There's one big unknown there, and that is how many amps will the motor actually draw at your chosen RPM. That is directly related to how much load is on the prop, which is not easy to calculate since it varies by boat speed and prop efficiency, and prop efficiency (or slippage factor) also varies with load.
Have you been able to find any drag test info, or did Mr. Bolger publish any estimates? That would be helpful.
I'm not that good at the math, but I think I understand the basic concepts pretty well. Basically my understanding is that, for this type of motor anyway, the motor will spin at a particular RPM for any given voltage, and as you load the shaft it will draw whatever amperage it needs to maintain that RPM. Thus amp draw is proportional to torque.
I need to chew on those numbers some more, but it's a start. I'll try and get my brother to look at it too. He has designed a lot of installations and is a whiz at this stuff.
So what kind of CNC router did you get, and what software are you using to lay out the cut files? A buddy of mine has a Shopbot which we used to cut out panels for one of my dinghies, and I've been fooling with Rhinocad to design some more. Cool stuff! Where are you located? I'd like to correspond off-line if you're willing.
Jim McMillan
> --- In electricboats@yahoogroups.com, "b504now" <b504now@> wrote:
> >
> > ok. Some better information. Now I am new to this game so any thoughts, help or fixes would be good.
> >
> > My prop is 10 1/2 dia with 13 pitch. I am not sure I want to go that way but lets start there. Oldish lower unit. My lower unit has a 7/4 ratio (drv Shaft to Prop) so my math works like this....
> >
> > If I want a cruising speed of 12 mph, 2 hr run time, at an efficiency of 50%, I'll need a prop speed of 1950 rpm. That translates to 3411 rpm at the drv shaft (allowing for gearing).
> >
> > If my battery bank produces 225 and operates to 40%, my run consumption needs to be about 1750rpm and based on the feedback, about 3.5hp, 67amps. (all 48v). So I will likely gear up 2/1 (emtr to drv shaft) which gets me to 3500. That seems close enough.
> >
> > everyone pick holes....I need all the help I can get.
> >
> > I also have built a few boats, both stitch and glue as well as traditional. But I just purchased a CNC machine capable of cutting 4X8 sheets. So my son's and my boat building is about to take a leap forward. The Sneakeasy will be the first one we do with CNC parts. (and also the first one my wife chose)
> >
> > Thanks
> >
> >
>
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