What boat is it and what do you want from it in terms of speed and or runtime?
I think .062" is a little big, racing boats this size we generally use 1.2mm if it needs any significant bend in it like a cat, a hydro, or a mono with a rearward mounted motor; but best for a mono is 1.5mm with the motor forward of the CoG, the thicker shaft is needed to eliminate shaft whip when there is little to no flex in the wire. .062 is 1.6mm which sounds like a marginal difference but it adds noticably to the wire's stiffness, and if you want to use it you will have to get the motor as low and as forward as possible to increase the radius of the bend. Flexshafts are not as efficient as a well set up wire, but an .098 flex will also work fine, and they are more tolerant of imperfect setups.
Don't pick a 3/16" shaft as 3/16" props will be too big for the boat. For maximum prop choice in the size range you will be using you want 1/8" if you are going to be balancing and sharpening your own BeCu props, or 4mm if you are going to be buying CNC alloy props.
If as it sounds like you don't have any rules to conform to, go for 3s, it is a bit easier on the electrics than 2s, and almost every model shop has a variety of 3s 2200mAh packs for parkflier planes. A 3s setup is also good in that you can buy a 2s pack and that backs off the speed a good chunk without having to do anything mechanical, which is great for letting learners have a go and build confidence/competence with much reduced risk of crashing and much reduced risk of damage if crashed.
Apart from on startup where an outrunners generally much higher pole count takes less angular movement before the ESC can work out which way round it is turning and starts making any meaningful torque, there is no practical difference in torque between an inrunner and an outrunner. I have raced with both and by far the biggest difference is in cooling, an inrunner can be water cooled much more effectively, and an outrunner can be air cooled much more effectively. As water cooling in the closed confines of a boat hull is more effective than air cooling, inrunners are usually the best choice for boats (maybe interestingly, the exception to that can be Mini and Micro boats, where the mass of the water cooling system can be greater that the extra mass added by an oversized ESC and Outrunner that don't need water cooling, thus resulting in a net weight loss). Motor KV, motor mass, and motor quality have a much bigger real world effect on torque than its type, pole count or wind. A motor between 60-90g is good for a boat that size, and due to the cooling differences I would go low to middle of the range for an inrunner or mid to high end of the range for an outrunner. 2800-3500 is good, if you want several minutes runtime go 2800-3000, for a minute or 2 of sprinting go 3000-3200, and if you only want a few passes go 3500. Props will be in the 27-32mm range for oval or general running but can go up to 35mm for SAWs.
Paul Upton-Taylor, Greased Weasel Racing.
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