detongueing/backcutting

[glenng]

Hi all, Dug an old scale out of mothballs after 20+ years,made a few changes and tried to run the boat at 4500 ft elevation(last run at 75 ft elevation). The boat used to launch easily with a X457/2 at the lower elevation. The first try @4500 ft. with a X455/3 the boat would bog down and cruise at about 5 mph. Would detongueing or backcutting the props make the boat easier or harder to launch? Thanks, Glenn

 

[anthony_marquart]

These are my opinions, not facts,.. Back cutting will unload the engine making it easier to launch,.. tongue removal can let the props turn higher RPM at the top end.

You might check the bearings in that motor if it was in storage too..

 

[glenng]

These are my opinions, not facts,.. Back cutting will unload the engine making it easier to launch,.. tongue removal can let the props turn higher RPM at the top end.
You might check the bearings in that motor if it was in storage too..

Thanks Anthony, Had the motor apart and it was in excellent cond.. I've seen backcutting done two different ways. 1. radius cut just outside the hub. 2. tapered from hub to tip-Any idea which works better?

 

[Russell Bear]

Try this...

http://rcboat.com/prop.htm

 

[glenng]

Try this...
http://rcboat.com/prop.htm

Thanks Russell, This is the article that got me wondering about back cutting/removing tongue from props in the first place-just couldn't find it again. I have a few extra X455/2 props. I'll try backcutting/rebalancing one, leaning out the needle a bit and see what happens Sunday. Thanks, Glenn

 

[Josh Stollfuss]

I bet the increase in elevation has big effect in HP. There is a rule of thumb that every 1000ft in evelation you increase you loose a certain % in HP. I don't remember the percentage off hand. Go down to a X452 and see if it will pull that prop, if it will, start increasing the prop size from there.

Josh-

 

[glenng]

I bet the increase in elevation has big effect in HP. There is a rule of thumb that every 1000ft in evelation you increase you loose a certain % in HP. I don't remember the percentage off hand. Go down to a X452 and see if it will pull that prop, if it will, start increasing the prop size from there.
Josh-

Thanks', Josh- I,m sure the elevation makes a big difference in performance. I just finished backcutting a X455/2(my first try) about a hour ago. If that doesn't get the boat up on plane

I was gonna try a prather S240 or a X452. Thanks for your reply Josh!

 

[Duck899]

I bet the increase in elevation has big effect in HP. There is a rule of thumb that every 1000ft in evelation you increase you loose a certain % in HP. I don't remember the percentage off hand. Go down to a X452 and see if it will pull that prop, if it will, start increasing the prop size from there.
Josh-

Usually about 3% per 1000'

 

[moparbarn]

not sure about hydros, but on my tunnels, lower nitro seems to be much easier to pull props with, at higher altitudes. i use it for initial setup, then start stepping up, & tuning as i go. props i could pull on 50% at sealevel wouldn't work higher up. 30% would pull 'em. i agree with anthony, back cut helps launch, tongue cut helps top end - although, back cutting will unload the prop at high rpm, too. rcu has some good prop stickies on their general boat forum.

 

[glenng]

not sure about hydros, but on my tunnels, lower nitro seems to be much easier to pull props with, at higher altitudes. i use it for initial setup, then start stepping up, & tuning as i go. props i could pull on 50% at sealevel wouldn't work higher up. 30% would pull 'em. i agree with anthony, back cut helps launch, tongue cut helps top end - although, back cutting will unload the prop at high rpm, too. rcu has some good prop stickies on their general boat forum.

Thanks Robin, Thats the plan- lowering nitro from 50% to 30%-get the boat up on plane and work from there. I had a extra X455 so I gave the backcut a try. If Rob is right about the 3% H.P. loss every 1000 ft. elevation the old O.S. .65 will only have about 2.25 H.P. @ 4500 ft. Thats not much to push a 15 lb, boat around with. I have learned one thing though-I drilled and tapped a piece of alum round bar(10/32 thread) to hold the prop so I could work on it and noticed one tongue was longer than the other(measuring from the bar stock to the tongues). Fixed that now I'll have to check my other props to see if any of the others are uneven. Thanks to all, Glenn

 

[Roy Munoz]

You might want to pull your pipe out a little too if that set up was for lower elevation...