Help! I have just completed a 1/200 scale model of HMAS WESTRALIA, a LEAF class Naval oil tanker from the Royal Australian Navy. As you can see from the attached picture, it floats well, is very stable and does not leak. My problem is that it hardly moves with the motor/propellor combination I selected.

I have used a 12v cordless drill motor and gearbox, giving me 500 RPM shaft speed. It has a single two bladed screw, 55mm in dia - not sure of the pitch. I planned on the model going quite slow (true to scale speed), however, a slight breeze overcomes the drive at the moment.

Should I just put a bigger dia propellor on, or a propellor with more blades, or a propellor with more pitch? I am unsure how to select a propellor to match the low output speed as I dont want to change the motor/gearbox combo. Any ideas where to get a new prop from, I would prefer plastic as my shaft dia is pretty small.

The boat is over a metre long, and weighs 6.5 Kg

thanks for any advice

Well I, personally, would go with a direct drive propeller of the motor. Then use an electronic speed control to keep the ship at dead slow. Then if you needed the power, you could increase the throttle to overcome water and weather conditions.

Direct drive off a cordless drill motor may be a bit extreme.
Most of our club ships use motors running about 9k-10k rpm
Our scale speeds are limited by pitching the propellers up or down.

If you can, add more pitch to the propeller, otherwise you need to increase the rpm.

You need to really start all over with your drive system
500 RPM's will get you nowwhere really slowly!
First off are you sure that that ship ran a two bladed prop? I suspect that it ran a three bladed prop. In the size you are running I would look at a 5cm three bladed prop.
I always and highly recommend the use of an ESC. This will allow you the ability to control your speed for those important manouvering competitions.
Last but the most important, loose the motor that you are using. With a boat that size I would suspect that you need a minimum of 2000 RPM's under load to get any head way if you run up against a breeze. I think that if you can find a motor like that listed on E Bay, item #2567071151 you will do well.
If you are trying to save money you can for go the ESC and make your self a manual speed control using a standard servo and three to four micro switches. If you need a picture of that I can post one for you.
So yank out that power sucking motor you placed in the boat and get a more efficient motor with somewhere around 5000 RPM's.
PS: Do not even bother trying to pitch the blades on the prop you have, you will not be able to enough speed out of that motor to make any difference.
Ask Umi what happens when slow boat meets slight breeze. Unless you have a few colas and lunch with you it is a long wait until that boat makes it back to the harbor

Very much thanks for your advice guys - yes I have been trting to do it on the cheap and looks like I am paying for it now. It is my first project so I guess its a steep learning curve. Very happy with how the rest of it turned out though, so not a total loss at least!
glen