44 Thousand RPM

[mm123521]

I have built up an Aeromarine SprintCat -- the small one - 29 inches long.
Running a Leopard 4074 motor @ 2000KV, on 6S.
That gives 44,000 RPM unloaded!
Using an Octura 442 prop - detounged, that should give speed somewhere over 70MPH.
I started out with a .187 flex drive, but ran into viabration and heating problems when I reved it up on the bench. Removed the .187 flex and installed a .078 wire drive. The viabration went away, but I still had a little heating of the stuffing tube.
Tried various lubercants, including luberplate, lithium grease, synthetic oil. I still had some heating, so I water cooled the stuffing tube.
I settled on a mixture of "luberplate" grease and Moble #1 oil. There is a resovour of lubercant in the adaptor (1/8 tube to the strut), so I only pull the drive to lube it, once every 3 runs.

I got about 8 runs with this setup and the motor, ESC, stuffing tube, and batteries are running cool.

The boat rides high and goes like heck! At 44,000 RPM, this thing "SCREAMS" when it runs.

I know that I am well above the normal RPM range, but everything is runing cool?

[mm123521]

Check out the water cooled stuffing tube.

Check out the motor mount. Its a block of insulating foam with a 1/8 inch plywood face. The whole foam structure is epoxyed down to the bottom of the boat. The motor mount is the flotation and the flotation is the motor mount -- real light, real strong, EZ to build, and dampens out the noise.

Running weight is 7.5 pounds with two, 3S, 5000MAH batteries stuffed into the sponsoons.

Mark

[Howard Lee]

I think high rpm's is the future in fe boats.
I'm not sure about reving it on the bench. The vibration is going to be different when the prop is under a load. If you are having a problem with heat its because of a clearance problem and not the size of the shaft. I'm running 55000 rpm's and a .187 shaft and don't have a problem with vibration or heat. I run teflon in my stuffing tube to cut down the vibration. I make the teflon into 2 pieces so i don't put it at the bend of the stuffing tube. I also grease my drive shaft every run and i clean the stuff tube after every 5 times out.

That motor has some torque on 6s so good luck with that small shaft.

[wparsons]

Wire drives can be run safely at much smaller diameters than a flex cable can, they typically run into problems snapping from too much of a bend rather than too much power (within reason).

[Fluid]

I was twisting 0.078" wire like spaghetti on 18 NiMH cells at 40,000 rpm years ago - good luck particularly if you go to larger props. Three-sixteenths cable is a far better bet for reliability and runs very smoothly in the water under load. Who cares what it does on the bench?

In reality you are not pulling 44,000 rpm under load, probably just under 40,000 but you'd need a GPS speed to get a closer idea. A Castle ICE controller would be even better. I'd guess you are closer to 60 mph, still a lot for that hull as it has a lot of lift with that wide tunnel.

Very high rpm as been used in FE SAW boats since at least 2002 when the 80 mph barrier was broken. The German JAGS team calculated they were spinning close to 48,000 rpm on the water for their 140 mph record, based on a sound recording of the prop noise. (No reliable data logging was available in 2004.) Most of the FE records through 2007 were set with prop speeds around 40,000 rpm or higher. Recently many record holding boats are using lower rpm and larger props for better prop efficiency, particularly with the larger boats capable of reliably running over 100 mph.

But then "lower rpm" is often still over 35,000. So the trend in the past few years is towards lower rpm and larger props. This makes sense as you cannot always count on flat water at record trials, and larger props will deliver more reliable power and efficiency in a slight chop. Same thing for oval racers, high rpm works if you race on flat water. Many do not, and larger props give higher speeds and better acceleration out of the corners.


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[6sHyper]

I have found the .187 flex will heat up really quick on the bench, but in the water its a different story, I run all mine with NO teflon liner, just an fyi

[keithbradley]

Where is it getting hot at? Its a result of friction so you have wire/flex hitting tube somewhere. It could be from an undersized tube, a sharp bend or improperly bent tube, or cable whip from an improperly supported tube or too much exposed flex/wire.

As far as running that high of RPM, it can be done, its just stressful on components and the more you stress them the more prone they are to failure.. I would say the most common thing for newer people to do in FE is try to run the RPMs that cars run. I did when I was newer to boating. I had a boat with a 1515-1y (2200kv) that I ran on 6s, and on a 120A esc! The setup probably pulled well over 200A, but it actually lasted quite a while. I ran an entire summer on that esc before it just quit. I have also had setups that lasted very short periods because they are overstressed. When you are running outside the specs of your equipment, you have to accept that it may all go up in smoke at any moment...or you may get lucky. You never know.
The question is, are you running outside the specs of your equipment? What esc are you running? Do you have any way to data log it so you know what kind of current you are pulling?
I run a m440 at just over 40,000(actual)RPM with a similar sized motor and its pulling about 3200 watts at full throttle. Different motor, different prop, different RPM...still, it should give you an idea of where youre at relatively.

[mm123521]

Ok, ran twice today. On the second run, the wire let go, Darn!
I got 10 runs out of that wire.
Just did a post-mortum on the drive line. The wire broke right where I had a small kink (an area of the stuffing tube with too much bend compaired to the overall curve).

Wire drives can be run safely at much smaller diameters than a flex cable can, they typically run into problems snapping from too much of a bend rather than too much power (within reason).
Today 03:53 PM

Wparsons -- I think you are right -- it broke right at the kink -- that was probably the source of the heat also.

