Kt value in FE calc ?

[sporthydrofan]

I did a search and did not find any threads on it...

what exactly does this value in the motor section stand for?

I'm assuming you take this value and multiply it by how many amps the setup will be drawing, then divide by 16 and that is the max weight, in Lbs, that the boat should be under...?

does that make sense?

[m4a1usr]

KT is the RPM/ Torque constant. You can find the KT for a particualr motor by dividing 1355 by the KV. So 1355 divide the KV= KT. And its a constant for all motors regardless of windings/magnets/ motor design.

John

[sporthydrofan]

can this value be used to determine the max weight that the boat can be per specific setup?
FE does not have an option to enter the boat net weight... and what is the In oz/A?

so for instance .60 in oz/A means that it moves .6 oz 1 inch per amp?

[m4a1usr]

Too many variables in hull designs, losses and what equipment being used to define anything accurately. Thats assuming one wants to know what will be a safe setup.

There is no such formula that I am aware of that one could come up with to define weight/ hull size/ speed/ for a given horsepower motor. Theres some safe assumptions that date back to nitro power only days. Based on cu. in. motors but still relative to horsepower output for todays FE builds.

John

[m4a1usr]

I'm on a computer right now that does not allow my to see FE Calc nor install it so I will have to pass on answering that question untill later. The ole brain aint had enough coffee this morning yet. Maybe some one will chime in though who knows.

Have you looked at this thread? http://www.forums.offshoreelectrics....ead.php?t=4159


John

[sporthydrofan]

any one else use FE calc and figure out a relation to drive train and weight limit or size it can push? for example a feigao 380-3500kv motor with 3S voltage and 435 prop can push up to a 27" mono hull... or 1.5lbs boat.. and does the "In oz/A" value have any bearing or relatin to maximum weight .. any insight would be greatly appreciated

[ozzie-crawl]

no read johns post #4

[sporthydrofan]

no read johns post #4

he did mention horsepower relative to boat size... is there a way to calc horsepower from the data fe calc provides? and if so .. is there a chart of HP vs. Boat size or weight...

ozzie..

[ozzie-crawl]

i maybe mistaken but i thought thats what john meant,there are to many variables with boat sizes,weights,performance to add that into FE calc.
i was only thinking a couple of days ago about power to weight ratio of our boats.
just as a quick calculation and i hope this is rite.
looking at say my tunnel hull it weighs 2.5kgs (5.5 lb) its average amp draw is 65 amps
on 4s, so thats around 962 watts or 1.29 hp
so thats a power to weight ratio of about 1/2 hp per kg of weight or 2.2 pounds. now this is just a rough calculation of its average amp draw but if its a 29" hull with a power to weight of 0.5 to 1
then maybe it could be worked out that a 40 inch hull weighing 5 kg would need
to produce a average of 1800 watts to get the same power to weight ratio

[sporthydrofan]

i maybe mistaken but i thought thats what john meant,there are to many variables with boat sizes,weights,performance to add that into FE calc.
i was only thinking a couple of days ago about power to weight ratio of our boats.
just as a quick calculation and i hope this is rite.
looking at say my tunnel hull it weighs 2.5kgs (5.5 lb) its average amp draw is 65 amps
on 4s, so thats around 962 watts or 1.29 hp
so thats a power to weight ratio of about 1/2 hp per kg of weight or 2.2 pounds. now this is just a rough calculation of its average amp draw but if its a 29" hull with a power to weight of 0.5 to 1
then maybe it could be worked out that a 40 inch hull weighing 5 kg would need
to produce a average of 1800 watts to get the same power to weight ratio

now we are talking!

so basically by what you presented here in USA we are looking at 1/4 HP (175 watts) per Lb. of boat... if you are under that and your at the max efficiancy levels on fe calc then it is safe to say go up in size on the motor (540 to 580.. or L to XL)

all we need now is to have some others chime in and confirm this basic theory of power/weight relativity..

thanks ozzie

[ozzie-crawl]

i havent looked greatly into it but if i use max amps instead of average it jumps to around
0.76 to 1
this boat runs low 40 mph area but in comparison my cat which is the same size and weight has a power to weight of 0.92 to 1 and runs around the mid 40s so slightly faster

[sporthydrofan]

ill post a new thread to see who else can chime in this is very interesting.

[ozzie-crawl]

good idea, can you ask about torque as well as this is some thing i cant get my head around with electric motors, some say that if 2 motors produce the same watts then performance is the same regardless of poles,rotor mass etc.
this to me sounds wrong

[Xfactor]

KT is the torque constant. Ive never seen the above formula for deriving torque constant or kt. You find it using this equation (KT)(KV)=1 so to find the kt you would divide 1 by the kv.

[Xfactor]

good idea, can you ask about torque as well as this is some thing i cant get my head around with electric motors, some say that if 2 motors produce the same watts then performance is the same regardless of poles,rotor mass etc.
this to me sounds wrong

Whoever said that is wrong wrong wrong