Johnnysplits

I was thinking of converting my boat from gas to brushless power. My question is, what is the reason for max rpm of 35K? I see that all the time in posts. Thanks.

*delete M i k e u p delete*

Good question and never read anything asking such a request. Thanks!

lohring

The right propeller rpm is a good question.  We have been trying to duplicate nitro engine performance with low cost electric systems so we match the nitro rpm.  However, what are the real constraints on rpm?

Power absorbed by a propeller is proportional to the diameter to the 5th power and rpm cubed.  Therefore increases in rpm need a much smaller decrease in diameter for the same motor power.  Because of that, the same diameter propeller will absorb close to the same power over a range of rpm.  Propeller efficiency should be related to Reynolds number, so small propellers should be less efficient.  Since our propellers are fully ventilated, cavitation isn't a limit. 

The best way to answer this question is to test a series of propellers with different Kv motors.  Careful data logging should help sort out the answer.

Lohring Miller

*delete M i k e u p delete*

kv of motor and lipo size x .80% for eff.

Johnnysplits

My boat has nothing to do with the question. Every post that I read about motors, prop sizes, lipo's all say that you want to shoot for a maximum of 35,000 rpm on the motor. Im simply asking why limit the motor to 35K?

srt10

My Feedback: (20)

not sure where your reading but there really is no "shoot for a max of 35k rpms" that I know of
what your wanting to do is really the first question IMO
Sport Boating
Club Racing
or setting up for SAW runs

sport boating, most keep things 28k and under
club racing like me, most my boats are setup at about 30k to 31k
I do have a Sport Hydro that I will try a setup that runs 38k with a smaller prop
and a gas size cat that I want to try a CC2080 motor on 8s 23k with more of gas size props, right now it runs 31k

and some run 45k+ for SAW runs

Johnnysplits

Ok let's go with club racing then. How come your's are set up to run at 31K instead of say 45K?

Got RPM

I too don't know where all these 35,000 rpm limits are found, but it doesn't matter. The true maximum rpm of most good BL motors is limited by the bearing quality. But that is not what we are talking about here. Max running rpm is determined by the motor Kv and the voltage. For most FE motors, multiply the Kv times the nominal voltage and take 90% of that value. This gives an approximation of the actual rpm on the water with a reasonable amp draw. So for the popular 2200 Kv motors on 4S: 2200 x 14.8 x .9 = 29,000 rpm. But rpm does not equal speed. I run an 1850 Kv motor in my P Sport Hydro and am just as fast or faster than the guys running 2200 Kv motors. Why? The lower Kv motor can swing a larger prop with more pitch. The larger prop is more efficient, top speeds are about the same.

Higher Kv motors are more sensitive to prop changes, meaning it is far easier to burn up a motor and/or ESC with a high Kv motor. I recommend lower Kv motors to newer FE racers because they are less likely to get into trouble, and if they are careful they can often equal the speeds of the higher Kv motors. Very high rpm props are usually not large enough to be efficient, so the upside is small compared to the downsides.

The right thing to do if you are a new FE boater is to follow the lead of others who have been successful. Do not try to reinvent the wheel, it has all been tried and found wanting. LOTS of gas boats converted to FE usually use 8S or 10S power with motors having Kvs around 750-800. They swing props between 60mm and 70mm in diameter, the lower the voltage the larger the prop. Several FE Hurricanes in my club reach the 70s running a 750 Kv motor on 10S with a pitched up x462. On-the-water-rpm is around 26,000.

So many gas boats have been converted to FE that there is really no reason to have to ask. Just do what others have done until you have more FE experience. Then you can try experimenting, although you will probably find that what you have done works pretty darned well with no need to try out-of-the-box ideas.




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srt10

My Feedback: (20)

45k is lots of heat in the driveline, and any vibration for 6 laps will tear things apart
P Spec class can only run stock motors and the UL-1 motor is the highest kv out for P Spec
My P Mono runs a big TP 4092 2100kv (I think) and that puts me at about 31k
some run a NEU 1900kv with a bigger prop to get similar speeds as my 2100kv

usually running higher rpms means more amps... more amps = less runtimes

Johnnysplits

Ok think I have my answers now. I saw someone on OSE that has the same boat as me running twin Leopard 4092 1390Kv , Turnigy 180a esc's, and x447 props on 6s lipo's. He says it does mid 60's so I figured to just go up a bit in Kv to get higher speeds.

srt10

My Feedback: (20)

try a different prop ( When you actually run it )
going up in kv on 6s in a big hull does not guarantee more top speed [:-]

Johnnysplits

Sounds good. Thanks!

srt10

My Feedback: (20)

yeah...
give us some pictures of your build

Got RPM

I think I understand part of the confusion. Many gas boaters seem to think that only one or two props will work on a particular hull. This is perpetuated on gas boat forums when folks ask "what prop should I use on an XYZ cat?". The answer is often interpreted as only one prop will work on that particular hull. So to go faster they need to spin that prop faster. This may feed the desire for higher prop rpm.

In FE we know that many props an be used on a particular hull, and with a reasonable motor we can prop up or down to achieve the performance we desire. High prop rpm can work, but it causes extra wear and is more finicky on prop choice. In Europe racers often use small props spun fast, but they use small hulls which cannot handle large props without handling problems. Any gas hull can handle big props with ease.


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Zenoahphobic

Got RPM I wish that was explained to me before going FE ,
I got the hype more Kv equals more spead initially, without being told that thrust diminishes very quickly going to smaller and smaller props, giving more revs but actual less boat speed to a point leaving a boat just sitting there with a screaming slipping and frothing prop.

 There is a need for good simple explanations like this for beginners/ convertors.

Johnnysplits

Got RPM- thanks!!

Old Sloppy

Gotta keep your guard up against hype .......there is a lot of it .....

Harry