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GAS FITTINGS
Peter Spreadborough,
of Southampton
Calor Gas Centre,
has 20 years in
the industry
PAINT AND
ANTIFOULING
Richard Jerram is
former UK technical
manager of
International PaintTRAILER-
SAILING
Colin Haines is a
design engineer
who has trailer-
sailed for 25 yearsELECTRONICS
Chris Ellery of
Greenham-Regis
Electronics is a
former Merchant
Navy officerTOILETS AND
PLUMBING
Gary Sutcliffe of Lee
Sanitation knows
about holding tanks,
toilets and plumbingYACHT DESIGN
Andrew Blyth is a
naval architect with
interest in stability
and buoyancyWOOD
Richard Hare is a
wood technologist
and long-time
wooden-boat owner50 of the most frequently asked boating questions are answered by our experts on the PBO website. Visit http://www.pbo.co.ukBOATBUILDING
Tony Davies has
been building and
repairing wooden,
GRP and steel boats
for 40 yearsSEA SAFETY
TACHOMETERS
PAINT AND ANTIFOULING
Indulging in tach talk
Q
I have an alternator
pulse-driven tachometer
which has five settings, none
of which are anywhere close to
the actual rpm of my Universal
diesel.Is there a small circuit
I could put in line between the
alternator and the tachometer?
Thecircuit input is the alternator
tach signal, and the output
would be the tachometer gauge
signal. The circuit would ideally
change the frequency (it’d be
nice if it was tunable), and
output a signal to the
tachometer to give more
accurate RPM indication.In my
case, the tachometer indicates
a bit over 6,100rpm, while the
engine rpm limit is about 3,600.
Edward Stoker
Richmond BC, Canada
DAVID BERRY REPLIES:
Frequency conversion is
fundamentally a difficult thing to
do, particularly when the input is a
varying frequency as in this case.
I’m not aware of any off-the-shelf
Out of
range
Q
What’s the protocol if a
small boat is forced out
into unfriendly and unsafe
water by firing range patrols?
Can a skipper refuse to
comply if he considers that
his boat is unsuitable for
those conditions? On a recent
trip back from Calais via
Dungeness to Eastbourne, I
encountered a problem of this
nature with a patrol boat.
David Hedley, by emailGUS LEWIS REPLIES: Most
range byelaws prohibiting
access to the sea area within
the ranges do not apply to
vessels (not used for fishing)
passing through the sea area in
the ordinary course of navigation,
and remaining in the sea area
no longer than is reasonably
necessary to pass through. In
other words, the range byelaws
do not permit range patrol
boats to prevent yachts from
passing expeditiously through
the sea area in the ordinary
course of navigation.
I assume that the difficulty
encountered by Mr Hedley off
Dungeness involved the Lydd
range. The Lydd range by-laws
contain a provision permitting a
vessel to pass expeditiously
through the sea area in the
ordinary course of navigation.
The same by-laws request that
vessels pass south of the
Stephenson Shoal to avoid
disrupting the firing, but this is
not a legal requirement.■ Gus Lewis is Head of
Legal & Government
Affairs with the Royal
Yachting Association.components or circuits that will
do the job: however, it is possible
with an intermediate stage. When
I wanted a remote tacho output on
my own boat I used a frequency-
to-voltage converter (an LM2917
chip) and fed the voltage output
(which varies as the frequency
input varies) into a microprocessor
called a Picaxe. I then wrote the
Picaxe software to scale the signal
so that it would output a calibrated
value that reflected the input
frequency. If you have some
software skills you could go
down that route, which combines
simple electronics with fine
(maybe non-linear) mapping
between the input and output.
Otherwise, it should be possible
to use a frequency-to-voltage
converter followed by a voltage-to-
frequency converter with some
scaling device between them,
which may be as simple as a pot.
It would be worth looking at the
datasheet for the LM2917 or
similar chips, which often include
circuit designs for commonapplications. There are also
pre-built circuits on eBay to go
from f/v and v/f, so they may help.
Some things to be aware of:
firstly, the frequency range of
the alternator must lie within the
frequency range of the chosen
circuit using sensible size
components (it won’t work
with large-value capacitors, for
example, due to their high leakage
currents). Secondly, be careful of
the peak voltage, which might be
outside the range of the chip. My
standard alternator output pulse
peaked at no less than 17V!■ Regular PBO contributor
David Berry is a retired engineer
with degrees in mechanical
engineering and computer
science. He worked for a
defence contractor designing
equipment for naval vessels.Q
I keep a 7 .9m (26ft) Dufour on Lake
Windermere, and although I have taken
her out of the water some winters and stored
her on land for the last two years, I often
keep her on the water and sail in the winter.
The lake is fresh water and doesn’t seem to
affect the hull: the boat is 30 years old, but
there is no sign of osmosis. Do you think it
is ok to keep her on the swinging mooring?
David Freedman
By emailRICHARD JERRAM REPLIES: The general
consensus is that vessels kept in fresh water are
more prone to develop blistering than those insalt water. However, whether kept in salt water
or fresh, it is good practice not to leave vessels
in the water year-in year-out, but to take them
out at least every two years to allow the hull an
opportunity to dry out. For blistering to occur,
hydrolysis usually has to take place within the
laminate. In simple terms, this is where water
reacts with certain elements within the hull
which are susceptible to moisture. As your
Dufour is now 30 years old and isn’t showing
any signs of blistering, there is a strong
possibility that you will remain free of this
problem. However, allowing the hull to dry
out will be beneficial, and a course of action
I would recommend.Mooring on a freshwater lake