## 63912Re: [bolger] Help with noise

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• Jul 9, 2010
Patrick,

The resonant frequency of a boat hull is not changed by weights.  Natural frequency is
solely a function of stiffness, k, divided by mass, m,  (sqr root (k/m)).  The magnitude of the response can be dampened by weights, but the frequency doesn't change.  Since stiffness is lower with longer spans the frequency can be raised by clamping the gunwales to the sawhorses.  This will profoundly alter the resonant behavior of the sides.

Control of the bottom and the chine can be achieved by setting building forms (I'm sure you still have them) inside the hull and clamping them in place.  You could make the forms taller than the depth of the hull and clamp them with ratcheting cargo straps.  You could also increase the stiffness by adding two longitudinals much like the inserts in a liquor box.  This may not get much since the the sander operates at about 10000 cpm.  Total elimination of resonance requires raising the natural frequency to about 60% of this value.   I have, however, reduced the ringing from a cheap Craftsman tank mounted air compressor by strapping a 2x4 to it

The underlying principal can be demonstrated using a ruler and a table edge.  Lay the ruler with 3 inches hanging over the edge and pluck it with your finger.  Now lay it with 6 inch overhang and pluck it.  You can also infer that forms will increase the stiffness from the write-up for Payson's Pirogue, where PCB instructs that the forms remain in place until after the gunwales and thwarts are fitted because the structure lacks stiffness otherwise,

V/R
Chris

On 7/9/2010 11:08 PM, Giuliano Girometta wrote:
 You are probably dealing with a natural resident frequency of the boat hull that is sincronous with the vibrating frequency of the sander (So the boat is acting like a bell). Just try to put a couple of weight fore and aft or one in the middle and that may take care of the problem. Otherwise may be a temporary diagonal brace between the sheers may alter the resident frequency of the hull away fron the frequency of the sander. Giuliano  --- On Sat, 7/10/10, Patrick Crockett wrote: From: Patrick Crockett Subject: [bolger] Help with noise To: bolger@yahoogroups.com Date: Saturday, July 10, 2010, 12:22 AM I'm doing some repairs on my Windsprint, and when I use the random orbital sander to fair the epoxy/fiberglass the whole boat resonates with a very loud, unpleasant hum. The neighbors finally complained today. I don't really blame them. I can't move it indoors, as I don't have a garage or shed big enough. Does anyone have any ideas for sound suppression? Patrick

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