Making Whistles is hard
Well, as mentioned elsewhere I have found it very hard to make a whistle that sounds nice (in fact one that sounds at all!) … the principal is simple enough, but the practice is much harder with quite fine tolerances and dimensional “magic numbers”.
Making a nicer whistle – version #4 (at least)
I had one noticeable success with a three-chime whistle for the 5″gauge loco, and the one for the Windermere Kettle is pleasant, but the main whistles for the boat just screech (very loudly, or not at all).
We need a Siren!
This idea has cropped up several times, but finding a set of drawings was hard, and with the “whistle experience” in mind, I really wanted a “guaranteed” design.
Well, as is often the case, the SBA provided an answer via their house magazine Funnel, with an article from 2007 (Issue 112) by Peter Cowie from Auz. This provided the basic idea and design for Befur’s Siren, which I can guarantee works! (Other boaters classify it as “deafening” even from many metres away!)
It does take a fair quantity of steam to set it off, on Befur it’s plumbed in via a 1/4″BSP ball valve and at 200psi that works very well. Here’s a video of it running on air (quietly!)
You should be able to download this article from the link above. I found the drawings provided hard to decipher, so with the drawings in this post and the pictures perhaps all will become clear. You will notice that mine is less shiny, and still awaiting a “trumpet”, there is talk of casting some in bronze!
Drawings
Pete’s original drawing from the Funnel Article
As I noted above the drawings in Funnel are quite hard to interpret (I think there were probably more sheets, which would have made it clear.
However, I decided the simplest approach was to re-draw in viaCAD, so there are links below to a viaCAD file and exported version in .dwg (Autocad file), and .png (graphics picture) file too…
(If you have trouble downloading the drawing files (WordPress doesn’t like them) then just message me (at foot of this post) with your email address and I will send them to you)
These drawings are metric (apart from the threads that are either BSP or TPI threads (because I happened to have the imperial change-wheels fitted to the lathe at the time!!)). The drawings show the components, a sectional GA , listing of the stock materials used to make the siren and a drawing of how the distribution plate was originally drawn (but not how I made it!).
Perhaps the key bit of understanding (which took me a while to spot) was that there is a steam space between the Porting Cylinder and the Body, which is supplied through the spaces in the Distribution Plate and delivers steam to the ports in the Porting Cylinder.
Manufacture
The material for the siren (from M-Machine metals in Darlington and on-line)
The siren is built from phosphor-bronze cored bar, all sourced from M-machine Metals in Darlington.
M-machine will take phone orders and are happy to supply small quantities for home-workshop user. The picture shows the stock used to make the siren, at a total cost of about £75. (in 2023)…
These are stock-sized cored-bar from their catalogue, and the sizes purchased are shown in the drawings I prepared.
The majority of the manufacture is simple lathe work. The larger threads are all screw-cut on the lathe. I usually cut the internal threads first, then cut the male (outside) thread, using the other part as a gauge.
The most challenging parts of the build are the Porting Cylinder and the Rotor.
Rotor
Siren porting cylinder and rotor assembled in the cap.
The Rotor slots are just slots cut vertically (radially) into the rotor with a slot drill, while the rotor is held in a rotary table on the milling machine.
The slots in the rotor can be seen in the photo on the right, inside the porting cylinder.
There are eight (8) evenly placed slots in the rotor.
The REALLY important factor for the rotor to work effectively is its fit within the Porting Cylinder, which needs to be quite loose. The bore clearance needs to be at about 5 thousands of an inch (sorry for mixed units!), and it’s axial clearance when the Porting Cylinder is clamped within the Cap is again about 5-thou. Clearances any tighter than these numbers will mean that the siren will not start, as the starting torque is quite limited, and any contamination or particles in the steam will jam it in place.
Porting Cylinder
The Porting Cylinder is, without doubt, the most complex component. It houses the rotor, with a retaining lip at the lower end, and the rotor is located by the cap at the top. The Porting Cylinder is held in place by the Distribution Plate below it, as the Cap is screwed down tight on top.
The porting cylinder has two sorts of slots/holes machined into it. The long slots (4 off) probably provide most of the sound. They are machined tangentially into the Porting Cylinder, this is so that the steam passing through these ports causes the rotor to spin and produce the siren sound, as the steam is released and cut off.
As it is possible that the Rotor stops with the above 4 slots covered, and thus no steam would flow to start the Rotor spinning. For this reason 10 “starting ports” (holes) are also machined into the Porting Cylinder walls. These are also tangential, and “ensure” that some steam flows to start the Rotor.
A spreadsheet showing the angular positions of the 4 slots and 10 ports in the Porting Cylinder
The Porting Cylinder is machined by holding it in a rotary table on the milling machine, while the ports and slots are machined by plunging a slot drill into the walls of the Porting Cylinder.
The location of the ports (10 off) and slots (4 off) are dimensioned on the drawings, but the angular positions are shown in the adjacent table. My maths was not good enough to produce a symmetric and even spacing, but actually I think the asymmetric spacing probably adds to the sound quality!
Tangential Positioning
The tangential billing is achieved by off-setting the centre-line of the Porting Cylinder from the centre-line of the drill, by 19.8mm. These dimensions can be seen on the drawings.
Final Observations
It is a very satisfying object to make and to use – even drawing admiring comments from other berth-owners at Befur’s marina. It is VERY loud, so some restraint is needed to avoid becoming a nuisance. Feeding it via a ball valve allows the siren to be played very effectively, from low growl to deafening howl. – who knows, maybe if we complete the Trumpet it will be louder yet!
It does generate a deal of condensate, so aim the outlet away from the the passengers, to avoid giving them an impromptu hot shower.
I’ll probably continue trying to make the perfect steam whistle, but for now we are happy!
Picture Gallery of Components
SY Befur 