Tuesday, 31 August 2010

Jet Unit - Part 2

After the first test run of the boat, the jet unit was removed and inspected. The rear impeller boss had sheared off the shaft resulting in the rear impeller not turning which could explain the lack in performance towards the end of the day!!! The jet unit had a number of other issues as well such as excessive tip clearance, slightly bent shaft, worn thrust bearing, and bent impeller blades on the rear impeller. The decision to fully recondition the jet unit was therefore straightforward.

 Rear impeller boss sheared (broken piece visible in stator tunnel)
(click image to enlarge)

The image below shows the shaft assembly for the UA jet unit. The item below is actually my spare unit which I didn't want to use as the rear bearing arrangement is different (bronze bearing) and the impeller blades were in worse condition than the current unit.

 Complete shaft assembly (thrust bearing and UJ coupling at front)
(click image to enlarge)

The thrust bearing is removed by removing the sleeve from the front of the shaft. This is located by a small grub screw. The allen key head was rusted out so this was drilled out. The sleeve then slides off. The thrust bearing can then be pulled off with a flywheel puller.

  Removal of thrust bearing and housing from shaft using flywheel puller
 (click image to enlarge)

The thrust bearing is a simple bronze casting with a sealed ball bearing retained by a large circlip. The bearing is sealed by 2 small oil seals at the rear of the bronze housing and one larger oil seal at the front of the housing. To remove the bearing the large oil seal is removed first. This was being replaced so it didn't matter if I damaged it whilst pulling it out. The circlip can then be removed using circlip pliers. The bearing must be pressed out, and cannot be pushed out by hand. I used the flywheel puller to do this.
Once the thrust bearing is out, the 2 smaller oil seals can be easily removed. All the seals were in bad condition and in need of replacement. I would suggest replacing the oil seals if in doubt as otherwise water and grit can destroy the thrust bearing.

Disassembled thrust bearing housing 
  (click image to enlarge) 

I ordered up new bearings and oil seals (water seals) from Nationwide Bearings http://www.nwideuk.com/.
The part numbers from Nationwide Bearings for the thrust bearing rebuild are:

  • Thrust bearing   63022RS
  • Small oil seal     OS125075025
  • Large oil seal     OS422507

New thrust bearings and oil seals

  (click image to enlarge) 

The rear bushing which supports the tail of the shaft is a marginally lubricated cylindrical bushing. The bushing is sealed by a further oil seal. This seal was also replaced. Again this part is available from Nationwide Bearings, Part Number OS352507.

Rear shaft bushing and oil seal (removed)

  (click image to enlarge) 

The jet pump housing was known to be in good condition as I had stripped it down before. Nevertheless, it had clearly seen some salt water use as there was some corrosion present. To prolong its life I decided to get the unit powdercoated. While doing this I also stripped down my spare unit and had this sandblasted to get an idea of its condition.

Jet unit (and spare unit) prior to blasting
  (click image to enlarge) 

 Jet unit (and spare unit to left) after blasting
  (click image to enlarge) 

Before powdercoating I had the stator section machined to incorporate an alignment recess (see image below). This was a feature of my spare unit and allowed the partially protruding nozzle section wear ring to slot into the stator section of the jet unit, thus aligning the two sections of the jet unit along the centreline of the shaft. Aligning the two sections is otherwise a matter of trial and error and is the most frustrating part of fitting the jet unit to the boat.

The unit was powdercoated by RPA http://www.bristolmotorcyclepowdercoating.co.uk/index.html who did a great job of the old casting. It had to be heated (gassed) twice to get the trapped gas out of the porous casting before powdercoating. They then applied a primer coat followed by the gloss black final coat as recommended for marine applications. All the bearing surfaces and thread holes were masked off beforehand. The final coating is hard as rock, looks excellent, and should last a long time.

  Jet unit after powdercoating. Alignment recess on stator section visible (bare aluminium).
  (click image to enlarge) 

During the powdercoating process, the rear bushing for the shaft got damaged by the heating process. The bearing is of the marginally lubricated type (bronze outer with plastic inner). The plastic bearing surface was melted by the heat so I had to remove the damaged bearing and order a new one (these are available from Ondrives http://www.ondrives.com/index.php Length 25mm, Diameter 25mm, Part Number: PM-2525-DX). The new bearing is pressed into the rear housing (nozzle section).

Damaged rear bushing. Replacement bushing must be pressed in.
  (click image to enlarge)

To improve the performance of the boat, the main repair to the jet unit was to get the shaft straight. This had been straightened before but not enough, resulting in a small eccentricity which prevented the tip clearance between the impeller tips and the wear rings from being the correct dimension. The eccentricity also caused the impellers to contact the wear rings occasionally which could be felt through the boat at certain speeds. I took the dimensions of the original shaft and had a new one made up by Hamble Props http://www.hamblepropellers.com/. The new shaft is made from ultra strong Duplex Stainless Steel and has a slightly increased diameter to further improve strength. The impeller blades were bent in several places and Hamble Props were able to straighten these out to the original shape.

New wear rings were made up from ABS plastic to match the impeller diameter by Plastic Machining Services, Telford http://www.plasticmachiningservices.co.uk/. A tip clearance of 0.2mm was specified. Any further tip clearance reduction was considered to provide insufficient clearance for the natural vibration and flexing of the shaft. The wear rings were pressed into the housings and are held in place through an interference fit.


  1. Hello,
    my housemate and I just bought a Spitfire hull in a pretty poor state and are planning on refurbishing it. Would it be possible to get in contact with you to ask you some details about stuff in common between Meteor and Spitfire?


  2. Hi,
    Sorry for the late reply. I havent checked this blog in ages. Fire me an email dom100@hotmail.com if you still need help.