MAKING A MODIFIED CORT NOZZLE
FOR SMALL AUXILLARY MOTORS
by Terry Lesh

I have a 1980 Johnson 4hp twin cylinder Sailmaster engine on my restored 1972 Baymaster 18 Cuddy Dory Sloop (Cayuga). It does fine the way it is, pushing the boat at about 5 or 6 mph by gps at ¾ throttle. That’s probably hull speed for this boat. A little disappointing since the boat I sold in order to get this boat was a 1982 Drascombe Longboat (22’), in which I had a 1979 Honda 9.9 Longshaft Sail model, and the Drascombe (Sowelu) did 8 to 10 mph with this set-up. I had a Dol-Fin rig on the Honda which helped keep the Longboat bow down, thereby increasing hull speed. The Longboat was a made a real fine launch under power, and also sailed quite well if the wind was good.

At this point your probably wondering why I sold the Longboat (digression here, I’ll get to the Kort Nozzle deal shortly). We loved her, but she was a crew boat and not an overnight cruiser for us (in our late 60’s). The opportunity came up at to get the Baymaster (I should say steal) at our local moorage. The Baymaster has a good sized cabin (see photo) and lots of cockpit room (which we liked on the Longboat). I have a lot more to say about both of these boats, but this article is about nozzles. If you want to talk about the boats e-mail me, or encourage me to write some relevant articles about them.

Cayuga

After restoring the Baymaster, I was quite happy until our local group (www.messaboutW.com) organized a messabout to Sucia Island in the San Juan Islands of Washington. Looking at the charts I saw that we would have to trailer the Baymaster (no problem) to either Bellingham or Anacortes. Then it would be about 22 nm to Sucia from either launch place. At 5 mph that’s 4 ½ hours on a good day, with the tide (can’t depend on winds there). Even though we have small porta potti on the Baymaster, we have never been on the boat more than a two or three hours at a time.

I’ve scared both myself and my wife more than once on our “voyages” over the years, so I started looking for a better way (more speed under power).

First I came across the “Powerthruster Nozzle” at trailersailer.com. Looked really good, “22 % more thrust, 30% savings on fuel, only about $79. I almost ordered it, but then got to looking around on the net about nozzles, prop guards, etc. Corresponding with one of our Messabout members revealed that the Powerthruster was not really a Kort nozzle at all, but some sort of knock off. Hmmm!

Back to the net on Kort nozzle theory. I discovered that it is really airfoil theory applied to hydrodynamics. The Kort nozzle idea is an airfoil shaped ring around the propeller that increases thrust and controllability of the propeller both forward and reverse.

There are many interpretations of this theory for both small and large boats. Just search the net for Kort Nozzles and prop guards. The advertised products range from the Powerthruster to just plain rings to stovepipes with holes around the sides and on and on.

My research revealed a more recent innovation of the Kort nozzle idea showing that a plain Kort nozzle is not as efficient as was thought due to internal cavitation pressures. This research showed that two airfoil shaped Kort nozzles in tandem with a pressure relief opening between them was a lot more efficient, even on small power applications.

Back to the net. CGM Products, Inc.(866 298 9359) Has a prop guard designed on the basis of this new theory for $125.95 including shipping. Model # = 9xp for small engines. https://www.propguard.net/

I came awful close to ordering this one. I may still if this project doesn’t work out. In Oregon we have lots of rainy winter days, so I decided to try to make one myself as a shop project

Here we go, finally! The 4hp Sailmaster has a prop blade diameter of 7 ½”. Carousing around hardware and lumber stores I found that ordinary woodstove pipe would work for just a tube. I could drill holes in it for pressure relief. Some of the advertised nozzles look just like that. Not good enough, I want the best result. Then I happened on some embroidery rings, just the right size (8”, (with some minor modifications), and only $1.19 each. I bought a bunch of these things and went to work.

The embroidery rings have an inner and outer ring with little bolt flanges holding them together. They appear to made of bamboo. The general idea is to glue a bunch of these things together in various descending and ascending circles to form an airfoil-like shape, fiberglass them together with struts and attach them to the cavitation plate on the lower unit of the engine. Two assemblies of the rings are required, spaced about ½” apart.

