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trontek
#1 Posted : Sunday, September 19, 2010 5:16:04 AM(UTC)
trontek

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Stumbled on this. May not be current but thought it interesting....

http://www.marinetteforsale.com/32E68BC.htm
Jim

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dougrose
#2 Posted : Sunday, September 19, 2010 10:38:43 AM(UTC)
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I looked at jet drive for my 28' since I was doing a conversion anyway, but I stuck with the velvetdrive and shaft. It is expensive to get a large jet that is efficient at reasonable speeds. It would certainly be nice to get the props out from under - especially in Florida.


1975 32' Flybridge Sedan, twin Perkins 6-354 diesels, 1:1.53 velvetdrives, 16 X 19 props. Merritt Island, Florida
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jimski2
#4 Posted : Sunday, September 19, 2010 10:38:51 PM(UTC)
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My brother has a 25 foot welded aluminum jet drive boat built by Grayling in Anchorage, AK. He upgraded the engine to a Ford 460 ci and uses the boat very little because of the gasoline prices. It is on the market now since he does not need the boat to reach his cabin on the Yehtna River for moose hunting. The jet drive is fine when running over a gravel bar, but several times the boat has ended up high and dry and a lot of sweat and muscle is needed to refloat the boat. The welded aluminum hull takes the groundings fine though.
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jimski2
#5 Posted : Tuesday, September 21, 2010 3:57:17 AM(UTC)
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The Grayling 25 footer jet boat sold yesterday for $16,000 to a couple who want to build a home across Cook Inlet from Homer, AK. 18 months on Craig's List.
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Doug Doty
#6 Posted : Sunday, December 05, 2010 1:50:17 PM(UTC)
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trontek wrote:
Stumbled on this. May not be current but thought it interesting....

http://www.marinetteforsale.com/32E68BC.htm


I followed up on your link and it was sold a while back !!
466 Bertram FDMY, " Easy Rider "
1984 39 Marinette sedan, " LIBERTY ONE "
440'S 19x22 wheels, 710 HOURS
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pfhlaw
#7 Posted : Monday, December 06, 2010 5:23:00 AM(UTC)
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Jet drives are not nearly as efficient as props when it comes to converting the engine's HP to forward thrust.
I have 2 jet drives. One is a Sea Doo Explorer with a 720 cc 2 stroke motor producing 85 HP at the shaft.
But Sea Doo says that at the pump it has only 35 HP propelling the boat.
The other is a Clinton outboard motor with a jet drive instead of a prop. The motor is rated at 7 HP but the jet drive de-rates the unit to about 3 HP.
Peter
1981 32' sedan bridge
twin Chrysler 360 cu. in. 250 hp engines
Raw water cooled
Nimbus II
Home port: New Buffalo, MI
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fastjeff
#8 Posted : Monday, December 06, 2010 8:57:52 AM(UTC)
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Most outboards with jet drive lower units instead of props are de-rated about 10 per cent.

Jeff
"...reality is not nearly as lovely as the world of Liberal Land. No wonder so many people want to go there." - Tom Sowell
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pfhlaw
#9 Posted : Monday, December 06, 2010 9:37:00 AM(UTC)
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Actually, according to Outboard Jets, it's about 30%.
Peter
1981 32' sedan bridge
twin Chrysler 360 cu. in. 250 hp engines
Raw water cooled
Nimbus II
Home port: New Buffalo, MI
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dougrose
#10 Posted : Monday, December 06, 2010 3:07:20 PM(UTC)
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You are always better off moving more water at less speed.

The force that you get pushing the boat forward is proportional to m (mass) times v (velocity) in the propeller slipstream. The amount of energy it takes to push that mass at that velocity is m times v squared (divided by 2). So, the energy it takes to generate a given force is proportional to mv squared divided by by mv or more or less proportional to v. When v gets smaller, the energy required per unit force generated in the slipstream becomes less.

This makes a larger propeller more efficient, up to the point where the increasing friction and form losses in the propeller get too large.

The problem with jet drives is that their impellers are very small, slipstream velocity is very high, and so efficiency is poor. Making the drive throat larger and the slipstream slower improves efficiency, but the larger size creates friction and flow losses even faster than for a propeller alone. It is hard indeed to make a jet drive boat that doesn't suck fuel.


1975 32' Flybridge Sedan, twin Perkins 6-354 diesels, 1:1.53 velvetdrives, 16 X 19 props. Merritt Island, Florida
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fastjeff
#11 Posted : Monday, December 06, 2010 9:59:20 PM(UTC)
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Got a headache reading that, Doug, but a great explanation!

Jeff
"...reality is not nearly as lovely as the world of Liberal Land. No wonder so many people want to go there." - Tom Sowell
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jimski2
#12 Posted : Tuesday, December 07, 2010 10:53:07 PM(UTC)
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Jet drives save you momey when you hace to replace props and lower units frequently. There are areas where they are necessary.
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dougrose
#13 Posted : Wednesday, December 08, 2010 2:37:58 AM(UTC)
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You bet there are areas like that. Last trip to the boat I put in a new strut, and promptly ran aground because I was paying attention to the cell phone and not the boat.... Now I have, at the very least, a rework on both props and I may need to replace them. A considerable expense for a few miles of boating....

You can buy a larger jet drive to benefit from the bigger prop, but then you are paying for a jetdrive capable of using 1000 hp when you only have a couple hundred. Just not economical.

There was a guy in Ft. Lauderdale who was making jetdrives out of plastic. (OK, engineering polymer) He had the impeller and the throat around the impeller in stainless, within the plastic shell, but the rest was injection moulded. Much cheaper than the huge aluminum castings that make up the usual jetdrive. I got a bunch of info from him, but he was looking for capital and apparently didn't get any, because he has disappeared.


1975 32' Flybridge Sedan, twin Perkins 6-354 diesels, 1:1.53 velvetdrives, 16 X 19 props. Merritt Island, Florida
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