Props

[mndsw]

I'm in the process of replacing the 55mm 4 bladed props on my twin screw boat with smaller 3 bladed ones because I have been informed that they are to big for the Graupner speed 700 turbo motors. After looking at a couple of prop web sites, is it true that inward turning contra rotating twin props are not as efficient as outward turning contra rotating props. If I swop my props around from one shaft to another what performance increase am I likely to get?

[Ghost 2501]

to be honest, it may be so on the full size models moving at high speed, but on a model it may not make much difference.

on a 700, use a 40mm 4blade as max size.

[pompebled]

Hi mndsw,

Ghost is correct, if your prop is fully submerged and is a high pitched race-prop, running a 700 on 12 cells that would be about as much as the motor will take without burning up. (watercooled!)

If the props are mildly or low pitch props, 55mm is on the edge, but if the motor will rev out and is watercooled it might work.

What hull do you want to power with those twin 700 motors?
Did you run it to see how hot the motors got?
What's the powersource, Lead, NiCad, NiMh, Lipo?

A picture would be nice.

Regards, Jan.

[pkboo]

Mndsw,

Not only you have a chance of burning your motor, if you don't replace the screws. Watercooling the motor will only move the problem to your ESC, cabling and/or batpack. I've just replaced my 35mm three bladed raboesch props with 31mm graupner speed props (2 bladed) 'cause I could fry an egg on my batpack running the bigger props. Even that small amount of change made a big diference. Not only do I have longer run times with my boat, I don't have the problem of heat any more and because the motor encounter less resistance it turns a higher rev which translates into a higher speed. The key to success is "test and test again" and props aren't that expensive compared to a burned out motor, batpack or ESC. I wish you luck!

[mndsw]

Thanks for the replies.
My boat is about 48" in length and uses a 12v sealed lead acid battery. The current props are by Prop Shop and are superbly made scale props. I only found out after fitting and securing them that they were too big for the motors. The boat runs really well and planes without effort but the motors do become quite warm.
I've attached a picture of my boat. It's what I would call a semi scale, part scratch built, freelance interpretation of an Italian Police launch. I wouldn't post a picture of it on the scale boats section!! Anyway new props it is then.

[steveciambrone]

Since they are scale props, you could also add a gearbox inline. This would reduce the load on the motors and decrease the current. Which reduces the heating. I would add watercooling either way you go.

Thanks
Steve

[green-boat]

Motors and ESC's should never get hot. Heat is nothing more than wasted energy that should be applied back into the system thru a change in props or the use of gearing.