Losing power after few mins

I have this setup:
36" fibreglass spearfish cruising hull
MFA850 torpedo brushed motor
4.5A/h 12v lead acid
Mtronicks viper 15A ESC
X52.5mm 2 blade prop

The motor is a recent upgrade from an older MFA800 which I had run it without incident for years. Essentially the 850 has twice the rpm, twice the current demand (10A Vs 5A) and similar torque, so I thought it would slot in without issue. Now, I only sailed it once but after a few mins on full throttle it started to slow down. If I stopped for 30secs or so and started up again it was fine, then started slowing again.

So, I'm wondering of candidates for this issue? The motor was quite hot, hard to know if it was too hot (didn't burn me, sort radiator temperature). Or could it be the ESC - I know it's rated not much above the motor at peak efficiency, and at the moment I don't know what current the motor is taking at full speed - could the ESC be restricting the current to protect itself? Perhaps try a smaller prop? Or could the battery be struggling to keep up with demand...any thoughts appreciated.

Thanks

Lead Acid batteries are not good at delivering high power for long periods. If the ESC was the problem I would expect it to stop the motor, then restart when the overload passed. For a highspeed boat an NiMh or LiPo would be better. SLAs are good for scale boats when the current demands are less and the weight saves buying lead ballast

Barrie

Mfa motors are hot running motors... there spec is also wrong!

A mfa 550 geared motor runs at constant 8amp.... however it takes almost 20amp to boot it around for the first few rotations till up to speed...

Action escs specifically we’re built around the 550 can size motors and recommend at least 20amp for continuous use....

Now that was the 500 size ... I have an 850 torpedo in a very very large old boat... it uses a 40 amp esc, on a 12v Lead acid, it’s another similar to action but I can’t recall the name this time of night...( I’ll check it in the morning with a photo)

So that’s why your cutting out the 15 amp esc you have is using its auto cut out safety feature due to overloaded current. Remember the stats on motor draw is when not under load... u physically have to double it to play on the safe side for under load rpm.

Change the esc to 40 amp and you’ll be hassle free. Money back guarantee on this one.

Thanks both. Reading about SLA, seems like the battery is under-specced, if the motor is drawing around 10A continuous that's over 2C which the battery cannot sustain. I've ordered a little remote energy meter gizmo to put in the circuit which records peak amps and power, be interested to see if it's near the ESC limit, then I'll decide what to do as a fix.

Thanks again.

You can check motor draw with a amp meter...

The lead acid being under spec that’s kind of a “meh” situation, but they don’t discharge like a lipo, so your can’t compare it to a C rating, leadacids are good for brushes systems same as nhmi’s but they have their limitations depending on use, I have a few boats still running lead acids, most are twin 550 geared mfa’s on 12v 3.2ah ( the slim line ones) I usualy get about 6 hours run time on those, i also have the graphner Wiesel, which runs 3x 400 brushes motors on 6v 4.5, and that I ran for an entire season without recharge the first year! I kid you not!!!

The 850 should be run with 30-40 amps the same as you run the T12’s as it’s the initial start up draw that’s causing your issues, I’m surprised you haven’t had the esc blow.... guess mtronics have upped their safety features in auto shut off... I’ve blown those with 400 motors in the past!

Let us know how you get on.

Aron

Energy meter finally arrived, wired it into the circuit last night and gave it a quick run. Great little device for £9, measures continuously but stores peak amps, minimum volts and ah and wh so you can get useful data on what your power system is doing. Turns out the motor hits 21A at full throttle (10A at 75%), so going to drop the prop size first and get that setup before I deal with the ESC or battery.

Going on my experience with a similar hull you will need a X45 prop, but the biggest problems are the motor and the battery. What you're trying to do with those is like asking a seven-stone kid to lift 300Kg after having had nowt to eat for a fortnight.
You'll get nowhere until you've consigned them to E-Bay or the bin.

Dave M

21 amp is still more than the 15 amp esc You have can provide.... hence the cut out... just swap the esc to a 25 and your sorted... but like said above if you go x45 then your gonna bump past 30 on draw... again my original advice was a 40 amp...

Spare props arrived today. Along with a 60A ESC out my car and a 3s lipo to compare so I did some testing. X52.5 was pulling 21A, X45 was 17A, X40 was 14A. S50 was also 14A. I then realised the S45 I bought was M4 not M5...so got an M5 S45 on order and a S40 to work out which will be best to sit the motor at about 11A max. But, on the S50 it was plenty fast enough, hard to tell if the 3S lipo vs 12v SLA makes much difference will compare when I can have a longer run with the correct prop. Motor was a lot cooler as well when I'd finished, unsurprisingly.