I'm currently in the process of putting together a catamaran style craft, as a kind of "proof-of-concept" for automatic floating debris retrieval. This is just a prototype of sorts.
What I'm currently stuck on in the design process is the propulsion mounting. The hull is going to be made of polystyrene, with the total craft dimensions being 200mm wide by 150 long by 150 high (at least those dimensions are what I'm thinking of at the moment).
The propulsion system is going to consist of two propellers driven by 6V DC motors (so pretty much based on differential propeller revolution rates). Propulsion-wise, I'm going with thrust over speed.
With the hull material, I can't really have the motors mounted inside the hull at water-level due to heat concerns and material structural integrity. So I'm sort of left with having a long shaft at a gradual decline into the water from the top of the craft OR using a 90 degree gearbox (but then there's the issue of the gearbox being in the water...).
So what would be best in this situation? I'm kind of leaning towards the "long shaft" idea at the moment, but my other problem with this is that I don't have much of an idea on how to extend the DC motor shaft (someone recommended I just solder on a long shaft, but that seems a little...unreliable).
My other question is then: how does one lengthen their motor shaft (parts, methods, etc)?
Just an additional point: I'm aware I will need counter-rotating propellers; having a little trouble sourcing them though. Only one hobby shop in my area has them, but they're only 2 blade (kind of wanted 3 blade). Would blade number really matter at this small scale? (Only wanted 3 blade due to more thrust with large diameter and lower RPM).
Sorry about the wall of text.
That's a really small boat and, just like a Faberge Egg, small isn't always the most economical or easy to work on. You may save money in regards to material for building the hull, but often run out of room inside. I would go with about twice the size, or 1.5X at least just to avoid getting myself painted into a corner later.
Most electric model boats have the motor as low in the hull as possible. Attached to the motor shaft is a coupling, propeller shaft with stuffing tube and propeller at the end. The coupling can be as simple as a piece of silicone tubing such as is used for RC airplane fuel lines.
The stuffing tube penetrates the hull and is rigidly glued and secured in place. By packing grease (usually vaseline) in the stuffing tube, water is kept from seeping into the hull.
If you have concerns about the motors getting hot, there are a few ways to counteract that. First, by using geared motors, they are better matched to the load and work a lot less hard as a result. They run smoother and more efficiently and are also able to swing a bigger diameter propeller.
Secondly, there are cheap and inefficient motors and there are good efficient motors. A good quality motor that is well tuned load-wise and allowed to run at it's max efficiency won't get excessively hot.
Worst case when all else fail, a motor can be cooled by plumbing water from a pickup just behind the propeller, going through a cooling jacket (heat exchanger) and out through an outlet.
Another, less traditional way to propel the boat would be to use a simple waterjet system. Most toy boats these days no longer have propellers, probably for liability reasons. Instead they have simple centrifugal pumps embedded in the bottom of the hull. They are constructed such that they are self priming simply by having the pump housing below water level. Steering is then taken care of by directing the nozzle at the stern.
In your case you could accomplish steering by having the water pickup face forward rather than down. This way your differential steering scheme would still work. Alternatively, you could install four waterjets - one in each corner.
It's hard to be brief, yet informative. I hope I helped you get unstuck in some regard.
Best of luck with your project.