Hi Coastal, We're all a bit crazy on Mayhem but that = enthusiasm
These motors fall into 2 groups. The JP Power 400 and Speed 400 suit 6 - 7v ish batteries, however, the remaining Mtroniks Vision and MFA RE385 suit 12v.
I think your choice of triple 400 size motors is ok but a decision on battery size/type will determine the motor choice and also the length of propshafts. The centre shaft could be shorter to allow the centre engine to be forward of the wing engines. The severe taper of the hull towards the transom/stern restricts space for mounting three engines in line using short 200mm shafts and it also means that the shafts will be steeply angled and this causes the props to waste thrust and lose efficiency. Its' important to keep the shafts mounted as horizontal as possible otherwise they are pulling water or 'wash off the bottom of the hull. Think of an optimum design e.g. an outboard motor where the thrust of the prop is horizontal. There's a lot to plan/think about but for me it's 60% of the fun/enjoyment.
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It's also worth thinking ahead. I started with sub 36" hulls and soon after decided that I liked the advantages of larger models, up to 50". Like you, my main interest is coastal forces craft WW2 and later. I like to transfer motors/esc's and batteries etc., between hulls - its a lot cheaper in the long run although the initial outlay is higher.
You mentioned scale size props - the real boats had quite large props with a steep pitch and revved at under 3000 rpm. Their marine engines were high torque slow revving water-cooled aero engines - max 2000 revs approx.