To shim or not to shim?

Author: Jon Mann

How do I shim my rear wing? What is the difference between base plate shimming and rear wing shimming? How do I shim for more lift? These are some of the many questions that crop up as riders seek to tune their foil set ups to their liking. The beauty of foiling is the ability to tune the equipment, though it is often very easy to dive into rabbit holes and get lost over thinking.

In this post I’ll try to give an overview of what the various ways are in which you can tune your foil and what to expect when you use different types of shims.

What is a shim?

A shim is just a spacer that you can use to alter the angle of you mast or rear wing. You can buy specially made shims for base plates, and some foil brands have specially made shims for their rear wings. But equally you can make your own shims using old cut up credit cards for example, so long as they meet the objective of allowing you to change the angle of your mast or rear wing.

In foiling we generally deal with 2 types of shim:

• Baseplate shim (or rake shim): This sits between your baseplate and your board and is used to change the angle of your mast relative to your board.
• Rear wing shim: These are placed between the rear wing and the fuselage in order to change the angle of the rear wing. The main consequence of changing the angle of the rear wing is either more or less lift (and therefore also drag).

How do I use baseplate shims?

My personal opinion on the principal reason for baseplate shims is to change the angle of your mast and therefore foil, with respect to the deck of your board.

Let us assume your foil, when flying at a steady state, flies with the fuselage level (which a lot of foils do). If the bottom of your board is flat, then your deck will also be level, meaning your fuselage and deck are parallel. 

The most common reason for base plate shimming is tail rocker on boards. Now let’s see what happens if you have tail rocker in your board. Fixing your deck level and looking at the foil, we see that the foil will be pointing down. Fixing the foil level we see that the nose will be pointing up. Depending on what your foiling level is you will either experience:

• Nose diving: If you are a bit more of an intermediate rider, know how to handle front foot pressure (and have some preferences already) and are very used to flying with a level deck, with the foil pointing down you’ll find that your board is constantly nose diving. 
• Front foot pressure: If you are maybe a bit less experienced, or not used to foils with front foot pressure, if the foil is allowed to fly level the nose of the board will be up making you feel like you have a lot of front foot pressure. Note: this is not really added lift and more of a perceived front foot pressure. This is also not wrong, many people adjust and get used to this and foil a little nose up. At the end of the day, moving the nose of your board up or down will still move the foil up or down, regardless of relative angle between the two. 

So, if we don’t like either of these, how do we address the issue? Base plate shims! A base plate shim with the thick part to the back of the board will allow you to compensate for the rocker, and allow the foil’s flight angle to be parallel to the deck. How much you need to shim really varies depending on how much rocker your board has, but typically 1 – 1.5 degrees are enough.

Base plate shims can be used for further fine tuning. Even with a flat bottomed board, some foils will have a slightly different angle of attack which may cause them to feel a bit more nose up or nose down and as such fine tuning with smaller shims can help optimise you set up. Though for the purposes of this post I won’t go into more detail so feel free to ask me any questions if you want to look at finer tuning of your set up. 

Why would I want to shim my rear wing?

Shimming a rear wing for more or less lift can really change a foil’s characteristics. It can improve the low end and improve the high end, change how much front foot pressure you have and even change how the foil handles. Often people think that they need a new foil for whatever reason (too slow, not enough lift, doesn’t pump well, not enough glide) but more often than not a bit of tuning with some chopped up credit cards can lead to different feels which might achieve what you want without having to buy a new foil!

How do I shim my rear wing?

Before we go into how to shim your rear wing, we need to understand what the rear wing does. The rear wing counters the downward pitching moment of the front wing. Which in layman’s terms means it pushes the front of your foil (and board) up. 

So what can we do to our rear wing?

• More angle of attack: We can shim it to create more lift. This will result in more front foot pressure but also more drag but combined with a slightly more stable feel. This is done by using either homemade shims or manufacturers shims that increase the angle of the rear wing. For top mounted rear wings that means a shim with the thicker part to the rear, or that lifts the back of the rear wing. For bottom mounted rear wings it means a shim with the thicker part to the front or that lowers the front of the rear wing. 
• Less angle of attack: This does the opposite. By flattening the rear wing you reduce the front foot pressure and reduce the drag (lets you go a bit faster) combined with a slightly looser feel and a bit better turning. This is done by using either homemade shims or manufacturers shims that decrease the angle of the rear wing. For top mounted rear wings that means a shim with the thicker part to the front, or that lifts the back of the front wing. For bottom mounted rear wings it means a shim with the thicker part to the rear or that raises the front of the rear wing.