Does back cutting a prop work on a mono v

[martin]

Running a small v hull on an Octura metal x435 at around 42000rpm for short straight runs. Ive recently de-tongued the prop which has let it unload more, Is back cutting advisable on a mono deep v hull or is that more suited to hydro use. Also would rounding the tips be benificial rather than the pointed tips or is that also more suited to hydros. Thanks Martin.

[TheShaughnessy]

spooning the tips and cupping them a bit will give the thrust a more direct path rather then sliding off the edge and creating more lift /prop walk. Spooning condenses the thrust cone. Back or bar cutting will further unload the prop and I don't believe it is suited to any one hull type.

[martin]

I have used bar cuts before & found it to be slower on electrics, Its probably better suited to ic motors. Martin

[TheShaughnessy]

I have used bar cuts before & found it to be slower on electrics, Its probably better suited to ic motors. Martin

I've only seen gas guys with the larger props using bar cuts.

[egneg]

Bar cut and back cut are 2 different types. Cupping the last 1/3 of the prop will help to narrow the thrust cone but add a bit more load - this is where the back cut comes in to help counter act this. You can also just back cut the prop to unload it a bit more as well. I have heard conflicting information about bar cuts for FE boats and have done it but it's not to the extent of the IC boats.

I hope this really muddies the water (pun intended).

[Diegoboy]

Thread moved

[TheShaughnessy]

they are a different style of cut, but essentially serve the same purpose.

[andym]

Back cut will increase rake and reduce lift, can be very good on a well set up mono. Bar cut does allow props to rev up a bit more, very use full if your prop is on the limit temp wise for your application.

[graill]

Back cut will increase rake and reduce lift, can be very good on a well set up mono. Bar cut does allow props to rev up a bit more, very use full if your prop is on the limit temp wise for your application.

Andy, your a prop god and i bow to you under 99% of the time but back cutting a prop WILL NOT change rake. I am with you on the reducing lift but rake is built into the prop from the moment its created and unless you cut the blades off and reweld them at different angles (sarcasm there) you wont change it. The only thing back cutting will do is change the exit angle.

Chasten me if you must. :smile:

[andym]

grail if blades where straight that would be correct, have a look at a large x4 prop and cut one blade then measure the rake, let me know what you find.

[graill]

grail if blades where straight that would be correct, have a look at a large x4 prop and cut one blade then measure the rake, let me know what you find.

If there is no rake originally built into the blade you will not create any rake if you back cut.

If any rake does exists and you back cut you will still not change the rake angle that was already there.

I wont post anymore on the subject, this was discussed three years ago too. :smile:

[Fluid]

Rake will not be changed by any form of cutting the blade - it is determined by the angle the blades are mounted to the hub:

http://www.propline.com/Propeller-General-Information/Propeller_Terminology.htm
Rake
Rake is the degree that the blades slant forward or backwards in relation to the hub. Rake can affect the flow of water through the propeller.

I strongly recommend NOT to Barr-cut the trailing edge of a prop. That is a fuel boat mod and does nothing for an FE boat. Every prop I have with the Barr cut - even those done by experts - has cracked at the bottom of the relief.






.

[martin]

:iagree: Barr cuts on a FE boat does nothing at all except make it slower, Tried that on some carbon props just so as i didnt mess up expensive metal props. Will back cutting an x series prop add any thing on a small deep v mono. Ive already detongued & that made a very noticable difference in a positive way. Martin.

[Doug Smock]

I strongly recommend NOT to Barr-cut the trailing edge of a prop. That is a fuel boat mod and does nothing for an FE boat.
.

Interesting...................................

[egneg]

Interesting...................................

Very .....................

I have done it but 4mm is the most I have done for FE and as long as the radius continues back up the hub I don't see why it would crack.

[martno1fan]

I strongly recommend NOT to Barr-cut the trailing edge of a prop. That is a fuel boat mod and does nothing for an FE boat.






.

Hmmm care to elaborate on this ?,it doesnt make sense to me :confused:

[andym]

Ask Paddy if back cut and bar cut do anything, each application is different but it can makee a large difference.

[andym]

Fluid look at your posted links picture, if I was to back cut that blade it would change the rake angle, thanks for the pics it confirms what I have been measuring.
On bar cut;I have seen a prop gain 4mph from bar cut, this prop is now over two years old and gets ran allot, no cracking
But as I said each set up is different,

[wparsons]

How exactly do you measure the rake angle? If it's based on a line drawn halfway between the leading and trailing edge, then back cutting would change the rake. If it's based on a fixed point on the blade (certain part of the curvature, etc) then that point wouldn't move with back cutting, and rake would be un-effected.

To me it would be like thinking that trimming the trailing edge of a plane wing would change the sweep of the wing, which IMO isn't the case.

[martin]

My personal experience on Barr cut on FE was running my boat then barr cutting that same prop at the lake & running again so running conditions were exactally the same . The boat was noticably slower & you could hear the prop had much less bite in the water. Where as de tonguing i found worked really well on X props. Martin.

[martno1fan]

My personal experience on Barr cut on FE was running my boat then barr cutting that same prop at the lake & running again so running conditions were exactally the same . The boat was noticably slower & you could hear the prop had much less bite in the water. Where as de tonguing i found worked really well on X props. Martin.

That doesnt prove anything,fact is theres too many variables involved to say it doesnt work,each setup is different so to just say bar cuts dont work at all is just silly in my opinion.Just because it didnt work on your prop doesnt mean it wont work on many others.
Mart

[wparsons]

IMO the big difference between FE and IC boats and bar cuts is that if you over prop by a bit an IC boat it won't rev out enough, and potentially won't even get onto the pipe. A FE boat (as long as it isn't WAY over propped) will just draw more current but still hit the RPM.

You may not get more speed with a bar cut on a FE boat, but I bet the amp draw would go down a bit.

[martin]

It proved to me on my boat with my set up which is all i was interested in when testing that the Barr cut prop was slower on my FE boat. Thats not to say it wont run better on another boat with a particular set up. But for me personally de tonguing & cupping a FE prop worked much better. Martin.

[martno1fan]

Someone posted that bar cuts dont work on fe and hes yet to reply and explain to me why.It doesnt make any sense whatsoever to me and as the last poster stated even if it gets the amp draw down then its a well worthwhile prop mod.As far as i know this was the main reason for doing such mods to props,not to gain more speed as such but to help unload the motor easier.While it might not be needed as much in fe every little helps,even if it just makes everything run cooler sureley its well worth it.
Mart

[martin]

I agree 100% if your getting to high on amp draw & high temps then Barr cuts will make a difference because of the noticable prop slip that i got when testing. I wasnt suffering from to high amp draw or high temps so certainly wasnt looking for more prop slip & lower speeds. This was just one of many mods i tried along with de-tonguing & cupping & found the latter 2 to be what i was looking for ie more speed. The reason i tried Barr cuts was perhaps even with a bit more prop slip taken into account i hoped i might of got more rpm at the top end & increased my speed because of the higher rpm making up for loss in prop slip. But as i said it didnt work for me & ran slower which is not what i was looking for. Martin.

[NativePaul]

Have you tried doing away with the barr cut by back cutting it to the same point? depending on the size of the barr it would be an extreme back cut but it might work and of the barr wrecked the prop you have nothing to lose by trying, I've back cut many props but never tried a bar cut because it seems weak and I've folded or broken enough blades as it is.

[martno1fan]

Hey Paul nice website,very informative :thumbup1:,i too prefer back cuts to barr cuts.
Mart