Pirate 21 with jet drive

Discussion in 'Jet Drives' started by ruurd, May 8, 2011.

  1. anthony goodson
    Joined: Mar 2007
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    Location: Dorset UK & Murcia Spain

    anthony goodson Senior Member

    Ruurd I have spoken to many people over the years who have been in the situation you are thinking of putting yourself in.They crop up on the forum ,convinced that a different gear ratio ,new impeller more power, trim tabs etc will transform their jetboat. The simple fact is ,that if this boat ,with fuel ,crew and gear weighs over 2000kg ,it will never perform as you want it to regardless of anything else.When you first asked your question I had in mind a boat similar to one which was returned to me by my son in law last year.This is a 21ft Dory it weighs 1100kg fuelled up. 130HP petrol engine ,top speed 36kts and it will stay up on plane with two onboard at just over 2000rpm . very economical and a delight to drive. Jetboats need to be light, because in my opinion they cope with mass ,worse than most other drives. When the 90 jet was first developed ,in "A "format. it was trialled in a 17ft Dory ,with a Ford V6 petrol engine Richard Parker was so pleased that this would plane with a ton [1000kg ]payload he subsequently used this fact in his advertising. The larger diameter mixed flow impellers used in Vospower/PP jets seem to cope with weight better than the multiple smaller diameter axial impellers of other makes but still not well.
     
  2. ruurd
    Joined: May 2011
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    Location: Netherlands

    ruurd Junior Member

    Hi Anthony,

    That's the good thing about the forums. Thanks for all the useful info.
    I think I have to search further for the "perfect" boat.
     
  3. capngeoff
    Joined: Dec 2009
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    Location: uk

    capngeoff New Member

    Pp90 Parts And Performance

    Hello all, many thanks for all the forum posts.

    I recently purchased nice little jet-driven vessel:-a Task Force 5.5M workboat with Mermaid turbo diesel and what I now think is a PP90 jet drive.

    The vessel was built by the Task Force factory for commercial use by Thames Water, so presumably was well specified and constructed at the time, in conjunction with Mermaid, Parker et al.
    She has a cathedral hull form, shallow draft, and narrow beam.

    I had expected the vessel to plane.

    When pre-purchase testing the vessel at sea NE Scotland, I was very concerned at lack of speed, circa 6 knots at 3000 rpm. I was consoled somewhat by the seller's assurance that the cause was her fouled bottom and jet orifices, and his assurance that 17 knots was once the norm.

    This was partly confirmed by beaching her to inspect underwater surfaces.

    Taking a gamble I purchased her, as the boat and engine seemed in very good order other than the lack of speed.

    After a long road trip to her new home on West of Scotland on a trailer, she was very thoroughly cleaned in all hull areas, also inside the jet, which was indeed partly obscured by mussels, weed and barnacle growth. All potential leakage orifices such as weed inspection hatch were carefully sealed up too.

    Sadly the speed did not increase much after this cleaning - only about 10 knots was possible even when revving the engine hard.
    My past extensive use & experience with many many shaft drive diesel boats with way more modest horsepower made me expect far greater speed from the Task Force, given the sterling efforts of the straining Mermaid!

    I tried many combinations of weight, payload positioning, trim tab settings, revs, in fact everything possible - but top speed achieved was only 14 knots OTG - even this was a little wishful, as she was heading downwind and surfing down a helpful Westerly F5 swell. Oh dear.

    Subsequently the PP's bucket detached from its right hand pivot, then its jet steering nozzle locked in full starboard helm at sea, giving this solo skipper some interesting problems to solve. Port was eventually reached using judicious juggling of throttle and see-sawing multiple zizag courses - astern.

    Inspection of the jet drive when the vessel was yet again dried out revealed that the alloy jet steering nozzle had lost its boss attachment, due to a combination of stress cracking and galvanic deconstruction of its boss casting's mass. This nozzle unit employs a stainless steel taper pin not unlike a bicycle pedal crank-to-shaft type fitting; sometimes called a Cotter pin, to achieve location and locking to the stainless steel pivot shaft.
    The whole pin containment boss area had decomposed and/or cracked off and disappeared with its pin etc., resulting in total steering failure.
    Presumably ( I feel certainly) this loss is/was caused by poor galvanic protection of the alloy parts. There was NO external copper connecting lead to this sensitive part, which is electrically isolated from its larger & well zinc-protected surrounding parts by its nylon pivot shafts' bushings.

    So now I need to replace the steering nozzle before any further experiments can be carried out, regarding speed over the sea, alas.

    Meantime I would really appreciate any advice , anecdotes , diagrams, or parts sources which any BDN contributors might have!

    I have already had some help re a PP65 from another member (thanks Monty ) but it appears mine is a PP90.

    A new or secondhand nozzle would be great.

    If all else fails I will have to manufacture a new nozzle.

    This is pretty urgent, as the boat should be in daily use .

