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
----- Original Message -----
Sent: Monday, November 16, 2009 12:53 AM
Subject: RE: [Electric Boats] Emerging technology - hopefully lower cost
Sounds perfect for a sail boat!
Seems to be pretty promising, possible lack of acceptance earlier in it's history was that when it was designed, it was easier to simply go with a bigger motor if you needed more power, today's energy situation tends to make the more power approach less appealing.
would anything else require building up to handle the increased thrust?
Ken.
> To: electricboats@yahoogroups.com
> From: ewdysar@yahoo.com
> Date: Mon, 16 Nov 2009 05:04:14 +0000
> Subject: Re: [Electric Boats] Emerging technology - hopefully lower cost
>
> Ludwig Kort applied for a patent for a Kort nozzle in 1938, so I wouldn't exactly classify this idea as an emerging technology. That said, 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
>
>
>
> --- In electricboats@yahoogroups.com, James Graham <osirissail@...> wrote:
> >
> > Here is a video showing the Kort Nozzles on a tug boat. Notice the shape of the propeller inside the nozzle. The maximum blade width is right at the interface of the nozzle and the propeller. Using a standard propeller that has not been modified in a Kort Nozzle shape is worse than no Kort Nozzle at all. The increases thrust at low speeds comes from being able to eliminate the 20-25% of the prop blade tip that is inefficient and using the nozzle to contain any off-tip water flow. For the same diameter propeller, the Kort Nozzle prop can have significant more surface area interacting with the water resulting in greater amounts of water being moved which equates to more power.
> > http://www.youtube.com/watch?v=QdSH6SBl5hY
> >
>
>
>
>
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