Ed,
Here is an excerpt from the sunfishclass.org, FAQ and two additional links
to articles addressing rudder and daggerboard humm. The first of the links
applies to aluminum blades and the second to plastic. I think the one for
plastic foils would also apply to wood and fiberglass.
Wayne
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From: sunfishclass.org
Frequently Asked Questions
Sunfish Tuning Guide for Racers
"Two operations should be performed to the daggerboard trunk to ensure a
scratch- and vibration-free board. First, sand the sides of the trunk, both
top and bottom, until any sharp ridge is eliminated. Secondly, add 13-inch
by 1-inch carpet or other protective strips to the fore and aft walls of the
trunk. Silicone glue or contact cement works well in applying the strips.
Before installing the strips, insert your board into the trunk and determine
the amount of extra space. Use enough glue [or carpet thickness] to
eliminate this play."
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Article from eZine, "On The Wire"
Eliminating Rudder or Dagger Board Hum By Gary Willcox
This was written for aluminum blades.
The J24 link for monohulls contained within this article is
no longer working so I have included the article at the
end of this post.
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Tech Tip from Hobie Cat
This is for molded plastic blades.
The flatted trailing edge should apply to wood and
fiberglass foils as well, but the next article by
Bill Moore adds another perspective.
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[Granted the J24 is a much larger boat than a Sunfish, but I think the
principle still applies]
Hummmmm
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by Geoff Moore - Shore Sails Ltd
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.
Hummmmmmmmmmmmmmmm!
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 received many different explanations why this occurs, the most
common being that the noise is the result of turbulence produced by an
unfair [uneven] 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 [or rudder or daggerboard]
splitting one big drop of water into two little ones. If the keel is
perfectly fair [smooth] 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
vacuum" 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.
Good Luck
Article found at www.j24class.org Tips and Tricks
