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        Discussion Highly accurate useless information (my 1930 Dumas Chris Craft.)

#1 KiwiDavid Feb 25, 2012 10:31 PM

Highly accurate useless information (my 1930 Dumas Chris Craft.)
 
Hi All,
Being a nice day I decided to take my (nearly finished) 1930 Dumas Chris Craft to the pond to check it's performance.
I have a GPS widget I designed some time ago to log max speed, avg speed, elapsed time and distance traveled. Battery used was a 2000mA.H, 11.1V LiPo.
Max speed 13.4kph (looked fast but a tad short of hoped for speed)
Duration 38 mins (usual somewhat heavy foot but also some cruising)
Distance covered 4.04 Km

I was impressed with the run time compared to my other boats with a 12-15 min run time, even if they are somewhat quicker at around 18-19kph.

The motor I'm using has the following spec-
No load rpm 11800 at 12v, 0.8A
Stall torque 2.7kg.cm at 26A
Max power 79W
At max efficiency - 43W, 9800rpm, torque 0.42kgcm, 4.7A
Prop (currently) Octura Y543P Diam 1.34". Pitch 2.05"

While the boat planes nicely I do wonder if the speed is just a bit lame. I have a spare Graupner Speed 500 which will drop in and I know this combo will drive me to around 18kph but only for around 12 minutes.

Should I leave well enough alone and enjoy the scale look and long duration or cater for the (elderly) boy racer in me?

Cheers,
David

#2 CaptCB Feb 25, 2012 11:30 PM

I will take run time, over speed, any day. I want to enjoy myself, otherwise, I would get into some sort of racing, and spend CUBIC DOLLARS. CB

#3 jeepers1940 Feb 26, 2012 12:56 AM

What he said.

#4 KiwiDavid Feb 26, 2012 03:05 AM

Hi,
Thanks for the replies. I think you're probably correct and run time is important for a scale boat.
I may drop the prop down a step in pitch just to see if revving a tad higher may help. Based on my other boats measured speed and the motor data I have on them this boat should have been nearer 20kph so obviously I have a lot more learning to do to get things right.

Cheers,
David

#5 der kapitan Feb 26, 2012 08:09 AM

David, be happy with the setup you've come up with---.

#6 Kcal Feb 26, 2012 08:13 AM

You could also switch back and forth as your mood dictates?

#7 KiwiDavid Feb 26, 2012 10:52 PM

Hi,
You're right. I should be happy with what I have and I can always switch the motor for added wow factor.
I guess I wanted to get an idea as to what was a reasonable speed for a 1/8 scale runabout. I've watched a lot of videos with inflated claims of speed - "30mph" boats being passed by ducks.
If I hadn't measured the speed I'd be happy - the look and the duration are fine and it's just the number that looks slow.

Cheers,
David

#8 woodybob Feb 27, 2012 03:22 AM

The 1930 Dumas runabout is a heavy one. Here’s a video kmot did of my 1930 C~C. Kmot introduced me to the group here. My runabout went through several props before I got a Raboesch Brass Prop. Not as fast as the old Octura props, but the runtime makes up for any lost speed.

#9 Sharky1dk Feb 27, 2012 05:02 AM

????? no pictures????

#10 frankg Feb 27, 2012 05:20 PM

:)Speed in the Dumas 1930 - 24ft runabout can be obtained but with big power and battery capacity. The propeller used in this model was one of the old Dumas cast bronze props, 1 3/4” diameter, 2” pitch.

http://www.rcgroups.com/forums/showthread.php?t=437635
;)

#11 KiwiDavid Feb 28, 2012 01:59 AM

Hi,
Those are impressive models and no doubt the standard by which all others will be judged. A tough act to follow!

I have uploaded a very short clip of my boat in the video forum section which I didn't know existed until tonight. I had never scrolled down there but did click on the video of woodybob's boat on the sidebar the other day - very nice.

I have the bronze Dumas prop and also a BeCu prop OCY534 which I'm too scared to process after dire warnings from a friend.
I'll drop the pitch down and see if the increase in revs can give it a slight improvement. If nothing else it should further improve the battery life as I move further away from the maximum power and back nearer maximum efficiency.
I can model these things in Spice (full electrical and mechanical modelling) but things like hull drag and air resistance are unknowns. I need a full wave tank and wind tunnel with lots of data acquisition.......or I could just go to the pond.

Cheers,
David


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