GlennShotwell has asked that we start a “non-commercial” thread that addresses thruster fins on short board applications. I know that we’ve touched on every aspect of this topic over the course of numerous threads. But, in the interest of keeping it simple and easy to find Glenn asked that we start a dedicated thread that hopefully can end up in resources.
A basic thruster 5’10" to 6’4" and from 18-1/2" to 19-5/8" and 2-1/8" to 2-3/4"
In general the fin positions are marked for a 4-1/2" fin base. Thruster fins range from 4-1/8" to 4-7/8" in base length.
These are the basic parameters for this topic that we’ll be discussing for a range baseline board. There are obviously alot of different shapes that fall within and outside of these parameter. But, we gotta focus somewhere.
Fin placement is marked from the tail of the board forward to the back bottom edge of the fin. Shapers each have their own preferences for their various models. But, typically the center fin is marked between 4"-4-1/2" forward. The side fins are marked 11-1/4" to 11-3/4". The trailing edge dot is usually marked approximately 1" in from the rail. The closer the fins are clustered together, meaning center fin forward and side fins back. The looser the boards will ride at the expense of drive. The wider the seperation the more drive and track your board will exhibit.
Is the angle that fins are set at relative to the vertical plane. Inorder to accuratley determine vertical, given all the different bottom contours that thrusters are available in, it’s often necessary to use a horizontal platform with four small legs that touch equidistant off the stringer to establish a reference plane. Typically the range is from 5 degrees to 7 degrees outward from vertical. In more powerful waves shapers tend towards 5 degrees and in less powerful regions shapers tend towards 7 degrees.
Is the angle the leading edge of the fin is positioned closer to the stringer relative to the trailing edge. Typically this range over 4-1/2" is between 1/8" and 1/4". Again, more power less toe less power more toe.
Is the shapes of the inner and outer surfaces of either side of the fins from the base to the tip. Foils are based up curves that attempt to optimize the flow of the water passing around the foiled surfaces for a given velocity range and angle of attack. Foil curves are described by the cord lengths, camber and position of max camber relative to cord length. Curvier fin foils have cambers that are a greater percentage of cord length. They produce more lift at lower velocities and can handle a greater range of angles of attack. But, they do so at the expense of drag.
Is the shape of the perimeter of the fin and is described by base length, depth and rake. Rake is determined by the steepness of the angle drawn from the leading edge base to the trailing edge tip.
Now that I’ve defined the terms and parameters I’ll let others opine on the what’s, where’s, why’s and how much I left out.
Ideally we want a set of fins that will provide us maximum control when we want control and maximum looseness we want to initiate a turn while minimzing drag all the time. Well life is full of trade offs and fins are no different.