My Experience as a Land Yacht Engineer: Part 2 – Fabrication
In this second entry, I am going to give an overview of the fabrication phase of the land yacht project—which may be summarised as follows:
Nothing costs quite what you want it to…
Nothing works quite like it’s designed to…
Nothing is finished quite when it’s meant to be…
Nothing Costs Quite What You Want It To
As this point suggests, there are design specifications to the project, one of which is a budget of £200. This means that whilst carbon fibre masts and a premium woven sailcloth are nice, they aren’t financially practical.
For our team, this primarily meant compromises in the wingsail material, where industrial insulation foam was used to fabricate the aerofoil ribs, PVC tube was used as a mast sleeve and as supports and an adhesive floor protector was used to cover the aerofoil ribs, giving the wingsail a solid geometry.
Moreover, this kept the wingsail extremely light and given that another key design specification was that the entire assembled land yacht’s weight had to be less than or equal to 30kg, this was advantageous. This material selection, however, was not without problems.
Nothing Works Quite Like It’s Designed To
The industrial insulation foam chosen as the material for the aerofoil ribs came with a foil coating. The initial intention was to use a hot wire cutter for accuracy in fabricating these ribs and as such, the foil was removed. Rigidity in the foam was immediately lost but it was hoped that once all ribs were affixed to the mast sleeve and wrapped, rigidity would return.
This seemed to be working out, until it was discovered that the adhesive floor protector that was to be used to wrap the ribs would not stick to the foam. This was a problem. An initial solution was to give the ribs a cling film skin, allowing the adhesive floor protector to stick. This was the result:
The initial wing structure before cling film skin…
… and after: one problem fixed; another created!
The cling film worked perfectly as an adhesive surface; however, due to the loss of rigidity in the foam from removing the foil coating, it also significantly deformed the outermost ribs. Unfortunately, this wasn’t the last problem.
As mentioned, PVC tube was used as a mast sleeve with the intention of slotting it over the mast, allowing for it to be fixed at particular angles of attack. The outside diameter of the mast was 19mm and the outside diameter of the PVC tube was 21.5mm with a wall thickness of ~1mm. This gave a clearance of ~0.5mm which, in principle, should present no problems. This, however, was not the case and when the sleeve was initially slotted onto the mast, it went 50mm before jamming up. As it turns out, 0.5mm clearance over 2m—the length of the mast and sleeve—is simply not enough, given that the slightest inconsistency in wall thickness or misalignment of the sleeve negates any clearance.
Finally, despite initial shaft calculations indicating a deflection of the order 1mm when loaded, the actual results were somewhat different:
Shaft deflection was 50 times greater than calculated…
Nothing Is Finished Quite When It’s Meant to Be
Using Microsoft® Project software techniques learnt in the project management module, the entire land yacht project was planned. Teams plan for the design phase, fabrication phase and any unforeseen delays. As is the case with projects, however, these unforeseen delays mount up.
At the very start of fabrication, the workshop was flooded. One week gone. Initial estimations of a two-hour wing assembly turned into 3 days. In the middle of the fabrication phase, ordered material was mistakenly appropriated by another group, requiring a new order to be placed. Another week gone. These types of issues are ubiquitous and provided a new set challenges alongside the afore mentioned technical issues.
Putting It All Together
Fortunately, any up-and-coming engineer is primed for problem solving and welcomes these types of challenges—at least in principle.
With regard to design issues, working solutions were found: namely, the aerofoil ribs were braced with correx and duct tape (a cliché, but for a good reason), which provided rigidity and a surface upon which the adhesive floor protector would stick; the PVC tubing was split longitudinally and wrapped around the mast, as opposed to slotting on top; and the shaft, despite presenting with a deflection fifty times greater than anticipated, never deformed plastically and so no additional solutions were required—which was nice for a change.
When it came to projected schedule overruns, Microsoft® Project gives a clear, visual representation of where critical paths lie and where slack in particular task exists; this allows for any appropriate adjustments to be made via reallocating tasks and resources so as to meet the required deadlines. It served as a good lesson in taking the time to apply the tools learnt in the project management module in order to avoid running into any major scheduling issues, as well as mitigating any last minute ‘panic’ solutions to design issues, such as resorting bamboo sticks for steering or, indeed, no steering at all… (again, actual examples from this year’s cohort).
Our team’s (almost) fully assembled land yacht is shown below:
In the final guest post, I’ll reflect on the race day and over all experience.
Jordan Davidson is a third-year Mechanical Engineering student on the Master of Engineering (M.Eng.) course. In 2014, he was awarded the BP STEM Scholarship and has interests in materials science and engineering mathematics. In a previous life, he was a musician and holds a B.A. (Hons) degree in Music.