Pic 1: aligning rings for gluing (epoxy)

Pic 2: joining rings for shaping

Pic 3: Showing airfoil shapes

Pic 4: Test fit

Pic 5: Alligning for fitting of ring struts

Pic 6: Finished nozzle

Pic 7: End view finished nozzle

How does it work? Stay tuned, I’m launching the boat next week and will do some tests.

Terry Lesh (5/1/04)

Postscript:

I finally got my Baymaster 18 launched and rigged at our moorage in Baker Bay at Dorena Lake, Oregon. I was very excited to make some test runs with the new nozzle. The plan was to do a number tests: Gps speed runs at different throttle settings, rpm readings at different throttle settings, and fuel consumption/speed runs.

My questions where: Does the nozzle make for higher hull speed? Does the nozzle make for higher hull speeds at lower throttle settings? How does the nozzle affect engine rpm at different throttle settings (all compared to running without the nozzle).

Equipped with a full tank of fuel, my gps and my Spirometer (a small handheld virbratach made in Germany for checking rpms from 800 to 50K on any rotating engine. It works like a tuning instrument for stringed instruments. It has a fine wire on a small reel with an rpm dial indicator. It works by placing the meter on the engine and turning the dial until the wire reaches its highest oscillation, then you read the rpm off the dial.)

With my wife Patricia, at the tiller we set out. First I did some static dock tests just see what the thrust might me like. My first impression was that when I engaged the gear at low speed the boat immediately kicked forward (I have only forward on my 4hp Sailmaster) with quite a noticeable lurch. Applying more throttle noticeably increased tension on the dock lines. Interestingly the engine would not rev up as quickly and easily as without the nozzle. I got more excited. This is going to be awesome! I figured the engine wouldn’t rev as freely in the nozzle mode as it did without because there was much more torque resistance in the water--?

I noticed this too as we got under way, inside the harbor it took less throttle to move out past the speed limit buoy, and the engine seemed to be laboring somewhat. After leaving the harbor I set about using different throttle speeds, reading the gps, reading the Spirometer, and making notes. Things were looking impressive, so I got impatient and decided open her up. Suddenly the engine started to labor a lot harder, there was a funny sound from the hull then a shudder then the engine took off went immediately to its usual full speed sound. I looked behind me and was aghast to see my beautifully crafted nozzle shattered in pieces and strewn out behind the boat!

We stopped, tilted up the engine, and yes there nothing left of the nozzle on the engine. Not a single piece. It had fairly well exploded and disintegrated totally. We picked up a piece out of the lake for diagnosis. What apparently happened is that the nozzle twisted so badly it got into the prop which tore it all to hell. Man there are forces down there I had no clue about. When I installed the nozzle I hand tested it for rigidity and twist stability. It seemed pretty strong, though it was a little flexible. It was securely mounted with 8 stainless bolts, two on each side of both of the rings to the cavitation plate on the engine.

Needless to say my notes were incomplete, my tests did not get finished, and I couldn’t make any comparisons. However my gps record showed a highest speed of 6.3 mph. We did some speed, throttle and rpm tests sans nozzle and found our speed was about 5.3 to 5.5 mph downwind (about 15 knots) and 4.3 to 4.5 upwind. The rpms were 3800 at about ¾ throttle and around 4100 at full throttle.

So what are the results and what did I learn from this. First I am not going through 8000 prototypes like the vacuum cleaner guy from Australia on TV to get the final model. However from 5.3 mph to 6.3mph is a 20% gain in overall hull speed. This is really all the test data have. Impressions are that the nozzle does make for an increase of thrust, and a greater torque load on the engine with a resulting increase in hull speed at lower throttle settings. It appears that messing around with this sort of thing also requires some experimentation with props. Lastly that there is a lot going on down there when the prop is working, forces I had not anticipated at all. I think the Spirometer is very accurate, and I see that my engine is not quite developing its full hp as it is rated 4hp @ 4500, with an operating range of 4000 to 5000 rpm. I’m within the range, but not peaking out.

I’m not going to go through the pains taking labor to make another one, it would have to be made of much stronger materials than wood and fiberglass. Would I buy one? If I am good and can save my allowance to shell out the $130 for a real one, I might, although Patricia says she doesn’t mind going slow (like 6.3 mph is fast?) as she can enjoy the view.

I would like to see articles from others who buy these things, or at least posts if anyone has tried them. I think they are worth experimenting with.

Terry Lesh (6/2/04)