single to twin gearbox

[believe the pipe]

i have a Zenoah 230RC and a Dumas Donzi on the way. I want to have it so the Zenoah turns both props. anyone know where i can get the gearbox that does this?

[CG Bob]

MACK Model Marine (http://www.mackproductsrc.com/Power%20p3.htm) has some.

[believe the pipe]

I was thinking of making a custom one. Set it up like a bicycle, with a gear on all three shafts then put a chain around them. Saves money, customizes everything to perfection, counter roataiont intead of right hand and left hand props. I was goning to go ahead with the plan but then realized i should consult the experts first :) What are your thoughts?

[CG Bob]

Set it up like a bicycle, with a gear on all three shafts then put a chain around them. Both props will turn the same direction as the engine with this set up.

counter roataiont intead of right hand and left hand props Counter roatating props is the use of a right hand and left hand prop; a right hand prop turns clockwise, a left hand prop turns counter clockwise;
There are two different types of counter rotating prop setups: two props rotating in different directions on a common shaft axis (like torpedos or a Volvo-penta duoprop system); and two props on two seperate shafts rotating in different directions.

The PT Boats of WWII had three props, all right hand pitch.

[believe the pipe]

yes CG Bob, you are right. I just realized that the shafts would all turn right. Then all I need is a right hand and left hand prop. But the advice i was realliy searching for was whether to go ahead with this custom gearbox plan. What are your thoughts.

[Ron Zee]

If the shafts are all turning "right", then the props will turn "right". If you put a "left" prop on the shaft turning "right", it will in effect be turning backwards. A "left" prop needs to be on a "left" turning shaft

[CG Bob]

If you use three gears or sprockets and a chain or timing belt drive, all of your shafts will turn in the same direction. If ALL your shafts are turning to the right, you'll need 2 right hand pitch props. If you want counter rotating (1 left hand & 1 right hand pitch) props, you'll need a gear box with an even number of gears. like in the attached drawing. The top drawing is based on your description of a chain drive. The lower drawing is a simple gear box for counter rotating props.

[OhioMike]

That dont look right Bob. That almost looks like a reduction on the starboard screw in reference to the Port screw. Maybe its just the drawing, but with a smaller gear to the stbd shaft, it will turn at a lower rpm than the port one right?? Am i right? Am i right?? LOL.

[CG Bob]

Mike,

If the two gears on the prop shafts are the same size, it doesn't really matter what size the other gears are. We'll assume that the gears on the engine and two shafts are 1 inch in diameter and have 80 teeth; the idler gear is 1/2 inch in diameter and has 40 teeth. One full revolution of the engine gear will cause one full revolution on the props, but two full revolutions on the idler gear.

[OhioMike]

Are you sure? Your output gear/motor pinion is turning at 100 rpm, if your direct to your shaft gears and their the same, their output to the props will also be 100 rpm. If you interject a secondary gear that is different, lets say half the size, then one or both of those shaft gears will then be at 200 rpms right? That smaller gear will have to travel twice as fast to match the motor output gear?? right??

[OhioMike]

wait....i got you...i think??

[CG Bob]

Mike - I made a little training device from some gears, rod, and wood. I'll bring it to the CMBC meeting (http://www.clevelandmodelboat.com/) on Friday to keep you amused. From the left to right the gears are: 35 teeth; 42 teeth, 32; and 35. If the motor is connected to the 42 tooth gear, and the props are connected to the 35 tooth gears; the gear ratio is 1:1.2. If the motor is connected to the 32 tooth gear, and the props to the 35's; gear ratio is 1.09:1. I used the set screw positions as timing marks, and highlighted them with some silver paint. The two 35 tooth gears are counter rotating at the same speed, even though the connecting gears are different sizes. From the top down the pictures are: gear timing markes aligned at 12:00; after 1 complete turn of the big gear;3 complete turns; 4 turns; and 5 turns.

[believe the pipe]

I agree with Bob. The question i have though is this: My shafts will be roughly 6 inches apart, so i think I'm going to need to use a chain. What kind of chain would i use?

[CG Bob]

SDP/SI (http://www.sdp-si.com/index.asp) is one source for chains, sprockets, timing belts and pulleys. You can probably use a timing belt and pulleys. They have several differnt styles and sizes of belts, pulleys, and all the other related irems.

[OhioMike]

Well now you show 3 different gear sizes, but thats cool. I understand where you were going with this now. Its really just a matter of crunching the numbers and you can come up with any ratio or final output. that is as long as there is hardware to use to achieve this. Thanks for the Engineering 101 class Bob...really! I copied the pictured for my reference library. I have a few projects with twin screws and 1 motor planned down the road and will need to better understand all this. I have found that the hobby shops carry a lot of various nylon gear sets for the car guys that should fit our usuall shaft sizes for model boats. Thanks again Bob. The RC tank guys suggest McMaster-carr and even tractor sales for chains, etc. They build 1/6 scale rc tanks that have paint guns in the turrents. Big drive systems, etc.

[CG Bob]

As long as the gears to the prop shafts are the same size, it doesn't matter what size the other gears in the system are - the props will turn at the same speed.