Model Boat Mayhem
Mess Deck: General Section => Beginners start here...! => Topic started by: sigfortunata on November 08, 2015, 03:44:28 pm
-
I have a 28" planing hull boat which in its initial set up runs 2 MFA torpedo 500 (ungeared) motors each with a separate ESC and battery turning a 35mm 3 blade prop. Batteries are NiMH 7.2V 42mah.
It runs well, planes at a "scale speed" but sucks power and subsequently doesn't run for long (10mins approx)
The chaps at my local club measured the amps under loads each motor pulls about 4+ at full power
They advise firstly downsizing to 400 motors but I'm not sure of which 400 size to buy as clearly the "cheap" MFA 500" were not the best choice.
I've seen JP and Mtroniks motors but I am confused as they seem to be directed towards aircraft.
Can anybody give me advice on what 400's would be suitable.
Please don't go straight to "buy brushess or get Lipo's", they are outside my current (no pun) budget.
Thanks
Steve
-
These are the nearest you'll get to a 7.2v Speed 400 these days. http://www.gliders.uk.com/Race-400-72V/productinfo/AERO7000.40/ (http://www.gliders.uk.com/Race-400-72V/productinfo/AERO7000.40/)
35mm props will be too big. No bigger than 30mm diameter; better with 25mm.
Dave M
-
Thanks Inertia I'll check out those motors. My next plan was to decrease the size of the prop, you think I should drop a full cm to 25 from 35?
-
42 milliamps? is that a typo for 4200.
-
oops guilty as charged.....4200 mAh...
have to say I'm still on the learning curve for electrics {:-{
-
Thanks Inertia I'll check out those motors. My next plan was to decrease the size of the prop, you think I should drop a full cm to 25 from 35?
Rule of thumb with brushed motors; Never use a prop which has a larger diameter than the motor case. Works for me.
DM
-
Hi, the motor change suggested was for 480 motors with a nominal 7.2v rating.
The motors you have fitted if my memory is correct( it gets worse just lately) lol,
We're MFA 500 rated with 4.5-15v, as you had car esc,s fitted I don't think you
Could run at more than 7.2v with them. With the 6v-12v nominal rating your
Rpm will be 7500-15800 so on 7.2v you will be on low revs even when flat out.
No doubt I will be corrected if I am wrong, there are some very clever people
On this site who know a lot more about running gear than me.
John
-
Oversized props will cause the motors to labour and produce heat rather than turning force. Smaller props will allow the motors to spin faster and push the water faster resulting in a faster boat that runs longer. A pair of smaller (preferably 2 bladed) props is probably the simplest and cheapest mod with a good chance of working.
Don't expect too much run time whatever you do - a small fast boat uses power and you can only store so much in a given hull size for the weight it can carry.
-
oops guilty as charged.....4200 mAh...
have to say I'm still on the learning curve for electrics {:-{
You and me Three reason I asked with a pair of 4200 in parallel and my 40'' PT boat 3motors pulling around 27 A total I get around 20mins on/off the plane cant see 4 amps only lasting 10mins . It was in the water when they measured it?
-
If you want power and Longevity I think you should consider a brushless/LiPo setup
With LiPos you will get more capacity/gram than Nimh or Pb batteries
And brushless motors are considerably more efficient than brushed
-
Nicads /Lipo battery weights
Nicads 1800mah=229 grams, 2400mah=301grams and 3300mah=339grams 7.2 volts.
Lipo 7.4 volts 2650mah=139grams( size 136x43x12mm).
Lipo 11.1 volts 2650mah=250grams(size 137x44x18).
3700mah=338grams(size 137x45x20).
The Lipo batteries handle far more amps out for a longer time than the Nicads.
As for brushless motors a 35mm(540 size) e.g. 3639-1100kv 800watt will turn 3 blade 37mm prop or 2 blade 40 to 50mm prop in a Spearfish or Swordsman for a speed about 30mph(1/2 to 3/4 of A HOUR) on a 5800mah Lipo battery.
I converted about 2 years ago, speed and run times are great.
Yes it was a big step but I would only go back old school for a tug boat.