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Results 1 to 6 of 6
  1. #1
    Join Date
    Apr 2008
    Location
    Johannesburg, Gauteng, South Africa
    Posts
    362

    Default "Hi all! I've seen many

    "Hi all!

    I've seen many posts about jet drives and would like to know the benefit of this system.

    I understand that it all boils down to the terrain of where the boat will be used, yet the jet drives seem to be scarce. I've yet to see one on a boat in my area. I'm mostly in fresh water.

    How would you change performance of a jet drive? On a prop motor, you'd go for a different pitch. Can a jet's pitch (pump) be changed?

    How is the fuel consumption (assuming same conditions and rig) of jet vs prop?

    Maitainance of jet drives costly vs prop? Spares availabilty?

    Do they also cavitate?

    Does HP output rating stay the same if I swapped my prop for a jet drive? I have a 1997 Mariner 115 ELPTO with 17" prop, what jet would I look at? I have a 18,5' bowrider that is used in fresh water, mostly cruising, seldom skiing or tubing. The performance is good, just looking at other options...

    Should I consider this option?


    Thanks!

    Benjamin."

  2. #2
    Join Date
    May 2007
    Location
    Inverary, Ontario, Canada - The Great White North Eh!
    Posts
    8,505

    Default "Benjamin, a jet d

    "Benjamin,
    a jet drive is one of those things that you don't get unless you absolutely need one.

    They are designed to run in very shallow water. The technology is exactly the same as is used on a jet ski. Water is pushed out of the (jet) by an impeller to create forward movement much the same way a jet engine on a plane blows pressure out of the back of it's engine.

    Much the same as you would use different pitched props on a conventional motor, there is different impellers depending on the particular application.

    A jet is a total gas hog often guzzling up to double what a conventional outboard would at the same displacement. I didn't say horsepower because if you take, say a 60 horse and hook a jet drive on - bingo, now you only have a 40 horse jet - about 1/3 of your inital horsepower is gobbled up in order to drive the jet.

    So if you want to see your fuel costs near double and turn your 115 into about an 80 horse and drop a couple of grand on the lower unit itself - plus re-rig it - then a jet may be an option - but unless you are running in water that is only an inch or two deeper than the draft on your boat with the motor up there is absolutely no reason to run a jet..."
    Graham

    A "professional" is someone who gets paid for their work - it doesn't necessarily mean they are good at it :)

  3. #3
    Join Date
    Jun 2008
    Location
    , nj, usa
    Posts
    51

    Default Great answer Graham! A fuel h

    Great answer Graham! A fuel hog indeed..........

  4. #4
    Join Date
    Jul 2008
    Location
    West Palm Beach, Crystal River, Florida, USA
    Posts
    8

    Default I have a jet drive Mercury opt

    I have a jet drive Mercury optimax and surprisingly does pretty good in the gas dept. About 4-6 mpg. Remember that the jet drive does not have a skeg and lower unit gear box and all that drag it presents when under way.. A jet drive virtually has a smooth bottom and no drag . This may compensate for the better fuel economy. The hull is the criteria you need to look at for fuel economy as well. Great in shallow water and tough to navigate in slow idle conditions with wind and current.

  5. #5
    Join Date
    Apr 2008
    Location
    Johannesburg, Gauteng, South Africa
    Posts
    362

    Default "Hi all! Thanks for the ins

    "Hi all!

    Thanks for the insight, I was just wondering what the pro's & con's were to the two options.

    Enjoy the day!

    Benjamin."

  6. #6

    Default "Benjamin: I've been runni

    "Benjamin: I've been running an outboard jet for about 10 years and before that I had an inboard jet.

    As someone said, although outboard jets are wonderful rigs when you need them for shallow running (I can get through less than 6 inches of water while on plane with my 21 foot boat with 200 hp engine, loaded pretty heavy), the inefficiencies make them - in my opinion - a poor choice for general normal depth boating.

    The outboard jet, because of the inefficiencies of the design, reduces your effective horsepower by about 25-30%. Therefore, for example, if you replace the prop lower unit of a 100 hp engine with a jet unit, it will be putting out at most 75 hp at the jet outlet.

    Associated with this is gas mileage. The engine will burn the same amount of fuel for any given RPMs, but because it's putting out less power at any given RPMs, it will be going quite a bit slower. Therefore your miles per gallon will decrease quite a bit. My V6 w/ jet burns pretty much the same GPH as my friend's similar engine at "cruise" speed (35-3800 rpm), but I'm going 25 and he's going almost 35.

    As for impellers, for any given engine (whch the jet unit is specifically matched for) there are not differnt impellers available as far as pitch and diameter goes. For any given rig, there are usually two options: stainless steel and aluminum. The stainless will be quite a bit more expensive, but more durable and a little bit more efficient because the blades are thinner and less flexy.

    All outboard jets - whether "OEM" factory installed, or aftermarket, are manufactured by the same company, Outboard Jets (www.outboardjets.com)

    From their FAQ on the website, some information on efficiency, impellers, etc. straight from the horse's mouth:

    2. What speed will I get?
    It depends on the boat and the weight. A good setup will get about 80% of propeller speeds when using mid range prop pitch. A light weight prop boat using a high pitch prop can run twice our outboardjet speed.

    3. How about a heavy load?
    A heavily loaded prop boat using a low pitch prop can haul much more weight than we can.

    4. Can we change jet impeller pitch for load or speed?
    No. Unlike a propeller drive, a jet can develop full HP at a standstill. We choose an impeller that gives rated factory RPM at full throttle. RPM hardly varies between a heavy load and running fast with a light load.


    There's quite a bit of info on the web about jet setups if you do a search."

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