Need help with excessive motor heat

[slowsl]

Hey guys, I'm getting a very hot motor after about 3-4 minute runtime. I had thought that I was fairly conservative with the prop size, but maybe I am missing something.

Leopard 3650 2750kv
zippy compact 3300mah 4s 1p 35c constant (45 burst)
swordfish pro+ 120amp esc
octura 1938 prop
boat is an outrigger, with the prop shaft above the waterline by about 1/8".

It's not binding & I don't have a thrust bearing. The only thing I can think of is to check the cooling system, but other than that I'm lost. Anyone have any ideas? I'm new to rc boats, so maybe I've completely missed something important.
-MIke

[martin]

To much pitch with a 1.9 ratio prop + to high rpm (40700rpm unloaded) for a 3650 motor.

[Fluid]

That is still a pretty high Kv for a 4S boat - even a rigger - I would guess that you have too much prop for four minutes of running. Try one with less pitch to let the motor spin up. Did you actually time the runs or just guess? All riggers are not created equal, some have much more drag that others, and the Leopard is not the most efficient motor out there. "Prop shaft above the waterline by about 1/8 inch" doesn't help much depending on the rigger and where the "waterline" actually is.

I run a Castle 1415/1Y (2400 Kv) in my JAE21 on 4S with an H-7 prop, it averages under 100 amps on the oval topping out in the mid-70s, but I have never run it for more than the 90 seconds it takes to run 6 laps with mill. The JAE21 runs With very little drag but is quite stable. Not many FE riggers do that.



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[slowsl]

Okay, thanks for the info. It was just a guess on the time. I knew I was going to be running hot with the setup, was going for speed when I bought the motor. Everytime I've pulled the boat out after a run, the motor has been barely warm, but I must have been light on the throttle for those runs, it was mostly just straight shots, then slowing to turn around for another pass. Looks like I will need to watch how long I stay on the throttle if I want to keep the motor alive!

[NativePaul]

I used to run a similar setup but with bigger 5800mAh cells,and I couldn't get the 5 minutes runtime I needed for racing, so I am surprised your getting 4 mins with only 3300mAh and a high pitch prop is that a timed 4mins or guestimated? If it was timed was much partial throttle running included in the time? I ended up using an x438 cut to 34mm detongued and backcut to get me about 4:30 on the oval, if you want to stay with the high pitch props try an N940 which was the fastest prop I found and its narrow blades need no detongueing or backcutting, though cutting it down to 35mm lost me little if any speed and got the run time up to about 4 mins.

Why is the shaft of a rigger 1/8" above the waterline? Riggers should prop ride, if rear sponsons are fitted they should be set slightly above the prop just to keep the back end up in turns should it be necessary.


Edit. Well done, I just saw you answered my questions while I was typing them, I suspect you have about 2 mins of flatout runtime in that setup. It is hot indeed, take care of your ESC too, I was logging peaks of well over 120A even after cutting the props down and at 38-40mm I wasn't averaging far short of it, I expect a stock 938 will be pulling over 120A average with spikes around 200A.

[slowsl]

I used to run a similar setup but with bigger 5800mAh cells,and I couldn't get the 5 minutes runtime I needed for racing, so I am surprised your getting 4 mins with only 3300mAh and a high pitch prop is that a timed 4mins or guestimated? If it was timed was much partial throttle running included in the time? I ended up using an x438 cut to 34mm detongued and backcut to get me about 4:30 on the oval, if you want to stay with the high pitch props try an N940 which was the fastest prop I found and its narrow blades need no detongueing or backcutting, though cutting it down to 35mm lost me little if any speed and got the run time up to about 4 mins.

Why is the shaft of a rigger 1/8" above the waterline? Riggers should prop ride, if rear sponsons are fitted they should be set slightly above the prop just to keep the back end up in turns should it be necessary.


Edit. Well done, I just saw you answered my questions while I was typing them, I suspect you have about 2 mins of flatout runtime in that setup. It is hot indeed, take care of your ESC too, I was logging peaks of well over 120A even after cutting the props down and at 38-40mm I wasn't averaging far short of it, I expect a stock 938 will be pulling over 120A average with spikes around 200A.

I just looked at a video from one of my last runs and checked the runtime. I went exactly 3 minutes, then pulled it from the water to check temps, which were a bit high IMO (could only keep my fingers on the motor for a few seconds). I should have been able to squeeze another 30 seconds out of the battery had it not been too hot. I was pretty much WOT except a couple times that I almost hit a buoy. I really like the way the 1938 (72mm pitch) prop handles all around, plus I clocked it at 50mph without the boat even setup correctly. So I guess my idea was to keep a similar pitch, and turn down the diameter a little to keep rpms up & load in check. I may try the N940. I see octura made a 2337 (85.1mm pitch) and 2035 (70mm pitch) which I was thinking may work okay without even turning down the diameter, but I can't find them for sale anywhere. It's very hard to find a small diameter, higher pitch prop to fit a 3/16" shaft. I have access to a lathe though, no biggie.
Oh, I thought as long as the boat handles okay, you want just the bottom 1/2 of the prop in the water. I guess I did notice being up that high, I was getting the high frequency hops. I thought it was due to my battery pack sliding forward and throwing the balance off.
Thanks for all the help, I feel better knowing it's not something wrong..... now, time to test different props!

