You could always use a drive dog fitting that way the prop has no thread. Not needed on a scale boat but often used on high speed models.

http://www.cornwallmodelboats.co.uk/acatalog/drive_dogs.html

Just examples of the breed

Props using dogs tend to be high speed racing props ( and SHARP ) very scale like if your doing a F1 offshore racer.

HI Les
Haverlock has already answered your question relating to the thread and I agree particularly with his advice to use a locknut.

The use of the correct thread is very important if you are running a very fast and powerful racing type of model where the forces acting on the prop are not small and are best used to keep the prop on the shaft, even with a locknut. The use of a locknut on a scale model where both forwards and reverse may be used frequently should help keep the prop in place.

As regards two props it is beneficial to use contra rotating props as the model will be more balanced under way and will also respond correctly to the rudder in both directions, as already advised by jgs43.

If you are modelling a true scale model of an original, say a lifeboat, then it will be important to replicate the full size arrangement on your model.

Most props whether left or right handed come with a right handed thread. The one company which does provide props with right and left hand threads is Graupner but only on some of the types and sizes - the thread direction is coupled to the handing of the prop so that they tighten on the shaft when running forwards. Does mean that you need to source a left handed thread on the prop shaft and I have only found one german shop/manufacturer which is able to supply M4 Left hand threaded shafts.
As said by other posters easier to stick with right hand threaded and use a locknut.
The props should rotate in opposite directions and the direction of rotation does create different effects on full size boats - e.g. raising or lowering the bow when running.
Outwards turning props (top of prop) are also supposed to improve manoeuvering and are generally used on tugs and the like. There is also supposed to be an effect on achievable speed but you would need to search for the relevant article on the web.
When the prop turns the boat tends to try and rotate in the opposite direction due to torque. Having the props on twin prop boats rotate in different directions cancels out this effect.
The pilot on single prop boats is normally positioned either right or left to help counteract this effect - there are some very interesting articles to be found on the web referencing these subjects with respect to full size boats

If the prop had a LHT the shaft would also need to be LHT.

in practice use a locknut!!! its no different having the left handed prop running a boat forwards than a right handed prop running a boat backwards.

Both right hand threads. Don't think anyone does left hand off the shelf.
Before you ask, I have my twin props with the blades angled inwards. To check this, rotate a blade to the 12-00 position, and both props should have the blades sloping in wards. I've based this on looking at those on lifeboats. But that's my preference.
It does also help if both motors run at the same speed.

The threads are both the same it the propellers are different right and left hand blades, and if your running to props they do have to run opposite direction or the boat will just go round and round, hope this helps
Lee.

I think that the thread is one way whether a right or left handed prop. The important thing is to use the lock nuts.

I'm sure I will be corrected if I'm wrong!

Edward