Who hears thumps in following seas?

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While doing 2 Pacific passages in Rainshadow, for weeks we listened to boat noises in a variety of conditions.  We noticed a pattern.

When we are sailing at greater than 5.3 knots in a following sea, we can hear regular low frequency thumps resonating throughout the boat. The thumps stop if the shaft is secured so the prop cannot spin. If the seas are on beam or forward, this thump does not happen.

I mentioned this in another posting, and I could not believe it when Czarina Blue (Duncan) responded that he hears the same thing.

So my question is, who else has heard this regular thumping in following seas at boat speed just over 5 knots?

If you are not sure, next time you are out under sail, with a following ocean swell (not just wind waves), listen for a regular thump when the prop spins freely. If you think you hear something, in the aft bilge, grab the shaft with your hand to stop the rotation. Does the sound go away?

Hearing such sounds always makes me wonder what is getting worn and ready to fail, (or what’s wrapped around the prop!) but I’m wondering if this is just a Nic 38 design “feature?” Similar to the drunken sailors “feature” we hear singing whenever we’re on a port tack forward of beam. (We figured out it’s actually the holes in the furling boom that turn the boom into a flute – it’s quite lovely actually.)

Really curious if anyone else has noticed these thumps from the deep. Because it’s audible throughout the boat as it resonates through the hull, it is confusing to figure out the source until you stop the prop from free spinning while under sail.

Marilyn, moderator

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