Sailing Instrument cables extensions?

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Hi, all,

I need an advice... Trying to install a Navman 3100 Multi on our new old boat. Problem is that both cables supplied with transducers are a bit short - about 5 feet. Electrician I am dealing with reluctantly proposed to insert pieces from standard cables of corresponding quality, but he is worried about double-splicing the wires. One of the wires is a coaxial RCA cable, the other is 4-wires shielded cable... To get an extension cable from Navman is apparently next to impossible in USA. Any opinions? Did anybody do something like this?

Thanks in advance!
Stan