Here is a breakdown of the current situation:-
Engine: Stuart D10 double cylinder, double acting, ¾” bore by ¾” stroke, driving the
Propshop’s 5 inch prop designed for the D10.
Boiler: Vertical Monotube, 50foot of 3/16 inch copper brake pipe wound in three
Boiler dimensions: diameter 108mm, Height 270mm plus 190mm stack.
Electric pumps (2 off): Feed rate 85cc/min each. Filtered Pond water is used.
Fuel: Butane/Propane in liquid form with evaporator on burner.
The heated gas is attemperated by a cooler fed with incoming feed water.
Burner: Jet 0.7mm dia., mixing tube ¾” dia., 100mm long, Stainless bowl 1¼ “ dia. with 1.2mm pitch mesh to prevent flashback to the jet..
Fuel consumption 4.5g/min at 530RPM.
Currently achieving 720RPM at 170cc/min feed and 45psi, 145degC steam conditions.
The exhaust steam is directed to an oil separator / feed heater (dia.50mm, height 225mm plus 108mm high vent pipe). The oil free water is dumped overboard.
PIC based microcontrollers monitoring the feed pumps’ strokes per minute, steam temperature and pressure and the gas cylinder pressure.
The demanded feed flow is set at the radio transmitter and the controller, via ESCs, ensures that the pumps maintain the flow independent of the boiler pressure.
The steam temperature has a manual setting that is amended by the demanded feed flow, such that, as the feed is increased, the desired temperature is raised, which in turn causes an increased firing rate via a servo gas valve, raising more steam and an increase in speed
– put simply more water in, more steam out and the faster the boat goes.
The controller has a start up mode such that the boiler is warmed up at a optimum rate to the boiling point without over firing and causing excessively high temperatures. Being a monotube boiler, it takes only a few minutes to raise pressure.
There are a few safety features: high temperature shutdown of the gas valve, high pressure shutdown of the gas valve and pumps, pump failure detection and standby pump cut-in. The boat automatically restarts itself if any of the above occurs.
Current problem: the stainless steel burner mesh crumbles away after about 5 hours running and is to be replaced by a ni-chrome mesh once I’ve woven it!
Was it worth it? The initial objective was to build a simple cheap boiler that didn’t require insurance or annual testing. Well the boiler is certainly cheap at £60 (£20 copper brake pipe, £20 toilet brush holder boiler casing, £10 brass stack, £10 sundries) perhaps a tenth of the price of a conventional boiler. Certainly it is very simple to make with only two silver soldered connections, plus one joining the two 25 ft lengths of copper. Not being a pressure vessel it is exempt from the test and insurance requirements.
At the project start, the best method of controlling the monotube boiler was a matter of debate and using a programmable system enabled many ideas to be explored without massive amounts of rebuilding mechanical and/or electronic controls. One thing you don’t have to worry about is the boiler water level!
So we have a boiler that is cheap, quick to start and runs as long as the fuel lasts (100minutes or more).
The controller hardware bits came to about £70 and took around a week to solder together, although it is transferable between models or even a full size boat.
The software is a totally a different kettle of fish, with thousands of lines of code, but, since completed, can be downloaded to the raw controller in minutes.
It took 2 years of experimenting (including pump and boat construction) to arrive at a practical solution with “Vital Byte” having it maiden voyage at Wicksteed in May 2009.
Where to now? I’ve got Selway Fisher plans of a 30ft steam launch and I’m just scaling them for the 5 inch prop; that determines that I build a 75inch launch to install the existing system in.
Now that I’ve broken the back of the problem, it would nice to know that other enthusiasts will take up the challenge and build a similar (perhaps better) monotube boiler system.