[Fluid]

You are going the wrong way in prop pitch. You are already going way slow for your setup, so either you are overloading the motor with too much prop and keeping the rpm down (most likely) or the boat is running too wet. With that Kv and prop you should be running in the 80s. The hopping is likely due to the high lift prop you are using and will both rob speed and heat things up. Screw on an m445 prop and get a baseline speed. I suspect it will be faster than your current choice, and will run a lot cooler.

What is the size of your rigger? A few photos would help, especially a side view of the transom/strut/prop.

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[martin]

Fluid I believe the rigger is the one you have posted remarks on under, Making an outrigger boat kit available for $35 shipped, need imput !. As been pointed out to the op the prop he is using is way to much & will cause to much heat on that set up.

[slowsl]

You are going the wrong way in prop pitch. You are already going way slow for your setup, so either you are overloading the motor with too much prop and keeping the rpm down (most likely) or the boat is running too wet. With that Kv and prop you should be running in the 80s. The hopping is likely due to the high lift prop you are using and will both rob speed and heat things up. Screw on an m445 prop and get a baseline speed. I suspect it will be faster than your current choice, and will run a lot cooler.

What is the size of your rigger? A few photos would help, especially a side view of the transom/strut/prop.

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I thought that larger diameter, lower pitch props were for heavier, more wet boats, and that small diameter, high pitch props were for hydros. I've tried a 1740 (42 mph), and an x442 (48 mph), but got the motor too hot after a few minutes. I have a few in my toolbox from my fathers 3.5cc nitro boat that I have not tried... (x642, 1747, 1750). Any thoughts on those? I will grab an M445 when offshore is back from vacation in a couple days and give you a report once I run that. Overall, from prop to front sponson is about 27".

Here is a quick video of one of my last runs, you can see how bad it's hopping. I didn't have that problem when the prop was deeper in the water, I will put it back down a bit.

and some pics

[slowsl]

Fluid I believe the rigger is the one you have posted remarks on under, Making an outrigger boat kit available for $35 shipped, need imput !. As been pointed out to the op the prop he is using is way to much & will cause to much heat on that set up.

Yes, sorry, It's hard for me to keep track of the forums/posts that I post in, this is the same boat. I started a new thread in order to get more attention to this particular problem, rather than posting in the original thread where there are only a limited number of people following it. larger diameter, lower pitch or smaller diameter, higher pitch..... this is my conundrum, lol. I don't want to spend a fortune buying up a bunch of props, but hey, if that's what it takes to get this design right, I'm all for it. I'm all ears for your suggestions. This has been an extremely fun journey so far, I've learned a lot, but yet I still know nothing.

[Fluid]

You have to match the prop to the motor, and a 1938 is not appropriate. The hopping is most likely due to the type of prop, not its depth. An x or m style prop is low lift and when set flat should stop the hopping. The rear sponson depth is questionable but may be okay.

Set the boat on a dead flat table resting on the sponsons. The centerline of the strut should be even with the bottom of the rear sponsons and flat on the tabletop. Set the CG 4" behind the front sponson ride surfaces. These should be close but each rigger design is different. Try a more reasonable prop with these settings and see how it runs.

Note that component quality matters for top performance. The Leopard motors are budget and run hotter than better designs. All the power in an FE boat is in the battery, battery quality matters. Some ESCs are better than others and will be more efficient. Just sayin'...




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[slowsl]

You have to match the prop to the motor, and a 1938 is not appropriate. The hopping is most likely due to the type of prop, not its depth. An x or m style prop is low lift and when set flat should stop the hopping. The rear sponson depth is questionable but may be okay.

Set the boat on a dead flat table resting on the sponsons. The centerline of the strut should be even with the bottom of the rear sponsons and flat on the tabletop. Set the CG 4" behind the front sponson ride surfaces. These should be close but each rigger design is different. Try a more reasonable prop with these settings and see how it runs.

Note that component quality matters for top performance. The Leopard motors are budget and run hotter than better designs. All the power in an FE boat is in the battery, battery quality matters. Some ESCs are better than others and will be more efficient. Just sayin'...




.

k, rear sponsons are 1/8" higher than front. Anyone care to suggest more props to try? Im going to order 3 or 4 tomorrow. I know leopard is a budget brand, but seems like it should perform a little better than it currently is. I was hoping to get a 50mph boat that isnt overheating after 4 or 5 minutes, but maybe I set an unreasonable goal. If I cant get the heat issue figured out, I may go with a better brand next. How would u rate the swordfish ESC's? I've also got the balance point 3/4" behind trailing edge of fin, so I will stick the esc in the trunk to get that corrected, it did seem to run wet on the front sponsons, especially around corners, hopefully that helps. Will update next wek with results and speeds. Again, thanks alot guys.

[jcald2000]

Try the 642 for 2 min.'s max!

[slowsl]

Try the 642 for 2 min.'s max!

I just noticed that I had accidently said x442, it was actually an x642 that I used. Again, here is what I've tried...

1938 (50 mph)
1740 (42 mph)
x642 (48 mph)
All run hot after a few minutes.

Do you suppose part of the problem is the battery pack? It's a zippy, and only 35c, and gets a bit hot after a run. Could the motor be struggling from the lack of discharge rate from the battery?

[Tylerfishsmith]

Any updates on this? I am having a similar problem with a 3650 @2750Kv with same esc and a X438 prop. Battery is 4s nano-tech 2250 65-130C discharge. Hull is very wet however, not an outrigger. Motor got very hot after only 2 minutes.