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Thread: Jet Motor on a cataraft

  1. #1
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    Default Jet Motor on a cataraft

    I have been with a guide in Vancouver Island that uses a jet motor on a small aluminium boat. It is great for heading up river in shallow water.

    Has anyone had any experience of using a jet motor on a cataraft to tackle shallow water?

    cheers

    Graham

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    Default

    This thread has a little information about using motors on cats, and even mention of jet units on cats. http://forums.outdoorsdirectory.com/...t=41706&page=2

    The first problem is deciding what to do about all the water spray flying every which way when using an outboard. The second problem is trying to figure out how to stop the cavitation. It seems the cat tubes create a lot of convoluted water in between the tubes. Mashing it all down to calm it creates more issues. It hardly works for prop motors without dropping the motor way down into undisturbed water, I can't imagine doing it successfully with a jet. But maybe someone knows a way to make it run shallow.

    My favorite idea involves strapping a snowmobile to the center of the cat, and letting a 2" paddle track provide the push. That way you could ride your sled year round on white water. -- Not sure how to steer it though.

  3. #3

    Default Arctic Cat a Raft

    Quote Originally Posted by Jim Strutz View Post
    This thread has a little information about using motors on cats, and even mention of jet units on cats. http://forums.outdoorsdirectory.com/...t=41706&page=2

    The first problem is deciding what to do about all the water spray flying every which way when using an outboard. The second problem is trying to figure out how to stop the cavitation. It seems the cat tubes create a lot of convoluted water in between the tubes. Mashing it all down to calm it creates more issues. It hardly works for prop motors without dropping the motor way down into undisturbed water, I can't imagine doing it successfully with a jet. But maybe someone knows a way to make it run shallow.

    My favorite idea involves strapping a snowmobile to the center of the cat, and letting a 2" paddle track provide the push. That way you could ride your sled year round on white water. -- Not sure how to steer it though.

    Jim I think you might be able to rig some type of rudder system to the Skis of the snowmobile! This system might solve some of the early and late season riding problems when the rivers are open but we still have snow on the trails.

    Jim King

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    webmaster Michael Strahan's Avatar
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    Default Jets on Cats

    Quote Originally Posted by Jim Strutz View Post
    This thread has a little information about using motors on cats, and even mention of jet units on cats. http://forums.outdoorsdirectory.com/...t=41706&page=2

    The first problem is deciding what to do about all the water spray flying every which way when using an outboard. The second problem is trying to figure out how to stop the cavitation. It seems the cat tubes create a lot of convoluted water in between the tubes. Mashing it all down to calm it creates more issues. It hardly works for prop motors without dropping the motor way down into undisturbed water, I can't imagine doing it successfully with a jet. But maybe someone knows a way to make it run shallow.

    My favorite idea involves strapping a snowmobile to the center of the cat, and letting a 2" paddle track provide the push. That way you could ride your sled year round on white water. -- Not sure how to steer it though.
    Hi Jim,

    There are a few folks out there running jets on catarafts, but back when I was assisting with R&D on this, it seemed like there was one problem after another. To deal with this you have to separate out the issues and deal with each one. Here are the dragons that have to be slain, as I remember them:

    1. Wake convergence. As the cat tubes are pushed through the water they generate a wake that converges about 2/3 aft of the bow. This creates an aerated water column aimed right at your outdrive. It is possible to flatten this water column enough to bleed most of the air out along the sides, if the "skid plate" is designed with this in mind.

    2. Cavitation. This happens with both jets and props, but is more common with jets, since their intakes are close to the surface. Cavitation results when: a) the outdrive is too high in the water column or b) the water column is too aerated to provide consistent motor performance. The workaround is all in the skid plate design. You have to get the front-to-back angle just right, and it has to be shaped properly from side to side. The ideal skid plate is turned slightly up along the port and starboard edges, to allow air to bleed out without coming at the outdrive. Some of the better skid plates also have tabs extending six or more inches past the transom.

    3. Skid plate support. There's a lot of hydraulic pressure pushing against your skid plate, so you need rigidity there. You can go with a strong material like aluminum, or you could use high density polyethelyne sheet material. If you go with poly, you are most likely going to have to brace it internally to prevent it from buckling under pressure. But it's not as easy as it sounds. If you use frame pipe for this, the temptation is to screw or bolt the plastic to the pipe from the under side. But the screw or bolt heads will create drag, trap air bubbles (leading to cavitation), and be subject to impact damage against rocks. This also applies if the plastic is to be secured to the bottom of the transom.

    4. Motor lift. Wake convergence allows you to run the motor higher than normal, once you are on step. But for this to be possible you must have an electric lift of some kind, since the convergence does not occur until you are moving at least 5-10 mph. Most folks are not using electric lifts on motors that would go on smaller cats, however on the larger custom boats it is a workable option. The largest we ran had an 88-horse on it. It was a custom-built 22' single tube cat made by AIRE.

    5. The spray issue. This is one of the biggest problems with power cats. Wake convergence forces water up under whatever floor system you are using, and unless you have a way to seal the floor against the tubes, water will squirt up between the floor and the tubes. You'll have twin rooster tails, one on each side of your floor. On the twin tube cats like the Super Leopard and the Cougar, you also get the same thing between the tubes on each side. If you can control this spray, you accomplish two things; you stay dry, and you push more water under the hull, generating more lift.

    6. The risk of flipping. Inflatable cats are not displacement hulls, they are planing hulls. As a result, they have a tendency to flip over in strong headwinds, rough seas, or at high speeds or any time air is brought under the hull and compressed. Some of this can be overcome by reducing speed, tacking, and balancing / securing your load better. But it is potentially dangerous and must be recognized.

