RPM question

[rcboatmanwithkids2880]

Correct me if I am wrong guy's , though I believe what Floatnturd is trying to achieve , can be achieved by using more appropriate specs for F/E Boating use , such as what may be suggested in this thread . In the process of using the advised type set - up , more reliability will be obtained . Again , experienced forum members would have to know a number of variables , such as type of hull & length , to help you in your pursuit of speed - hopefully with reliability as your main aim or big $$$$ will be wasted. .....

Brad
Oz

[Boaterguy]

yes that is important, but either way 58K unloaded is way too much.

[rcboatmanwithkids2880]

58k = $$$

Brad
Oz

[Fluid]

An rpm range of 30,000 to 40,000 is really too high for most applications. Far better to be in the 25,000 to 30,000 rpm range for most users - prop efficiency will be better, wear and tear will be reduced, and the chances of over-amping damage is reduced. My oval race boats spin in the lower portion of this rpm range and they are plenty fast. IME the 35,000 - 45,000 rpm range is best left to SAW applications only, this is just not needed with most models.

Realize that just because you have a 1600 Kv motor on 8S does not mean you will be spinning 47,000 rpm on the water. Depending on the prop and hull, 40,000 to 42,000 is more likely under load. This is still too high for almost any model. When I set the P and Q Offshore SAW records last year, my rpm on the water was logged in the 39,000 to 40,000 range. I simply did not need more prop speed, I had room to prop up instead. Excessively-high rpm means small props and reduced prop efficiency.




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[sailr]

I always thought between 30,000 and 40,000 rpm was the best place to be. And around 30k for sport then closer to 40k for racing. Or have I just made that up? Sure I read it somewhere.

Absolutely correct, but staying closer to 30,000 rpm is the best goal. If you want to run 8S, then the KV for 30,000 rpm would be 1000KV. The lower KV motor can usually swing a bigger prop. Super High RPM would equal a tiny prop and probably blow the motor anyway. Very few motors are truely rated at 50,000 rpm.

What you want is brute torque to swing the best prop selection. Stay with lower KV and bigger prop!

Also, lower KV usually means lower AMP draw as well.

[martin]

Fluid what type of bearing/bushing do you us in the strut for SAW runs at 40000rpm. do you use the floating brass type ie Speedmaster style or something different. Thanks Martin.

[Fluid]

The strut I used had lead/teflon bushings in it. Most all of the SAW record holders at that event used L/T bushings. I don't see why a brass sleeve setup wouldn't work as long as it is well lubed. Realize that for SAW runs the motor is only 'on' for well under a minute total, heat buildup is not an issue.



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[floatn turd]

Thank you all for the help, I understand a lot more how all this crap works, I never new all this math was involved the FE world. In my 5th scale gas Baja, I mix, I go, I break, I replace.... thats it!!!


thanks again

[martin]

Thanks Fluid, yes the L/T bushings seem to have less friction than the brass floating bushes. The thing that was of concern with L/T at over 40,000 was excessive wear, How do you think L/T compare with ball bearings at high rpm or is the L/T more than good enough.

[Fluid]

IME the L/T hold up fine. I made 50+ passes at over 80 mph and the bushings seem like new. For oval running they will need to be replaced from time to time as the teflon coating wears off and the hole opens up, that is one reason why the sleeve bearings are used - less maintenance.

Ball bearings sound like a great idea, but they are so sensitive to damage that to maintain any small perceived advantage you will have to service and/or replace them a lot. Some seem to like stainless bearings - but the problem there is the balls are not as round as steel balls are. Ceramics have their own issues. We have so much power available today that the tiny amount of drag advantage ball bearings might have is of no consequence. Having to worry about one more thing (bearing maintenance) when trying to set records is never a good thing. KISS.



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[martin]

When you say sleeve bearings i take it thats the standard type L/T bushings. yes ive used many ball bearings over many years in boats & model ic engines & would never ever entertain stainless only carbon steel. The stainless ones are not hard enough.

[Fluid]

No, the sleeve bearings are the perforated floating brass kind. I've used them for many years on most of my oval racers with great success. They seldom wear out as long as you use adequate lubrication - and grease isn't it.

http://www.offshoreelectrics.com/pro...s-spds-014-bsf



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[martin]

So are the L/T used on SAW because of less friction & faster over using the floating sleeve bearing.

[Fluid]

I think that the L/T bearings are used because many do not trust the sleeve bearings at very high rpm. I have seen the latter work at over 35,000 rpm, but I believe that most SAW racers have more faith in the L/T product.



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