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Triple waterjets

Discussion in 'Technical Discussion' started by CaptPKilbride, Jul 15, 2018.

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  1. Capt J

    Capt J Senior Member

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    It's hard to make a direct comparison with my experience. Reason being, the boats I've run with inboard diesel jets have been fairly new boats with new engines. The Arneson boat's I've run have had engines like 8v92 TI Detroits underneath your feet. So the engines themselves have created lots of noise and vibration, not the propulsion. I did run a 2015 55' Uniesse with Arnesons for over a 1000 NM on a delivery, so let me take that back. The Arnesons have vibration when you're getting up on plane, but when running at speed they were very smooth as well inside the boat. Smoother than straight inboards, I would even venture to say as smooth as jets. The Arnesons are a good 4-5' behind the boat, so any caviation they create doesn't hit anything (hull above prop). The only vibration you feel is if any is transmitted through the drive itself. The jets I've run, wake action REALLY tends to slow them down and then it takes several seconds for them to speed back up to where they were cruising at. For example I've put some miles on the fast Hinckley.....The 34' I think it's called the runabout......cruises at 34 knots......you go over a 1-1.5' boat wake......and it falls down to 32 knots......then takes a good 30 seconds to get back to 34 knots...….the slower ones are far worse in speed drop and time to recover. Plus the jets have nothing to make them track straight, so in a sea they really swing the compass (when you're trying to go straight!)
  2. GhostriderIII

    GhostriderIII Senior Member

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    Arneson drives are more efficient than Hamilton jets. And you're figures are a bit skewed. Most crew boats have three engines and those big engines use 100 gal/hr EACH = $1000 hr on US prices. 500 hrs/yr = $500,000
  3. CaptPKilbride

    CaptPKilbride Senior Member

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    How about kamewa jets? Or MJP?
    My numbers aren't skewed because I was throwing out a round number for a comparison of apples for a twin engine pleasure craft. Oh, and a lot of crew boats these days have 4 engines.
  4. GhostriderIII

    GhostriderIII Senior Member

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    Well, the figures are still right except adding an additional engine = 400 gal/hr = $1200/hr = $600,000.
    My old ship had twin 1900 hp engines that would crank up the speed to around 30kts and burn 400 gal/hr. I didn't need to go that fast - but it was only in an emergency - like escaping pirates in the Seychelles.
  5. CaptPKilbride

    CaptPKilbride Senior Member

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    Interesting, thanks for sharing your experience. I have seen reports done on an Arneson boat using extremely accurate scientific data collected with accelerometers and real time spectrum analyzers that show quantitatively that the surface piercing propellers and drive system do generate and transmit a fairly significant proportion of the overall noise and vibration experienced in the boat.
  6. olderboater

    olderboater Senior Member

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    One thing the jets we're referring to have benefitting them even further when it comes to noise is they are coupled to MTU engines.

    Much of the inefficiency talk is not at all true or relevant to the boats we're talking about with Kamewa or MJP jets. There's a lot of difference between a Rotax or Yamaha in a 20-25' boat vs. larger boats with Kamewa or MJP. If you want a 116' boat to run 50 knots, then jets are by far the most efficient propulsion choice. So much is matching to the purpose and intent.

    As to fuel on such a boat with triple MTU 2600's, at cruise you're probably talking something in the range of 150-300 gallons per hour depending on how you define cruise, whether 44 knots or 40 knots or 35 knots. The AB 116 uses 297 gph at 44 knots. At 35 knots it would get more nmpg that we get doing 20 knots on a 130' Westport and would be within 10% of what we get on an 85' Pacific Mariner at 22-23 knots.
  7. GhostriderIII

    GhostriderIII Senior Member

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    Last edited: Aug 24, 2018
  8. olderboater

    olderboater Senior Member

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    I used the numbers in my calculations for AB, but it's 500 nm at 90% based on 16000 liters or 4226 gallons at 44 knots. We all know Kamewa is RR. As is MTU. I'm very familiar with MTU 2600's. That's what Westport uses in both their 125 and 130 now. We have 2895's.
  9. Capt J

    Capt J Senior Member

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    Jet's are only efficient in a narrow window of RPM like cruise rpm and +/- 5-% of that RPM. It depends on if they were set up for WOT or Cruise, what nozzle size and impellor and pickup that was fitted. You have to understand that the jet nozzle size is what determines the velocity of thrust it puts out and it has an optimum RPM......over that rpm and the nozzle size restricts the thrust or volume, under that and velocity of thrust goes down incredibly and efficiency goes right into the garbage. The window is so small, that below that you just CHEW through fuel.....like hull speed where you end up running more than you think, even if it's only going from marina's to inlets, jets are horribly inefficient, the least efficient form of propulsion and that has been proven over and over again.

