Forse non e' quello ma e' facile provare quindi tenterei con la regolazione della sensibilita' che potrebbe essere stata alterata accidentalmente nelle varie "manovre".
Lo strumento ha una regolazione della sensibilita' che imposta a quale profondita' (in pratica quanto tempo dopo il "Ping") deve iniziare l'incremento di guadagno per riuscire a rivelare echi provenienti da lontano. Se tale impostazione e' settata su livelli troppo alti (o bassi) si ottiene una risposta scorretta percependo echi troppo vicini non reali (turbolenze dello scafo p.es.) o percependo preferenzialmente echi lontani. E' probabile che non sia questo il caso ma vista la semplicita' dell'impostazione io ci proverei.
Fra l'altro ho dubbi che si tratti di disturbi poiche' in tale caso i valori letti dovrebbero essere quasi casuali, saltando da indicazioni molto basse a molto alte e non restando solo nella fascia alta, immagino anche (non e' stato detto ma stiamo parlando di barche a vela) che il difetto sia presente anche a motore spento e cio' escluderebbe interferenze elettriche (quasi tutte generate dall'alternatore).
Infine una bella misura di tensione sui morsetti di alimentazione dello strumento (proprio sui morsetti, non sul quadro o sulle batterie) non guasterebbe..
Dopo questo tentativo, se vano, penserei (purtroppo) a un guasto.
Il manuale di installazione (
http://www.bluemoment.com/manuals/nasacruiser.pdf ) dice questo:
SETTING THE SENSITIVITY THRESHOLD
Echoes received from near objects will clearly be larger than from more distant
objects. Echoes from keels or turbulence under the boat may be substantially greater
than echoes from the sea-bed and may cause the sounder to alternate between the
sea-bed and something more local. To overcome this problem, the Cruiser sounder is
fitted with variable swept gain. This reduces the sensitivity for local objects, progressively
increasing the sensitivity as depth increases The point at which the gain starts to
rise is called the sensitivity threshold. The sensitivity of the unit at depths below the
sensitivity threshold will be considerably reduced whilst above the sensitivity threshold
the unit rapidly becomes more sensitive with increasing depth.
The sensitivity threshold is factory enter to 0.0 metres. To view the sensitivity
threshold press ENTER The value can be altered by using the up and down
keys. The value is then fixed in memory by pressing ENTER.
E il manuale operatore (
http://www.cactusnav.com/files/clipper%20depth.pdf ) dice cose analoghe:
CHANGING THE GAIN DELAY SETTING
The Clipper Echosounder measures the depth beneath the
transducer by timing the delay between the transmitted ultrasonic
pulse (the “ping”) and its returning echo from the bottom, The echo
from nearby objects is much stronger than from more distant
objects, and so the Clipper Echosounder has swept receiver gain
sensitivity so that weaker (more distant) echoes are detected as
reliably as those from short range. However, reflections from
nearby turbulence or bubbles may sometimes be confused with
those from the bottom. To prevent nearby reflections from being
misinterpreted, sweeping the gain can be delayed until a pre-set
depth is reached. The delay is called the sensitivity threshold, and
can be viewed at any time during normal operation by pressing the
SET button. Pressing SET again returns to normal echosounding.
The sensitivity threshold can be changed if required to minimise false
detections at short range. The factory pre-set sensitivity threshold is
zero, but it can be increased in 0.1 m (0.5ft.) steps, depending on the
configuration of the unit, up to a maximum of 5m (1 6.5ft.). Press SET
to enable sensitivity threshold changes. The DEEP and SHALL
buttons then make the sensitivity threshold deeper and shallower
respectively within the available range. Pressing SET again to return
to normal echo-sounding also stores the revised sensitivity threshold
in memory, so the unit always operates with the new setting.