Counter rotating twin propellers

[stduck54]

My 28 inch scale tugboat has twin propellers. However, I always thought that these should counter rotate inwards: in other words, the LH propeller runs clockwise and the RH runs counter clockwise. In this case though, they run outwards, so LH runs anti-clockwise and RH runs clockwise. I could swap the propellers (they are a mirrored pair) and reverse the running direction on the transmitter but it is not easy to do and involves a lot of disassembly/assembly on the boat itself. Is this an issue, does it affect performance? Does anyone know what are the pro's and con's?

Duck

[nick_75au]

Hi Duck
From your description they are correct,all boats these days have the tops of the propellers rotating outboard.

There is a very slight efficiency advantage with them the other way, which was utilised in WW11 on a certain class of British warship. However the captains of those boats got a reputation for being poor skippers due to the fact manoeuvrability suffered. They were difficult to handle around the dock as the paddle wheel effect of the prop at low speed counteracted the rudders.

For your tug better to have the advantage of manoeuvrability over the very slight efficiency gain.
Regards
Nick

[Ghost 2501]

I would have thaught that props turning in-board at the top would mean that the part of the ship on the outside of the turn would have an upthrust acting on it, reducing roll

[toesup]

I could swap the propellers (they are a mirrored pair) and reverse the running direction on the transmitter but it is not easy to do and involves a lot of disassembly/assembly on the boat itself. Is this an issue, does it affect performance? Does anyone know what are the pro's and con's?


It would be interesting to see if 'reversing' the rotation of the props makes a difference.

Yes, you could swap the props on their shafts... but why not just swap the + and - leads to the motors.. that will reverse the direction of rotation of the props... and reverse the channel that the motors run from on the transmitter... ;)

[Umi_Ryuzuki]

Seems like someone did some of that testing and reported back here...
Can't seem to find the thread though.


Here is another thread on propellers.

http://www.rcgroups.com/forums/showthread.php?p=8843655

[woodybob]

In a nut shell... http://www.rcgroups.com/forums/showpost.php?p=8846499&postcount=15

[CaptLee]

Have tried them both ways and it comes down to the Inboard Tops seems to dig a hole in the water and the stern squats more. Most run with Tops outboard for a better riding boat. Do not understand the Hydrodynamics but that is the results.

[jeepers1940]

This same conclusion was reached some time back on a forum at Model Boat Mayhem (UK). No one professed to understand the hydrodynamics, but based their observations on experience. It was said that rudder control was supposed to be better with the propellers - observed from the rear - turning over and outwards, the left prop turning over to the left, the right over to the right.

[ScottOram]

Take a look at this. I got this from the Coast Guard Academy. It has a lot of information regarding Prop directions, single and twin prop navigation.

[Ghost 2501]

perhaps we should have a "how does your boat handle" thread?

[rmdesignworks]

with the props turning inboard the props actually push water out from under the craft making it light in the stern area,,turning the props outboard pulls water up under the boat lifting it,,,,inboard rotation also pulls the water in from the sides where the water is generally already cavitated as opposed to fresh water from under the boat that is heavier having less cavitated air bubbles in it which also allows better bite by the rudder,,the increase in manueverability..

[stduck54]

with the props turning inboard the props actually push water out from under the craft making it light in the stern area,,turning the props outboard pulls water up under the boat lifting it,,,,inboard rotation also pulls the water in from the sides where the water is generally already cavitated as opposed to fresh water from under the boat that is heavier having less cavitated air bubbles in it which also allows better bite by the rudder,,the increase in manueverability..

Thanks guys. It seems that outward rotating is the way to go, which is good, it saves a lot of mucking around disassembling/re-assembling. As the model has only limited access to below decks, its like building ship in a bottle anyway.