Experience of A.A. Leak's compounds

[malcolmd]

Hi Mal,

I am building one of these engines currently. I have 'graduated' from a model engineering background - sold my steam lorry and got stuck in.

I don't know where you are based, but I am in the UK, so became a member of the SBA. They're a nice group of guys who are really knowledgeable, friendly and encouraging.

As always, there are positives and negatives to any design. The book states 390 lb/hr (not 290), giving a water rate of around 35 lb/hp/hr. To achieve 11hp, I have calculated that I would need a BMEP of 114psi in the HP cylinder! Can someone verify this for me? With the mother of all boilers at 200psi WP this might be possible???? I am looking at using a published design for a 3 drum water tube (Roberts type?) boiler, but I'm way off worrying about that just yet! The advice I got was to open up the back of the balanced LP valve, as the movement will throttle the passage to exhast at the as drawn dia, and also fit a strongish spring. Also fit a spring in the valve on the Edwards air pump (none shown) to help things. I was also told that there may be wiredrawing on the HP valve, so I set out the geometry to check it in line with one of my textbooks, and I think the eccentric rods may be too short. Can anyone advise?

From what I have seen on the water, the boats have smaller boilers and live with the fact that they are only getting maybe 7-9 hp out of it (anyone?), which is obviously adequate for the hull sizes, as they seem to go alright.

I am also looking at Selway Fisher's designs around the 25' mark.

C.

I am also in the UK and just joined the SBA, but am yet to meet anyone, just twaling the web site and Funnel magazines.. check out the pictures in the link I posted below, a rather more "exhibition quality" version of the engine than I would aim at!

[malcolmd]

Hello

A friend of mine owns a set of castings for this engine and could sell them to you
If you are interested please contact him by mail or phone
Bruno Martin-Neuville
bmartin.neuville@noos.fr
+33145864033 mobile +33682416034
(he speaks english )

Kind regards

Thierry

thanks Thierry, I have sent him an email ...

[Laurence]

Hi. I am also building one of these engines. A while back I had a chat with John Griffiths, owner of Tenacity (which is featured on the back cover of Camdens booklet about building the engine). He gave me some very useful advice about the building of the engine, errors in the drawings and modifications necessary to get it running nicely. His details are in the SBA membership list. I would reccommend giving him a call before starting any work.
I have also spoken to other owners using this engine and it seems to be quite a rugged design with good sized bearings.

[Midship]

Hello Malcom

I was with Bruno during the WE, he was checking all the castings
you can see him steaming on his boat Mélusine this WE for a meeting of our club
http://chaloupesvapeur.blogspot.com/
best regards

Thierry

[malcolmd]

Hi. I am also building one of these engines. A while back I had a chat with John Griffiths, owner of Tenacity (which is featured on the back cover of Camdens booklet about building the engine). He gave me some very useful advice about the building of the engine, errors in the drawings and modifications necessary to get it running nicely. His details are in the SBA membership list. I would reccommend giving him a call before starting any work..

I will certainly do this - thanks Lawrence

[malcolmd]

Hello Malcom

I was with Bruno during the WE, he was checking all the castings

Thierry

Excellent Thierry - I think I have persuaded the "houshold powers" that this is a good use of funds!

[Midship]

He explains me the option he proposed to convince your Ministry of Finance , we will be very happy to see you and show you our boats and workshop, and Paris and/or Versailles where I live:)

[Mjolnir]

Hi Guys, I have built one of these engines and am currently building the boat to go with it. So I can't actually comment on how it works out in the boat yet. I built mine from the drawings as published in the Model Engineer around 1983, there are a lot of mistakes in these. One in particular wa the length for the pins that the radius arms for the valve gear go on. The pins are not long enough, fortunately I spotted that one before I had made any. There were other mistakes too, but no castings were spoilt. I don't know if Camden have made any corrections in their drawings, but I would suggest checking them carefully. I made my own patterns and had them cast locally (New Zealand) since shipping costs from the UK tend to be a bit prohibitive. The hull I am building is thirty feet long, with a six foot beam, so I expect the engine should drive it along OK.

