About to Repeat a Mistake?
Well, not a mistake per se, but rather a compromise.
The first MPR (Mini Pool Racer) was nothing more than a scaled-down version of a VERY OLD design. Jay's original design had a turn fin located in the center of the hull. I copied this layout on the MPR (a.k.a. Prototype #1) and onto Prototypes #2, and #3. It was only a couple of days ago that I got to thinking and wondering why the fin was located at the center as it was. It would seem that the longer the moment arm between the turn fin and the outside ride surface is in relation to the height of CG, the less inclined it should be to want to flip over in a turn.
My guess is Jay put it there so the boat would turn either way. Power systems were marginal at best back then and counteracting torque and propwalk was less of a concern as it is with today's high-power systems.
I was just about to cut the hull bottom doubler piece when it dawned on me that I should move the turn fin as far to the inside of a right turn as possible. I have no plans to turn left with this hull. All future prototypes and the "Final Build" will incorporate this feature.
Stay tuned for photos another day.
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Prototype #4 Progress Photos and Comments
I built the motor mount from the laser-cut parts. I installed a motor, coupler, and short stub shaft on it and used this assembly to locate the through-hull hole in the step for the stuffing tube. Then, I made a small scrap plywood support to hold the stuffing tube at the right distance from the bottom of the hull. There were some minor adjustments to the motor mount legs to get the right angle with the motor and the shaft.
Once everything was right, I tacked the motor mount in place with thin CA, then used polyurethane glue for the stuffing tube/hull joint. Finally, I held everything in place with blue tape as the stuffing tube joint cures. After that, I'll re-glue all the plywood parts.
Interesting things to note:
1. I re-located the servo. The height of the servo arm in its previous location was always an issue as was the bend I had to put in the linkage to access the mounting screws. With this lower-profile design, I needed to do something...... I moved the servo to a new location just forward of the step and modified the mounting. The ESC should now occupy the space formerly filled by the servo. I may have height clearance issues with the Rx too, so I may have to get creative.
2. I have cannibalized the turn fin from the MPR and installed it in a new, inboard location.
3. A GREAT way to hog out Styrofoam from the inside of the hatch is to use a a Dremel tool with a wire wheel. It absolutely vaporizes the foam. Interestingly enough, the Silkspan and sealer is much tougher and you can grind the foam away leaving nothing but the covering. This is exactly what I did and I can actually tape down the hatch without interfering with the motor. I may still have to build a blister for the motor as the skin is REALLY thin above the motor.
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It's Not the Ride Pad That's Offset....
..... Although it looks that way, but view it instead as if I hacked away the non-trips on the starboard side. The water doesn't see what's above the ride pad, and since I won't be turning right, I don't need those non-trips.
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Thoughts on the geardrive
When Fast Electric was in its infancy there were extreme limitations on power systems. With only a few European Super-Motor exceptions, the only readily available motors were those intended for RC cars/buggies running on 6~7 cell NiCd batteries. In order to up the cell count (more power) in marine applications, a rather clever fellow by the name of Ed Hughey figured out a way to run two motors on 12 NiCd cells spinning a single prop.
How you ask? Simple: He realized that the electric motors of the time were far more efficient at higher rpms, so he decided to put two motors in a gearbox driving a single shaft and select an appropriate gear ratio to allow the motors to spool up while spinning a larger prop at a lower rpm. This allowed the motors to spin at higher rpms when running (better efficiency) and put twice the power into the propeller. He kicked a lot of FE butts back then with his geardrives.
But then the brushless motor revolution occurred, then the LiPo revolution occurred, then the low Kv outrunner revolution occurred, and the need for the complexity of a geardrive is just a distant memory.
I toyed with the concept using a couple of Speed 300 motors in a "mini" Hughey gearbox decades ago. I've attached a photo of that dual gearbox with a single motor in it (first photo). I still have it in my inventory, so in the future, it might get a couple of those Surpass motors and go into something fun (maybe my 16" cat...:confused2::olleyes:)
This mini Hughey gearbox ran remarkably smooth and quiet, but back in those days the selection of gears was limited, so I only had a few ratios to play with. As things went along, I built another geardrive to put into my diminutive H&M Mini Shovel to try it out (second and third photos). The results were excellent, and it too ran like silk. That gearbox ran with great success. I then built one for my MHZ Gecko, but it placed the motor higher in the hull, and spun it the opposite direction which limited the hull's self-righting feature. It currently resides in my "spare motor" parts bin waiting for a new home. With that gearbox/motor combo, the motor would contact the 9/32" tube that contained the bearings. The solution was simple, just cut away some of that tube (fourth photo).
Because I'm doing things no one else has done, and trying to engineer a way to use existing, off-the-shelf equipment, I find myself in the same position Ed Hughey was all those years ago: Finding a motor of the right size and Kv is limited. I may have to consider a way to pick a motor and make it's Kv bend to my will.:bowdown: So I'll be making the Kv of that Surpass motor whatever I want it to be by using a geardrive.
Depending on the gear ratio I finally select, I may have to cut away some of the brass bearing support tube on this one as well, we will have to wait and see.
:cool2: