Engine selection

[Peter Webster]

I have 2 steam engines which i am deciding on which to use for my P.S. Pevensey from the Float a Boat plans. The first one is a 2 cyl. vee oscillator and the othe is the Graham Industries TVR1A which is a Twin cylinder, double-acting, reversing (Hackworth gear) steam engine both are similar capacity 1/2 " Bore 5/8" stroke. What boiler should I use as the minimum, ideal size (if such a thing exists). I want to get about 15 minutes on the water each time.
Thanks in advance
Peter W 

[derekwarner_decoy]

Hi PD's......Peter....here is a link to an image of the boiler & engine from the Pev.......

http://en.wikipedia.org/wiki/File:PS_Pevensey_Engine.jpg

It appears that the boiler top itself is approx 20" above deck level
The FaB plans @ 1:24 provide a good sized vessel of Pev ....you could consider a boiler diameter to fill the hull & having the boiler top in a similar scale relationship

Are you going to have the boiler locally home built? or go for a proprietry unit? ....the Macsteam WEB site provides steam output values for their respective sized boilers 

If I applied the same thought process to my intended build of Oscar W @ 1:24 from the FaB plans I could only fit a 3 1/2"diameter horizontal boiler

Either the 2 cyl. vee oscillator or the TVR1A Twin cylinder would require suitable gearing down........what have you considered?........Derek

[derekwarner_decoy]

Hi PD's....I am sure I am not breaching any copyright here as it is open information on the Macsteam WEB site.......& just by chance the example shown is 1/2"bore x 5/8"stroke which is the same as your selection Peter...............& also the consideration by our newer member Bruce from QLD in OZ.......Derek........

To calculate the size of boiler required for an engine simply divide the steam output of the boiler by the swept volume of the engine. This will give you the maximum speed in revolutions per minute that you could expect with that boiler/engine combination. I know some people will argue that this doesn't take valve cut off into account but it simplifies the sums and any cut off will only improve performance.

The swept volume of an engine (that is the volume of steam used per revolution) is 3.142B x S x C x 2 where B = the bore radius squared, S = the stroke, C = number of cylinders and x2 for a double acting engine (leave out the 2 for single acting engines).
So for example, if you have a twin cylinder double acting engine of 1/2" bore and 5/8" stroke the swept volume per revolution is:

3.142(1/4x1/4)x5/8x2x2 = 0.49 cu ins/rev

If you wanted to operate this engine at 40psi, then the 3 1/2" boiler will make 711cu ins of steam at 40psi

so 711/0.49 = 1451rpm This is the maximum continuous speed at which your engine will operate at 40psi.

The above figures are approximate but should give you a good idea of the performance you could expect. You can increase the performance very slightly by lagging the boiler, engine and steam pipes properly.

[Peter Webster]

Derek,
           I am hoping to build the boiler with the help of Tom Prior, a fellow club member, who incidentally also has an "Oscar W" of around 1/26 scale. He builds all his own engines & boilers and on the Oscar W he has also built a clutch system to use if the steam runs out in the middle of the lake.
           The engine I think I will use will be the Graham Industries unit as it looks better and yes I will be gearing it down considerably.
            Thanks for the boiler selection info, it appears useful
Peter W

[kno3]

Hi, may I ask why you think gearing down is needed on either engine? The Graham TVR1A is a powerful one and should be able to directly drive paddlewheels of 10-13 cm without problems. Gearing it down would make it spin faster than needed and consume a lot of steam vs. direct drive.

[derekwarner_decoy]

Hullo kno3...you may well be correct here....it is just that I have not seen a similar example of a 1/2"x5/8" twin wobbler nor a Graham TVR1A engine in a direct drive application under load....or  rotating from start or low RPM......to the nominal 120 RPM @ the paddle shaft .....

Naturally if you could point to any example would be a great help for many PD members.........Derek 

[Peter Webster]

I realize the power of the Graham engine and maybe you misunderstand my meaning of gearing down. I mean to slow the paddle shaft speed down by using a small gear on the TVR1A engine and a larger one on the wheel shaft, maybe I should have said to gear it up but don't forget I am from downunder and every thing here is opposite . 
Peter W

[kno3]

Gearing up or down.... I mean you shouldn't use any gears, but direct drive, so that engine shaft speed = paddlewheel speed.

[kiwimodeller]

Peter, not sure about your oscillator as I only have experience with the PM Research V twin but the consensus for the Graham seems to be a horizontal boiler 3 & 1/2" diameter and 6" long. I have run one on a 6" by 3" successfully but it was in a relatively slim easily driven hull. Graham only reccomend 30 to 35PSI which is pretty easy to maintain but the volume of steam needed is governed by the revs used. Not sure that I agree with Kno about direct driving. The engine would probably do it and a heavy loading actually reduces the volume of steam needed but when you load an engine like that you then need higher pressures to get the engine to self start. It seems from lots of discussions like this that a reduction of around 3 to 1 up to 4 to 1 is needed and that revving the engine to around 400 to 600 gives better throttle control. The exception is very long stroke engines but again these need more pressure to take advantage of the long stroke. My large (78") side wheel paddler Waimarie has a big single cylinder engine and it has a reduction of 3.3 to 1. It is possible to rev the engine too high and get the wheels cavitating but the advantage is that I have very good control over the engine. Too many projects on the go at present but one day I would like to try a paddler with two very long stroke small bore engines coupled directly just like some of the originals were. Hope this is of assistance, cheers, Ian.

[kno3]

Kiwimodeller: A single cylinder model steam engine often needs some reduction, otherwise it would be indeed hard to control at slow speed, because of the dead spots. A well built twin cylinder shouldn't really have this problem and be easy to start, especially if the friction is low. I was able to run my Graham TVR1A very slow reliably after a pretty short running in period.

And regarding the boiler: 3 & 1/2" diameter and 6" long doesn't say anything about its steam production capabilities! Not every 3 & 1/2" diameter and 6" long boiler, regardless of maker, is going to produce as much or as little steam as the Maccsteam unit you are probably referring to. One needs to know the heated surface and the burner type & input in order to make a meaningful affirmation about what type of boiler is good for what engine.
I have also heard the Maccsteam 3 1/2 boiler being referred to as adequate for a Graham TVR1A, but I have no idea what heating surface it has (flue and Galloway tubes surface). Can anyone shed some light here? I'm wondering if it's comparable to the old Cheddar units such as the Puffin?

[tonyh2]

Hi there,

I hadn't noticed a later posting on the same/similar subject so here goes.

Has anyone any knowledge of any plans for an engine, similar to that suggested by Kiwimodeller, of  long stroke, small(ish) bore, and suitable for direct drive, run as a pair on opposite sides of the hull?

The boat is about 1.1m x 20cm, weighing about 10kg with effectively a single stern paddle.

Thanks

TonyH