    She is otherwise a great wee vessel, and the jet drive's beaching capability is a huge advantage for my sort of sea-going: proper harbours are a luxury for me; sand or gravel berths are the norm, with lots of thing like creel lines, ropes and jetsam which would love to tangle in a "conventional" propellor setup. So, I am prepared to live with the perceived inefficiencies inherent with jets and hulls which might not plane readily.

    Kind Regards, CapnGeoff.
     
  4. ruurd
    Joined: May 2011
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    Location: Netherlands

    ruurd Junior Member

    Hi CapnGeoff,

    I ended up buying the boat with the same toughs as you had.
    I was a bit more lucky concerning the speed. In the previous posts you can see the boat was doing only 14 knots with a 165hp mercruiser diesel and a pp90 jet drive according to the seller.

    After the boat arrived I had to do lot's and lot's, and more cleaning. In the last years the boat was not handled very well and a mix of oil and water had filled up the bilges and other separate compartments.
    Also the turbine blades where stuck in the diffuser. This was only because the engine was not running for a long time. On the first test run the boat came in plane for a few seconds but before I could measure the speed the filters where blocked. I already drained the tank and changed the diesel but still there was a lot of hidden debris.
    At the second test run she was able to reach a speed of 19 knots trough the water. This was not expected because the seller told me the boat only would do 13/14 knots.
    Later I found also trim tabs in the fore-peak that where removed. Also the jet housing was leaking on the transom. After installing the trim tabs and repairing the leakage speed was the same.
    The max revolutions of the diesel engine is 3800rpm. I can't find a curve for the pp90 jet but I think the jet can handle a lot higher speeds. This might be the reason for it to sial only 19 knots.
    Next summer I continue the work on the boat but I'm happy it does 19 knots instead of 14.
    Maybe it's possible to increase the engine speed a little bit. I'm not sure if the engine really goes to 3800rpm because the indicator is connected to the dynamo and is not so precise. In that case increasing engine power will bring the speed up to max. The clearance on the impeller is also an important factor. If this is to big, the performance will be effected largely. In my case impeller looked good and clearance minimal.

    Originally the boat was fitted with a petrol engine with unknown power output. Usually a petrol engine has higher rpm's than a diesel engine so maybe the speed was higher with this engine.
    The old hour counter was still installed in the console which displays 1000hrs. The diesel engine has a digital counter which only read 73 hours.
    Although the diesel engine is a few years old and not handled very well it is not used so much. The engine paint shows no marks of any tool has touched it before.

    The anodes of the boat where painted and ground wires broken. The jet unit especially the alu bucket shows some corrosion marks and needs some attention.

    I'm also looking for a pp90 manual and performance graph. The only manual I have is for the pp65 that was available on this forum.
    I read on the forum that pp jet's was taken over by Vosper Thornycroft (UK) Ltd.

    Maybe I will give them a call to find out if there is more info available.

    About the beaching capability, this is indeed a advantage of a yet system but as you probably know sand will grind your impeller resulting in a smaller diameter an larger tip clearance if you do this regularly with running engine.
     
  5. baeckmo
    Joined: Jun 2009
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    Location: Sweden

    baeckmo Hydrodynamics

    Hello Ruurd, so you went for that boat after all! A quick comment on your thoughts here: DO NOT TRY TO INCREASE RPMS!!!! It is a misconception that jets need high revs, they do NOT! The problems you see with a highly loaded jet (i.e. full power at low speed) are due to cavitation. For a specific operating point (power and speed) the determining factor is inlet tip relative velocity. This is proportional to rpms times radius, and if you try to increase rpm, you will have an increase in cavitation intensity with a consequent loss of performance.

    In fact, with this engine, having a fairly flat power curve between 3500 and 3800 rpms, you may even have better performance with a slight increase in impeller pitch plus a reduction of nozzle outlet diameter. Also, it is extremely important that there are no disturbances to the flow in the inlet sector, i.e. an area ~40 % wider than the inlet and all the way up front. Check that the plate holding the cleaning "fingers" is blending smoothly into the hull without any sharp edges.
     
  6. ruurd
    Joined: May 2011
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    Location: Netherlands

    ruurd Junior Member

    Hello baeckmo,
    Nice to hear from you again. I have some picture's of the boat in action this summer. I still have to do some work to make it reliable but now it's winter.

    I was searching the internet for the Mercruiser performance graph but was only able to find a graph for the newer mercruiser 165.
    Also performance info for the pp90 jet is hard to find.

    Do you know where to get parts for the jet? Like impellers with different pitch settings etc..

    After the work on the boat is finished to make it more reliable, I will experiment a little. I think it's a good idea to purchase a Motronic cable to read out engine data. Then I have a better view on what the engine is doing.

    I'm really interested in the jet technical data. Recently I followed a course(still do) in centrifugal pumps and turbines. One will learn about absolute and relative fluid speeds at the pump blades generated power etc etc. I think a jet pump has a lot of similarities.
    It is difficult to check for cavitation on the jet before it does damage. You should be able to hear it but with the engine noise it's not really possible.

