1/4 Scale Cracker Vid

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Lou

having the charge figure it does look like it was pulling 30 amps which is a very good level for this motor on 24 cells. It is the current at maximum effciency - but from the motor sound and boat speed it isnt there yet. BTW output from cell tests and rated efficiency at hose amps loads is approx 620 watts.

I, would not go to 30 cells yet. Even at the amp draw you have now your draw will rise to approx 69 amps, give or take 1 or 2 amps.

IMO your priority of COG is correct. The hull is running very wet. The speed is down from where it should be if the motor was winding out on 24 cells.

Ie right now 30 amps (the current at the max efficeincy point ) is being drawn at well below the max eff rpm. If you can loosen the hull up you should be able to run faster without additional current draw.

Try the other props but the current prop might do you for a while given the current draw. Once you get it down a bit then start loading it up with extra cells, 2 at a time. Then recharge at the pond if possible to see how u are going with current draw.

BTW how are you cutting the prop - all the toungue off, the tips rounded the back edge tapered to a point like a wing instead of a wedge and the prop back cut from the hub to give more of a scimitar shape? The scimitar shapes are very efficient. But dont loose the cupping on the prop that is important
 
Thanks Andrew, My prop is cut as you describe but I think I could cut more back at the hub(more simitar). The back edge is still a wedge and I want to make sure I know what your talking about before I start Grinding. Do you mean the trailing end of the blade and do I sand the back edge(edge facing the bow) to create the wing shape.

Thanks

Pete
 
Pete

The edge facing the bow of the boat is the leading edge (where the tongue used to be). The trailing edge (facing the stern) is squared-off on a surface drive prop. You want to taper (sharpen) this edge the same as you do to the leading edge. All of the metal removal (leading & trailing edge) should be done from the back of the blade. The back surface of the blade is the surface facing the front of the boat (I know it sounds confusing) and is convex shaped. The face or front surface of the blade is the concave side and should be left untouched other than any polishing that you do.

I hope this helps to clarify matters some.
 
Thanks Doug, I will do this. It seems like a very simple change. Does it make a big difference in amp Draw?
 
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Pete

Subsurface drive props

Cutting down the blade area certainly should have a significant effect on the current draw. I'm not so sure that airfoiling the blade trailing edge would make a measurable difference although it certainly should increase prop efficiency.
 
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Lou just to add to Doug's comments

that square edge on the back is designed to pull air with it in a surfacing application

Under the water its trying to pull water with it so its just creating drag that edge is moving 53mph at the baldes mid point and over 106 mph at the tip. Smoothing the shape should help a little

Andrew

www.fastelectrics.com
 
Heres the link to a new vid. Its 19meg and the site seems slow. http://www.lousfastrc.com/Video/Big...d/morpfinal.mpg

I ran 26 cells today and hit 23mph. The motor was warm but I could hold my hand on it indefinatly. I had only let it cool down for about 5 min after a 24 cell run. On 24 cells I hit 22mph. Only 1mph more for 2 more cells. Will 30 just create heat? Should I be sticking with 24 cells and increase prop size?. I will try and figure amp draw tonight when I can time the whole vid.

I pulled the motor and the brushes are not even fully seated, the com is perfect and there is no indication of any heat problems. I am going to install cooling on the can for the next test runs. I should have an emporor buy the end of next week
 
I think given the speed I would stick with 24 cells and get speed up before i did any thing

The hull seems to be runnign wet comapred to the 1/10ths

Check the bottom for hooks and flatten it if there are any

Move the COG toward the transom

If you can use a shorter connector to shorten the prop shaft this should also help - the nose will go down as the prop moves toward the transom / deeper

BTW is the shaft log streamlined?
 
Hi Andrew, I figure I was pulling 38 amps using the cut down 646 what size do you figure will be best. I also have a v937 3 blade I could try. I want to see how the motor holds up at 60 to 70 amps for 90 seconds. I'd also like to see how fast It'll go to :rolleyes:

Thanks
 
With that much power ie 730 watts out - it should be faster

if you want to see how fast it will go - you need to loosen it up - these boats run aired out

running wet it will not go fast.
 
Hi Steve, I used Adobe premire and hacked as I went along learning. :D
 
Pete

now that is informative! What a fantastic development tool.

This is the first real onboard data I have seen so its worth its weight in gold

Were you using the cut down 646 or another prop?

Are u doing more runs with different props?

What cells were they and what stage of the run shown was that? The cell voltage seems low for 30 cells at 35 amps given what the dischargers are getting. Are any of the cells hot?

The hull is starting to loosen up which is good. The hull should run so you can see the skeg regularly.

Your prop slip is 56% (for the 646) that is rather high

Also I am wondering if you can induce more lift. - maybe some rocker in the hull would help or a wedge under the front of the hull to keep the minimum angle of incidence consistently higher?

Also what is the all up weight? As an alternative to getting lift up are there any areas u could reduce weight?

Looking forwar to seeing this develop. The data recorder is ceertainly a helpful tool.
 
Hi Andrew, I was using the 646. Heres the 2 props I have. Any input would be great. http://www.lousfastrc.com/props.htm

The short vid was the end of the run and there may have been a bad cell in the pack. I think the 462 prop should slip less.

Weight is around 12lbs with 30 cells. Reducing weight is not an option. More Power is :D The data recorder is helping to confirm my prior amp draw estimates and the data I'm getting from it is consitent. It is not waterproof so percautions have to be taken not to get it wet.

Thanks for the input

Pete
 
Pete

cool

well first i would check the cells and ensure there are no weak ones - the voltage is low but maybe thats just the end of the run - it is higher in the earlier part

The 462 prop should slip a lot less. But I would be concerned about it drawing more current and hurting the cells around a poor cell (if there is one).

If you cant reduce weight go for loosening it up. The hull should be out of the water - try and uncut 646 2 or 3 blader if u can get your hands on them

Try and keep the prop small to start with and work up slowly - can u get the graupner cf props a cheap way to get closer to the right size. just go up in size till the slip[ is down then start going up in pitch - or at least removing the low pitch part of props

The data recorder is excellent - cant wait to see more from it

IMO you just made it a must have for a lot of racers and modellers.
 
OK this is good

4thrunrpm32cell646prop.jpg


40 amps and 27mph :D

Here a comparison of amp draw between the big prop run and todays run

compare3rdto4thampsfull.jpg


Heres some Vid that was really fun making

http://www.lousfastrc.com/Video/BigCracker...ellrun27mph.mpg
 
Looks like the chop loosened the hull up a bit. Looks good. Which prop did you make this run with?
 

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