I Ripped out the stuffing tube -- recurved it and the glue is curing right now. While that is curing, I'll build another drive shaft. Will be back on the water tomorrow. I'am gonna give this wire drive another try.

Howard, thanks for the info, If I break this next wire, I'am going back to the .187.
If you can make it work at 55K then maybe I can get it to work at 44K.

Keith, yah, I know the RPM's are very high.
When I went into this, I knew I might be busting some equipment, but wanted to see how far I could go with this hull. Thanks for the troubleshooting help!

Mark

[mm123521]

OK, I got another problem.

There is an instability in the hull at the very top end.
The hull will rock over onto one sponsoon then rock back to the other one.
This goes on untill it flips, sometimes ten times before it goes over.

At first, I thought it was play in the rudder, so I built a whole new rudder.
This one has zero play -- it is very "tight".

Now I am scratching my head as to where the instability comes from.

If I look at the trailing edges of the sponsoons, where the water exits off the hull.
These edges are not square, they are slightly rounded -- could that cause the instability?

I tried raising the strut -- same instability at the last 10% of the throttle.

Also, I have not yet adjusted the COG untill it goes wet. I been slowly moving the COG forward. I started at 30%, not sure where it is now. I have seen straight "blow overs" and I dont thing that is what is happening.

Look at the video, I dont give it WOT untill the last 10 seconds of the video.
Its hard to see but it rocks from one sponsoon to the other.
You can see the instability in the far streightaway -- I get off the throttle twice, before it flips.

http://www.youtube.com/watch?v=s7Tklt-1ulc

Mark

[Brushless55]

I was twisting 0.078" wire like spaghetti on 18 NiMH cells at 40,000 rpm years ago - good luck particularly if you go to larger props. Three-sixteenths cable is a far better bet for reliability and runs very smoothly in the water under load. Who cares what it does on the bench?

In reality you are not pulling 44,000 rpm under load, probably just under 40,000 but you'd need a GPS speed to get a closer idea. A Castle ICE controller would be even better. I'd guess you are closer to 60 mph, still a lot for that hull as it has a lot of lift with that wide tunnel.

Very high rpm as been used in FE SAW boats since at least 2002 when the 80 mph barrier was broken. The German JAGS team calculated they were spinning close to 48,000 rpm on the water for their 140 mph record, based on a sound recording of the prop noise. (No reliable data logging was available in 2004.) Most of the FE records through 2007 were set with prop speeds around 40,000 rpm or higher. Recently many record holding boats are using lower rpm and larger props for better prop efficiency, particularly with the larger boats capable of reliably running over 100 mph.

But then "lower rpm" is often still over 35,000. So the trend in the past few years is towards lower rpm and larger props. This makes sense as you cannot always count on flat water at record trials, and larger props will deliver more reliable power and efficiency in a slight chop. Same thing for oval racers, high rpm works if you race on flat water. Many do not, and larger props give higher speeds and better acceleration out of the corners.

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good stuff

[Jacked1]

From the pics and the sound of the motor in the vid I would try lowering the strut some. The prop sounds like it is porposing so I am thinking the difference in rpm from when the prop is in and out of the water is causing torque roll. I am prolly wrong but might be worth a try.

I have always loved the sprint cats!

[Fluid]

This typically happens when the prop is run too deep, lifting the back of the sponsons and causing the instability. But it could be the strut angle, or several other things. What did you change between the last time and now? A pic of the transom from the side would help in the diagnosis.



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[Howard Lee]

In racing all around the world they are pushing for rpm's for a reason.
The smaller the prop the less of a load your putting on the motor and drive. Big prop big loads on everything. So the more rpm, the smaller the prop the faster it will go. Also you can run a smaller prop deeper in the water with less power lost.

[mm123521]

Fluid:
Its all in pieces now, so I cant give you a good side shot of the transom.
I did have a lot of negative in the strut, for todays run, 2 or 3 degrees at least.
Also the rear end of the strut was slightly below the sponson level.

What you say makes sense. I got the loctite curing on a new driveshaft right now.
It will be ready for another run tomorrow afternoon. I'll put the strut at zero degrees for the run tomorrow.

When I observed this before, the boat was very light on the water.
It looked like 95% of the boat was out of the water.


Thanks!!
Mark

[Fluid]

Flattening the strut should help, as may raising it. The "x" props are low-lift props but they still have some lift. They do not like to be run deep on most cats, the result is lifting of the transom and overall boat instability. I have de-lifted some "x" props but on most cats it is best to raise the prop a bit to settle the transom into the water.

The Aeromarine cats will run fastest when they remain in contact with the water but are running on the back of the sponsons. Most of the time this means running the centerline of the prop above the bottom of the sponsons. See my avatar photo, that 34" cat is running on the back of the sponsons very fast. Different hull designs will need different attitudes, but the AM boats seem to like this one.

Remember that you are running an electric motor, not a fueler. Fuelers gain power by increasing motor rpm: Torque = rpm x average effective cylinder pressure; HP = torque x rpm / 5252. But that is not how electric motors work, their max torque is at near zero rpm where amp draw is highest. As rpm increases past a certain point efficiency and power drop off, largely due to 'commutation' problems. The ESC can't keep up - and internal electrical losses climb, basic physics.

40,000 rpm is high rotational speed, but it's doable if you realize the limitations...stress on components, high amp draw until the rpm is reached, and sensitivity to prop sizing and boat setup. These latter make over-amping the ESC and packs relatively easy. Just be careful and have fun .




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