jet drives reconsidered.

Discussion in 'Jet Drives' started by Frosty, Aug 12, 2012.

  1. Frosty

    Frosty Previous Member

    Newtons 3rd law says to every motion is an equal and opposite. No problems there,

    But-- if that be so then why are jets almost always fitted under the water level of the boat. This places the unit in submerged position suffering the consequences of horrendous corrosion and maintenance problems. We have all see the white encrusted corroded units,--enough to put any one off a jet

    If it were to be up above the water and ejected higher the same equal motion would exist but the unit could be washed , lubricated. adjusted and maintained from the swim platform or the marina dock.

    The only problem I can see is the priming of the pump to work from the start.

    The benefits of an out of water unit begs consideration.

    Does it not?
  2. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    Only on poorly designed installations. We always opt for GRP ducts. Never have problems :D

    So lets say it is the same height as the mast...what would occur...the head is too large, thus losing thrust.

    There is a reason why the centre line of the jet is generally coincident with the running water level or static water head!
  3. Frosty

    Frosty Previous Member

    Well I was'nt thinking quite as high as the mast. I did say just above water level,

    I know of no jet units made in GRP,---It was'nt the duct I am concerned about but the reverse bucket and SS impellor and housing made from alluminium that just rots away for static placed boats in sea water.

    For such as a cat with shallow draft at the stern a jet would be a little more than 1 foot higher than standard yet clear of water when static. As you say generally in line.

    Hong Kong Boeing hydrofoil jets dont have a problem with water line ejection,
  4. latestarter
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    latestarter Senior Member

    The main problem would be where will the jet of water go.

    Passing astern of the jet boat in an open boat would be like going through a car wash in a convertible.

    You could hire it out to clean the topsides of other boats. :D
  5. Ad Hoc
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    Ad Hoc Naval Architect

    I know. :)
    I was just pointing out the basic reason why, with a simple illustration.


    We also designed and made our own ducts from GRP years ago too (900mm dia)....not one of these boats have experienced any corrosion..anywhere in and around the WJs.

    No, but it uses a lot of energy in doing so. The fact the boat has low resistance when up and running offsets this. Except that it is still a tad thirsty with those GTs.
  6. DCockey
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    DCockey Senior Member

    Could any problems occur with priming the pump and starting the water flow if the impellor is above the static water level?
  7. Frosty

    Frosty Previous Member

    The shipyard near me has a couple of ferries rotten with huge Ke Ma Wa jet units looking like white encrusted cheese. Similarly the car park at the yacht club last year had half a dozen seed doo and jet skis in a similar condition. These cheese like lumps of alluminium can not be repaired,--they can not even be dismantled.

    Alluminium is next up the list from Zinq that we use a sacrificial anodes.

    Outboards that tilt are made mostly l of alluminium, they are not left submerged in see water, but these small engine are not suitable for larger craft.

    But is this really necessary just because it the way we do it.

    The grill of course can be made of GRP but thats not what I would call of mechanical consequence.

    The Boeings shift 13.500 gall per minute. Why you say it uses more because it is ejected higher will need some explaining --well at least to me.

    A jet pump sited just above the static water line( 3-4 inches would be sufficient) would mean the pumps life would be extended equally by its owners intent.

    Corrosion is the pits of marine equipment and a lot of money is available for improvement.
  8. Frosty

    Frosty Previous Member

    Yes there will be,-- I mentioned that. It would seem a small problem to solve in exchange for easy maintained jet that does not need barnacles scraping out of the tube and multi blade impelors.

    What a pleasant prospect to sit on the dock adjusting bucket control and lubricating bearings after a nice soapy wash knowing it will be in perfect servicable condition a month later.

    What is now the worst drive to keep sevicable could be one of the best.
  9. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    Yup, that's what all of them use. They have small anodes dotted around the waterjet body and on the hull. You need to be aware of the dissimilar metals all in close proximity. Removing the duct, and making it GRP, helps significantly in corrosion control.

    Perhaps those that you see have not bothered to maintain/replace the sacrificial anodes?

    You're mixing apples and pears.

    The GTs for Beoing jet foil are very thirsty, as all GTs are. It needs the GTs to work, ie the power to get up off its foils and move fwd. When running the discharge from the WJ is high above the static water level.

    If you have a hose pipe, open the tap...water flows out at a constant rate. Move it around you horizontally....only the direction has changed, the flow rate is constant. Now slowly raise the end of the pipe higher and that flow rate the same....and there's you're answer.
  10. Frosty

    Frosty Previous Member

    The jet is not fired in the air or even up at all. I thought you were going to suggest the extra power needed was to get the 13,500 galls of water up to the 10 feet say above the water line the the Hydrofoil works at.

