Hi, I am new to this hobby so please anyone helping will be well appreciated. I'm not sure how bad but I think I have a temperature problem.
I modified this relatively cheap boat (http://www.raidentech.com/1scspxrareco.html) and added a real rudder with a servo and another servo mechanism to deploy a fish line. The extra hardware and a base made of wood to hold the servos, receiver and an ESC, as well as a plexi-glass for cover to make it water tight, so this added quite a bit more weight. See pics.
The twin motors are supposed to be 380 type but I think they are a little bit bigger (I measure their length to be 44 mm, Mabuchi spec sheet have the 380 at 38mm). The battery pack that came with it is a 7.2v Ni-CD 1800 mAh.

So, in it's maiden voyage I continuously run the boat at full throttle for 8-10 minutes. The boat seemed to be going at 5-6 mph rather than the racer type speed of 20 mph or more. After opening the cover and touching the motors and the small heatsink on them, they were extremely hot, I could barely keep my finger on it more than 3-4 secs.
What surprised me more though is how hot the battery got. I could hold it in the palm of my hand but was very hot. The ESC got barely warm.
I've read that batteries and especially motors get hot that's why they employ water cooling, but what bothers me is the battery.

Has anyone experienced this kind of heating problems, are the motors too small for the extra weight, is the battery stressing to support the current demand, is this because it's a cheap battery and it's internal resistance is high. Do I need a better quality battery? How am I going to ran the motors and battery cooler.
Thanks in advance!!

Hi Yiannis,

Welcome to this forum!

You've joined the boaters that are modifying toy boats into 'real 'RC boats.

You have several issues:
- the boat is way too big for the two tiny motors, adding the extra weight isn't a problem, the fact that the props now run completele submerged, is.
- the ampdraw of these motore is already substantual, if you let them turn the (oversized) props under water makes the ampdraw surge up.

The motors will play along for a while but eventually overheat and lose magnet power, or/and they'll burn up.

You mentioned maiden voyage, which suggests, you did not break in the brushes prior to loading the motors heavily; the contact surface between the brushes and the commutor on a brand new motor are tiny, if you squeeze the current through such a small area, the sparking will cause the commutor to burn in and damage the motor, chances are the motors are alredy damaged beyond repair.
If you can look into the motor onto the commutor, check if the commutor has black marking and/or grooves, if so, the damage has been done.

The NiCad battery doesn't nearly have enough 'oompf' to power both motors and will heat up, as long as you can hold it, it's not a big deal, NiCads are rugged, but the thing connecting strips between the cells may glow and burn through, after all, it's toy-grade.

If you can find them, use smaller props to get the amp load down
Make the motors (specially the brushtabs) watercooled
Get a better battery.

Better still; think about converting the boat to a single 700 motor on 12-14 cells NiMH, or 4S Lipo.
A 700 motor is still very modest for this size hull, but infinately better than torturing small motors into submission...

Regards, Jan.

Thanks for the quick reply and suggestions.
I've looked thru the motor vent hole and the commutator does not appear blackened. I had ran the boat before the mods for maybe 5 minutes total, it run faster.

I'm waiting for a new battery pack 5000mAH, 6 cell Ni-Mh anyday from Ebay, a Chinese brand.
I heard that Japanese batteries are better quality than Chinese brands, is that so?

I'll go look for a smaller prop. But is it going to be able to propel the boat? I don't really care for speed, as long as its able to cruise between 5 to 10 mph I'm fine.

I was thinking to get two 540 size motors with which little mods would be necessary to mount them at the same spot, and add a bigger heatsink. Would this be a good enough fix to run the boat for 15 minutes at a time.
I would resort to better stuff on my next project, and thanks to a great forum like this you can get great help. But for now I'm pressed for time, I have to take the boat overseas to Greece next month and I should be thinking of ways to ship it there, not redesigning.

Does re-greasing the stuffing tube help a lot? Who knows how long the boat was sitting on some shelf.

Regards,
Yiannis

Hi Yiannis,

If the boat has been shelved for a long time, the grease may have hardened and will add drag.
Replacing it will help a bit in this case.
Do not pack the stuffing tube with thick grease, it will make the stuffing tube watertight, but also increase the strain on the motor, causing more heat.

If you can replace the 380 motors with 500 types, the larger motor will have more torque, but only if you choose the correct type; a high revving 540 will be bogged down by the submerged prop and draw a lot of current, building up heat.
A 5-pole low revving 550 would be a more economical choice, longer runtimes and still decent speed (not on the plane, mind you), depending on the prop chosen.

Due to the longer runtimes with the new battery, installing brushtab cooling is the way to go, taking away the heat where it's generated, at the brushes.

I've added a few pictures to show what I mean.
Brushtab cooling in combination with can cooling should keep most motors cool.

Regards, Jan.

