Stevens's Marine Screw Engine Recreated

[Jasonb]

This one can be firmly blamed on Sanjay as a few months ago he posted about the restoration of an original Stevens's Model Dockyard "Direct Acting Marine Screw Engine" which caught my eye. I do have a reprint of their 1919 catalogue but the image of the engine is not really clear enough to see what is going on but his photos were enough for me to have a go at recreating one of these engines. I went for the No1 size which at 3/4 x 3/4 or in my case 19mm x 19mm is a similar cylinder size to the Stuart "10" range.





As usual I drew it up in metric using Alibre with the intention to use a 3D aluminium print for the main frame with the rest cut from solid.



There was not much to do to the 3D printed frame as I included most of the holes in the file that was sent for printing. I just had to open up and ream the hole for the crankshaft bushes and mill off a small machining allowance on the foot and valve chest "cover"



The main bearings were turned from bronze and the reamed hole used to gauge their size.



The cylinder was roughed out from some cast iron bar



Before finishing the decorative bead with a form tool



A flat section was milled to accept the separate port block



All the holes and passages were machined into the port block prior to bonding into place with JB Weld



The valve chest was just turning and milling from a rectangle of cast iron





Valve milled from a bit of bronze block

[Jasonb]

The eccentric strap was machined from some flat brass repurposed from a shower door hinge



The outside was mostly done with the rotary table before the vice to finish off the lug for the clevis.



I did not take any other construction photos as it is all much the same as other engines. Trial assembly ready for a quick test run..

[Sanjay F]

How wonderful, it looks gorgeous and I was wondering when you were going to make it as I remember you getting the frame 3d printed. The finish on the frame is fantastic, looks the part with the grainy texture

I definitely think its one of the more interesting and unusual engines; there are so many similar looking engines in these old catalogues by the various manufacturers of the period, but this one sticks out.

An amazing job as usual and really demonstrates what you state about being able to construct things from scratch .........I'll get there one day ...........

[steamer]

Where did you get the frame printed Jason?

[Jasonb]

Thanks Sanjay, ye s the "cast" finish is just right and as you say its a bit different from other engines which is part of the appeal.

Steamer, I have had all mt 3D metal printing and some PLA done using Craftcloud. They are base din Germany but give prices from around the world and I tend to opt for ProtoTI in china who are usually the chapest option. It's easy to get a price, just upload an STP etc, choose material and finiah. Make sure you check the part size once uploaded as it may need scaling up or down.

[Chipswitheverything]

This fine engine is a remarkable demonstration of the way that completely modern technology and capabilities can be combined with design from long ago : and allied also to the familiar and traditional processes of workshop model engineering.  Interesting and impressive in all aspects of the build. Dave

[Jasonb]

Thanks

It is interesting to see how much more compact height wise the engine is compared with a Stuart 10V, which shows why the Scotch Yoke was used to eliminate the length needed for a trunk guide and conrod. Ideal if you want to get it all below deck or just keep the ctr of mass low in a hull.




Well as you can see from the above image it has now had a coat of paint and been mounted on a small base just to stop the flywheel touching.







Quite happy with how it runs. It is not the easiest engine to set up as the port face is on the cylinder which means you can't set the valve to that as it is not solidly connected to the engine once the valve chest is open. I settled for checking that it had equal movement within the valve chest and did a rough 90 +30deg setting for the eccentric. Just needed one adjustment to that to reduce the lead slightly and off it went.

<a href="https://www.youtube.com/watch?v=JO3-BPAoWqU" target="_blank">http://www.youtube.com/watch?v=JO3-BPAoWqU</a>

[Sanjay F]

Quite happy with how it runs. It is not the easiest engine to set up as the port face is on the cylinder which means you can't set the valve to that as it is not solidly connected to the engine once the valve chest is open. I settled for checking that it had equal movement within the valve chest and did a rough 90 +30deg setting for the eccentric. Just needed one adjustment to that to reduce the lead slightly and off it went.

Love the colour and and yes, needs a base as it keeps wanting to fall over! Set up was tricky, I had actually given up on this engine as I couldn't set it up blind, but used Jason's technique for roughly setting it up and it worked. It's a powerful little engine given its size and I can see you having to hold it down before it escapes of the bench! 

[Sanjay F]

For a bit of fun, here's a similar photo to the one above, a MkI Stuart 10v next to the Stevens' Marine Screw engine, I think mine is the smaller, baby version for sure!!

[Jasonb]

Yes that scotch yoke going up and down makes for a "lively" engine. I think yours is the same size, if you raise it up so the crankshafts are level like in my photo there is not much difference.

[Krypto]

Great build!  The fillister head screws look good on a design of that vintage.

[Jasonb]

Thanks, I also like the look of them on these engines. Hard to get metric ones so I end up making my own.

[PaulR]

  Very nice. Did you use a slot drill, end mill or a bit of chain drilling for the yoke?

[Jasonb]

I drilled the corners 2mm. Then ran a series of 5mm holes along to remove most of the waste and then milled it out with a 3mm cutter which was the smallest I had with 8mm+ long flutes to cut right through. Finished the corners with a file.

I think Sanjay's just had a round pin running in the slot but I added a square bronze block

[PaulR]

I drilled the corners 2mm. Then ran a series of 5mm holes along to remove most of the waste and then milled it out with a 3mm cutter which was the smallest I had with 8mm+ long flutes to cut right through. Finished the corners with a file.

Cheers. Does the yoke rock slightly relative to the crank disc? I've made four or five engines with yokes and that happens to all of them and I've never figured out why although it doesn't cause any problems when running. I suppose a guide rod at each end would stop it if necessary or maybe having the square block solves it?