve is to convert the analog plans for a 12’ John Welsford SCAMP sailing dinghy into digital. I’ve included a photo of one of the bulkheads. Obviously, I require only 2D images, in millimeters.

How can I create an (x,y) coordinate system that allows me to measure from a baseline/centerline? Ideally, an ability to establish an (0,0) origin point that will allow the (x,y) coordinates to appear either next to the cursor or in the measurement box in the bottom right corner as the cursor moves would be very efficient.

Alternatively, is/are there a better way to make this digital conversion?

Thanks for your time.


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Switch to Camera (menu) > Standard Views (submenu) > Top

Then use the ORIGIN (the intersection of the axes) as the intersection of your baseline and centerline.

For the bulkhead (above) you can draw half of it, then copy and flip or mirror it.
(A mirror plugin may make it easier.) To use the built-in “Flip along” commands you need to group the half before the command will appear of the right-click context menu.

After the two half groups are mated correctly, explode both and regroup as one.

@thorviking, drawing this way you can hover over the vertices (and geometry) with the ‘Tape Measure’ tool and it will show X, Y (and Z) values next to the cursor.


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If you are using your scans of the sheets, be aware that things may not line up correctly.

Thanks, Dan, that did the trick. I left out some details that relate to cuts that depend on dimensions that will be determined later (hatch doors, stringer cutouts), but it was a great first attempt.

I think I’ll pursue that plug-in option you mentioned just to see how a mirroring operation differs from a flip operation. I also want to fair that curve at the cabin top. The curve appears to dip down as it meets the centerline and it was more apparent after the copy-flip operation, so creating fair curves is my next effort.

Thanks, again, for your help.


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@g.h.hubers, thanks for your help.


@DaveR, yes, even to a newbie, distortion in scanning (though this was a smartphone photo) or the possibility the paper plans were not accurate/to scale occurred to me.

Thanks for your help.


I got a mirror plug-in and I think it is more straight forward than the flip operation.

Speaking of the Extension Warehouse, what curve plug-in would one recommend to create a fair curve to connect the offset points seen in the attached photo? This is the bottom of the boat and, of course, will require mirroring.

Thanks, again.


Take a look at Bezierspline from Fredo6:

(For all Fredo6 extensions, you must install the ‘library’ as well)


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@MikeWayzovski, thanks for the tip.


Thanks, Mike. The Catmull Spline tool in the Fredo6 collection did great.

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Until now, a confluence of pandemic and other life factors preceding it forestalled my efforts to progress in converting paper sailing dinghy plans to digital with the desired end to convert them to gcode for the purposes of cutting out boat components on a Maker Made M2 CNC. So, here I am again.

As I consider fair curves and smooth gcode tool paths, it occurs to me that the concept of a “fair curve” in Sketchup is theoretical and that curve “fairness” is relative to the number of straight line segments which can be specified. As I zoom in to study these straight line segments, I can detect them relatively quickly in the curve of the bulkhead (used Fredo6 ‘Bezier curve’ plug-in), and, yet, the curve(s) of the boat’s bottom piece (using Fredo6’s ‘Catmull Spline’ plug-in) shows no apparent straight line segments, even at higher zoom rates. Reminding the reader that my newbie capacity remains in effect:

  1. Does the Catmull spline tool, in fact, create curves from straight line segments?

If so…

  1. How does one call up ‘entity info’ or other parameter to monitor and specify that input?



Like any other curves in SketchUp, Catmull splines are made up of a number of straight edge segments. By default there will be 7 segments between control points. Here is a Catmull Spline with precision set to default 7 segments. Entity Info shows there are a total of 30 segments. The endpoints of the segments will have a cyan dot on them while the curve is being drawn or edited.

You can change the number of segments (precision) before or while you are drawing the curve or when editing the curve after it’s been created. Simple type a new value for precision followed by s and hit Enter. Here I’ve increased the Precision. You can see the increase in the number of cyan dots and the increase in the segment count in Entity Info.

My “entity info” box (I’m using Sketchup Pro '19) is not like yours, Dave. I was, however, able to stumble my way to specify “30s” and learned that was the upper precision limit. As you can see in the photo, the curve at each end is missing some segments. How might that have happened?

And on this second try, a variation on a theme…still problems at each end of the curve.

I have no idea what caused the problems noted above, but I started over, including new endpoints. I flipped the orientation, too. I specified “30s” so there are 330 line segments in each curve. The centerline is 3,250mm so each line segment is about 10mm.

I expect the difference you are seeing in Entity Info is actually a PC/Mac thing. It hasn’t changed from 2019 to 2021.

How long is the boat? Is it possible the centerline wasn’t 3,250mm long? Maybe you were running into a short edge segment thing. Hard to tell.

I hope you don’t mind me asking but what kind of self-respecting Viking builds a boat with one transom like that? They build double enders. And at 10 feet long, it ought to be a proper færing. :smiley:

…I love my MAC until such times as these…

Though the plans are in metric, the boat is listed at 11’11". As I recall, the design brief called for a boat not more than 12’, unstayed mast, water ballast, quick and easy to launch/retrieve, hard to capsize, easy to recover, beachable, seaworthy and suitable for camp cruising. Small Craft Advisor magazine commissioned noted Kiwi small boat designer John Welsford, and the design (SCAMP - Small Craft Advisor Magazine Project) has become quite popular with plans and precut kits for the homebuilder as well as a fiberglass version produced by Gig Harbor Boat Works. Hull #1 first splashed in November 2010 and is the perfect boat for ambitious expeditionary sailing as done by noted solo adventurer Howard Rice. I understand the magazine has sold over 600 plans and kits, so it is a design with a broad international profile.

I’ve been sitting on plan #151 and have been delayed in proceeding with the project in ways that sometimes make me wish I’d purchased a kit. As I already invested in over $800 in okume marine plywood (with more sheets to procure), and with a willingness to be “ground up” in my approach, I find reasons to be content to learn Sketchup and get the paper plans, as I describe, digitized and capable of being executed by the M2 CNC I am also attempting to create, understand and command.

As to “self-respecting Viking,” I do fancy the double ender Caledonia Yawl design which resembles a proper færing and have thought about ditching the SCAMP in favor of building that boat. But the SCAMP design offers some performance/safety advantages that will serve me better and, besides, I’ve already got the plans and much of the materials.

Thanks for all your help, Dave!



I like John Welsford’s designs. I had thought about building one of his. I also like the Caledonia Yawl and pretty much everything design by Iain Oughtred. If there was room in my shop I’d build one of his boats. The CY would be ta bit too large but I’d like to do Elf.

I’m all for you learning SketchUp but I’m going to tell you to get off your computer and get out to the shop. That boat won’t build itself. I do know what you mean, though. Years ago as a first boat I built a Stevenson Weekender II. I spent a long time pouring over the plans and making drawings and notes (before I had SketchUp) before I finally made the first cut.

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