Thread: F 161 and fin
View Single Post
Old 19th February 2007, 10:59 PM   #6
Roger
Dream Team - School Guru
 
Join Date: Aug 2006
Posts: 2,112
Default RE: F 161 and fin

Hi Guest,
Getting rid of the "super thin" trailing edge (TE) on the fin should stop the noise and vibration.
If doing the above procedure once eliminates some of the noise, but not all of it, repeat the procedure.
Another thing to look for is a little bit of "concave" just in front of the trailing edge.
If you have a little concave on both sides of the fin, just in front of the TE, you probably need to "blend" the concave in with the rest of the foil.
The fin guys use a flat diamond impregnated file to take care of this problem, but you could use a good sanding block and get about the same result.
Get a straight edge (or better still a hardened and ground dowel pin) and check the back part of the foil on both sides of your fin.
Ideally, if you place the straight edge (or dowel pin) perpendicular to the vertical axis of the foil, you should not see any light as you slide the straight edge (or roll the dowel pin) down the fin.
If you see an area of light (where the straight edge or dowel pin does not contact the foil surface) you have a concave area in the foil.
If the concave is on both sides of the fin, then the foil can be "too thin" to prevent vibration/noise even though the TE is thick enough.
So, work out any concave areas between the high point of the foil (usually about 30% back from the leading edge (LE) and the TE of the fin.
This may shorten the overall width of the fin slightly, so don't overdo your filing/sanding. Just work the little concave areas out until there is a straight line of smooth foil surface from the high point to the TE.
Then do the blunting procedure above until your noise/vibration goes away.
Normally these little "concaves" have resulted from the hand shaper (the guy who finished the fin when it was manufactured) gettting a little too agressive when shaping the back of the foil.
This can occur on CNC fins as well as hand shaped fins as the CNC machine does not do all the finish work. A hand shaper must go in and "blend" all the little "lands" between the CNC tool cuts.
On molded fins, this normally does not happen, and there is very little finish work done on the foil of molded fins after they come out of the mold.
Some CNC fins require very little "foil finish work" as this is a function of the "feed set over" used.
The more "passes" the machine makes, the less blending of the lands between the cuts or passes is needed.
Custom CNC machined fins from craftsmen like Dennis Parton at Tectonics require almost no blending, but the fin stays in the CNC machine 2 or 3 times longer and machine time=more costly, but better quality with a more precise foil.
Hope this helps,
Roger is offline   Reply With Quote