Tuesday, January 14, 2020

Follow the Designer's Plans

There are many types of personality that get involved in boatbuilding. We each do it for our own reasons. Most are not qualified to design that boat, so buy a set of plans that comes closest to meeting their needs, from a designer of their choice.

I have a question for you, as an amateur boatbuilder preparing to start a new project or already into it. It is a rhetorical question, so please don't bombard me with responses. The question is this. If you selected your plans from a reputable designer who knows his craft, why are you wandering off the path chosen by the designer and building it differently, why are you changing important aspects of the design?

You may think that the change that you make is of little consequence but can you be sure? You may be placing the whole project at risk, also jeopardizing the time and expense that you invest in it.

I have been working with a client who can serve as example. He is building a dinghy, of stitch-&-glue construction. Simple enough, in fact so simple as to be the simplest form of boatbuilding. He is an engineer and engineers readily admit to the tendency to redesign anything and everything that they can lay their hands on. It is the difference between a tinkerer who must fiddle first before getting on with the job at hand and a person who just gets down and does what is needed without fuss. It is seeking perfection beyond the point of perfection, where it sometimes becomes so complicated that it no longer works.

In this case he has seen the beveled chine details that are commonly used by another designer and has applied them to my design, which uses square edges on the panels. The wheels came off when the bulkheads didn't fit correctly because the hull panels were narrower than they should have been, due to the loss of width to two bevels at each chine. I referred him to my chine details, clearly shown on the drawings. He had not considered that the change would affect the panel widths of the CNC kit that had been supplied to him.

Another issue developed at the same time, in attempting to fit the bulkheads into the hull. As an engineer, he had meticulously measured the bulkhead locations on my construction drawing and attempted to set them up in those positions. But they didn't fit. I referred him to the building instructions, which clearly tell exactly where to place the bulkheads, without measuring from the drawings. He admitted to having not read the instructions and was working only from his measurements from the drawings. The problem is that this was a hand-drawn design from more than 30 years ago, before CAD, and designed from a small cardboard model that I built by trial-and-error then dismantled to measure the panels and transfer them to the drawing. Somewhere in that process the drawn transom angle differs from the boat that results from the hull panels.

I have gone for more than 3 decades not knowing that the transom angle was wrong and many hundreds of boats have been built to the design, with the builders happily following the instructions to build boats slightly different from the drawings but all very much the same as each other. If this builder had read and followed the instructions I may never have realised that the transom was drawn at an incorrect angle on that drawing. The drawing will stay as it is and boats will continue to be built that way, following the instructions.

I drew this design primarily for first time amateurs, with basic woodworking skills and no knowledge of intricate boatbuilding. My advice to that builder was to take off his engineer's hat and to replace it with the amateur boatbuilder's hat.

There are, of course, ways that a builder can modify the boat from what is shown on the drawings. Simple ones like changing a rubrail detail don't need to be cleared with the designer. Speaking larger boats, anything major, like an interior layout redesign or anything structural, should not be changed without first getting agreement from the designer that you can make that change. Something that may appear to you to have little significance may actually have structural, stability or other implications. Extending the stern or increasing the freeboard changes the distances that structural components have to span and may invalidate aspects of the designer's structural calculations.

Most of my larger monohull sailboat designs have the engine located in the middle of the boat, where it has the optimum location for stability, trim and pitching characteristics, with a galley counter built over the engine. Modifying the interior layout has to be done around that engine location because moving that heavy weight will change the trim of the boat.

I had one client who had the unbalanced skeg-mounted rudder modified by a builder who thought that he knew better and said that a balanced rudder would be an improvement. Yes, a balanced spade rudder will give a lighter helm due to reduced loads. But, and it is a very big but, don't place a balanced rudder behind a full skeg or a full keel unless there is a lot of separation between them, to allow the water to close properly behind the skeg or keel and give the rudder clear flow onto its leading edge. The result of this bad modification is a rudder that pushes its leading edge into the water flow to one side of the obstruction when the helm is turned but has it hidden when on centreline. The result is unpredictable rudder response because the flow onto the rudder blade is very disturbed and irregular.
The unbalanced rudder before modification
The rudder and skeg after modification. Don't do this.
Another engineer client added many fancy features to his boat, features that would help a very expert sailor to draw more performance from his boat, as well as some intended to make the boat easier to sail. He is the ultimate tinkerer and the boat looked very impressive when completed. But so many of the changed details just did not work. I had designed for him a boat with relatively simple rig that would be easy and efficient to sail. The boat that he launched was much more complicated and difficult to sail. The extra features have gradually been removed and he is now unmodifying (the spell-checker says that I can't use that word, it clearly doesn't understand) his deck layout to bring it much closer to my design.

Builders of my boats will know that I am very supportive of their projects and am freely available to advise on changes that they may want to make. If they heed my advice we are good. If not, they are responsible for the results themselves. The designer of your boat certainly knows more about the design and what you should be doing with it than you, as the owner or builder. The best option is to work together with the designer to adapt the boat and your needs, rather than your wants, to produce the most suitable boat.

We have a very wide range of designs, covering many styles, materials, aesthetics and capabilities. See them on our main website or our mobile website.

3 comments:

Johann Mendelsöhn said...

Very true words. I'm now in year 15 of studying the Argie 15 with all the hardwood items made and the first two sheets of plywood cut out. I'm actually very glad it took so long. I now k ow the plans and details off by heart a d enjoy every minute. Also fully understanding why you designed it the way you did! Thank you again!

Losloper said...

During teaching Fellows complicated surgical procedures, I often tell the learners: "The enemy of good, is 'perfection'". It is that 'last' stitch, or that last dissecting cut or slice, which gets you in trouble and cause the complications.
Don't do things you don't have to do. Know when to stop! I think that is as important for a surgeon than any other piece of knowledge or skill, maybe the MOST important thing.

I think it is an apt analogy to what you are describing, from my world....

Magnus Murphy

Eduart Wolf said...

Any engineer should know it is the figures in a drawing or the instruction that count. Never take a measurement from a drawing you did not do yourself.Basic law.
I often used old drawings from the ancestor of current projects with new measurements, some pieces are simply cheaper and faster to build than to draw. But maybe that engineer was not of the "building" kind, they come in all sorts.