Jet vs. Prop Efficiency

Discussion in 'Jet Drives' started by Michael1, Oct 6, 2007.

  1. Man Overboard
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    Man Overboard Tom Fugate

    Gerr's Propeler book has an explanation if I'm not mistaken. I am at my studio, so I can't double check.

    Let me see if I can remember this;, super-cavitating propellers induce cavitaiton through their design. I can't remember the exact design parameters, but cavitation is induced at the leading edge of the foil. this cavitaion (water vapor) blankets the entire surface of the suction side of the propeller this reduces friction, thereby freeing up more power to generate thrust.

    Super-cavitating propellers are similar to surface piercing propellers, but they are not the same, as they operate 100 % beneath the water surface.

    I believe Super-cavitating propellers are made with special alloys to eliminate erosion from the suction side of the impellers.

    It would be stand to reason that Super cavitating propellers would operate more efficiently over a broader range of RPM's than surface piercing propellers. but yet, it would also stand to reason that they would need to operate at a higher RPM then a normal propeller, at least to be efficient.
     
  2. Pericles
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    Pericles Senior Member

  3. Man Overboard
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    Man Overboard Tom Fugate

    Pericles,
    Hey, thanks for that article. This is what I had in mind. I was surprised the fuel consumption for the original jet ski was 11 gallons per hour at full throttle; and for the finished scaled down lobster boat 6 gallons per hour at 2/3 throttle (2100 lbs. @ about 20 knts) The jet drive is defiantly not as efficient as a prop drive.
     
  4. Pericles
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    Pericles Senior Member

    Man Overboard,

    The fuel consumption of a 140 hp Suzuki, which would deliver similar performance on that hull, would be around 4 gallons per hour at 20 knots, saving 2 gallons per hour. However, one grounding and the lower unit repair costs will easily outstrip the 2 gallons per hour fuel saving.

    There are so many variables that each case has to be considered individually. For me, the most important benefit is the lack of underwater projections and if taking the ground is imperative, the jet has it all, it would seem.

    The major manufacturers of jet drives will continually upgrade their products, so occasions where vessels have not performed as desired, are becoming far fewer. By getting the JD manufacturer involved at the start of the project, success should be assured.

    Pericles
     
  5. Pericles
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    Pericles Senior Member

  6. Man Overboard
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    Man Overboard Tom Fugate

    Pericles,

    Thanks for the link. I followed the links in the other thread to the New Zealand jet boat site. One of the posters did not recommend using a drive from a personnel water craft, but these guys are used to running V8's at 40+ knots. I just don't need that kind of speed. I would rather have a lighter boat, lower CoG and more room inside the boat. One of the purposes of building the boat is to get used to working with Kevlar and carbon fiber.

    The lobster boat used a Kawasaki unit, but I think I would rather go with Sea-Doo. Their motors are strong reliable units, and they have an enclosed cooling system.
    I contacted See-Doo; getting tech info from them may be difficult, but a local dealer is helping me out.
     
  7. Michael1
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    Michael1 Junior Member

    You may want to check the PWC forums, such as pwctoday.com. When it comes to reliability, most of the complaints I've seen are how unreliable Sea Doo engines are. You might want to check out Yamaha.

    Michael
     
  8. speedboats
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    speedboats Senior Member

    CDK, if that is a pic of the transom of your boat I can see what the problem would be. How many different jet pump manufacturers had you looked at before 'jumping in' to the twin berkely install? How much jet pump tuning did you do? What RPM are you pulling. Do you install an outboard and put a 24 pitch prop on it an expect the boat to go fast? Come on, the engine probably won't make any RPM, will burn way to much gas and parts (due to high load/RPM on the engine). Big boat like that I'd've gone with an axial flow pump rather than a mixed flow type.

    Jets definately have more manouverability, you just need to leard to drive with the throttle and reverse bucket (which on those Berkelys is a joke). Dial up American Turbine and look at their HTRII bucket. A jet is different, and as such must be driven in this manner. Don't like it and don't want to change? Imagine if that was our attitude in an automobile, have you driven a Model T lately? (I kinda prefer the newer two pedal method... you know, gas on one side and the brake on the other)

    You didn't use a grill? Geez, this is one of the most important things on the bottom of a jet, it aligns the water ready for the impellor, blocks off water that isn't needed, and yours lets rocks through? We build ours with about a 9/16 gap between bars. Don't like getting your bowline caught in the drive? Either cut it short so if it is in the water it won't quite make it to the drive (carry a spare longer one for those occasions when you park on a beach), or tie a figure 8 in the end so it won't pass through the grill, or try this real novel idea, when the rope is overboard, pull it inboard.

    It seems that you guys are falling into the same trap that we fell into in the '80's, same small jet pumps into bigger and bigger boats with bigger and bigger engines and wondering why they make lots of noise, go nowhere, and burn heaps of gas. Ever wondered why big passenger ferries don't use 50hp eggbeaters? Would be real economical considering that the 50's don't burn much gas and go so well on my dinghy... Let's get thinking... fit for purpose???