Do you have a brand of motor that you prefer and some you don't? If two motors are wired in series rather than parallel does it cut the speed of the boat a lot?

Hi Chuck,

Most brushed motors originate in one or two factories that produce millions of them, often to specs of retail sellers.

I select a motor to fit with the boat in question, no particular brand.

Two motors wired in series usually doesn't work well, due to the differences in drag in the driveline, both motors will not run at the same rpm, the motor with the smoothest drive line will run the quickest, whilt the other motor may not run at all...

Regards, Jan.

Hi Jan,

I suppose one of these factories is Mabuchi Motor Co. They have a list of motors, see here http://www.mabuchi-motor.co.jp/en_US...ct/p_0304.html
However in the spec sheet they don't state the number of poles nor the number of turns as many ESC's call for. How do you figure all that. And then looking around on Ebay or other online sellers some say some don't, what number of RPM falls in the low revving ones of the 550 type?

Mabuchi shows two models (http://www.mabuchi-motor.co.jp/cgi-b..._ID=rs_555pcvc) one of which is RS-550PC at 7.2v, 15300rpm no load, is that ok.

Isn't driveline drag the same in either series or parallel connected motors. since the same current must flow in both motors at some point both motors will turn, granted not at the same speed.

On most of the rc boats, water cooling is the way to go, but for speed boats. What about my boat at 5 to 10mph, is that enough speed to pull in water and cycle it around a tube??

Will a heat sink like this one work just well enough.


regards,
Yiannis

Originally Posted by pompebled

Two motors wired in series usually doesn't work well, due to the differences in drag in the driveline, both motors will not run at the same rpm, the motor with the smoothest drive line will run the quickest, whilt the other motor may not run at all...

Wouldn't that happen more in parallel where all the electricity can go through the easier turning motor? In series it has to flow through both motors regardless of how fast either is turning. In parallel it can take the path of least resistance and totally bypass the second motor (in an extreme case).

Hi Yiannis,

Yes, the one in your link should work, or the 12V one on the same sheet, on 6 cells you have sufficient torque to turn a big prop (rule of thumb: Ø motorcan= Ø prop).
The other 555 types are torque monsters that would work well in a tug turning a relative big prop with a low pitch.

As for the motors in series: get two identical motors and do a test, you'll see what I mean.
Running idle without load you'll see/hear a difference in rpm, putting on a load will amplify that difference.

Fancooling is designed for cars, where there's a lot of air around, in a closed hull the heat dispersed will heat up the interior, only a little will dissappear via the contact of the hull with the water.
Watercooling is far more efficient.

If you position the inlet behind the prop and the outlet in the bottom as a slanted backward tube (which will create sucktion) you'll have flow in the cooling system as soon as the props starts to turn and the boat moves forward.

Regards, Jan.

Hi Jan,

Again thanks for your replies.

I actually gave you the link to the wrong motor.

Mabuchi shows two models (http://www.mabuchi-motor.co.jp/cgi-b..._ID=rs_555pcvc) one of which is RS-550PC at 7.2v, 15300rpm no load, is that ok

.
I was looking at the 550 motor and gave you the link to the 555, sorry. Those are 12 volt and much less RPM (4800 rpm) and current. This is the link I meant, (http://www.mabuchi-motor.co.jp/cgi-b..._ID=rs_550pcvc), This is much higher RPM and a lot more current for 7.2 volt operation.
I wander what sort of RPM, current, torque relationship I should look for 7.2 volt operation?

I got the new 5.3aH Ni-mh battery and I'll try it, in series and parallel motor combinations.

The inlet behind the prop should it be facing the prop or away from the prop, and can the outlet be above the water line from the side.

regards,
Yiannis

In series the motors will spin half as fast, but you'll get more runtime. In parallel they'll spin faster, but runtime will be reduced.

Indeed it's slower, verified today a little more than half as fast. Seems motors slightly cooler and higher capacity battery (5.3aH) slightly warm.

It'll be back to the normal speed if you wire them in parallel, but since it's higher voltage per motor those temps might rise again.

Originally Posted by yiannis2u

The inlet behind the prop should it be facing the prop or away from the prop, and can the outlet be above the water line from the side.
regards, Yiannis

Hi Yiannis,

The inlet behind the prop should face towards the prop, so thr thust of the prop pushes water into the cooling system (as the boat is relatively slow).

The best position for the outlet is under the hull, close to the motor(s) in the form of a slanted tube (aluminum Ø 4mm), facing backwards, this creates sucktion and helps to keep the flow going even an low speed.

As the props wil work submerged in your heavier boat (please confirm that they do work fully submerged now), the 5 pole motors I suggested, with the lower rpm and the high torque is a better solution in terms of ampdraw and runtime than a high revving motor like in your 'correct' link.

Regards, Jan.

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