Steve_Harrison
New member
I have a Seamaster 27 fitted with a Smiths speed log which appears to work by vacuum. Does anybody know anything about these and why mine isn't working? Any info would be much appreciated. Thanks, Steve.
Smiths speed log
- Thread starter Steve_Harrison
- Start date
schilde
New member
Maybe it isn't working because it doesn't work on vacuum!
Our old Smiths log worked like a modern one with a paddle wheel impellor and two wires that led to the speedometer. I still keep it, because I put my vhf license behind the glass. Much neater than sticking it in the window.
I have seen a mechanical log where the impellor turns a rotating cable that goes into the back of the speedometer like the speedo on a car. These work by having a spinning magnet on the end of the cable and a disc near it connected to the needle, that gets dragged around (against a spring) by the magenetism. Very unreliable in a boat.
Why do you think it works by vacuum? I would be very interested to learn about it if you are right. What do the works look like?
Steve
s.childe@rya-online.net
Our old Smiths log worked like a modern one with a paddle wheel impellor and two wires that led to the speedometer. I still keep it, because I put my vhf license behind the glass. Much neater than sticking it in the window.
I have seen a mechanical log where the impellor turns a rotating cable that goes into the back of the speedometer like the speedo on a car. These work by having a spinning magnet on the end of the cable and a disc near it connected to the needle, that gets dragged around (against a spring) by the magenetism. Very unreliable in a boat.
Why do you think it works by vacuum? I would be very interested to learn about it if you are right. What do the works look like?
Steve
s.childe@rya-online.net
Steve_Harrison
New member
It has 2 rubber air tubes connected to it. If I remove one and blow into it, the needle goes round the dial. If I remove the other one and blow, it goes back to zero. Conversely, if I suck either tube, the opposite happens! So air definitely plays a part somewhere.
Steve
Steve
chippie
New member
I think it may work by air pressure in the line, is there a tube with an open end beneath the water?
steve_cronin
N/A
Pitot tube speed indicators....
are simply a coiled tapered tube with a needle fixed to the centre of the coil coupled to a flexible tube which goes to a rear facing nozel fixed to the transom of a boat. As the boat speed varies then the pressure depression in the tube varies and the coil either winds up further or tries to straighten out which alters the position of the needle on the scale. When calibrated correctly this gives an indication of the vessels speed through the water. They were very common in the 60's & 70's but due to their inherent inaccuracy at low speeds were not very popular on sailing boats. No distance readout was normally given and if it were it was notoriously inaccurate.
Smiths Industries also made a cable operated "Log" which was fed by a spiral impellor under the boat, the cable driving a car type speedo head in the cockpit (or anywhere you could mount it without too sharp a bend in the cable). These were standard fit to early Westerlys like the Centaur and Konsort. Again accuracy wasn't their greatest virtue and there were superceeded when Autohelm (now Raytheon), Cetrek, B&G and others brought out logs with small linear response impellors and more reliable electronics.
Pitot tubes of the coiled tapered tube variety are still the most common type of small aircraft Air Speed Indicators and are universal in gliders. In aviation applications the requirements for a clean medium flow and strong maintained pressure ( they are in the nose facing forward in aircraft) are more easily attainable.
Steve Cronin
are simply a coiled tapered tube with a needle fixed to the centre of the coil coupled to a flexible tube which goes to a rear facing nozel fixed to the transom of a boat. As the boat speed varies then the pressure depression in the tube varies and the coil either winds up further or tries to straighten out which alters the position of the needle on the scale. When calibrated correctly this gives an indication of the vessels speed through the water. They were very common in the 60's & 70's but due to their inherent inaccuracy at low speeds were not very popular on sailing boats. No distance readout was normally given and if it were it was notoriously inaccurate.
Smiths Industries also made a cable operated "Log" which was fed by a spiral impellor under the boat, the cable driving a car type speedo head in the cockpit (or anywhere you could mount it without too sharp a bend in the cable). These were standard fit to early Westerlys like the Centaur and Konsort. Again accuracy wasn't their greatest virtue and there were superceeded when Autohelm (now Raytheon), Cetrek, B&G and others brought out logs with small linear response impellors and more reliable electronics.
Pitot tubes of the coiled tapered tube variety are still the most common type of small aircraft Air Speed Indicators and are universal in gliders. In aviation applications the requirements for a clean medium flow and strong maintained pressure ( they are in the nose facing forward in aircraft) are more easily attainable.
Steve Cronin
Steve_Harrison
New member
Re: Pitot tube speed indicators....
That's the closest description to mine yet. The only difference being, mine has 2 tubes and they connect to a fitting on the hull just forward of midships. The principle you describe would seem to be the same as mine and it suggests the reason mine isn't working may be something to do with weed or some foreign body blocking the sensor. I guess I won't know for certain 'till I lift her out of the water next. Thanks for your comments, Steve.
That's the closest description to mine yet. The only difference being, mine has 2 tubes and they connect to a fitting on the hull just forward of midships. The principle you describe would seem to be the same as mine and it suggests the reason mine isn't working may be something to do with weed or some foreign body blocking the sensor. I guess I won't know for certain 'till I lift her out of the water next. Thanks for your comments, Steve.
