JonD
08-31-2011, 04:24 AM
Seems that the question "which is the best motor for my boat ?" comes up time and time again on this forum, and everyone has their own thoughts and recommendations. I am currently going through the same exercise for my Genesis. Lot's of reasonably priced motors out there that appear to offer good reliability and performance, based on what I read on this forum.
However, seems to me (and please correct me if I'm wrong), first thing to find out is what "rated RPM" best suits your boat. I gather that some boats work well with a bigger diameter prop at lower RPM while others (like the Genesis, according to several postings I have read) work better with a smaller prop running at higher speed. Is this correct?
If so, then does it make sense to first decide on a target RPM? (i.e. rated motor KV x battery voltage) e.g. I have heard that 28,000 - 30,000 RPM is about right for the Genesis. I realise that this is only "rated RPM" based on theoretical motor KV and battery volts, and not necessarily actual achievable prop RPM, but it gives a starting point or basis for making decisions. Am I right so far?
This "target" RPM can be achieved by various combination of motor KV and number of LiPo cells (battery voltage). e.g. a motor with a low KV with say 8 cells, may result in the same RPM as a motor with a high KV and say 4 cells.
I guess, where I am leading with all of this, is how do you decide which way to go i.e. low KV motor and high battery voltage or high KV motor and low battery voltage? Or doesn't it make any difference?
e.g. If I look at Leopard motors, say in the 4074 range, the 1050KV with say 8 cells (about 29.6V) would give 31080 RPM, and the 2150KV motor with 4 cells (14.8V) would give 31820 RPM - almost exactly the same.
A low KV motor with higher battery voltage would result in about half the current of the high KV motor solution, which I would imagine would reduce overall temperatures, especially in the ESC. Also, reduced current would reduce the "work" that the ESC main capacitors have to do in filtering the voltage ripple caused by high current.
High or low KV motors cost around the same. Lower current rated ESC's are generally cheaper (as long as they can handle the higher voltage), but higher voltage 6 - 8 cell LiPo's tend to be more expensive than 4 cell LiPo's.
So, what is the general consensus? Or am I missing the point completely ? :confused2:
Cheers,
Jon
However, seems to me (and please correct me if I'm wrong), first thing to find out is what "rated RPM" best suits your boat. I gather that some boats work well with a bigger diameter prop at lower RPM while others (like the Genesis, according to several postings I have read) work better with a smaller prop running at higher speed. Is this correct?
If so, then does it make sense to first decide on a target RPM? (i.e. rated motor KV x battery voltage) e.g. I have heard that 28,000 - 30,000 RPM is about right for the Genesis. I realise that this is only "rated RPM" based on theoretical motor KV and battery volts, and not necessarily actual achievable prop RPM, but it gives a starting point or basis for making decisions. Am I right so far?
This "target" RPM can be achieved by various combination of motor KV and number of LiPo cells (battery voltage). e.g. a motor with a low KV with say 8 cells, may result in the same RPM as a motor with a high KV and say 4 cells.
I guess, where I am leading with all of this, is how do you decide which way to go i.e. low KV motor and high battery voltage or high KV motor and low battery voltage? Or doesn't it make any difference?
e.g. If I look at Leopard motors, say in the 4074 range, the 1050KV with say 8 cells (about 29.6V) would give 31080 RPM, and the 2150KV motor with 4 cells (14.8V) would give 31820 RPM - almost exactly the same.
A low KV motor with higher battery voltage would result in about half the current of the high KV motor solution, which I would imagine would reduce overall temperatures, especially in the ESC. Also, reduced current would reduce the "work" that the ESC main capacitors have to do in filtering the voltage ripple caused by high current.
High or low KV motors cost around the same. Lower current rated ESC's are generally cheaper (as long as they can handle the higher voltage), but higher voltage 6 - 8 cell LiPo's tend to be more expensive than 4 cell LiPo's.
So, what is the general consensus? Or am I missing the point completely ? :confused2:
Cheers,
Jon