Personal Background

Me and my little brother, Nicky, have been going to the beach our whole lives. Over the years, we've built award winning sandcastles, boadyboarded with dolphins, established a rainbow-loom bracelet empire, and of course, surfed! Ever since I've left for college in 2021, Nick has been surfing even more and was looking into some board upgrades. Unfortately, FCS and futures have a complete monopooly on the fin market so a good set of carbon fiber fins cost around $150.

Technical Motivation

While I love Surfing and bodyboarding, the engineering part of me reels back whenever engaging with the surfing discourse around board and fin design. Almost all of the guides or differences between different boards and fins are characterized heavily by "Bro Science", making almost all decisions based on trial and error and feel. This always rubbed me the wrong way from an enginerring mindset, but I also didn't have the time, hundreds of dollars to test a variety of different fin designs and styles, and access to an ocean in Ithaca, NY or Milford, MI.

Coming into this project, I had extensive experience working with Carbon Fiber and FEA analysis through my project team, so I settled on some goals for this project: Give my little brother a kickass birthday present (Due Date: Sept 21st), Design a brand new fin using engineering design principles, Conduct more CFD and Sturctural Analysis on Surfboard Fins than seemingly anyone else before, Manufacture these myself and inexpensively, Do this all while working full time during my Internship at General Motors. Seems easy!

Fin Design

Nearly all fins, are designed around the FCS or futures fin box system. However, the fin boxes themselves are fixed to the board so normally, once you choose FCS or futures, you're stuck with them for the rest of your surfing days. Since I was designing for my Brother's favorite board, I went with the FCS fin box design. Also, almost every surfboard fin uses one of the low digit NACA Airfoil types, but I thought a thicker airfoil could perform better at high Angles of Attack so I ended up designing a Fat Fin and a Skinny Fin to compare against each other in my CAE analysis. After establishing my master model, I then created my fins using surface modeling tools in Fusion. I confirmed G2 Surface continuity through the use of the Zebra tool.

CAE Analysis

I first analysed the aerodynamic performance of my fin under the extreme conditions of turning on a massive wave. As failure during this use case, would be the most catastrophic, I wanted to test fin aerodynamic performance during this case, ensuring the surfer can still control their board and manuever properly. However, I also wanted to identify the pressure behvaior on the fin during this extreme circumstances. Thus, I exported the pressure information from Ansys Fluent and imported this into Ansys Mechanical. This allowed my to analysis to model the forces on the fin in a much more accurate manner.

Through my analysis, I found that the Fat Fin did not perform signifigantly betetr than the Skinny Fin during this analysis. At 25 m/s (~50mph) the Fat Fin generates 0.88 N of Drag force and 1.9 N of Lift. The Skinny Fin generates 0.92 N of Drag and 2.76 N of Lift. This result surprised me as the Cl/Cd information available at airfoil tools showed otherwise. After confirming the Skinny Fin could withstand the loads during my extreme case with a FOS of 6.25, I decided to move forward with manufacturing my Skinny Fin Design. This is particularly impactful as the volume of the Skinny Fin is ~½ that of the Fat Fin, resulting in ~50% reduction in material costs.

Manufacturing

To achieve my goal of inexpensive carbon fiber (An Oxymoron) surfboard fins, I decided to manufacture these using the Forged Carbon Fiber Method. This method using chopped carbon fiber tow, an ambient-cure epoxy, and a 3D Printed Mold. This method was developed my Lamborghini and I used the guide developed by Easy Composites, an absolutely amazing Composites Company and Youtube Channel. They output legitametly fantastic, technical videos and I highly reccomend looking into their stuff.

Accordingly, I designed Split Molds based off my fin design using lots of boolean tools, along with various offsets and addint draft angles and chamfers where necessary. I also added holes for whatever mounting hardwear I could get my hands on. In this case, some larger than ideal bolts and nuts. To withstand the pressure during forging and demolding, I printed the molds at a high density infil of 75% and used PETG as the filament as it won't bind to the epoxy I'm using.

The first Successful Fin Prototype!

General Fin Design Parameters. I created a Master Model based off these parameters for rapid design iteration! Presently, I alter my designs based on these parameter, airfoil type, and base length. Zebra analysis of Surface demonstrates G2 continuity throughout Fin, ensuring smooth fluid flow over surface Ansys Workbench Workflow: CFD Analysis in Fluent and then mapping pressure contours into Ansys Mechanical for Sturctural Analysis. Pressure Contours of Fin in Ansys Fluent Stress Analysis based on mapped Pressure Contours Fin Layup Fin Extraction