Hull vibration from fiberglass fin

I rebuilt a 18 and ran it the first summer without a fin. Bought and installed a fiberglass fin this spring. Now there is a hull vibration at all speeds but it is really loud around 15-20 mph. I know it is from the fin since the noise was not there before. Anyone else have this issue? My other boat has an aluminum fin and it has no vibration. I'm thinking of sharpening the trailing edge first, and if that doesn't work, cut some small vertical grooves in the fin to make the water turbulent as it passes. Any words of wisdom from the masters? Thanks. Mike

 

Mike,

Is the fin a solid big one, or is it the inboard style fins? My Dyne had the two inboard style fins. I did not have your problem though.

Greg

 

That is one beautiful boat. How does it drive pulling a short line skier with the tow bar that high. I replaced the fin on a 77 model I had once by making a copy using 1/8" stainless the same size and shape of the original. I didn't notice a difference.

Vann

 

If it were mine, I would try shortening it an inch and see if that helps. I shortened mine about 15 years ago to help with trailer clearance and I don't notice any performance difference.

The obvious fix is a metal fin.

jim

 

The vibrating fin is the single large outboard style. It is a sunk fund at this point so I'll carve on it a bit and report back my findings. Last resort would be to make one out of SS or AL.
I kept the original Bartlett tow bar mainly for looks. I use the boat for barefooting and the height helps. It would not be good for shortline slalom but boat #3 is a Nautique.

 

I use to have Skip's contact info. He has the two inboard style fins that my 18 had on it. He can probably get them for you. Dynebob owns the boat now. Maybe he can get a picture of what the fins look like while it is on the trailer.

Greg

 

Wondering if you ever found a solution to the vibration? I put a new motor on our 18' last year and immediately noticed a whine (we questioned hull vibration as the cause as it definitely isn't coming from the motor) the first time out. It peaks at about 18-22 mph.

 

I have not messed with the fin. Boat doesn't get used much and usually pulls footers over 40mph. Like yours, the vibration is loudest around 15-20. Above that, the engines make so much noise, I wouldn't be able to hear the fin anyway. My other boat has an aluminum fin that doesn't vibrate. The aluminum is thinner and most likely stiffer, so it is some kind of fin-natural frequency-resonance thing.

 

I have been rebuilding an 18' Hydrodyne. I am unsure what model it is, since I haven't seen a photo of one just like it.
I put it in the water last year and the fin vibrated, producing a tone throughout the boat. It turned out the fin was poorly mounted, since the wood was rotten and the bolts partially pulled out and someone just siliconed it all together.
So I rebuilt the floor with all new fiberglass and the fin is extremely stiffly mounted. Now it produces 4 different tones, depending on boat speed and trim :-(
The fin is aluminum, almost rectangular, about 21" x 10".
I think I will take a grinder to the fin and give it a much more rounded shape and sharpen the leading and trailing edges to reduce the water turbulence that may be initiating the vibration.

 

That is a pretty deep fin. Do you know what year your boat is?

jim