    There are probably other issues I'm not thinking of now, but these are some of the ones we encountered. It would be fun to tackle this again some time. I have some ideas that we never tested, which I am *sure* would work!

    Best of luck!

    -Mike
    LOST CREEK COMPANY: Specializing in Alaska hunt consultation and planning for do-it-yourself hunts, fully outfitted hunts, and guided hunts.
    CLICK HERE to send me a private message.
    Web Address: http://alaskaoutdoorssupersite.com/hunt-planner/
    Mob: 1 (907) 229-4501
    "Dream big, and dare to fail." -Norman Vaughan
    "I have climbed my mountain, but I must still live my life." - Tenzig Norgay

  5. #5

    Default Jet on cataraft

    The gentleman that had the post ahead of me pretty much nailed the problems on the head, but in my experience it is simplier to deal with.

    I ran a 40 hp on my 16' Aires for 8 years and could make 23 mph by gps up 20 mile and 18 up the big Su, not as efficient as a small jet sled, but pretty dam good.

    Made a hull out of white plastic that dog sleds runners are made of and to stop water from coming up between it and the tubes had 1/2" rubber bolted on that to the plastic and forced up against the tubes, no water coming up at all. To stop cavitation I put 1"wide by 1 1/2' plastic strips approx. 1' inboard of the tubes, this forced water with lots of air to go outside of the plastic strips and pure water into the jet unit and no cavitation.

    Go see Tracy at Alaska Raft and Kayak and talk to him, he used to make them. I got a picture years ago from the shop he worked at, and made one myself. I am off to South America in the morning for a month or I would try and dig out pictures, I am pretty sure Tracy can help you out. Good luck. Oh yea, only drawback was I needed more water to get on step than a pure sled did, once on step could go in 6" water.

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    Default Funnel up a tunnel

    http://www.jpwinc.com/index.php?page=photo&photo_id=167 is another link to Jacks Plastic welding showing a cataraft planing. I cannot quite work out in which direction the plywood hinges.

    I wonder if a tunnel hull built into the skid plate would help with the shallow water. The flats boats off the coast of Texas can run in real skinny water with tunnel hulls.

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    Default I remember those skid plates-

    Quote Originally Posted by mad View Post
    The gentleman that had the post ahead of me pretty much nailed the problems on the head, but in my experience it is simplier to deal with.

    I ran a 40 hp on my 16' Aires for 8 years and could make 23 mph by gps up 20 mile and 18 up the big Su, not as efficient as a small jet sled, but pretty dam good.

    Made a hull out of white plastic that dog sleds runners are made of and to stop water from coming up between it and the tubes had 1/2" rubber bolted on that to the plastic and forced up against the tubes, no water coming up at all. To stop cavitation I put 1"wide by 1 1/2' plastic strips approx. 1' inboard of the tubes, this forced water with lots of air to go outside of the plastic strips and pure water into the jet unit and no cavitation.

    Go see Tracy at Alaska Raft and Kayak and talk to him, he used to make them. I got a picture years ago from the shop he worked at, and made one myself. I am off to South America in the morning for a month or I would try and dig out pictures, I am pretty sure Tracy can help you out. Good luck. Oh yea, only drawback was I needed more water to get on step than a pure sled did, once on step could go in 6" water.
    Mad,

    The design you're talking about originated with Paul Jobe at the now-defunct "Wild Alaska Rivers Company" (WARCO). He was my hunting partner for 17 years, and the guy I had nearly all my R&D experience with (I was never an employee, I just spent most of my free time down there brainstorming and testing ideas). Several folks took the design and, either because they were trying to save money or just wanted to tinker with it, built your own. Perhaps you are one of those folks? I remember those floors; the plastic is UHMW, and was purchased in Anchorage at Cadillac Plastics over on Post Road. The method of bolting a rubber strip along both sides of the skid plate was one of the most effective means of eliminating the spray issue, however it did nothing for the double-tube boats (the Cougar and the Super Leopard). The original Cougar and Panther design had some hull design idiosyncrasies that created spray off the front of the tubes, however the new Cougar design, with its gradual curve at the bow and stern, has eliminated this issue (alas, the Panther is no more).

    I was not aware of the longitudinal strakes you mentioned, but I can see where this would help with the aeration issue. It would not eliminate it, but it could reduce it. Of course, it increases the cost.

    At any rate, WARCO attempted many experiments with this, and many of them failed. Some boats used full-length UHMW sheets, others just used a skid plate at the back, etc. There was talk of an aluminum hull to go between the tubes, but I don't think this was ever done. Regrettably, most of Paul's designs and modifications were done on customer's boats and because the delivery schedule was often tight, no blueprints were ever really developed or retained. For that reason, nearly every rig was a custom job.

    There were some ideas that were never tried, and I think they are perhaps the most promising. One that really should be done is an inflatable floor that laces to a flange on the tubes. This would certainly control the spray, and would also duct the aerated water where you want it to go, provided the bottom of the floor starts in a vee shape.

    I don't think a tunnel hull would work because it aims that aerated water right at your jet intake, or at your prop if you're running one. I believe you'd get a lot of cavitation.

    You are correct that if you're working with a shop, Tracey is your man. He used to work for Paul at WARCO and that's where most of this started.

    Hope it helps!

    -Mike
    LOST CREEK COMPANY: Specializing in Alaska hunt consultation and planning for do-it-yourself hunts, fully outfitted hunts, and guided hunts.
    CLICK HERE to send me a private message.
    Web Address: http://alaskaoutdoorssupersite.com/hunt-planner/
    Mob: 1 (907) 229-4501
    "Dream big, and dare to fail." -Norman Vaughan
    "I have climbed my mountain, but I must still live my life." - Tenzig Norgay

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