Thruster fins; placement, cant, toe, foil, etc

General Discussion
1

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.

Placement

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.

Cant

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.

Toe

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.

Foil

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.

Template

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.

2

This is great. I’m in need of a Thruster 101 course.

Can you please offer some general guildlines on placement,cant and toe for a grom board (5’-5’10") and what about a big-guy board (6’4"-7’)?

I know this is outside the basic parameters you layed out for the discussion but may just a general guideline on how things change as you go smaller and larger on boards.

thanks

3

I mentioned some numbers on placement (fore-aft) to a shaper friend - he told me 10 1/2 and 3 1/2 for all shortboard thrusters, and move the rail fins forward a little if the tail is under 14 inchs wide. He then called another friend shaper on the North Shore to confirm (which was confirmed) - their opinion was that 3 1/2 and 10 1/2 was the competition thruster standard.

Choosing to be difficult, as I am, I measured the boards in his shop, from the used rack. The “Kelly Slater Pro Model Merrick” signed board on the wall came down. 10 1/2 and 3 1/2. The Rusty from the used rack - 10 1/2 and 3 1/2. Also, about 1 1/4" off the rail for the rail fins.

Wanting to learn something, I measured my buddy’s boards. He is a competition shortboarder and really finicky about boards, but has no idea what measurements are. All his boards were 12 and 4. He bought boards of all measurements, apparently, but the only ones that felt really good to him were at 12 and 4. All his boards were 6’3" to 6’4" with tails very close to 14 inches.

There’s no right answer…just differences in performance based on position.

4

Seems to always boil down to compromise and preference.

My shortest boards around 6’3"-6’4" i like 3 1/2 center and 10 7/8 front with around 1/4 tow…I like to take a straight edge and point my fins 1 1/2 off the nose.

Cluster tends to get wider as the board gets longer but again this varies.

Base I like a fin thats 4 5/8- 4 3/4 same size all around or a smaller trailer for smaller waves…oh yeah combine it with Herbs superchargers and the board really comes alive and carries through turns and floaters.

Cant i like em at 6-7 degrees. Less than that felt drivey but a little stiff and surfed sorta flat, more than that was loose but had a delayed response but again preference is key.

Foil i usually go for thinner foils and stiffer fins especially when the waves have more power but again this can vary some.

Rake medium, more than that feels stiff and hard to snap and less loses drive and stability in juice.

Love adjustable fin systems. Every personal board I shape has em and i like to play with the positioning while in the water to fine tune the board but you can get alot of strange looks and questions from people watching you mess with your fins every 15 min in the water.

5

Hi guys

I agree with Troy adjustablity is the way to learn and have fun

If you like doing it all your self check my post on (vac forming fins and adjustable fin boxs)

They are quick and easy to make,I like em ,you might to!

have fun Mike

6

Thanks Tom!

I agree with Blakestah and have found the same specs out here.

Merricks boards tend to have tighter clusters. GLoehr goes with 11 x 3.5 as a std. I remember when I was hammering you guys about the criticality of toein and Blakestah hammered the point and then some…super critical. Since then, I have taken the time to do toein perfectly…started using slow set epoxy to set my fin systems in place…no stress no mess and perfect.

Just to add the the info…you can have drive from a slightly tighter cluster if you move the cluster back. I have found that as you move the cluster back you get better bottom turns (drive, speed, projection) and as you move the cluster forward you get a touch more pivoty top turn. I have set boards with the cluster forward (11.6 x 3.8), rode them, and have reworked the cluster back to 10.9 x 3.5. The difference was obvious…better bottoms but slightly harder tops. Im not going to compromise my bottoms so Im happy with the results. Im also using adjustable centers.

I do notice foot placement is more sensitive with a tight cluster and thats a bit of a problem for me cuz I dont hit my foot sweet spot everytime cuz I ride so many different boards…it takes a few waves to get a good rythm going.

PS - so Im making my first EPS blank today…sucker weighs only 1.25 lb before shaping!!! An equivalent Clark weighs about 4.5lb.

LOOKOUT!

7

Thanks Tom! Now I’d sure like to hear how variations of all these affect the ride of a board. Also as far as “Placement” what about the distance the side fins are from the center of the board? Or is it more common to measure how far the side fins are from the rail? Just saying “3 1/2 and 10 1/2” is a pretty vague description.

8

Tom mentioned the rear edge of the rear fin as 1 inch inside the rail.

I’d heard 1 1/4" as more standard.

9

Generally side fins are 1-1 1/8 off the rail on a shortboard.

Cluster variations: In general terms a wider cluster gives you longer turn, larger sweetspot. Tighter cluster smaller sweetspot but tighter turns and better snap.