From memory the centre prop was used in harbour or for station keeping at sea. when required the "wing" engines were started. I remember many years ago watching one leave Yarmouth harbour on the isle of Wight and the sound was incredible.
This Sea King was bought me as an anniversary prezzy over 40 years ago and was run up to about 11 years ago when it was stored away in the cellar. Unfortunately we had a flood and as the boat was wrapped in a binbag it could not float out and all systems were wrecked. I now intend to re-fit. As you can see she needs new rails and external fittings.
[Score: 5/10] 30" SEA ROVER Capable of 3mph and a runtime of 20mins Single Propellor (3 Blade 30mm) Direct Drive to a Mabuchi (3 Blade) Powered by Lead Acid (6v) 2Amp/h Batteries Controlled Through N/A (5Amps) ESC - Comments: Model was Sea Rover, built about 50 years ago with my father on the living room floor. Power was a Mabuchi 6/12 volt running on two PJ996 lantern batteries or two 4.5v bell batteries wired in series. Well before R/C. Finally gave ot aay to neighbours son and switched to aircraft for 40 years. Now living in Portugal and glow fuel hard to get so changing back to water craft.
Your speedboat is an "Intercepter 650" and came in two versions, Ready to run with installed radio gear or as a bare bones and fit your own running gear and radio. I have the ready to run and has a decent turn of speed.
As stated, the results are saved in the TX. From experience with R/C aircraft you would get positive results by disconnecting motors and switching on both units(unless you need various throttle settings)in which case make sure the props are clear of anything. That way you can see the results before saving.