Development of a Jet Drive for Small Boats
using a Commercial Water Pump - Part 2

by Steve Hansen

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Water Trial 1

The photo below shows the jet pump and its accessories installed in Candy's 12 foot tin skiff. I was hoping that the Evinrude could be used for comparison and backup but I was informed that it hadn't worked all summer. The oars did work.

Please note the piece of green suction hose hanging over the side. This was to prove problematic.

After rowing out into the pond a bit we fired up the engine. The bloody thing actually worked. At something just above idle with about 10 psi at the nozzle we were moving at about 1.5-1.7 mph as measured by GPS. Going to full throttle we accelerated to around 5 mph (30 psi on the gauge) but couldn't go in much of anything resembling a straight line. A big problem was posed by the 9 ft. length of suction hose which didn't really want to stay underwater and was causing quite a bit of drag. Nonetheless it worked.

Two exciting action shots are shown below.

Disaster strikes! The welded pvc fitting just above the pump outlet gave way producing a rather impressive geyser. Henceforth we called the jet drive "Geyser King." Notice how Candy got the worst of it.

Nothing damaged but we were a bit wet upon return to the dock.

Here is the offending fitting. Candy is pointing out my flawed plumbing technique. Amid a few epithets she said something about needing threads.

Water Trial 2

Time for lunch and to make some repairs. I was mystified as to why the fitting let go but I cleaned it up and applied more solvent and glue and let it set while we had lunch.

Another item to take care of was the pesky suction tube. Ideally we'd have a flush through-hull fitting with strainer but Candy had something against putting holes in her boat. So we elected to try to constrain the hose as much as possible to the side of the boat. A couple of rope-and-clamp schemes failed so we elected to duct tape a brick to the hose. Ever resourceful, Candy had a roll of 6" wide duct tape (eat your heart out Red-Green!).

The photo below shows the fully dressed and weighted suction hose.

Time for the next test.

Under way again the suction tube was still making a lot of drag (it wasn't possible to row the boat much faster than 1.5 mph) but with the tube closer to the boat, directional control was much improved and we were actually able to go in straight lines and make predictable turns.

For those who think that the nozzle would work better immersed the answer is absolutely not. Immersion caused cavitation and a reduction of the top speed to around 3.5-4 mph. And it doesn't look near as impressive.

Just as we were getting comfortable with the thing, the second disaster. The same fitting let go. This time Candy had a hard time reaching the throttle and we both got even more thoroughly drenched. Notice how you can't see Candy.

From the shore all that could be heard was crazy laughter and shouts of "kill the throttle, kill the throttle."

Please note the swim goggles on Candy. Of course, she wasn't wearing them when the pump let go.

Enough for today, we figured another attempt at repair would be futile. Time for dry clothes, beer and wine.

Summary and Next Steps

First, in case you wonder where my wife Chris was, she was playing the part of the smart older sister, staying dockside and taking the pictures. No dummy she.

I think that this was pretty successful, enough to warrant outfitting a boat properly for this simple jet pump. I've already figured out how to adapt it to my stalled Shell Swifty-14 rebuild project in a way that won't damage the boat should I eventually abandon the project.

Some tentative conclusions:

  • In my backyard test setup I was able to achieve about 40 psi pressure with the 24mm nozzle. In the boat I was only able to obtain about 30 psi. This may be due to the longish suction hose as compared with the backyard suction tube which may have acted to limit the capacity of the pump.
  • The nominal top speed was largely due, I believe, to the hull speed of the boat and the drag of the suction tube. When rowing the tube and brick did have a noticeable drag. A longer, cleaner hull should do better.
  • It's certainly been a fun project thus far!

I plan to have the Swifty outfitted with the jet drive by the spring of 2004. Stay tuned.

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