motor and drive shafts

by Marcus McConnell
(Westville SC, USA)

I'm trying to scratch build three semi-scale model boats: two that are going to be identical single prop, 13 inches long by 6 inches wide by 2.5 inches high from keel to deck.

The third model boat is a dual prop, 48 inches long by 12 inches wide by 8 inch from keel to deck.

I would like to know what would be the best way to set each boat up for motors and shafts and couplings. These are my first scratch built model boats and really don't know where to start. I've got all the plans drawn out and built but have no idea how to do the motors and shafts.

I would like to do a direct drive on each. I've attached the plans for the small boats. My large boats plans were drawn directly on the board and I have no paper record of them.


I have some difficulty answering your question because you make no mention of performance goals or budget.

Here is what you can do though:

  1. First, draw up a profile of the boats full scale

  2. Figure out what you're doing for a rudder. Most performance boats these days have the rudder and prop well behind the transom. If you have the funds, you can purchase pre-made high performance rudders or, if you're on a shoestring budget, make your own.

  3. Once you know where the rudder will be, you can determine where the prop should be, in order to determine the length of the prop shaft. The longer you can make it, the shallower angle it can take going into the water. The shallower angle, the more of the thrust is propelling the boat forward. You need some downward angle to keep the nose from lifting.

  4. Lay in the motors and coupling on your layout.

    Figure out the length of your shafts. It may require drawing up a profile of your boats and measure. Make sure you've got room for a rudder behind the prop.

  5. Pick your motor. The easiest way is to compare your boats to similar existing designs. See if those are within your budget.

  6. Google "Graupner USA" (without quotes) and buy the shafts, couplings, a handful of props in different diameters. For the dual prop boat you want counter rotating props, so get a left and a right hand of each size you think will work. You can also get battery packs, ESCs and motors from them if you can find something you like.

Best of luck!

Petter



Comments for motor and drive shafts

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Just a few comments...
by: Petter

OK, I see. I'm having a hard time seeing where you're thinking of mounting the motor, so disregard my comment if it doesn't make sense. I'd try and make sure of two things, at least for the smaller boats:
  1. These hulls are so short, you need to flatten the shaft angle as much as possible for best performance. In order to do that, the motor may have to be moved further forward than what may seem natural. However, it'll provide much better performance.

  2. When installing the motors you want the motor shaft lined up as well as you possibly can to the prop shaft otherwise you'll strain the motor and coupling unnecessarily and create excessive friction which leads again to poor performance and a strained motor.


When running motors direct drive they often run too hot. If you can't hold your hand on them for three seconds (after having run the model boat for a few minutes) they're running too hot. A motor that is run too hot will fail prematurely. If that is the case, you will need to install a water jacket on it/them and water pick-up and a water outlet.

Motors Already Selected
by: marcus mcconnell

I already have the motors, I canabalized my RC trucks for them. All I know about them is that they are 3v max capacity. I have made my own rudders and i have a $60 budget right now. I plan on mounting them through the hull. I'm not going for a speedy boat but a little more torque like a tug boat. the plans have the mounting spot for the motor drawn in them in one of the ribs. Thank you for all the info and site.

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