Was wondering if anyone used any particular ratio or method for scaling fins up or down in terms of height versus chord (Y versus X in the image below).
Just as you would when scaling a board up or down in terms of length versus width whilst trying to keep the same “feel”.
For example, scaling a large single fin down to a height to be used in a thruster set normally results in the base being too narrow. Conversely, scaling a thruster fin up to single fin height results in the base being too wide.
I’ve tried scaling fins with a photocopier by % reducing the height down to the 4 1/2 inch height of a lot of thruster fins and then stretching the base by % but it never results in a shape that looks similar to the original or is pleasant to the eye.
Don’t forget to scale the thickness too, or you’ll possibly increase the drag of your fin (this is assuming that you already use an optimised foil profile, which is very unlikely).
In theory, you cannot scale foils (the common name for aerofoils & hydrofoils, read wings & fins) and keep the same characteristics since you are changing the flow regime (different Reynolds number). But I would just scale all dimensions, not forgetting the thickness.
Are the guidelines from the big fin companies a good starting point for basic fin area versus rider weight? Are there better rules of thumb for this?-J
Fin companies have no idea where your fins are placed , how wide the tail is or type , all big factors in any fin choice and recomendation information .
So here’s the fin shown above scaled down for thruster use, but then the width scaled up by 20% in relation to the height to give a better base length.
Does anyone else here do this sort of digital scaling to make big fins out of small fins and vice versa? And do you have a rule of thumb you use or do you just do it by eye, or adjust as required to get the ratio of base length to height that you want?
Depends on how good your “eye” is - I believe mine is OK, but I could be wrong.
The fin I’ve shown is just something I’m using as an example for discussion. It’s not necessarily something I’m intending to make.
Ultimately a fin template is only a piece of paper and a pencil away. I don’t tend to accumulate them because of this fact.
The reason for this thread though, was just to gauge if anyone had a more scientific approach for this which would retain the characteristics of a fin when resizing it. Along the lines of Greg Griffins scientific approach to his preferred fin template itself.
Huie is correct about fins for surfboards. Ever wondered why badly foiled fins work?
Just because it doesn’t matter that much.
For windsurfing this is a completely different story, here fins are extremely important. The fin has to cancel out the entire lateral force induced by the sail. Fins really make or break a sailboard.
I guess there could be a scaling function, much like that used for scaling boards in programs like Shape3D. You can scale board length, but keep width and thickness the same.
My way would be to draw up rectangles on a page and transfer the outline from squares to the rectangles. Scale the base and height of the squares by the ratio of actual to desired. I.e. 6" actual to 4" desired base would give rectangle base 4/6 of the square’s base. 10" actual height to 5" desired height would give rectangle height 1/2 the square’s.Then mark points and do curves, I suppose you could do this in a graphics package by playing with ratios.
If your question is, “How do I decide if a 4” or 4 1/2 base is right for a scaled fin", I have no idea. What Ben says, I guess.
BUT one thing we’ve learnt from scaling boards is that it doesn’t work over more than a couple of inches in length. Scale a good 6’0" to a 7’0" in software and you don’t end up with a good 7’0" board, but the 6’0" will go to 5’10" or 6’2" fine. A single fin to a thruster may be asking too much.