One point about this design, it has what may be regarded as a short stroke by steam engine standards. This seems to be a direction that the English designers started to go back in the days of steam launches. A guy here in Auckland has a lovely little Simpson Strickland triple that also has quite a short stroke. The reason they did this is probably to get more power out of a more compact engine. For the same inertia loads on the bearings, and the same piston speed, if you shorten the stroke you increase the maximum RPM in the same proportion. If you do the maths you will find that this allows you to get the same power, albeit at a proportionately higher speed. This then allows a relatively smaller prop to be used. My Leak will run happily at 600rpm. Now, this will not necessarily suit guys who want to see those gleaming steel connecting rods slowly turning, but it does help to make a more compact installation, with a smaller propellor size. (Relatively speaking, I will be starting with a 21" by 21", which is still not a small propellor.) The higher speed may lead to slightly higher friction losses, and possibly slightly lower propellor efficiency, but the thermal efficiency should be better since the heat loss from the smaller cylinders should be smaller. One might note that the same process has happened over the years with car engines, eg shorter strokes.

I'll post some pictures later.

regards
John

[malcolmd]

One point about this design, it has what may be regarded as a short stroke by steam engine standards. This seems to be a direction that the English designers started to go back in the days of steam launches. A guy here in Auckland has a lovely little Simpson Strickland triple that also has quite a short stroke. The reason they did this is probably to get more power out of a more compact engine. For the same inertia loads on the bearings, and the same piston speed, if you shorten the stroke you increase the maximum RPM in the same proportion. If you do the maths you will find that this allows you to get the same power, albeit at a proportionately higher speed. This then allows a relatively smaller prop to be used. My Leak will run happily at 600rpm. Now, this will not necessarily suit guys who want to see those gleaming steel connecting rods slowly turning, but it does help to make a more compact installation, with a smaller propellor size. (Relatively speaking, I will be starting with a 21" by 21", which is still not a small propellor.) The higher speed may lead to slightly higher friction losses, and possibly slightly lower propellor efficiency, but the thermal efficiency should be better since the heat loss from the smaller cylinders should be smaller. One might note that the same process has happened over the years with car engines, eg shorter strokes.

That sounds like a set of well reasoned arguments John, as you say this is the general direction of engine design all the way to Formula 1!, and its good to know you have a working engine.

It leads me to ask twosuppliemental questions

1 - I note that some people have gearing 'twixt engine and prop, is this to up prop speed, or reduce it, and how do you figure the optimal speed for a given prop?
2 - What Boiler have you built for your engine and what size is it (heated surface, grate area, lbs/hr etc.)? (I am thinking of using the larger of the SBA's Watertubeweld-less designs with 60+ sqft of heated surface...

Mal

Malcolm

[Mjolnir]

Hi Mal, Gearing could be applied either way...if you had a long stroke low speed engine and a hull with a small clearance for the prop. you could gear the speed up to allow a smaller propellor. Apart from the loss in the gears themselves, the smaller propellor would be less efficient. Or if the engine is a really high speed one, you could gear it down to suit a more efficient prop...as is almost always done with steam turbines. But actually, if we already have the hull, and it has a limited prop size, we could design the engine to turn a prop that would just fit at a speed that would provide enough power. That is not as good as running a bigger prop...the bigger the better in general, but it doesn't help the efficiency much if they are chopping into the boat or dragging along the bottom of the canal.

The boiler I am building is a three drum Yarrow style of thing, an 8 inch steam drum and two 5 inch mud drums. Five inches is too small for easy expansion of the tubes, they should have been at least six, but the design was done and approved before I came on the scene, and I didn't know that until I tried anyway. There are 126 tubes, each about 5/8 diameter and about two feet long. (They vary as they are bent.) The design has been approved to ASME standards. I haven't fired mine yet, the pressure parts are nearly ready for a pressure test, then I have to finish the furnace and so on. The prototype has been steamed and produces plenty of steam, so I am not going to have any trouble with that. I will be oil firing, but the space betwen the mud drums would allow a grate area of about 2 square feet.

Incidently I did make one major modification to the Leak, I fitted a piston valve to the LP as well as the HP. Since the LP is balanced in the original design I am not sure that this was justified, but hey, one does these things. If anyone in NZ wanted to emulate this, I still have the patterns.

regards
John