Going from 7.2v to 8.4v...questions.

[tim slocum]

Ok, question, all things being the same, what kind of top speed increase should I see if I go from 7.2v to 8.4v. For the record, the boat is a Proboat Classic Runabout, the motor is a Titan, direct flex drive, Octura 430 prop. Also, what kind of increase would there be for 9.6v?,12v? Math is'nt a strong area for me, help me out. What effect would this have on runtimes,considering you stay with 3300mah for all applications. Discuss.

[patmat2350]

For smallish changes like this, it'll be close to proportional (which means twice the voltage, twice the motor/prop speed, and half the runtime).
But "twice" is not a "small" change... you'll run into other things, like how the prop needs not twice, but like eight times as much power when you double the speed, and because you'll start hitting cavitation limits, etc.

But 7.2 to 8.4 is small enough for a first cut estimate. So get you calculator: 8.4 / 7.2 = about 1.17; motor will go 1.17 times faster, or a 17% increase... and about 17% less run time for the same milliamp-hour cells.

Pat

[Ghost 2501]

here's 2 pics to compare, both of my najade

pic 1 Linlithgow Union canal, running on 9.6v

pic 2, Branston waterpark, running on 8,4

any help??

both occasions Espirit D'Atlantique was running at full power, you will probably notice that the hull is at a higher angle of attack on 8,4v as she hasn't been able to ride up on top of the pressure wave generated by the hull. on 9,6 she has a more level angle as the pressure wave has moved further aft, allowing the boat to sit on the wave, and so get on the plane.