    Jet units are a mix of Alluminium and stainless fasteners as is my car engine but it does not stand in sea water.

    Of course GT use lots of fuel. Thats the choice of the builder not the machinery, and I think Boeing might have a few kicking around the workshop.

    The point is that the benefits of getting this machinery out of the water might not be so difficult but would result im a massive improvement on halting or slowing corrosion.

    I am up to my limits of disgust at the monthly clean up of barnacles on my drives, it starts to re occur in just 3 days.

    The higher line of thrust could be accommodated in the design of the hull. Something needs to be done as its getting down the simple decision of boat or no boat.

    The point is that raising out of the water does not take from --'to every motion is an equall and opposite',--so why do it, apart from the minimal problems offered so far
  11. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    You've lost me??

    The inlet for the water, is is at the base of the aft foil. It goes up through a duct and is discharged above the waterline. The water having to change direction from horizontal at inlet to vertical going up the duct and then again to horizontal to the outlet.

    The Boeing Jet Foils often had the GE 501-KF GT which was rated at 3800PS. The Kawasaki type water jet was rated at 90m3/min. It delivered about 5.2tone of thrust.

    The discharge is above the static waterline.

    The jet, or pump has to produce a head. Which is simply:

    H = H(PO) - H(PI) + (rho).g.h

    H(PO) is the head at the outlet
    H(PI) is the head available at the pump

    Both have the term h(i/j) the head from the centre line of the shaft, as it is simply Bernoulli's equation.

    And then finally to produce the head also has 'h', the distance from the static waterline to the centreline of the shaft.

    Thus changing the h/h(i/j) affects the performance. To maintain the same flow rate the power must be increased as the head is increased. That's what I mean by the hose analogy.

    Does this mean you can't do it no. You can..but you must accept the consequences. You wont find too many jet suppliers willing to let you have discharge above the waterline, just for your maintenance preference, owing to the consequences of such; which will always be blamed back to the WJ supplier when things go wrong.
  12. Frosty

    Frosty Previous Member

    Right --we are going off track here, I am talking about raising to just above the water line of a planing boat. I used the Boeing as an example of firing water above the water without loss of Newtons 3rd law.

    The Boeing can now be forgotten about but yes moving water UP needs power thats understood.

    Ive opened a thread about this before, how boats have remained the same for 50 years. same same. We had corroding jets and outboards in the 50;s

    Cant you design a jet boat with the jet only getting wet when running. You can have it as close to the water as you want just not touching it at rest. Why should Hamilton care apart form compromising the inbuilt obsolescence.

    The only prob I see and was mentioned earlier by some one else is priming it other wise what is the problem.

    Again on a shallow draft cat 1 foot higher is all thats required.
  13. Frosty

    Frosty Previous Member

    So a 100Hp 15 foot boat--- 1 ton has now the pump 1 foot higher in the hull --how much extra power would be needed.

    Yes there are consiquececs and im sick of them growing all over my drives and Ide be interested what other negative consiquences would be in terms of power but what are others apart from the massive horrendous improvements.

    Im not a mathamatician but 'ille bet its not 5hp more.

    Ille take those consiquences --any more?
  14. CDK
    Joined: Aug 2007
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    CDK retired engineer

    Although my own experiences with jets were a real nightmare, I've also seen very good ones.
    The large catamaran ferries connecting this island to the coast have stainless steel jets and hydraulic cylinders for steering and trim. Because they make several fast trips each day there is no marine growth; they look like they were installed last week whenever I see them. It is all a matter of how much money you are prepared to invest.

  15. Frosty

    Frosty Previous Member

    My thougts are that it would work and I am thinking of a grid placed correctly in the boat (GRP) with a bowl above taking up to water level. The jet would be bolted to that and fired out of the back.

    The shaft would be connected in the conventional way with some flexible shaft.

    The pump would not prime like this so-- A flap of some sorts would be attached the the outer side of the impellor housing that shut with some kind of electric solenoid --(remember we are not under water). The flap would seal the unit temporarily while an air pump pulled air from the unit filing it with water. The engine would then start and prime, This would open the flap door from its own pressure. The jet would then be operational.

    After use the entire unit would be washed and be no where near sea water. It would be in perfect condition to be used again in an hour or a month.

    Nothing at all on this drive system would touch the water at rest.

    Any better ideas on the prime
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