The saga continues . . . .

Both motors that I'm looking at require a reduction ratio somewhere between 2.5:1 and 2.1:1, which can be achieved using gear ratios of 12:30 (2.5:1) through 12:26 (2.166:1), assuming that I'm aiming at around 4000rpm. This is a nice friendly set of ratios, that I can achieve with some small and cheap gears. For example: this 12 tooth gear driving this 26 tooth gear directly attached to the prop shaft, to give a 2.166:1 ratio, or this one with 28 teeth for a 2.33:1 ratio, or this one with 30 for 2.5:1.

Depending on the actual maximum power output of the motors I can go to higher ratios (and a lower target rpm), but too much higher and the increased size of the main gear will start to interfere with the keel - I'd have to take a chunk out of it to fit the gear. As it is, with the 30 tooth gear there's only about a milimeter's clearance between the top of the keel and the bottom of the gear. Putting a millimeter deep groove in the keel to acomadate a larger gear won't be a problem, but I'd rather not have to.

This approach has a couple of other characteristics: firstly, if you take a look at the specs for the 12 tooth gear you'll see that its bore is only 2mm. Both motors have much larger diameter shafts - the 2770's is 3.17mm, and the 2776's is 5mm. Unfortunately, I can't get gears with that bore without going to a larger pitch (the pitch is basically the number of teeth per milimeter diameter), and that would mean a much larger main gear, and all sorts of issues connecting the gearbox to the prop shaft. So, instead of all that I'm looking at making a gearbox with two shafts: the prop shaft, on which is mounted the main gear, and a second shaft on which the 12 tooth gear is mounted. Then the motor is attached to the second shaft by some kind of joint (I'm thinking a small rubber link, since that's easy to make, flexible, and it's easy to get it to link two shafts with differing diameters).

The only remaining question (aside from what ratio to go for) is how in /hell/ do I make sure that the gears don't slip on the shafts? None of them have any kind of locking method (the larger ones have equivalent gears with a brass insert and a locking screw, but they're three times as expensive), so the only reason they won't slip is because they're made with sufficient precision to fit /exactly/. The shafts I'll be using (this for the secondary shaft, and probably this for the propeller shaft) are precision ground, so they'll fit very snugly, but I don't know how much force that'll take . . . . I guess I'll just have to see.

To make sure it all spins freely, I'll also be using bearings: these for the prop shaft (and stuffing box) and these little beasties for the secondary shaft. In packs of ten, since it's /much/ cheaper that way. And yes, those dimensions /are/ in milimeters - the smaller ones are 5mm in /outside/ diameter, and 2.3mm wide. Grab a ruler and marvel at the fact that they're full-on bearings, two of which could fit on the nail of your pinky. And about 90c each ;-)

For the stuffing box I'm planning on using the larger bearings to seal both ends of a length of pipe - this provides a near perfect seal as well as adding support to the prop shaft. It might not work as well as planned, though, depending on how well sealed the bearings are - if they end up being very susceptible to water I may need to just use a simple plug fitted as well as I can manage around the shaft. That has the advantage of being easy to remove so I can replace the grease, but it won't seal anywhere near as well.

So, how much does all that end up costing? As described above, about $48, including the tube the shaft will exit the hull via, and leaving me with some spare bearings of both sizes. Going with a large gear that has a set screw, so that I'm not worrying about the torque causing slippage, would add another $9.

Going to higher ratios, the next two viable steps are 32 teeth (2.67:1) and 35 teeth (2.9167:1). That would work if the target rpm was somewhere in the low 3000s, which I'd expect to be the case if the available power was around 40-45 watts. They cost only about ten or twenty cents more, and only the 35 tooth gear would necessitate any work on the keel to allow space.

The final decisions now depend on the power curves of the motors - Jaycar apparently doesn't have them, but I've sent them an email asking them to try and get hold of the datasheets from their suppliers. Here's hoping they comply . . .

himi