από Geoff Moore 



by Geoff Moore 

One of the strangest noises you will ever hear on a sailboat is a mysterious humming noise. Usually, it happens when
the boat starts to plane or surf, but sometimes it happens even at moderate speeds.

I must have been very young when I first heard it because I never questioned why it was there. It probably made
sense to me in the same way an airplane, or a big truck made noises when they went fast. So why not sailboats.

It wasn't until a non sailing friend asked me what that noise was that I even gave it any thought. You would think that it
takes energy to cause this vibration. That energy must be coming from somewhere. So wouldn't you think it is robbing
you of speed? That was when I started asking questions. It amazed me how few people cared. If it is adding drag it
isn't very obvious. I have been on some very fast boats that have had some very loud hums. Some people actually
grow fond of this high frequency hum. Maybe it has some kind of hypnotic calming effect on certain individuals.

I eventually recieved many different explanations why this occurs, the most common being that the noise is the result
of turbulence produced by an unfair centerboard, keel, or rudder. Another popular story is that the rudder vibrates as
it strikes turbulence from the keel or centerboard. Other people will swear it is the slack in the rudder hardware. I
have even heard so called experts say that it is caused by air trapped under the hull.

I am sure there are some very smart "technical type" people who know exactly what is happening, but this is what
makes sense to me. First, almost every underwater foil has a squared off trailing edge. Why square? Well, razor sharp
edges are difficult to maintain and in the case of a capsize they could be down right dangerous. Round is out because
the eggheads tell us that water releases better from a sharp angles. So that leaves us with square.

Actually a perfectly square trailing edge has a better chance of vibrating than an unsquare trailing edge. Imagine the
keel splitting one big drop of water into two little ones. If the keel is perfectly fair they should both reach the trailing
edge of the keel at exactly the same time. When they get there they both can't exit the keel together or there would be
a void in the middle. Since "nature abhors a vacum" one side has to pop over to the other side as it exits. Since the
keel is symmetrical there is nothing to keep the water exiting from a consistent side, so the sides alternate. The
resulting vibration is the mysterious hum!!

At least that is the way I picture it. As evidence I note that the vibration never happens up wind...and if you file the
trailing edge, or one corner of the trailing edge, so that the keel, or rudder, is slightly asymmetrical the hum will either
be eliminated or greatly reduced.

* Geoff Moore *
* Shore Sails Ltd *
* 7 Merton Road *
* Newport, RI 02840 *
* 401-849-7700 *
* fax 401-849-7952 *
* gmoore@shoresails.com *