    We build our boats to suit either jet or egg beaters, the later is built lighter as it has no requirement for impact resistance as the jet does. When both types of the same hull are matched for speed, the egg beater has a smaller hp sticker on it, but, and here's the clincher, they both use about the same amount of fuel. Depending on pump choice or setup the jet will make it ontop first. Anyone here seen a jetsprint boat, damn things will out accelerate any supercar to 60mph.

    Expensive to maintain? Get a new mechanic, what we see alot of around here it that if the marine mechanic dosn't know about jets, he'll keystone the parts, and charge you the labour it took him to figure out how to do the job. eg, mainseal in a Berk, they'll take off the reverse and steering controls, remove the nozzle, remove the bowl, remove the impellor nut and remove the impellor, then see that the mainseal isn't accessed from there, or worse, remove engine, thrust bearing and housing and then the mainshaft out the back of the pump. How many hours to remove and re-install all that? and who's paying for the labour? All that is needed is to make sure boat is out of water, remove the two locking bolts behind the thrust bearing, remove the retainers and old rope seal, install new seal, replace retainers, don't overtighten retainers as they will pinch and wear the shaft (they are designed to weap), put back in the water. How much was the rope seal? About $45, labour? about 1 hour (min charge)

    Ever had to change a gearbox on a jet?

    Bollard pull. Had the national jetboat marathon here recently, couple of guys sunk their boats after launching themselves off pressure waves, the rescue boat, fitted with a ZZ383 and HJ212 waterjet lifted and continued to pull a water laden 21' race boat onto the plane. That's pretty good pull.

    Packaging a jet into 14'? Simple, it's what we do.

    500hp to do 90 mph in a jet? done that too (about 600 to 650 is 100mph)

    Jets do require a design difference and consideration before the boat is built, and simply converting to jets isn't always going to yeld the best performance from either the pump or hull. This was another trap we fell into in the '80's. With people continuing to do this without doing homework, and the sales man's need to sell numbers, jets will continue to get a rubbish name, and will possibly go into obscurity for a decade or so.

    Why don't the big manufacturers make jets? Well what do they sell? short answer... powerheads. Yamaha, Suzuki, Mercury, Johnson/ Evinrude, Honda and whoever make billions of dollars off selling easy to interchange, easily packaged, eggbeaters. Why start into building jets and lose engine sales? Engine sales would start going to the likes of Mercruiser, Marine Power, Kodiak, Indmar etc... or even worse for them we'd marinise our own GM, Ford or Mopar powerplant... So... why sell jets?
     
  9. USCGRET/E8
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    USCGRET/E8 Senior Chief

    I loved my jet with the 460 Ford & Berkley. It was a major hoot throwing it into reverse at 40-50 mph and come to a sreeching halt, water shooting up into the air, eventually sliding around backwards as she fell off plane. Then punching it and taking off in the opposite direction! Anyone in the boat at the time would soil themselves and those on shore would do a jaw drop, then shake their heads in disbelief. This was years ago, before it was ever done on a jet ski. As far as manuverability, I learned how to actually walk the boat sideways up to a dock. Ya just have to remember to turn off the engine once there, as there was no standing still when running!! Incase you're wondering, the boat was a '79 Rochelle. Jets are good at what they do, just like anything else.:)
    [​IMG]
     
  10. RANCHI OTTO
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    RANCHI OTTO Naval Architect

    submerged propeller vs waterjet efficency...
     

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  11. PSG-1
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    PSG-1 Junior Member

    While a prop drive may own the ocean, or rough water, and it may have more torque for towing applications, when it comes to shallow water, high speed, and high maneuverability, the jet drive OWNS IT.

    http://www.youtube.com/watch?v=6tLhEzMgYrQ

    And:

    http://www.youtube.com/watch?v=_I0Tod7-FXY



    That's a Duracraft 1648 SV johnboat hull, doing 180 degree spins at 30 MPH.

    Try any of these stunts, such as going through 4 inches of water, or doing 180 degree spins, with a prop drive on the same type of hull, and let me know how it works out.
     
  12. FAST FRED
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    FAST FRED Senior Member

    Jet vs prop,"efficiency" ???

    Does anyone count the cost of purchase , installation, up keep and repair? Or service life?
     
  13. cyclops2
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    cyclops2 Senior Member

    They do not know what is more efficient ?

    Most people link low operating expenses to more profit & efficiency.

    The largest cruise ships & super tankers do not know as much as the jet drive companies.

    Is that a true fact ?

    If you want to show off & be odd. Get a jet drive. :eek:
     
  14. MechaNik
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    MechaNik Senior Member

    Cruise ships and tankers are clearly not suited to jet drives. Why would that mean they can't offer an efficient application?
    Many commercial operations favour jets over props which again doesn't mean that relates to yachting, but you would have to compare apples to apples.
     

  15. cyclops2
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    cyclops2 Senior Member

    What camparison are you doing ?

    They are NOT both jets or props, apples. They are VERY DIFFERENT. The facts stand.
     
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