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[davejay]

Hi peeps, I'm building a 1/24th scale Lowestoft steam drifter with a length of 47" x 9" beam based on a GRP hull, was wondering what would be the best brushed motor and speed controller combination for this, I have two 12v 7ah lead acid batteries (will be using only one, the other for ballast!)........previously I was building a 51" coaster which I had to abandon due to a badly moulded GRP hull and on that particular build I was going to use a MFA torpedo with a 6 to 1 gearbox on it and a 15amp Mtroniks speed controller, I wasn't sure whether using a 6 to 1 gear ratio was better than using a direct drive, I would have thought that having the motor turn 6 times for 1 turn of the prop would have consumed more juice from the battery thus reducing run time, so is direct drive better than gear drive?
If you can help me choose the correct motor and speed control for the drifter I would be so very grateful.

P.S The prop would be approx 50 to 55mm and three or four bladed.

All the best and kind regards, Dave

[malcolmfrary]

Electric motors are strange beasts, and often counter intuitive.
A motor designed to spin fast, if forced by its load to run slow, will run very inefficiently.  Putting a big prop on a small motor will overload it, so a gear reduction to a big prop from a fast spinning motor will allow the motor to work at its best.  Otherwise it would need a bigger, slow revving motor to produce the torque needed to spin the prop.  Rule of thumb is that the motor diameter should not be less than the prop diameter.  It should also have more poles than the prop has blades.  Thats for direct drive, gearing effectively gives a bigger diameter motor, and lots more poles per turn of the prop.

[inertia]

Your choice of the 6:1 MFA motor's not a bad one, but one of these on direct drive and 12v might be pretty useful, too. Certainly quieter! http://www.componentshop.co.uk/775-dc-motor.html
DM

[davejay]

Thanks Malcolm and Dave for the replies, I obtained a MFA 6:1 919D motor, coupling and propshaft, in the process of being fitted and aligned.
A couple of quick questions come to mind however (as always), firstly, is it worthwhile putting some kind of rubber/foam between the motor mount and the ply plate to which the motor will be attached to in order to deaden/minimise any noise/vibration from the motor being transferred to the hull of the boat? And secondly, how do you identify which is + and which is - on the connections of the aforementioned MFA motor? Probably a very simple answer will be forthcoming and will hit me squarely on the forehead like a plank!!!

All the best, Dave

[Buccaneer]

Hello Dave,
I'm a bit late with motor suggestions but it may be useful for somebody else. I use a MMM T12 on my 48" x 9" LLoydsman with a 2 1/2 inch 4 bladed prop. Plenty of go there. If you are using 2 batteries why not have two charged ones connected through an Action P103 Parallel Power Board. I've used these on a couple of Tugs where I needed the weight so used two batteries instead of extra lead. I charge mine in situ so have to remember to charge them both!
Regarding your motor connections I have always understood that connecting a simple DC motor the 'other way round' just makes it go 'the other way round'.
If it does not go round the right way for your prop just swap the leads over, either at the motor or at the ESC connection.

Yours
John

[davejay]

Thanks John, I knew the answer to my connection question would hit me like the proverbial plank in the forehead!!!..........I should have listened more in those physics lessons rather than playing with the van de graph static generator that made the hair on ones bonce stand on end....no good for me in my case now!!
Makes sense really, swapping connections reverses the rotation...ahh well, lesson learnt..........off now to hide in a dark corner of my shed/workshop/doghouse/boathouse!!
Before I go though, I had contemplated connecting the two batteries in unison but was not sure how to go about it, I was going to have them both in a state of charge but swap the connections when the one being used had depleted itself, I can see myself ordering several components from AcTion in the near future (although there are other suppliers of fine electronics elsewhere! [wouldn't want to upset other people!!!]).
Right, I'm signing off now to skulk away to the dark recesses of my wooden home in order to redeem myself.....somehow!!!

Regards for now and happy modelling to one and all. Dave