>I have get cavitation at about 1100 revs (tickover 850 revs, shallow
>canal
. Ususally on
> rivers and very deep canals at higher revs the vibration disapears.
> Having
This doesn't sound to me like cavitation in the strict sense as that
ought to *increase* with higher rpm.
I wonder if it's a different problem - air getting sucked under the
base-plate of the counter? PRESIDENT used to have this at one time -
lots of air in the prop stream and a rumbling noise, which could
disappear at higher rpm. What was happening was that the counter was an
inch or two out of the water at rest but at higher rpm the 'suction'
(yes, I know) of the prop sucked it deeper into the water (following her
up the Severn the water was at the level of the centre rubbing strake of
the counter, i.e. she was about 9" deeper in the water. We cured the
problem by ballasting so that the counter baseplate was under water when
she was at rest. ADAMANT had similar problems, again cured by
reballasting.
The 'sinking' in the water is much more noticeable in a steam boat with
its bigger propeller but I suggest this might be the explanation if your
counter baseplate is at or above water level at rest or at low speeds
(this only needs to be the case on one side to have the effect). If you
can get someone to observe from the bank or another boat (it's not very
safe to lean over just above the propeller to check) you ought to be
able to tell whether the disappearance of vibration coincides with the
counter sinking below the surface.
Alternatively you could put some moveable ballast (e.g. a number of
people) at the stern to see whether this solves the problem at low revs.
I would have thought this possibility was worth checking first as
reballasting is likely to be cheaper than any other cure.
Sean
>
> Terry
> nb Hakuna Matata
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