Shim Angle Naming Convention

New topic to get this out of the Severne Red Wing topic.

PatK wrote:

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Moses/SAB says their fuses have minus incidence angle for the stab. The 900 has -1.5 deg:
sabfoil.com/en/products/Fuselage-900-High-Strength#datasheet
They are selling a set with shims, they are all minus:
SHIMS -0,5/-1/-1,5/-2/-2,5/-3
With my Moses 790 kit i got two 3d printed gray unlabeled shims, both increase the lift, so i think they are minus too like the one in the set. For me it is much more logical to count the angle as the sum of the built-in value with the shim. E.g. -1.5 of the fuse and -1 of the shim ends in -2.5 deg. This makes more lift. To reduce the lift i add a plus angle, e.g. -1.5 of the fuse and +1 of my self-made shim ends in -0.5 deg. This is easy to calculate and not confusing. For me it is important to know what the built-in angle is, so i can imagine what happens when i deal with a shim.
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Yes, the 900 fuse has -1.5 deg stab angle built in. Stab down angle is the same as front wing up angle (angle of attack = AOA). The shim numbers refer to the change of front wing AOA. The shim naming convention refers to whether the stab increases the front wing lift (positive numbers) or decreases it (negative numbers). It took me months of research to figure this out. Sailworks says this, and all the SB-using guys in Seattle say this.

Therefore, the stock -1.5 deg stab down angle is the same as a front wing +1.5 deg AOA.

If I use a -0.5 deg shim on my 900 fuse, the result is +1.5 - 0.5 = +1.0 deg AOA. This is less lift than the unshimmed +1.5 deg AOA. If you want to use straight arithmetic, use it for the AOA, not the stab.

If you mike your negative shims you will find that the back end is thicker than the front end. All of mine are.

Also, I can confirm with lots of experimentation, that using negative shims REDUCES the lift. That is precisely the naming convention, used by SAB and Starboard. I don't know about the others.

The thing that matters is the difference between front and rear wing orientation. And a difference is generally defined by subtraction.

ie. If the front wing is travelling at +2 degrees angle of attack on the fluid and the back wing at -2 degrees on that same fluid then you've got a 2-(-2) = 4 degree difference. If you want to shim it for more difference, (ie. more inclined to lift at low speed, and you want to stand further forward at high speed.) then shim the back by -2 to make it -4 . Then you've got 2-(-4) = 6 degrees. That'll get you going!

It makes sense that if the shim points the rear wing more downwards relative to the front wing we call it a negative (-) shim.

Of course you could always argue that differences should be found by addition.

That's why I started this thread, to get the discussion going so that we all come to an understanding about the naming convention for shims.

Like what is shown in the picture above I also have that set of SAB shims. They are labeled with numbers that do not have either positive or negative signs. That means they are positive degree numbers. I can prove this below.

At Boeing they say, "In God we trust, all others bring facts and data." Here below are some facts and data.

The first photo shows my -0.5 deg shim mounted under a 450 stab. I use this for my 950 wing.

The second photo shows, from left to right, a -0,5 deg, -1.0 deg, and the SAB-provided 1,5 deg. All were mounted such that up is the forward direction on the foil. The squared end is forward, the pointy end is aft. Note that the thickness is greater on the aft end for the negative shims, and that the thickness is greater on the forward end for the positive shim.

Now let's show some measurements, taken with a vernier caliper. Sorry, metric folks, these are in inches. I'm a yank. The third photo shows those measurements. The gray and white shims I got from Sailworks, and the black shim is from the SAB kit. Again the measurements show that the forward end is thinner than the aft end for the negative shims, and the reverse for the positive shim.

This proves that a negative shim REDUCES the down angle of the stab, This, therefore, REDUCES, the AOA of the front wing. This REDUCES the lift at a given speed. Examine the first photo again. If the aft end is thicker than front the down angle is reduced. In the photo this would rotate the stab 1/2 degree clockwise. Since the stab is on the bottom of the fuse, clockwise in the photo is a reduced down angle.

After two years of on-water experimentation with shims, I can testify that a negative shim REDUCES the lift and keeps the foil down during virulent gusts. Reducing the lift lets me use more back foot pressure to control the pitch of the foil.

QED

nice vid @ArtemN how much do you weigh if you don't mind my asking?

SAB lists the angle of incidence on their fuselages as -1.5 or -2.3 degrees depending on the fuse (negative b/c of pointing down?). All their shims are labeled w/ negative angles, so adding shims (or a negative to a negative) gives you a bigger difference relative to the fuselage.

Adrian Roper also explains his shimming naming convention: skip ahead to 26:50 as i can't embed timestamps w/ youtube. His rationale is difference in angles b/w the front wing and stabilizer are what matters, so positive shimming would be adding to this delta.

Slingshot Hoverglide foiler here. I measured my stock stab AOA at -3.5. At higher speeds my i76 had too much lift so I placed a plastic tab from a bread bag under the front of the stab (it's on top of the fuse) then a piece of packing tape on the leading edge to cover the gap created. Can't exactly recall but I think it took the AOA to -2.5. Much better ride. I know of several foilers more skilled than I who do this. Very simple hack.