I have a hobie 18 (1980) When I get to a certain speed I start to get a humm from the back I think.
What is it
Is it bad
What need to be done to eliminate it if it is bad?

Thanks
Dave

I use to have a H-18...that sound is from the rudders...try sandng down the trailing edge some...its not a bad thing ...my FX-One does the same thing.....

Also make sure you have the bushings installed on the rudder pins.

If not, you can pick a pair up at any hardware superstore... look in the lawnmower wheel area.

Dave,
I had the same thing on my H16. I used a knife and scraped a sharp edge to the trailing edge of the rudder blades. The hum went away completely. Others have said that they miss the hum after they redo the blades, since it's always a good indicator that you are clipping along nicely.
Jack

no - it's not bad...you should hear the Hobie Tigers when the wind is up - they sing.

Dont know the reasoning, Im sure its a flow thing, but evry time Ive gotten the "Hum Job" I have been flying. Seems the boat was working at optimal speed. Dont know if its bad, but it never seemed like a bad thing to me.

David Mosley

Guys,
Humming is caused by cavitation, which is caused by uneven flow across your daggerboards or rudders. I agree it does sound cool, but it is slow.

Sand the trailing edge of your boards and wet sand the sides starting a couple of inches back. That should get rid of it.

Bill

Don't forget to clean up the leading edge of the boards too. Many factory boards have an ugly seam on the leading edge that needs to be cleaned up.

Will

I disagree...the humming is caused by a well balanced trailing edge. There doesn't need to be any cavitation (and this word is often misused; see below). The same kind of 'humming' can happen to airplane control surfaces if they are shaped very evenly. The foil, if balanced so well, begins a very fast and short oscillation (hum). In the case of the airplane example this can be severe flutter causing damage. If you just taper, very slightly, one side of the trailing edge of the dagger or rudder it will be ever so slightly unbalanced as the water leaves the board and it will settle down.

Cavitation is caused by very high mechanical pressures of which a sailboat is not capable of producing. If you shock the water with a huge drop in pressure, water vapor bubbles are formed. Shortly after that, the water would encounter normal pressures again and the bubbles would collapse with a good deal of force. Cavitation is common on poorly designed water pumps (causing long term damage to the pump or plumbing) and on boat motor props...Often sailors look at their rudder during a turn and see bubbles in the water trailing the rudder and call it cavitation. However, this is nothing more than the water curling on the lee of the rudder and drawing in some regular 'ol air from the surface. This is still not good since it decreases rudder effectiveness and is usually a sign that you turned the rudder too hard too fast.

Cavitation: (from dictionary.com)
1- The sudden formation and collapse of low-pressure bubbles in liquids by means of mechanical forces, such as those resulting from rotation of a marine propeller.
2- The pitting of a solid surface.
3- Medicine. The formation of cavities in a body tissue or an organ, especially those formed in the lung as a result of tuberculosis.

Humming,

As in "We'll really humming", or just "Humming Along".

[color:"orange"] [/color] thought that was sort of a audible knot meter, the faster you go, the louder it is!

There have been some good responses, I'm just killing time here at work. just sort of humming along.............. [color:"orange"] [/color]

My 6.0na hasn't hummed yet - although I've had her at "Full ramming speed" on several occasions! My 5.2 used to sing like crazy in a good blow and whenever I had someone new on the boat and they asked what that was, I told them "HANG ON!!! THAT'S THE FAT LADY!" Swooosh.

Jake

Hahha that had me laughing out loud here at work.

Using a sharp pair of good scissors (on the trailing edge) is the best way to get a clean even edge. Followed by some light sanding. Be sure the leading edge is nice round and smooth as it is more important than the trailing.

Do not get them too thin or they will break easily.

AJ
Nacra 6.0 Express

You should fix the trailing edges per Jake's instructions,
as the flutter may delaminate the trailing edge.

Nobody has mentioned that along with the humming you get a really nice vibration that causes an interesting sensory experience when you are sitting on the hulls. I was really annoyed when Rick got rid of the hum on our Hobie 18.

Sensory experiences are cool. Come to think of it, I will keep my hum.

David Mosley

I had this explained to me somewhere in my past. In Bethwaites book, High Performance Sailing, there is a diagram of water coming out of a tap onto the side of a glass and sticking to the glass side until it gets up a ways on the other side. This shows that water that is moving always wants to stick to a smooth surface up until the surface speed drops to zero, or a critical speed is reached where the corner is too sharp for suction to overcome the force required to bend the water quick enough.

The water wants to stick to the sides of the smooth foil until a certain speed is reached and the corner becomes too sharp for the waterspeed and the water can no longer stick. The hum is caused by the foil reaching the speed where the water can no longer stick to both sides of the foil. The nearly flat blunt back part of the board allows air to be sucked down at the back and the humming is caused by the water flipping back and forth between sticking first to one side of the board and then the other side. This is slower than a sharp trailing edge because it drags more water along behind the trailing edge of the foil. Rick White likes the sound of silence because it is faster. Any of you guys that think this is BS, I encourage you to leave your boards as they are, especially if I get to sail against you. Whatever turns your crank!

One other way of making the back corner less critical is to have a real rough surface on your foil. This makes the boundary layer thicker (this means you drag even more water along with your foil) and you do not get nearly as much suction on the sides. But this is not a good state because it causes your board to ventilate even sooner than having that blunt back edge. If you have ever had one of your rudders ventilate when you are cranking along at over 20 knots, it is a real adrenaline rush and is probably because you either picked up a weed, or you have rough boards, or you have a real blunt trailing edge. The leading edge is critical on your foils, andso is the polish. the weeds you pick up introduce roughness, and really lower the ability of the foil to give good turning ability. THe same with all those old hobie rudders that we have all seen with the black paint killed by the sun and the fiberglass fibers showing and real rough. Guess how good they work! Food for thought, isn't it?

Good or bad I don't know, but I know the leading edges of rudders and boards are really beat up, and so are the trailing edges. Come to think of it the whole boat is really beat up.
Oh well price was right.

Dave

Here is a link regarding the cause and cure for Humming.

http://www.hobiefleet929.org/id17.html

then scroll down to "Hobie-Rudders" and click on "Correcting Hum"

Nige

I always thought too much Rum caused the humming? Or was it the beer?

AJ
Nacra 6.0 Express