Ah, but there is a difference Paul! Modern ships are often diesel electric and equipped with several sets of diesel engines so they can bring on line as many as are required for the current power loading which obviously is very different when the ship is at sea compared with when it is in port. It is also useful as a set can be shut down for maintenance without taking the ship out of service.

However, there are usually still only two actual propulsion motors fitted. The diesels are the equivalent of batteries in a model boat.

Colin

Paul, yes I know, I have travelled aboard her. She needs four to reach 30 knots but she is the only ship that has the four pods. They are the biggest ever made as it is and have suffered problems from overloading which is why she crosses the Atlantic at a rather more sedate pace than originally.

Colin

Paul – the QM2 prop pod arrangement is ingenious and obviously will provide great manoeuvrability but must also involve large cantilever loading – how do they deal with that cantilever loading inside the hull ?

Ian

Probably with a lot of heavy engineering. If it was simple, everybody would be doing it, but the classic layout keeps being used because it works. Obviously a great idea for a large cruise ship allowing it to maneuver in places where tugs might not be available and having the ability to design the power supply arrangements around the ship requirements, rather than the other way round. Did Mongomery Scott once say "The more they overthink the plumbing, the easier it is to stop up the drain"? Simpler is usually more reliable.

I can see that tugs benefit from having two props, but not four. Just too much expensive complication where a simpler and cheaper answer works just as well. If a BIG tug hull is wanted, try – http://www.mobilemarinemodels.com/ft-xb-2615-p.asp

Q. If you had room, I would go for 4 rudders as this would look very cool. Rudders between the props will be less efficient.

A heavy duty servo should be ok, if the rudders are balanced (pivot point part way back from front of rudder). The rudders need not be overlarge, 4 rudders of modest size I think would work ok and not strain a single servo .

This setup will be drawing considerable current. .Heat is always an issue with powerful ESC, getting rid of it. One high power esc would do but would need water-cooling, or use one smaller esc per motor and get away from this issue….?

I would say that a decent air flow through the engine area would be helpful as well.

Ashley

I wouldn't subject a single P98 to the combined current from four MFA 850's. I would suggest one for each pair of motors port and starboard, so that you can then take advantage of differential throttle to assist maneuvering. Controlling the port motors from the LH stick and starboard motors from the RH stick would replicate the way tugs are steered – or used to be before 360­° thrusters and the like were introduced. It's often referred to as "tank steering". There's a current thread somewhere which describes this, or you could have a read of the article "Do they both go round, mister?" here **LINK**

DM

Q. Consider that an ESC has a red and a black wire out, to go to one motor (or whatever colours it might use)

Simply double up the red and black wires so that each motor receives one red and one black. The voltage will be the same for both motors.

PS if using screw connector strips please hide these from DM as he disapproves of them.

Ashley

Like this. I've included a couple of twin indicator fuseboards P95/2 for safety in case any of the motors become stalled.

Totally agree.

To quote from a previous post a couple of yeas ago – Did Montgomery Scott once say "The more they overthink the plumbing, the easier it is to stop up the drain"? Simpler is usually more reliable.

Q

Not wishing to dampen your enthusiasm but you might find that 3 850s draw a lot more than 10amps especially in such a large and potentially very heavy boat.

I would strongly recommend that you calculate your boats total displacement from which you can work out how much power will be required to move it.

If your tug will be pulling a load then you will also need to factor in this mass into your power requirements.

For example my Thor tug is only 3ft long but its 'all up' weight is 80kg and needed far beefier motors than the MFA 850.

Paul

Q

Reducing the size of the motors isn't the answer as the 800s might only move your boat at a snails pace and still become banging hot whilst they eat through your batteries.

Just like my esteemed fellow nutcase (Ashley) I seldom follow convention and I tend to build big boats with big motors and in your case I would be looking for larger 12-24volt motors such as golf trolley or wheelchair motors, these motors have lots of torque and would be ideal for a 5ft tug, these motors along with their big batteries will have the added bonus of contributing weight to the ballast.

Paul