Development of a Jet Drive for Small 
                  Boats  
                  using a Commercial Water Pump - Part 2 
                  
                  by Steve 
                  Hansen  
                Back to Part 
                  1 
              
              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. 
                
              Back to Part 1 
              Visit Steve 
                Hansen's Boat Page 
               
  |