Motor KV Compensation

[riverman]

Building a AeroMarine Titan 33 Hull that I would like to be in the 55 -60 mph range. I know that the recommended range of rpm is in the 30000 range. Is this "in the water" rpm or the "free in the air" rpm? Do I compensate for the motor efficiency rating (e.g 10% )and/or the water slippage factor a (approx. 25 % for a mono hull)? In other words do I design the motor/battery (KV x battery voltage) to be about 35 % higher so as to bring it down to the 30 - 32 thousand rpm in the water? Thanks in advance.

[martin]

30000rpm as you say is unloaded rpm, motor efficiency & prop slip is taken off of the 30000rpm. If you end up with 30000rpm at the prop after factoring in motor efficiency & prop slip the motor would probably be at around 45000rpm unloaded.

[Scott T]

Prop slip is more about the boat speed due to the rpm, not the unloaded vs loaded rpm. So to aim for a loaded rpm range, just use the unloaded rpm and the expected motor efficiency. To calculate the boat's speed, factor in the prop slip and loaded rpm speed (with prop choice of course).

Use the calculator at the bottom of this page for rough calcs, it's quite useful - http://www.offshoreelectrics.com/conversion.htm

[steveo]

U should make a thread of your boat in the build section, I have been looking into the titan 40 I would like to see your project

[riverman]

Thanks to all. So I will just adjust for motor efficiency and not hull slippage.

What is current thinking on motor brand? Want to do this only once. Neu - Castle - Leopold - ? -

Advantages of going 4s or 5s? With 4s i could do 2 in parallel and be able to adjust balance easily (tuck them on either side out side rails) and of course get more run time, but use a higher KV rated motor to get target rpm. 5s will get me a lower KV rated motor. With 3 -5 minute run times, what is better for heat build up, knowing that prop has a lot to do with it.?

Thanks again

[Fluid]

We have five racing Titan 40s in our club - most run the Neu 1521/1.5D motor on 4S2P. Speeds are between 50 and 55 mph depending on the prop and water conditions. Much more speed than this and the Titans will chine walk. Most put the motor in back and the packs between the rails ahead of the motor.

If you just want a sport boat then 6S would be easier on equipment and give longer run times. A 1200-1400 Kv motor will give the same speeds on 6S but at considerably lower amp draw for less heat and longer running. Of course you can prop up for higher speeds but heat goes up and run time goes down.



(Kv x nominal pack voltage x prop pitch x 0.9 x 0.75)/ 1058 = boat speed in mph assuming the boat is trimmed out well with good quality packs and an appropriate prop.



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[riverman]

Thanks to all - I am going to start a thread over in the build section - I need advice all along the way.