    116' really is above the size that you'd use Arnesons in, I don't think they have a drive big enough to handle that. Jet's aren't the most efficient choice, they're simply the easiest choice for both mounting and keeping COG aft. The World is not Enough will do almost 50 knots with it's twin props and it's a heck of a lot bigger than a 116. 10 gallons a mile isn't exactly efficient especially for an express purpose built to go fast. AB just threw tons of fuel and horsepower at the boat to get 50 knots.....nothing revolutionary or efficient about that.

    You want to see revolutionary, look at Jim Smith and his early boats. He was building full outfitted 55-60' SF with full tuna towers and they were pushing just under 50 knots with 2-760HP 8v92's which would be rated less than 700HP by todays standards and this was in the 1980's, boats like Speed Merchant...….and bushwacker

    I've worked the numbers and 1 Arneson boat I was dealing with, we had the props down to 1% prop slip...….1%. It was underpowered but 2 8v92's pushing an old, very heavy 59' express 27-31 knots at 55 GPH

    The only boat I know of that offered both jets and Arnesons in the same hull and configuration was Azimut with the 86S.
  10. GhostriderIII

    GhostriderIII Senior Member

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    Bigger problem with jets in shallow water is sucking up the bottom. The screens get covered with mud, weeds and ev
    Is no one using turbines?
  11. Capt Ralph

    Capt Ralph Senior Member

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    Long ago, usually a center stinger.
    I can imagine a few reasons turbines have faded from a small niche.
  12. Capt J

    Capt J Senior Member

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    A few manufacturers tried that as a main engine, the turbines throw a ton of heat, but in a boat as main propulsion they're pretty much undriveable/undockable. You do see turbines in fast yachts as a center engine, connected to a jet only run when you want to go really fast. The advantage of the jet in a center engine is there is little drag when it is not running.
  13. CaptPKilbride

    CaptPKilbride Senior Member

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    This is the crux of the question I posed in my original post. Wondering how feasible and tractable a triple waterjet setup is in a scenario where a 40 knot boat will be spending a fair amount of time operating in the 18 -22 knot range. (Thinking of nighttime operations here)
  14. GhostriderIII

    GhostriderIII Senior Member

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    Yeah, right. Think about containers lost at sea when you're clipping along at 22kts. I know the crew boats operating in the GOM try to avoid nighttime ops for that single reason.
  15. CaptPKilbride

    CaptPKilbride Senior Member

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    When is the last time you were in the GOM?
  16. GhostriderIII

    GhostriderIII Senior Member

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    2011 after Deepwater Horizon accident
  17. olderboater

    olderboater Senior Member

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    Most jets or other fast boats don't run a lot at night. Not consistent with their purpose or normal use. Perhaps it's because they already got where they were going in half or a quarter of the time.

    As to running 18-22 knots, first, the design of most of these boats does allow efficient running at those speeds because unlike some other boats, they can maintain plane at those speeds. However, not the norm for them to run so slow. If you can cruise at 45 knots, a slow cruise for you is more likely 30-35 knots. We have two boats that cruise at 35 knots (WOT 41-42 knots) and our slower cruise is generally 30 knots or so. We don't run them at 20 knots. And for those talking about inefficiency of jets at slower speeds, which isn't true necessarily for the boats we're discussing, our two 40 knot boats, 35 knots cruise, have conventional propulsion systems and are not efficient at slow speeds. The larger one gets .34 nmpg at 36 knots, .35 nmpg at 28 knots, .36 nmpg at 21 knots, .35 nmpg at 15 knots. The smaller one gets .66 nmpg at 37 knots, .67 nmpg at 29 knots, .69 nmpg at 22 knots, and .71 nmpg at 17 knots. We went to Key West on Thursday in the baby one. Left home at 7:00, docked in Key West just after noon, just in time for lunch.

    When we chartered the AB in Spain, we boarded in Barcelona, had lunch in Valencia, had dinner and spent the evening in Palma.
  18. captaintilt

    captaintilt Senior Member

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    In terms of Jets, they are very smooth in operation as long as you have the correct gear and motor mounts, and in terms of efficiency they are efficient at higher speeds than a prop. The jet boat I just delivered was burning 2 gals / mile at 38 knots and the similar prop boat at that weight would burn 2.8-2.9 gals / mile at that same speed.
  19. CaptPKilbride

    CaptPKilbride Senior Member

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    True, but the reason I asked about the slower operation at night is related to a use that involves a lot of hours accumulated running in darkness, and in inclement weather with reduction in visibility.
    The statement made that crewboats operating in the GOM try to avoid operating at night just plain made me laugh out loud. Check out the Marine Traffic website for Port Fourchon sometime. Live AIS tracking of a very busy port supporting the oil and gas industry, which is a 24/7 kinda gig. Those guys might shut it down if it's pea-soup fog, but if you tell the charterer that you want to avoid running because it's dark you will be unemployed.