Tuesday, September 1, 2015

[Electric Boats] Another Response to noisy outboard in general

 

In rereading your very good letter, I see what you are getting at.  A friend and I worked on the OB unit today, and tried a few things.  He brought two SS props, god-awful heavy, ugly, so we didn't use those.  The 12X12 prop I have gives me cruise speed right at under 10 amps, or about 15 with a passenger or two, loaded is under 20 amps on smooth water.  I don't think I can do much better than that 4 to 5 mph hull speed at cruise for battery life.  Yep, I can hit 150 amps floored, my top speed doesn't do planing, but it shoves a huge bow wave to get that speed.  In an emergency, I could do pretty well, at a high drain on the pack.  This speed part is from earlier notes on the lake, we worked on land today.
 
However we did find that the lower gear case is most probably the noise culprit, not caught in the rebuild – darn it.  Will discuss that with the shop later and for sure.  The reduction is 2.0 to 2.34 to 1, or close to that as the exact model is not known, it came with the motor.   I would think I might try a different prop, a 12X8 or similar would be more flexible as far as speed control, foot throttle is pretty sudden and could use some smoothing out on the bottom end, just a bit.
 
Just about decided to add four more Interstate SRM-27s, as a second bank with crossover switch.  That's about @ 10 amp draw, and guessing 8 to 10 hours each pack, for a decent week end.  With this size my personal batteries are available at a great price, I don't think I will switch to the bigger sizes, just use the  A—B bank setup, easier on the charger too.
 
Thanks for your input, it really helps ---------  Cal
 
 
Sent: Friday, August 28, 2015 11:51 AM
Subject: [Electric Boats] Re: More on noisy outboard in general
 
 

Oh no, I don't think you should step up the ratio. You want a reduction. The foot should have adequate reduction already so direct drive to the driven shaft should be in the ballpark, but if you want to get seriously into the engineering aspects you can calculate the ideal total ratio for a given prop, based on the optimum power/speed curve of the motor. Electric motors hate to be driven under load at insufficient speed. They tend to overheat, for the main thing. Of course you don't want to overspeed the motor, either. Usually there is a constant published for the motor, and multiplying it by your system voltage will give you the ideal speed to run the motor at full power. For a displacement hull, if the motor is sufficiently powerful, full power setting should be about hull speed with a clean hull and prop. So if you know what prop RPM will give you that speed, you can calculate the desired reduction ratio, keeping in mind the ratio you already have in the foot of the outboard.

I am not using an outboard. I simply replaced my old Atomic 4 with electric. My reduction gearbox is fully enclosed and made by Baldor. They only make whole number ratios, such as 2:1, 3:1, 4:1, etc. Mine is a 2:1 unit. Thrust bearing is integrated into the gearbox. This unit is fairly compact but It it probably not what you want for an outboard.

You do need a tachometer of some sort to figure out where you are at on motor RPM.

Adjusting prop size and pitch to match the load (hul) and motor properly is probably going to be much simpler than diddling with the gears.

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Posted by: "cal" <h20dragon@centurytel.net>
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