NavList:

   

Reply

Keith,

Thank you for the extra details.

By "relatively high frequency", I only meant that it is not the sort of
low-frequency (on the edge of human hearing) which does seem able to
transmit from water to air if the sound waves strike a calm surface
perpendicularly. (I don't even have any hard evidence that sound can be
transmitted from water to air with reasonable efficiency under those
circumstances but I think it can).

It is interesting that what you have heard happens with a clean hull and
sounds so like bubbles. Could it be bubbles? Eel grass can be so dense
that its respiration at night depletes dissolved oxygen to the point
that it can kill animals in the grass beds. Perhaps by day it might
saturate the water with oxygen. Could that come out of solution when it
contacts the surface of the hull, as the surface water cools at night?
Alternatively, could it be gas released from bottom sediments as the
tide falls (reducing pressure at the seabed)? Release of seabed methane
is by no means uncommon.

I'm groping for an explanation but also trying to illustrate the wide
range of phenomena which might be responsible.


Trevor Kenchington


You wrote:

> To clarify - I have only heard this noise when I have been down below in
> a boat. The noise appears to come from all parts of the hull below
> water. I can be (almost!!) certain that the noise appears even on a
> newly-antifouled boat, so feeding on algae could be contra-indicated by
> that.
>
> My boat is a cold-moulded wooden boat with epoxy-glass outer skin. I
> have not heard the noise from within a steel hulled boat, but I have
> from within a glassfibre boat. I assume this is because I haven't spent
> much time in metal boats.
>
> The noise is not very high frequency by standard human measures, almost
> exactly what I make by making a clicking noise in my mouth; and the best
> description is clicking - certainly each sound is not a scrape (ie, each
> noise is short); I mentioned explosive bubbles because that really is
> the best description. Like having a champagne supply under the boat.
> Keith Williams
>
>


--
Trevor J. Kenchington PhD                         Gadus@iStar.ca
Gadus Associates,                                 Office(902) 889-9250
R.R.#1, Musquodoboit Harbour,                     Fax   (902) 889-9251
Nova Scotia  B0J 2L0, CANADA                      Home  (902) 889-3555

                     Science Serving the Fisheries
                      http://home.istar.ca/~gadus

   

Reply