RPM?s

[Jack6835]

So almost everything I see about boat shaft RPMs is 20 to 30,000 RPMs? So the new tracks and 41 I just purchased which runs 6S Lipo is pushing 40k rpm?s at the prop minus drag Coefficiency. So? which is it? I?m currently building a couple boats and trying to calculate motor size/kVA only to find out that the formula must be complete BS! This is pretty much the same exact set up as the Spartan. So now I?m wondering if I can actually run a higher KV motor since I?m using an SK 180 ESC and cooling isn?t an issue. Any tips, thoughts?

[Fluid]

The Traxxas boat is a compromise using the same motor for 4S and 6S. The problem is, it’s a bad compromise with excessive rpm on 6S for most beginners. This isn’t Traxxas’ first time making the same mistake, they do it as a marketing ploy to appeal to 1) the folks who don’t want to spend a lot of $$$ on batteries and 2) to those who are impressed with speed above all else. Yes, the boat will run 50+ mph…..but for how long? They get around using a compromise motor by using a tiny 42mm prop - dedicated cats of that size usually use 50-60mm props which are much more efficient. But put a slightly larger prop on it and you’ll pop that ESC.

For most uses targeting around 30,000 rpm is a good choice. The boat will be less sensitive to prop changes - a slight upsize won’t burn down the boat like a 40,000+ rpm setup will. Lots of newer folks seem enamored with high rpm, but with today’s props it isn’t needed to go fast. I was asked this question over a decade ago, and the answer hasn’t changed too much since then. For general sport running and longer run times, 20,000-25,000 is a good target. For oval racing around 25,000-30,000, and for SAW racing with just a few passes 40,000 or more. That said, I currently hold the NAMBA Q Sport Hydro SAW record of 95 mph set turning just 28,500 rpm. You don’t need high rpm to go real fast.