Denny,
Your results sound right to me. I was also thinking that because of the extra drag, the Kort nozzle would have to be mounted on retractable system, like an outboard leg, to eliminate the performance hit while sailing. Permanently installed under the hull would slow the boat down even when clean and probably catch all sorts of stuff (lines, weeds, trash, etc.).
Even though electric drives are a developing technology, people have concerned themselves with drive efficiency for a long time. I remember reading over 20 years ago about people installing Kort nozzles on ICE outboards for sailboats, but the results didn't justify their use for typical pleasure craft.
Now if anyone is building an electric tugboat or cargo barge, a Kort nozzle should be right up their alley...
Fair winds.
Eric
--- In electricboats@
>
> As with most things in life - you have to do the numbers. Gerr's book states the extra drag of the nozzle out weighs the advantages of ducted flow if the Power Factor is < 25 and kort nozzles do not offer a significant advantage unless the Power Factor is > 35.
>
> Power Factor = ( square root of shaft HP times RPM)/( wake speed in kts raised to the 2.5 power)
>
> Wake speed is the speed of the prop through the water, in our slow boats figure 90% of boat speed.
>
> For a heavily ballasted sailboat the nozzle would have some advantage. For a displacement launch (unballasted) the power factor goes down because the speed goes because the boat is so much lighter than the ballasted sailboat.
>
> My own tests with with my 25', 6 hp launch showed the nozzle to have no effect on speed vs. amps consumed at anything over 3-4 mph. (This with the 6 blade prop designed for a nozzle, however). It made a huge difference power loading the boat onto the trailer.
>
> Denny Wolfe
> www.wolfEboats.
>
>
> ----- Original Message -----
> From: kendall bonner
> To: electricboats@
> Sent: Monday, November 16, 2009 12:53 AM
> Subject: RE: [Electric Boats] Emerging technology - hopefully lower cost
>
> Sounds perfect for a sail boat!...
>
>
> Ken.
>
>
> > To: electricboats@
> > From: ewdysar@...
> > Date: Mon, 16 Nov 2009 05:04:14 +0000
> > Subject: Re: [Electric Boats] Emerging technology - hopefully lower cost
> > .... Kort nozzles can provide a 25-30% improvement in thrust at low speeds, the shrouding causes extra drag that negates their advantage at speeds over 10 knots. These devices may not provide much of an increase in speed over un-ducted blades, but they will be able to move heavier boats, which may be an advantage after adding 1000 pounds of batteries to electric conversions.
> >
> > Fair winds,
> > Eric
> > 1964 Cheoy Lee Bermuda 30 - Serenity
> > Marina del Rey, CA
> >
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