Been playing with rail fins a lot lately. I was quite happy at the improved performance of cutaways in center fins.
So I initially made rail fins with cutaways.
Then I made them without.
Wow, what a difference in fin action. The rail fin with a cutaway across the first two inches always goes whoosh through turns…not bad, but the increased hold/drive from using a non-cutaway made me re-think things…my guess is the flow profile (vs depth) changes a lot from a single to a rail fin. On a rail fin, the fastest water flow, in a turn, is very near the hull. And it probably decreases as you increase in depth. For a single, the transitional layer adjacent to the hull is a lot deeper, so a cutaway can work well…but put a cutaway on the rail fin and the water on the high pressure side will run through the cutaway and eddy up the fin on the low pressure side, and its all whoosh and no drive…
Even making the fin surface area larger didn’t help the cutaway fin achieve the same drive I could get with a much smaller fin with max chord length at the hull.
HTH helps someone…I didn’t explore really small (half inch) cutaways, still makes sense they would work on a rail fin, but you’d need max chord length where the water flow is fastest in a turn, probably by no more than 1/2 to 3/4" up from the hull.
How deep are the rail fins? Same “butter knife” outline? Did you test in a 2+1 set-up or thruster (equal depth fins) or twin? How much toe-in? Does the single like a fixed box as much as your rotating one?
How deep are the rail fins? Same "butter knife" outline? Did you test in a 2+1 set-up or thruster (equal depth fins) or twin? How much toe-in?
The rail fins are 4.5 inches deep.
The one with the cutaway was the “butter knife” design.
The one without was
If you line them up, you would see the leading edge is almost identical in the two. The grid is a 1 square inch grid I use in all my fin designs.
For the second fin I took chord length as a function of depth, and fit it to an ellipse, and then scaled so that the mean chord length was comparable to an FCS G5. It is smaller in surface area than the first one, but much more powerful. Right now I have the board set up with one fin on each rail (good for kicks and lots of weird stares).
It’s been tested in a thruster, but the center fin (unchanged from stock in this board) is actually 0.25 inches deeper. The toe-in varies…the fin boxes rotate. I tested a bunch of different “lockout” angles, and settled on what is equivalent to 3/16" toe-in for each fin. The effect of changing toe-in is very similar across the two.
Quote:
Does the single like a fixed box as much as your rotating one?
That single has more rocker than I like in a fixed single. The looser tail seems to go well with more rocker…or maybe it is because it is so much faster through a bottom turn that the greater rocker feels better. The alterations in rockerline were the biggest changes from fixed to rotating box…the greater rocker just went better. But its too slow if it is used with a fixed fin.
