At Last I Have A Dowty (photos may not be visible to none regisitered members)

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The Dowty has been on hold for a while and now I am in truoble with the Tool Maker, because he broke his broach on a keyway he was milling into a sleeve of a stainless bearing. Never mind he has a modified method now and it should be done this afternoon.

Think very carefully if you restore a Dowty jet unit, they are not easy and not cheap, but are a challenge. The good thing is that most are OK and mine was a real desperate case. My other unit is fine, but it leaks around the seal, so it has to come out sometime.

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OK, so the toolmaker has broken two broaches now making the bearing sleeve. This is going to be one hell of a fight to fit, but I will start on Wednesday.

Picture below shows the shaft and bearing, plus sleeve, spacer and impellers. This has cost a fortune to have remade, but never mind I can see light at the end of the long tunnel now 

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All but the impellers and bearing housings are new. However, the stainless steel shaft section that goes through the rubber bearing (top right between the impeller blades) (pict above), does not fit the shaft, it is too precision made, so back to the toolmaker to open it out a little. I might have to throw my hat in the door first 

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Progress 

The turbine section of the jet unit all fits and is loosly assembled. Now I have to work out the tollerances because the impellers have 1/8" movement in total and I know the measurement is taken from the inside of the 'tube' from the front, but I am not sure how I assemble the spacer washers, to give me 1/16" clearance on both ends of the shaft for the impellers, it is so fine for such a high spinning rota. There are so many unknowns at the moment 

The only way is to insert the main roller bearing 'bucket' in the main jet housing at the front without gaskets, then measure the movement of the shaft to allow for 1/16" tollerance. However, by placing a gasket in the housing, that will alter the tollerances at the front and rear, so I need a very thin, but oil proof gasket (they used cork), but I cannot find any thin enough.

Then replace all and tighten up slowly. The items are also keyed onto the shaft and it is not easy to move them, what a terrible design  :(This outlines the problem of the stern then, because the unit does not use the stern other than to extend through, so that has to also have a gasket and when tightened will have to be exact otherwise the stern will buckle and leak as it is a very thin stern wall.

This jet has certainly tested my patience, because so much of it has been remade to make the tollerances as they were in 1958.

Pictures below show shaft, impellers and large alloy spacer assemblies. Ignore the temporary holding tube at the far end with the blue nut, that is to hold it all together for now. I will polish the rotas when all dimensions are worked out.

Oh there is one other challenge when I replace the rota tube (stainless steel) into the main jet unit (alloy and not perfect), it has to line up so accurately with the alloy housing, bearing at the front and cutlass bearing  because it has to form a water tight seal that takes all the jet pressure otherwise the boat sinks   

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I think I have an idea to get the exact gap for the impellers.
If there are any engineers with ideas, please let me know, because I plan to place a piece of cardboard 1/16" thick between the impeller and stainless jet housing and assemble the unit leaving it inside. This is because it will be a trial and error (hit and miss affair) otherwise. It can stay inside until I fire up the unit or even turn it over by hand when it will fall apart. Because the stainless impellers and tube can chew up stones and grit, I dont think the tempoary spacer will cause it any problems and will disintegrate in seconds once used in vengance 

Can anyone see any unforeseen problems with my simple solution ?

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Impellors polished, they do look good now, but no picture as they are the same but shiny 

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I have found that the tollerances on the shaft are less than I thought, so that helps a lot. Still some finding out to be done, but now I have the cork gaskets and just need a very large and very slim rubber seal for the jet to outlet seal.

I have some fibre spacers, they clearly came from the jet unit, but as to which end I have forgotten. However, I need to have some made, so back to the toolmaker again.

Looking back at the first few pictures on pages 1 and 2 of this board, they do not come from the rear, so they must come from the front, which is a little wierd, because that implies that the bolts on the activation tube, may not be as tight as I thought, which would allow (very) slight movement of the internal cutlass bearing, manybe I have solved my problem. They also act as central locators for the bolts, though I have no idea why the holes were drilled larger?

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So progress is moving slowly along. I have now assembled the propshaft and it is ready to go in the housing, but I need a 21cm x 9mm x 3mm rubber gasket first and that is the quest this afternoon.

Below is the assembled shaft, outside the 'activation' tube. I still have to work out how to set up the 20thou tollerance, with a rubber gasket involved?

From the left, is the drive shaft coupling, then the bearing 'pot', then the alloy (sacrificial) shaft anode cover and the flow unit, the 1st stage impellor, then the stainless bearing which goes through the 'cutlass' bearing and the 2nd stage impellor. The 5thou spacers sit on the shaft and go into the bearing pot.

The installation is tricky, because it all has to be assembled (once I have the tollerance set), then all the seals have to be installed and the rear (very heavy) activation tube has to be installed with its cork gaskets and sealing compound all without moving. Then bolted up and all the tollerances still in place 

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I have a gasket being specially made that should be with me by Monday from V Seal in Halifax 

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I have been having a thought, because I had no idea what the little hollow 'top hats' were for (see above), it has now all fallen into place:

When the 'activation tube' is installed, it has a seal around the end, which appeared to serve no purpose at all. In addition, the clamps holding the activation tube to the intake housing had the little top hats which the fixing bolts go through.

Answer:
To stop the electrolytic action between the alloy intake housing and the stainless steel activation tube and the propshaft, which has components from alloy to stainless and even brass to rubber, they are all insulated by the pointless (in my thoughts at the time) seal and top hats, they are no longer pointless.

This now makes me consider a sacrifical anode on the rear of the second stage impellor, just to be sure. Amasing modern science, if you have to work it out like me.

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Just a pictures of the plumbing, from before and after.

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Another step forward. I have found Walker Rubber on 01603487371, who will sell 1 foot of moulding or whatever you wish. Not only that, they will make a mould to your design. Obviously the mould will cost, but I am happy with the rubber mouldings I will receive from them next week.

Thanks Walker Rubber 

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Everything is happening again, I have very large 'O' rings arriving, the toolmaker has produced, yet again some great bushes and I now have a sandblaster to sort out the tail pipes of the jet unit. Not only that the sandblaster man lives 100 yards from me and I never knew.

I have lost count of the components that I have made for the jet unit, but it is a lot.

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Robert (toolmaker) has done it again, the old anode was a little distressed, so a new one was required. I think the old one was magnesium, but for use in rain water a soft alloy is best apparently. I am out of my depth on this, but that is what it is made of. The good think it is so easily changed that I have no worries, unlike the assembly of the rest of the unit.

Also see
www.dowtyjetboat.co.uk

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Due to a bit of ill health I have not progressed with the Dowty for some time, but yesterday I fitted the fender rubber from Walkers and it fits like a glove and looks brilliant. Just have to make up the rear clamps now from stainless steel.