Twin props

[Peter Smith 24Participant @petersmith24]

Is there any good articles, for a layman, on the topic of twin propellers and the argument about counter rotation.

The simple method, I am told, is to reverse polarity. Then I read on to find that on some motors this does not work. My logic says that there must be motors produced for this purpose but who am I.

Peter Smith.

[Peter Smith 24Participant @petersmith24]

Counter rotation

[harry smith 1Participant @harrysmith1]

Hi Peter

Reversing polarity on motors does not always work with brush motor as some rev faster one way than the other.

Brushless motor rev the same both ways and to reverse them I swap the two outer wires over.

The torque from the props can cause the boat to roll over sideways!!

I see a Fantome (which is a very large boat) do is.

Every type of boat is a bit different in setting up, so it's down to a lot of testing.

Harry Smith

[ashley needhamParticipant @ashleyneedham69188]

Hi Peter. Whereas Harry is correct about some motors going faster one way than `t other, the difference is likely to be very small.|

Generally twin prop boats would have props going in different directions, but it makes little or no difference in efficiency with a model which way this is (props going up in the middle or down). HOWEVER if building to scale then it would be nice to get the rotation right as per the original.

As Harry says, a bit of experimentation might be required.

Some very old brushed motors will not reverse, but it is unlikely you have one of these types. All modern boat brushed motors will reverse (yes yes someone will come up with an exception).

Ashley

[Charles OatesParticipant @charlesoates31738]

It's not something to worry about Peter. Thousands of models have been built with twin brushed motors over the years, and as Ashley says the differences are minimal. I've never had do do more than tweak the rudder, and that's with fast scale and sedate models. If, like Harry you're making a very fast boat that runs near its limits, that's another matter, but I presume you're not, so go ahead and just reverse the motor connections on one motor.

The most common reason to have contra rotating props on a model, is so that the turning circle is more even in each direction, the amount of difference varies between types of boat / ship. There are other reasons but I'm not opening that can of worms when you just need a straightforward answer.

Out of interest, what model are you building?

Chas.

[Malcolm FraryParticipant @malcolmfrary95515]

The non-reversing motors referred to earlier are field wound types. Way back, permanent magnets with strength and long life were difficult to make. An electromagnet was the alternative, giving a motor that was also happy to run on AC, but only in one direction.

All modern permanent magnet motors turn both ways. The vast majority are designed symmetrically, so, withing a small percentage point, respond equally in both directions. Differences are down to manufacturing tolerances and wear.

Some motors intended for ultimate performance have adjustable brush gear which allows the timing to be tweaked. This improves performance in one direction at the expense of performance in the other. Any motor suitable for use in a "normal" boat won't be one of these.

[Martin Field 1Participant @martinfield1]

The real boats actually ran in the same direction, surprisingly , perhaps.

I wonder how they handled in a turn.

Martin

[Martin Field 1Participant @martinfield1]

The real boats actually ran in the same direction, surprisingly , perhaps.

I wonder how they handled in a turn.

Martin

[Colin BishopModerator @colinbishop34627]

The real boats were probably fitted with twin identical motors which may have only run in one direction with a geared drive for reverse. If the props were relatively small there may not have been much of a tendency to turn in one particular direction while at high speed and at lower speeds it could be easily compensated for with the rudder(s) and even taken advantage of when berthing/unberthing the boat as is the case with moderm sailing yachts.

There may certainly have been a slight difference in the turning circles between port and starboard but again, probably not enough to matter.

It's nice to have symmetry but not essential.

Colin

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