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View Full Version : barr cut amp reduction.



TheShaughnessy
03-05-2012, 12:46 AM
i did some runs this weekend in my ms geico. have the stock motor with a t120. Tested 3 props a b225 a m445 and a x645 that I modded to a m series. the 225 pulled 90 peak amps and seemed the slowest which i believe is because the prop has too much lift and needs some cupping.

Next the m645 pulled 98 peak amps and seemed to be the fastest.

the m445 pulled the least out of the three at 85 peak amps and didn't seem far off the 645.

So the question i'm asking is... Can i expect a 10% drop in amp consumption if i were to bar cut say the x645 dropping amp consumption to around 90 amps. This will be my first attempt at a barr cut. Is there an approximation for how deep/wide i should make the cut.

Hopefully one of the prop gurus will be willing to share some secrets on prop modding. I have a pitch gauge on the way so hopefully you guys can give me some tips to get started.


Darin if you happen to read this. I'm getting 98 amps on a 645 i remember you saying you ran a 230 and it still wasn't enough so you went bigger. My watts up meter is limited as it doesn't tell me an average amp consumption but only the peak values. Boat seemed fast, but not fast enough.

I was wondering if you had any eagle tree numbers on what props in this range are pulling at wot. I feel like with numbers that high im gonna be using too much battery and the motor will be getting pretty warm with the california sun beating down in mid june.

TheShaughnessy
03-05-2012, 04:15 AM
Blank on purpose

Steven Vaccaro
03-05-2012, 08:03 AM
im guessing i'll be on my own with the prop modding. Seems this knowledge is too valuable for the forum.

I think its just impossible to tell you that unless someone has tried that exact thing on that exact setup boat.

siberianhusky
03-05-2012, 08:06 AM
Posted at 11:45 pm complaining at 3:15 am! People sleeping maybe? Only the middle of the night and a Sunday.

jcald2000
03-05-2012, 08:24 AM
Peak values occure at start up and are not an indication of motor load at speed. Get an Eagletree data logger, it is far cheaper than motors, ESC and batterys!

srislash
03-05-2012, 08:59 AM
I'm just signing on to this as I just started the prop mod thing as well.I do have an Eagle tree.Now if I could get a decent day.

Gary
03-05-2012, 09:40 AM
The only way to know is to do it yourself. Then you will know for sure.

egneg
03-05-2012, 10:06 AM
I prefer a back cut it over a bar cut - just my .02

TheShaughnessy
03-05-2012, 03:15 PM
I was a little anxious I guess u could say. Couldn't sleep, just kept thinking about props. Lol. I'll work on my patience skills.

TheShaughnessy
03-05-2012, 03:22 PM
I think its just impossible to tell you that unless someone has tried that exact thing on that exact setup boat.
Not looking for exact like, oh yeah you will drop 10 amps for sure. Just something like I generally shave off 5 amps or 15 amps. Just a general approximation.

TheShaughnessy
03-05-2012, 03:24 PM
The only way to know is to do it yourself. Then you will know for sure.. True to some extent but the right info or bit of advice has the potential to drastically reduce the learning curve.

TheShaughnessy
03-05-2012, 03:27 PM
Peak values occure at start up and are not an indication of motor load at speed. Get an Eagletree data logger, it is far cheaper than motors, ESC and batterys!. I know that is why I listed as peak and not constant. I was rolling on the throttle in an attempt not to spike real hard but I know eagle tree is needed. It is next inline. Just spent a bunch on other parts though So its a couple months out.

martin
03-05-2012, 05:29 PM
I beleive Barr cuts work better on ic engines than electric due the the torque issues at lower rpm which electric dosnt suffer from as much. Ive used Barr cut on electric, cant give you any figures other than it unloaded the motor noticably with more slip slower excelleration & slower top end. For me detonguing & back cutting works better on electric, if you want lower amp draw then i would think Barr cut would probably give that judging by the experience i got.

TheShaughnessy
03-06-2012, 04:53 AM
I believe Barr cuts work better on ic engines than electric due the the torque issues at lower rpm which electric dosnt suffer from as much. Ive used Barr cut on electric, cant give you any figures other than it unloaded the motor noticeably with more slip slower acceleration & slower top end. For me detonguing & back cutting works better on electric, if you want lower amp draw then i would think Barr cut would probably give that judging by the experience i got.

thank you for an amazing response. Now i can look further into back cutting since i'm not familiar with that.

Fluid
03-06-2012, 08:04 AM
The Barr cut reduces blade area, so it will reduce amp draw. I prefer back cutting too, but at the LA SAWs last year I needed to reduce blade area quickly without any tools other than a small Dremel - the Barr cut was easy to duplicate on both blades. It freed up the prop and gained me ~3 mph, setting the old Q Offshore record. Later discussions with Mark Grim confirmed that prop loading should be cut noticeably with this mod. But depending on the hull, reducing blade are could reduce prop efficiency enough to slow the boat.



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TheShaughnessy
03-07-2012, 11:31 AM
thank you guys that helps a bunch, i just needed a starting point.

ron1950
03-08-2012, 08:05 PM
hate to sound stupid but what is a barr cut?i know what a back cut is never heard of the barr cut

martin
03-08-2012, 08:30 PM
Its easier with a pic & im sure someone will put one on, basically if you take a round file & file into the lowest part of the back edge of the blade so you get a semicircle cut out of the blade.

ron1950
03-08-2012, 09:25 PM
ok thanks ive seen some square cuts like that....at least i can follow what ya all are talking about now lol

properchopper
03-08-2012, 10:11 PM
This is a Barr cut. Can be done with a small round file.


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A tip-cut (not pictured) is done with a scalpel and a Rabbi

ron1950
03-08-2012, 10:42 PM
A tip-cut (not pictured) is done with a scalpel and a Rabbi


ouch!!!!! more infomation than i wanted , thanks tony btw...ah no pictures? lol

TheShaughnessy
03-09-2012, 05:06 AM
hate to sound stupid but what is a barr cut?i know what a back cut is never heard of the barr cut

I know what a barr cut is but not sure what a back cut is. I remember there being a real neat prop article that briefly goes over some cuts and cupping but i haven't been able to find it lately. I'll try the tips section now.


Speaking of tip cuts... I think you guys are a little confused, were talking about props, not ........

egneg
03-09-2012, 10:24 AM
I know what a barr cut is but not sure what a back cut is. I remember there being a real neat prop article that briefly goes over some cuts and cupping but i haven't been able to find it lately. I'll try the tips section now.


Speaking of tip cuts... I think you guys are a little confused, were talking about props, not ........

A back cut takes away from the trailing edge. More so at the hub and tapers towards the tip.

osprey21
03-09-2012, 05:53 PM
hate to sound stupid but what is a barr cut?i know what a back cut is never heard of the barr cut
Barr-backcut

http://rcboat.com/prop.htm

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