cowana

I have just started work on my next project - an RNLI offshore lifeboat at about 1/17 scale (80cm long).

The power source will be 2 Robbe 13 turn-700 motors, running in parallel off a 9.6V NiMh.

I am trying to decide what props to start off with. As I don't have enough money for metal props, I am sticking with plastic. For this model, I want it to go as fast as possible. The options are:

2 Blade: 35, 40, 45, 50, 55mm
3 blade: 30, 35, 40... 60, 65mm

The two blades are £0.80 each, the 3 blades are £2.50 each, so I could try a few twos or only one three (per motor)

What do you think?

cowana

mfr02

Think two blade. Make sure the diameter is smaller than that of the motor (rule of thumb). You want to let the moters rev freely. More blades and diameter will work against this, but smaller props will shift less water backwards.
Welcome to the black art.

cowana

Motor Stats:

(Robbe)
Powerful motor for all kinds of boat models with 7-10 NC cells.
Diameter: 44.00 mm
Weight: 320 g Length: 67 mm
Voltage: 8.4-12 Volts
Rotations: 12800
No. of cells: 8
Current Amps at max. efficiency: 13.00 A
max power: 25.00 A
max. efficiency: 75.00 %


Does this mean I should run a 40mm prop?

Poppa

Hi there If it's any help to you i just completed a 1/12th scale model of a Brede class RNLI lifeboat totally from scrap materials.
It should have two props but i decided on one 40mm prop run by an old 540 motor.
Actually the model is 3 inch under scale as the real thing was 33 feet so 1/12th scale would be 33 inches, my boat is 30 inches.
As luck would have it that 540 motor, and the 40 mm prop give it a fair turn of speed, pretty good scale wise, so i reckon that 2 props of 45 to 50 mm will do fine for your model, though i don't know how heavy it will be.
Here are 3 pics to give you (I hope) some idea)

cowana

I have now decided that I will run two Graupner 600 ECO motors, running 40mm 3 bladed props.

Poppa

Ok i hope it works out for you...I would be interested in seeing pics of the model.
Is it a kit model or are you building from scratch like i do?. I love to build my models from Scrap, and odds & Sods then see them work, to me its more satisfying than kit models, and is what modelling is all about in my book, though i have built kit models

cowana

Scratch.

Foam 1/2 scale model
Scaled it up to a 'inverse' plank on frame boat
plank
fill
paint
mould release
gell coat
fibreglass
resin
more resin
remove from mould
... and that is so far...