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Triple waterjets

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

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  1. olderboater

    olderboater Senior Member

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    I understand what you're talking about and very well aware of the crewboats as I have an engineer who worked in that area. However, what you describe is again not a normal use of a high speed triple jet. I'd also say that a jet is not significantly less efficient running slower at night than any other high speed boat. You design a boat to run 50 knots or even 40 knots and it won't be as efficient at 12 knots as one designed to run those speeds.

    As to the areas and conditions you describe, we would never run in those situations. We don't have our boats in charter but any charter captain who runs in conditions he considers unsafe is the one who should be looking for another job. We do occasionally run in the dark but never in high traffic areas such as the gulf ports. In fact, we will slow down on a long crossing to such an area to delay our arrival until morning.

    Now, a triple jet will gain efficiency at slower speeds and the comments about it's inefficiency are a great exaggeration and don't reflect today's set ups. On the other hand they'll never be as efficient at slow speed as a lower hp displacement hull. That's true of many other boats though. We have twin MTU 2895's in our WP 130's. Many owners of the same boat do run it at 12 knots. We seldom do but will if conditions require. It is much more efficient at 12 knots than at 20 knots. However, it still is going to use more fuel than a full displacement hull and much smaller engines running the same speed.
  2. Capt J

    Capt J Senior Member

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    A yacht like you're describing with jets is INCREDIBLY inefficient at hull speeds. It has to do with physics and the nozzle size. The nozzle size is setup for cruise rpm/wot. At slow speeds the water velocity out of the nozzle is very low and very inefficient at propelling the boat forward. Has nothing to do with hull design versus trawler etc. It has entirely to do with the physics behind how a jet operates.

    In a jet boat you need to run a lot higher rpm to maintain the same hull speed or slow speed as a propeller or Arneson boat.
    Last edited: Aug 27, 2018
  3. CaptPKilbride

    CaptPKilbride Senior Member

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    Actually, my comment was more directed towards this post:
    I do not believe there is any crewboat operating in the Gulf of Mexico that avoids nighttime ops for the single reason of containers lost at sea.
  4. Indigo_Bay

    Indigo_Bay New Member

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    I have a question on the maneuverability of yachts fitted with three water jets during slow speed ops. In this case I observed a Codecasa 50s which was on inbound course through the Simpson Bay bridge into the lagoon in St. Maarten. Maybe by coincidence or on purpose she was the last unit in line after a few Benetti Super Veloce 140's and larger cats, weather conditions were fair, 15/17 knots trade winds from NE. What I noticed that she was struggling to build up sufficient slow speed momentum and struggling with her bow thruster to keep course with the slight turn port before going the through the hole. Quickly there was actions on the fenders required as she got through diagonal.

    What I could not assess if this was a problem with the bow thruster not building enough pressure on time, lack of competence on the bridge (which I doubt) or if there is a disadvantage of maneuverability with triple water jets under slow speed.

    Could somebody clarify?
  5. Capt J

    Capt J Senior Member

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    Slow speed maneuverability with jets is troublesome. Take a garden hose and try to clean your driveway without a hose nozzle and compare the effectiveness with a good hose nozzle. Bow thrusters are only so powerful, depends on the installation.
  6. Indigo_Bay

    Indigo_Bay New Member

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    That makes sense, thanks for this. This probably also explains why that particular yacht previously had used tug assistance for the passage...which at this location generally is a very rare thing.
  7. BlueNomad

    BlueNomad Member

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    Yes there is a major disadvantage of manoeuvrability at slow speed. As there are no rudders, steering is via vectored thrust and is typically optimised for high speed. Going through the bridge at 40+kt is probably not a good idea, so there isn't much magnitude to begin with. The transverse component of which is even less, even with the buckets hard over.
  8. Capt Ralph

    Capt Ralph Senior Member

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    I do remember some private experiments with attaching a fin (near rudder sized) to the bottom of the nozzle.
    I never got any follow up if this really helped. Anybody else try this?
  9. Indigo_Bay

    Indigo_Bay New Member

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    There's a clip which is covering the scene during an inbound approach for the first time. But also there's (unreleased) footage showing this remarkable yacht struggling when on her own without assistance from a tug boat.

  10. rtrafford

    rtrafford Senior Member

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    Had a similar thought recently when contemplating water jet propulsion for a large sloop, whereby one could employ retractable thrusters with rotation qualities that could be deployed for slow speed maneuvering and precise control. Watching her clear the bridge reminded me of walking on black ice.