20k rpm and bigger prop the way to go?

[martinsson]

Hello, this is my first thread start here at OSE, the reason is that I wanted get you thoughts on something that has been on my mind for a while now.

I'm a part of a group of fun loving amateurs the meet up on a yearly basis to "compete" with homemade hulls, this is not at all that serious but of course winning is the game.

The rules are:
Max 1m hull length (drivetrain and rudder not inlcuded).
Home made (DIY hull), no limitations on material or hull design (mono, cat, outrigger etc.).
Electrical propulsion (although the number of motors to be used is not specified).
Max 4S. (the number of batteries to be used is not specified).

My contribution for the last couple of years has been a 750mm long home made aluminum rigger (race photos below), glued 0,5mm thickness, SSS 2970-2250KV, Flier 120A ESC, drivetrain from a dragon hobby sword (stiff axis), this boat is fairly light and surprisingly seaworthy since it has quite high sponsons and main hull, the prop's used has varied a lot and this bring me to my question.

Swapping the 37mm three blade dragon hobby stock prop for a significantly larger 51mm Grauphner two blade made a lot of difference to the better, the boat sprints better, is less nervous, and achieves higher top speed, but it is noticeable that I do not get the same top revs out of the engine, I understand this puts a lot of strain on the setup (ESC, engine, batteries) but it held up fairly well the one day I used it ( ~4 runs) and the temperatures did not seem excessive.

So now I'm thinking that I should take this (less revs more prop) further, and do it a bit more correctly (perhaps) when designing a new outrigger 1m long in aluminum.

I'm currently looking at some motors from SSS, both 4092 size category, one is 1350KV (4D) the other is 1800KV (3D), they would provide roughly 19548 and 26064 rpm unloaded respectively on 4S, both of these are significantly slower (in terms of revs) but at the same time vastly more powerful in terms of torque compared to my current motor (2970-2250KV) which would give 32500 rpms unloaded.

My thinking is to go for the extreme and try the 4092-1350KV (19548 rpm unloaded) hoping that it will not drop as much in revs once loaded, and pair it with a ~60mm high pitch 2 blade prop and get the speed that way, it makes sense to me go this route and I really want to try it out (please note, I'm an amateur), of course I need to step up in terms of ESC and drivetrain, perhaps go for a 180A and a 1/4" system.

Please let me know what you think of my reasoning, I want to try the lower rev bigger prop route, if you have any better suggestions (most likely :) ) please let me know, but I would like to reside in the ~20k rpm range.

All the best / Anders Martinsson

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