One thing that a reduction gear does handily for us, is to transmit the axial force of the prop shaft to the hull, with its integral thrust bearing. Consequently, most DIYers pretty much automatically figure on using a reduction gear of some sort, regardless of the need or lack of need for a reduction. Early this year I performed a little experiment. With my motor, voltage, prop and hull, it appeared that my 2:1 gearbox should be closer to 3:1, so I bought one and my efficiency actually dropped somewhat. Efficiency being regarded as watts per RPM. So hmmm... I tried replacing the reduction gear with just a short jumper shaft and a thrust bearing. Because of limitations built into the mounting system I built, I had some alignment issues that I could not resolve. Nevertheless, I saw, surprisingly, an increase in efficiency but had no overheating problems in the motor. So in my case, direct drive shows considerable promise of increasing my range/speed. Therefore, one of my near future projects is to completely redesign my mounting system with an adjustable thrust bearing mount and bearing assembly through which my new SS shaft will be fixed. A separately adjustable mount will serve as either a direct drive motor mounting plate, or for mounting any standard C face gearbox.
Alignment is always the 600 lb gorilla in the room when you are talking about optimizing any propulsion system. Good alignment will increase motor life, reduce vibration and noise, and increase efficiency leading to increased range and/or speed given a particular battery bank. While it is great to get alignment spot on, steel does flex. Bearings do have play in them. I figured that what was needed was first for the shaft to be held in perfect radial, angular, and axial alignment, secondly that the motor be likewise aligned with reasonable precision, and that thirdly, some means of dealing with inevitable minor misalignment and transient misalignment be implemented.
It occurred to me that a splined connection somewhere between thrust bearing stabilized prop shaft and the motor would take care of this. Sometimes just playing around and showing off, we put incredible amounts of torque on our drive trains, so just any splined shaft would not necessarily be adequate even though we are often only dealing with 5 to 12kw motors. Just browsing online for splined shafting and coupling resources, I ran across an old friend... the 1-3/8" 6-splined tractor PTO standard. Hmmm... nominally rated for 1000RPM, torque I forget but it is sufficient, and not too expensive. Well...
The problem of course is adapting a 1-3/8" splined shaft to my 1" prop shaft, and another to my 7/8" motor shaft. Joining them with an internally splined coupling sleeve would then of course be simple. I decided that the most expedient way to do this was to buy an extension shaft splined at the ends, cut it in half, and turn down the unsplined end of each half to the appropriate size so I could use an ordinary 2 piece shaft coupler to attach the part to its shaft. Below is a pic of a shaft similar to the one I bought. Unfortunately I did not think to take a picture of the one I bought before machining it. The one I got was 12" long with about half its length unsplined, and each end splined for 1/4 its length, more or less. Conveniently, the ends were center bored, making lathe operations relatively simple.
http://www.growleymonster.com/eboat/pics/1-3+8++6T+Shafting+Splined+Both+Ends_L.jpg
So then I eventually found a contractor working on my ship willing to turn down the shaft in his spare time. Otherwise I would have had to buy a lathe which actually would have been pretty cool to have anyhow, but this saved me about $600. And here is the finished assembly:
http://www.growleymonster.com/eboat/pics/SlipJoint01Assembly.jpg
http://www.growleymonster.com/eboat/pics/SlipJoint02CouplingSleeve.jpg
http://www.growleymonster.com/eboat/pics/SlipJoint03Components.jpg
I also cut my spare prop down to 13" from the original 14"x10, and bought a nice stainless shaft to replace the bronze one that is a little bit eaten up and none too straight. I will be doing a haulout to replace Cutless bearing, shaft, and prop, and then I will completely redesign the motor mounting system to accomodate a thrust bearing and a motor mounting plate that will accept either the current C face gearbox, or any C face motor. So I will be able to go direct drive or use the existing reduction gear and bolt the motor directly to that. Did I make a mistake somewhere that lead me to believe that direct drive is the way for me to go? I guess there is one way to find out. But I don't think so. Should I find it best to use the gearbox after all, no biggie. Easy swap. But I think a separate thrust bearing and the splined joint will help in either case, and the new mounting system will make the propulsion setup much more flexible in terms of component changes.
Just thought I would throw this idea out there for anybody contemplating a DIY repower, and for any ideas and comments from the group.
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