    I cleaned the inlet grating but when the boat is out of the water I can make some detailed pictures of it.
    Are you suggesting that a PP jet was delivered with different nozzles and inlet shapes?
     

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  7. baeckmo
    Joined: Jun 2009
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    Location: Sweden

    baeckmo Hydrodynamics

    I will collect some data on this engine and the PP90 jet for you. The fact you are entering the world of turbomachinery turns your project into a perfect lab study. What level are you?

    There is a very easy and efficient way to get a hold on the unit's cavitating performance. In pumping, we use the s.c. "specific speed" (Ns=N*Q^^0.5/H^0.75; N=rpm, Q=m3/sec, H=maq) to describe the balance between speed, flow and head for a certain pump. In the same way we use the s.c. "suction specific speed" (Nss=N*Q^0.5/NPSH^0.75; where NPSH is Net Positive Suction Head). NPSH is the head margin over the actual vapour pressure for the fluid in the inlet. The value of the Nss is practically constant for any speed (rpm) of a specific pump.

    To measure the Nss, drill a pressure tap, 3mm dia, 100 mm in front of the impeller inlet plane (no burrs in the inlet and no chamfer) at the 2 o´clock position, connect to a precision manometer reading absolute values. Then hook up your boat to a tension meter ("dynamometer"), and the other end to a bollard.

    Keeping the boat straight in line with your rope with smallest possible steering manoeuvres, note rpm, pulling force and inlet pressure at say, 300 rpm intervals from 1500 rpm. You will notice that from ~2600 rpm and upwards with this jet, there is a change in the trend. Ideally, without cavitation, the thrust is proportional to "constant*rpm^2". Then you can draw a diagramme with measured force asf. of rpm plus ideal force asf of rpm.

    While still in the boat, make a few test runs (free runs), also with stepping rpms as before. Measure speed and inlet pressure at each rpm.

    When the measured static thrust is deflecting from the ideal trend, cavitation is degrading the pump efficiency. If your engine is not too noicy, you will also note sharp rattling sounds in the jet (use a wooden stick between the jet inlet and the bone behind your ear, and it becomes very clear!).

    Next, you have to calculate the flow at each rpm. You get it from your measurements as follows:

    Q=(A*T/density)^0.5: where A is the effective nozzle area (in m2), T is thrust (in Newtons) and density of water (roughly 1020 kg/m3).

    Now you can write the expression for the Nss with your values. The PP 90, if correct, has a critical Nss of roughly 270 to 280 rpm. Above that the performance is suffering from cavitation. With some care, the 90 jet can be made to operate fine at over Nss 300rpm. Any operating condition causing Nss to rise above the critical value is causing cavitation in your equipment. In many cases it is obvious also when you draw the speed versus rpm diagram, that there is a change in trend, when the critical Nss is reached.

    I use this procedure as a first step when involved in cases where jet performance is below customer expectance. Most problems are detected this way. I'll see if I can show a few pics from actual cases later.
     
  8. ruurd
    Joined: May 2011
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    Location: Netherlands

    ruurd Junior Member

    I'm working as a ships engineer. Besides the job learning for Bachelor Maritime Officer with a technical specialisation. This type of calculations are part of the education,we also had to calculate the NPSH for centrifugal pumps so it's nice to get some professional assistance and info on the subject. Maybe I can even use it in a assignment.

    I must confess that during the last trails a loud rattling sound could be heard from the yet cover. This was heard close to the cover. Cavitation crossed my mind but I dismantled the yet just before the trial and thought the impeller was touching the outer ring at full power a little. Maybe it was cavitation.

    A test setup like you propose sounds like a nice way to check this.

    It's really nice if you can send me info on the engine and jet.

    I'm not unhappy with the speed of 19knots but in my opinion 25 knots should be possible. If cavitation is going on then this will damage the jet so it must be resolved anyway.
     

  9. Murray Peterson
    Joined: Sep 2009
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    Location: Kuttabul, Queensland, Australia

    Murray Peterson Structural Engineer

    Jet Boat Performance

    Just a comment on performance. I have a 20’ Swiftcraft Explorer that weighs about 1500kg with an old AMEC jet drive and it does about 15 knots at 1400 rpm before it gets up onto a plane. At 2900 rpm it is doing 32 knots. I have the engine (Repco 200HP V8) out of the boat at the moment because it is not running well and I know there is a fair bit of wear on the stator veins (30 years old) and other performance reducing issues, but it was still performing quite well compared with equivalent prop driven boats.
    In my opinion, if a jet boat is performing very badly compared with an equivalent prop boat, there are significant maintenance or design issues that need to be corrected.

    I have owned 2 jet boats and 1 outboard powerd boat. When I rebuild my current Jetboat I will probably sell my current outboard. I very much prefer the Jetboats.
    I guess it comes down to taste and your own priorities - the main difference in the feel of the boats is the outboard steers more like a car, where as the jet steers more like a hovercraft.
     

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