Lots of good responses here.
In concern to the variables remember that in any equation the number of variables is very important to its complexity, you said "these variables are fairly small in magnitude" and individually that is true yet together it becomes much more complex with prop dynamics, load conditions, heat dissipation, current limits, voltage fluctuation and let's not forget current (ocean) and wind conditions and even life that partakes of your hull and prop that makes understanding changes to a system difficult. (I did not even list all the variables involved)
One thing that I have noticed with my boat is that if you over load the motor by using a prop with too much pitch, your current usage goes up and your speed stays the same where if you have a prop with less pitch, not overloading the motor, efficiency plays a much larger part in the power and speed equation. Same speed less current usage with an optimized size and pitched prop. Even a 2kw motor at 24v will only draw 60% of current limit at speed in calm conditions, most electric motors should.
In going from a 5kw to a 10kw you will not gain anything but efficiency if the 5kw moves your boat at its hull speed. If it does not then the 10kw, depending on power conditions proper for the motor, may gain you some speed but only by a small increment. It all depends on what you feel you need I guess. If your 5kw moves you over 70% of hull speed great but moving to a 10kw will only decrease a few losses and increase your speed by a few percent. If your 5kw moves you under the 70% line of exponential power usage then you will see greater benefit moving to a 10kw if all other conditions remain the same but you could see the similar changes if you optimized your prop (if it were causing slip or some other loss in propulsion) so as others have said, it depends on your system and how it currently performs.
Weight of the motor may or may not be important and that depends on your boat and what you have on it… Depending on the motor involved doubling the power it can produce physically should double its weight so again it depends on your boat and what it can carry as well as other systems like reduction and cooling that is standard to the motor that add weight.
Just remember the ocean always wins, keep safety in mind,
Kevin Mannell
General Manager
Advanced Marine Electric Propulsion
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