What shall I build?

[Julian]

Hi,

I have a friend who is building a "Clyde Puffer" boat. He wants me to build him a small steam engine to power it when its on the pond. I have come to the conclusion that the simplest but best to build will be a twin cylinder double-acting oscillating engine.
Question:
1. Should the flywheel be large or small to give power?

2. What gives the most power/torque...large bore and short stroke or small bore with large stroke?

3. What rpm's should I be aiming for?

4. Should the cylinders both work together ie. both push/pull together or should they be 90degrees apart?

I will also be building the boiler but do not yet know if it will be vertical or horizontal.

Julian.

 

[Jasonb]

Due to the need to get the propshaft line low in the hull you will need to kep the flywheel quite small in diameter but make it bulky eg a bit of length and no splkes, just solid.

RPM will really depend on prop dia and pitch, so don't want anything too fast as it will look wrong skimming over the tops of the ripples

I would set them 90deg apart for easier starting and reversing

Jason

 

[Tin Falcon]

2. What gives the most power/torque...large bore and short stroke or small bore with large stroke?

two different animals but related. power is a function of displacement times rpm and pressure .or work and time. like 1 joul per second =1 watt.



torque is a funtion of how much force is applied to the piston. This is function of piston area and pressure.


4. Should the cylinders both work together ie. both push/pull together or should they be 90degrees apart?

Yes the best configuration for a marine engine is a double cylinder double action that way it will be self starting ie 90 degrees apart.
Tin

 

[Blogwitch]

As Jason said, because it is a flat bottomed boat and the propshaft is horizontal rather than inclined as in normal ships, you need to keep the crankshaft as low as possible.

Contrary to popular belief, a flywheel on a small marine engine is not really required. If you make a long stroke engine, it will run plenty slow enough and should give you more than enough power. Of course, it all depends on the size of the model, that will dictate how large an engine and boiler needs to be made.

You don't really have a choice of boiler, the layout of a clyde puffer really means that it will be a reversed layout. Put in a long propshaft so that the engine ends up in the hold, this will also help in sealing the tube, if the engine is mounted in the rear casing, the prop tube would most probably only be a couple of inches long at most. The vertical boiler should be mounted in the engine casing to the rear of the ship.


Bogs

 

[fcheslop]

What size is it?, if its Vic Smeeds at 17 1/2 inches long mine has a 1/2 bore 3/4 stroke single oscillator with a variable pitch prop the boiler is 2 1/2 dia 7 inches long smithie type . The steam plant is housed in the cargo hold area and the cover is double skinned alloy made so air can flow .The burner is similar to the bowman wick type and i also have a gas poker type but there not enough room for a commercial gas tanks.Its a very tight fit so keep a eye out as you build.Wish id thought of fitting a vertical boiler good luck.
regards Frazer

 

[Blogwitch]

Goodness me Frazer, I thought we were talking about a model ship, not a thing to play with in the bath.

This is the one I scratch built on a 4ft long glass hull. Fully loaded and sunk to waterline using two internal ballast tanks, it weighed about 1cwt (50 kilos). Unfortunately, not steam driven, but could have been.


Bogs

 

[fcheslop]

Hi Bogs , that's large for me my smallest live steam boat was 4inch long 1 inch beam 1/8bore and stroke piston valve engine with a 3/4 dia by 1 1/4 vertical boiler .Open launch but was sadly lost with all hands due to an engine room fire but currently thinking of something smaller :big: sorry for intruding on the thread
regard Frazer