Been a lurker for decades… love the site, especially the treasure chest stuff, all the building tips, alternate methods materials, and other “how to” tips etc… and the exchanges and discussion on new ideas.
Myself, I built and launched myself an epoxy lapped ply version of a LHF rowboat from Gardner Classic boats back in about 1998 and have been enjoying fishing and rowing both fixed seat and sliding seat on both large and small lakes ever since. It’s been used and abused since …. well the abuse actually started during construction and continues to date, but that is a whole other story.
All along I was thinking about designing and building something similar but bigger for many years now. There’s been many many false starts in that regard. The early free version of the Hulls software was great, but it only runs on Windows 95 and while I still have an ancient laptop with 95 & Hulls loaded, it’s a battle capturing and moving all the design data from that old system onto newer computers running newer operating systems and then printing it out. Hulls is also somewhat limited with respect to the number of frames and the extent of the offset table etc.
Then along came the initial 1 - 4? boat building articles that Barend Migchelsen submitted and are still archived here. I have built a rowing dory or two using those concepts around 2000 or so, but I was always dreaming something with more shape like the LHF or even stretching a Shellback dinghy out to 18 or 19 ft. There were innumerable false design starts in Hulls that regard, until approximately 3-4 years ago, when I discovered the rest of Barend’s series as well as Jim Mcgovern’s further work on it.
Jim’s work was especially interesting because he’d created an excel spreadsheet with Barends formulas within it. A penny dropped at that point, in that both Mcgovern and Barend were using their math to create a single strake via two specific curves that defined the top and bottom edges/chines. So if you want a more strakes and more shape, just create more curves to define the additional strakes or chines to generate a multi-chine boat.
Jim’s spreadsheet provided a great starting point because he’d built in some formulas etc. But significant changes would be needed. He used set station locations like “Sheer Line Width at Station 72" instead of formulas to define the station locations. In addition, he entered those actual measurements for the design into the formulas ie the 6, 12 and 18 inch station locations. The design was also double ended, so defining stations based on their location from the boats max beam sort of makes sense for ease of calculations, but it doesn’t reflect normal station #ing practice - ie, a snip of Jim’s spreadsheet.
Being familiar with Excel, I could change his station locations to formulas based on boat length, and directly link them to the design parameters; being Canadian, (ie familiar with the metric system & how it mimics and easily calculates Percentages and vice versa), I reconfigured things to calculate the station spacing based on percentages of boat length using typical station #ing while taking the lazy way of avoiding conversions from feet to inches or 0.129 of an inch to the nearest fraction.
I also realized that the upper data portion of the spreadsheet was effectively breaking down each curve into the desired rocker it should have, as well as it’s desired beam. The reconfiguration ended up looking like this and is more typical of what an offset table might look like. All the offsets (red circles) are calc’d via Barend/Mcgovern but are for an upside down boat.
The above spreadsheet structure defines a single chine, but obviously one can copy, paste and then edit additional curves to create as many additional chines as desired. Excel also lets you graph the results, so use those graphs to refine the design and line off the planks to get a proper appearance, but keep in mind Excel graphs are not to any set scale!!!! so what it shows is likely stretched or compressed vertically or horizontally is some way or another. The gird lines on the graph for equal values should generally look square if you are close, but you can measure your x and y axis’s/gridlines and adjust them to exactly the same scale if you really need to see the true shape or have access to a plotter and actually want to print out and trace out the molds versus lofting them.
Here’s various graphed views of the 5 strake design, I did up in Excel using this approach. (The Bow on view is relatively true to scale, while the other two are blatantly stretched vertically.)
1) You don’t have to use exactly the same curve for each chine. That’s how I created the tumble home in the sheer strake at the stern in the above plan view;
2) you can see the successive shifting of each chine at the stem mentioned earlier ie to create the rake of the stem;
3) I used the shortened boat above to create a buoyant bow for this design then locked the position of the first 3 or 4 stations, but manually spaced stretched the rest of the hull as fairly as I could to length.
Please use all the above and the attached spreadsheet with a huge dose of salt , I’m not a trained designer in any way, just a guy who’s played around in “Hulls”, doesn’t have CAD or other boat design software but was familiar enough with math & Excel to build on what Barend and Jim started. I simply blundered on how de/reconstruct a portion of what real boat design applications can do based on the information in Barend and Jim’s articles.
PS – Jim also had the following issue that he couldn’t resolve in excel “The only thing I could not figure out was the length of the bottom panel. The profile view gives the chord of the arc, but I could find nothing for the length”.
I found the formula for length of the “arc” here and used it to figure out roughly the maximum length of boat I could build from 2.5 sheets of plywood scarfed together depending on whether it was a 2x ender or not… (see the “cord length” tab in spreadsheet) it’s a great formula… It however needs something to account for the length of a person’s garage, I had to shorten my design accordingly. See what I mean about a dose of salt…