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Old 22-06-2009, 09:16   #1
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Jet Drive Dinghy Propulsion

My teeange son thinks the the dinghy is his jet ski. He spends more time in it than anyone and thus he has by default become the dinghy captain. The problem with this is that we have broken our fair share of shear pins and props and I fear that on our next trip we may run out of either or both at an inoppurtune time. Has anyone had experience with jet drives on their outboards? They use these commonly here in the western shallow streams and rivers to avoid the problems that I mentioned. We do our sailing in the Caribbean and often beach the dinghy and motor in shallow rocky anchorages.
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Old 22-06-2009, 09:28   #2
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A few years back I saw an RIB with a jet drive, weighted about 700 lb, but it was big, steered with handle bars, really cool, and expensive.
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Old 22-06-2009, 09:34   #3
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RIBs with jet drives are increasing. This last cruising season we saw about four or five of them in the Bahamas, of course they were all off fairly large motor yachts. Heavy and expensive.
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Old 22-06-2009, 10:43   #4
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There are some outboards that use what are essentially Kort nozzles. They are less efficient at higher speeds. I don't know what horsepower they go down to or if they would be any more resistant to damage caused by running aground. I think the better and less expensive solution might be to change how your son operates the dinghy.

Kort nozzle
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Old 22-06-2009, 10:57   #5
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I bought a jet boat (a Bombardier) for our lake, thinking that it would be better in shallow places.

As I result of my ignorance about the principles of jet drive, I was totally wrong and deeply regretted not buying an ordinary boat with outboard. Jet boats (and jet skis, too, by the way), as it turns out, are extremely sensitive to trash in the water (like floating seaweed), which gets sucked up into the intake, clogging it, and are extremely sensitive to sucking up rocks from shallow places, which destroys their impellers.

Besides that, they are much less efficient than prop drives. Our Bombardier with 240 horsepower sucks gas like the Space Shuttle -- you can almost watch the fuel gauge falling if you have any throttle on, although it is not quite as fast as regular outboard driven boats with half the horsepower. It does have a couple of advantages -- tremendous manueverability (you can put the helm hard over at top speed, and the thing just spins around -- it's a blast!), and if you are pulling water skiers, you don't have to worry about cutting off anyone's legs with the prop. But that's about it; every other aspect is a minus.

It's been for sale for three years, but so far haven't found anyone as stupid as I was.

So don't do it! Better buy him a nice RIB with a center console, and a nice powerful two-stroke outboard. And instruct him on avoiding contact between a moving prop and the ground, which is a pretty basic dinghy seamanship skill.

I sympathize with the dinghy-as-runabout idea. There's a lot of territory you can't explore in the mother ship. It would be fun to have a dinghy which is comfortable for ranging about further afield while the mother ship is at anchor.
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Old 22-06-2009, 11:04   #6
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I know of cases where garbage bags shut down jet drives. When it happens to a propeller, quite often you can go in reverse to remove it. Not with jet drives whose impeller cannot go in reverse. You either have to hire a diver or get the boat hauled.
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Old 22-06-2009, 11:31   #7
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I have seen them on old 15hp Yamahas. Too bad someone doesnt come up with an inexpensive lightweight inboard motor/ jet drive for dingys. Keeps the weight low, no lower unit weight etc required, might be kinda cool if well thought out.
Regarding your son the dingy driver: it sounds like he needs some lessons in caution! I spent 3 years in the Caribe with one prop for my 15HP Yamaha. A lot of time in the bahamas too...granted I was probably lucky... but munching a prop should be a pretty occasional thing if proper precautions are taken shouldnt it?
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Old 22-06-2009, 12:33   #8
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Time for your son to start buying props and shear pins
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Old 22-06-2009, 17:16   #9
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Solution: frankenjet..... A RIB with a fiberglass bottom, cut out the bottom and fiberglass in the essential components of an old jet ski (kawasaki 300, 440, 550). These can be found for under $200, $100 for the 300 which would also be the most fuel efficient. I don't know what sort of seat would be used, maybe a box covering the entire back end which would be the drivers seat. I've seen this being controlled by a joystick (forward left and right). This would be mounted like the stick in a plane.
There is also such a thing as an outboard jet drive, This can be a bolt on to an exiting outboard but a dinghy motor may be too small for this, google it
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Old 22-06-2009, 17:48   #10
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Thanks for all the advice guys. I especially like the advice about cautioning a teenager. They take your advice until they turn the corner in the cove! I have owned several jet skis and even the high performance internal units are not very large and couldn't weigh that much, whereas I agree that by the time you go to a large enough outboard to power the jet drive you have a weight issue. I would be interested to see a rib with an integral engine.
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Old 22-06-2009, 18:06   #11
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Propeller guard?

Have you considered a propeller guard?

They look to come in various shapes and sizes and are mainly designed to protect swimmers but a robust one could do the trick for you.

A few links:-

PESA
Solas
getaprop
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Old 22-06-2009, 19:04   #12
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There are two types of jet units available - inline/inboard, and outboard. Most of the above comments are aimed at the inboard units, with which I have zero experience.

Jet pumps (a lower unit replacement) are available for outboards down to 20HP, both as aftermarket parts and as factory outboards. You generally lose about 30% efficiency, and the factory-installed options are often labeled accordingly (eg, a "20HP" may be a 35 powerhead).

Weeds are a huge issue, as is rough water. I don't think you'd be happy with the outboard version in saltwater.

I have a prop guard from Cabelas that works exceptionally well on a 15HP, except in small rocks which get stuck between the prop and guard.

Cabela's -- Mac's River Runner

This appears to be an even more robust variant:

Cabela's -- Rock Hopper Motor Guard

Motors have been known to violently depart boats when hitting underwater obstructions - make sure you have a safety rope from the motor to your boat if making a habit of running into hard things!
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Old 23-06-2009, 01:15   #13
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Quote:
Originally Posted by rjrendemd View Post
Thanks for all the advice guys. I especially like the advice about cautioning a teenager. They take your advice until they turn the corner in the cove! I have owned several jet skis and even the high performance internal units are not very large and couldn't weigh that much, whereas I agree that by the time you go to a large enough outboard to power the jet drive you have a weight issue. I would be interested to see a rib with an integral engine.
Here you go: Seaport Jet

There are various RIBs with jet ski propulsion systems available. This one, from Avon, could be carried on board some of our boats.

But again -- a jet ski impeller is much more expensive, and much more difficult to replace, than an outboard shear pin. This is NOT a solution to that problem, in my opinion. You would not want to run that thing up on beaches if there are any pebbles in the water, just like you wouldn't do that with a jet ski. A regular outboard, on the other hand, is no problem, if you practice basic dinghy seamanship, tilt the engine up when needed, don't back into rocks, etc.

Plus I think fuel would be a problem, unless you're only cruising in places with fuel docks. That thing will burn through 45 liters of internal fuel pretty fast.
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Old 23-06-2009, 03:35   #14
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This might be an option

I agree, the Jet ski propulsion is too sensitive to garbage and junk in the water to be very useful for mounting on cruising dinghy...
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Old 23-06-2009, 05:54   #15
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We have a dolphin fin that attaches to the very bottom of our dinghy motor. It is supposed to make the motor more efficient. Can't speculated on that, even though I have had it on for several years. I can say however that I KNOW it has saved our prop on a number of occasions! How the heck do you run aground in a vessel that has an 8 inch draft!? Repeatidly!
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