Vospower PP100 Jet + 300HP in 7.5 m RIB - a reasonable combination?

Discussion in 'Jet Drives' started by Wrong Way, Jan 28, 2020.

  1. Wrong Way
    Joined: Jan 2020
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    Location: Germany

    Wrong Way New Member

    I am considering buying a 7.5m RIB with a (Vospower) PP100 jet driven by a 300 HP Caterpillar 3116 engine. This is the original configuration from the builder.

    The craft is a 1996 commercial offshore RIB, 7.5 m long * 2.9 m beam, weight approximately 2400 kg according to seller.
    Craft appears to be in good order and has been in commercial operation until two years ago as a safety/training RIB.

    This old thread hints that the PP100 might not have been the right choice for the craft but I don't know enough about jet drives to really understand why.

    The top speed - again according to seller - is around 30 knots. I find this surprisingly low, and before I travel halfway across Europe to have a look I would like to ask the forum whether a PP100 is a reasonable choice, and 30 knots a reasonable top speed for this craft configuration?

    Thank you,
    WW
     
  2. bajansailor
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    bajansailor Marine Surveyor

    Welcome to the Forum WW.
    Do you have any photos of the boat which you could post on here, or maybe a link to her online?
    Re the reported top speed of 30 knots being rather low, what sort of cruising speed do you need to achieve with the boat that you buy?
     
  3. Wrong Way
    Joined: Jan 2020
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    Wrong Way New Member

    Thanks bajan :)

    I don't have any photos of the jet if that is what you are looking for.
    Regarding speed, I am looking for a loaded cruising speed at around 25-30 knots, but that would be unachievable/uneconomical with a (presumably ligthly loaded and with a tail wind) top speed of around 30 knots.

    It is my understanding that we need to look at the specs and thrust curves for the PP100 and from that someone more knowledgeable than me can assess whether the speed is as expected or it's an unsuitable jet model, or something is wrong with the hull, intake, impeller, engine or other.
    Is that correctly understood?
    Do you know anything about the model?
    Alternatively, do you know whether it is acceptable to take the PP115 thrust curves posted here and scale them down by say 35%?
    And are there any rules-of-thumbs/tables or other for determining the expected drag of a planing hull like a RIB?

    Thanks,
    WW
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    30 knots top speed seems pretty good to me for a 25 foot diesel powered planing boat. If 25 knots max cruise is too slow, then I think you will have to look elsewhere. What is the hull ? Deadrise ?
     
  5. bajansailor
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    bajansailor Marine Surveyor

    Thanks for the update - a photo of the actual type of boat that you are considering would be useful.
    What do you intend to use the boat for?
    It might be easier to look for a boat with an outboard motor (or two) if speed is such a priority?
     
  6. baeckmo
    Joined: Jun 2009
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    baeckmo Hydrodynamics

    Afraid you are misinterpreting the discussion in the old thread. The problem there was a combination of high displacement, short waterline length and a so called cathedral hull. The low slenderness number [length/(displacement)^(1/3)] caused a very high hump resistance, and the hull shape caused a collection of air at the waterjet intake opening, i.e. a serious reduction of the jet thrust. It simply was a totally unsuitable situation for waterjet propulsion.

    In your case, assuming the hull is a straightforward V-shape, without any disturbances (spray rails, transverse steps etc.), and given the displacement figure is correct, the PP 100 is very well suited for the operation. But, as has been noticed again and again: the waterjet is sensitive to weight increase, leading to increased hump resistance. The Caterpillar is a heavy lump of iron in relation to its power output; I had preferred a lighter engine.

    The performance cited for your specific case is "normal" for the boat type. RIBs often drag parts of the collar in the water or in the spray, and they often have quite high V-angles, leading to proportionately high "basic" drag. That said, there might be a scope for an upbrushing of engine and jet. My experience with a couple of workboats with PP 100 unit is very good; it is a good performer and a simple and rugged piece of engineering. Only pity is that Vosper Thornycroft, who bought the design from Parker engineering, did a lousy job.

    Forum member Anthony Goodson is a valuable source on jets in general, and PP/Vosper in particular; he might have additional info to share.
     
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  7. anthony goodson
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    anthony goodson Senior Member

    Good evening baeckmo and all. The PP100 is rated to 3 tonnes in old money and 350hp but I always felt the upper weight limit was very optimistic,as baeckmo says jets are very weight sensitive. Does the weight quoted include fuel ,gear ,and even crew ,it all adds up. The jet itself is well made and designed and should have a long service life. I think I have a data sheet for this somewhere I will have a look and will post it if I can find it.
     
  8. anthony goodson
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    anthony goodson Senior Member

    Here you go ,sorry about the quality,but if you enlarge it it is legible. In my experience ,jets almost always perform best somewhere in the middle of their designed payload range and never too much above that. Many times on here we see jets not performing well because they are overloaded for their size, economics and marketing have often played a part in this.
     

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    Last edited: Jan 31, 2020

  9. Wrong Way
    Joined: Jan 2020
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    Wrong Way New Member

    Thank you for all the replies - they are super useful!


    Thanks for the info. The upper rating of 3 tons and your judgement on the real-life viability of it, is the key info I was looking for. Btw, I can't read the hp numbers on the thrust curves diagram, but I don't really know how to interpret them anyway. Is there a good guide on how to interpret these thrust curves somewhere?

    The 2400 kg is the dry weight, so with a full tank (200 l), 4 crew and camping gear I will go over the rating. Even downgrading the engine will have me continually pushing against that limit.
    The hull is a "deep-V" hull, heavily built but with the tubes out of the water when at rest.
    So I have decided to pass on this one.

    Yes, I feared that I didn't quite understand the discussion, me not being a NA, or trained in hydrodynamics, or an engineer, or in the trade, or even technically minded :)

    What's an "upbrushing"? Is that tasks like ensuring the water inlet and impeller are working optimally? As I want to buy a jet diesel RIB I will need to look into that.

    Thanks for all the help,
    WW
     
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