Oops, typo in my last paragraph. RPM is not 785, it's 745 like I said in the second paragraph.
Fair winds,
Eric
Marina del Rey, CA
--- In electricboats@yahoogroups.com, "Eric" <ewdysar@...> wrote:
>
> Hi Robert,
>
> I would not run 48V through a 36v controller. If the controller is rated for 48V, then it should be OK (I mention that because many controllers will accept a range of voltages, I just don't know anything about your specific controller).
>
> As far as the prop, my 30' 10,200 lb ketch (sound pretty close to what you've got) is running a 13x15.5 four blade prop. At full throttle, the boat motors at 6kts (92% of hull speed) drawing 5100W from the batteries with a motor RPM of 1900 and prop speed of 745rpm (2.55:1 reduction).
>
> Do all the math and you'll see that I'm getting 37% slip at 6 kts. This is much better that most propellor manufacturer's estimates of 55% for auxilary sailboats. Here's an excerpt from a previous post I made about prop slip in heavy displacement auxilary sailboats: "With the old prop and the diesel engine with 2:1 reduction, engine rpm was
> about 1200 (shaft speed of 600) at 3kts and 3200 engine rpm at 5.5 kts. With the new prop and electric motor with 2.55:1 reduction, the motor speed is 830rpm (shaft speed of 325rpm) at 3kts and 1900 motor rpm at 6kts. Doing the calcs, the old prop showed 39% slip at 3kts and 58% slip at 5.5kts. The new prop calcs to 27% slip at 3kts and 37% slip at 6kts."
>
> We can see that the old diesel with the factory recommended propellor did slip almost 60% at 85% of hull speed. Of couse, having the prop in an aperture doesn't help, but even sailboats our size with fin keels and exposed props don't do much better. One need to keep in mindd that all boats act differently, while a ski boat might have only 10-15% prop slip at high speeds, typical sailboats can have much more slip. We can also see that slip is a function of speed, slower speed have less slip, so it's difficult to pick a single number and say that "this prop slips this much" or "this boat has this much prop slip".
>
> Since your boat is so similar to mine, feel free to use my configuration as a target for your calculations. To repeat myself, I'm running a 13x15.5 four blade prop at 785 RPM to achieve 6kts at 5100W (6.8hp). Your 11hp motor should be able to generate that type of power, you just need to figure out the reduction ratio for your setup.
>
> Fair winds,
> Eric
> Marina del Rey, CA
>
> --- In electricboats@yahoogroups.com, "Robert Jorgenson" <falcolnxp@> wrote:
> >
> > 48v battery supply
> > 36 v controller
> > 10 hp General electric DC motor
> >
> > can the golf cart controller handle the load?
> >
> > what size Prop will be most efficient????????
> >
> > fire hazzard lithion ion batteries =factory sealed 24 v battery packs in heavy alluminun box.
> >
> >
> > what size prop should work well??
> > I realize I will have to experiment but i need a starting place.
> >
> > was not getting consistent voltage even though the battery cells read fully charged.
> >
> > battery terminals factory sealed with foam.
> >
> > removed foam from top of connectors
> > two of the nuts on top of batteries required a 360 degree turn to bring them to he same torque as the other 14 9/16 nuts.
> >
> > this is why I have had an electrician guide me thur such a simple p
> > ocess as battery instalation.
> >
> >
> > Capt. Bob 435 678 3436
> > Shockwave
> > Golf of Mexico
>
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