How does my log work?

[garethdeb]

Hi folks
I have just bought a cheap little 14ft Marina speedboat for a bit of fun on the Solent. I'm fitting her out with bilge pump, kill switch and a few other Ought to Have's and generally tidying her up but I can't suss out how the log is supposed to work. It consists of a trailing arm arrangement on a bracket screwed towards the bottom of the transom with a clear plastic flexible tube running through the transom and all the way up to the speedo which it plugs into the back of. I can't however see how it's suposed to work. There is a reasonably stiff sprung plunger in the bracket but the tube is pushed onto the trailing arm so I can't see how the arm lifting up could put pressure into the tube, which is how I assume it's supposed to work. Can anyone tell me?
Many thanks for any help and advice.
Regards
Gareth

 

[Jonrob]

If you look at the leading edge of the trailing arm, you will see a small hole, the unit works on air within the plastic tube being compressed by the water, the faster you go, the greater the pressure which pushes on a diaphragm in the back of the log and lifts the needle, very similar to a tyre guage,simple but effective.

 

[rafiki_]

The techie term is a Pitot Tube. Works on pressure drop as previous note. Quite common on small mobo's, but not as accurate as the paddle wheel log.

 

[pheran]

The techie term is a Pitot Tube. Works on pressure drop as previous note. Quite common on small mobo's, but not as accurate as the paddle wheel log.

Agreed, Pitot Tube system - which has a number of other applications including on aircraft. Don't know about lack of accuracy but that wouldn't seem to fit very well with requirements in the air.

I've had such Pitot Tube systems and have found the main problem is keeping the little hole clear of crud which seems to build up in next to no time.

 

[garethdeb]

Ahhhh, splendid, many thanks for that. So I take it the trailing arm lifts up and down so that it can be stowed out of the way for recovering the boat onto the trailer to avoid damage yes?
Many thanks for that. I will have a look tonight and clear the little hole if needed. Is there any way to test it, stick a tube into the hole and blow hard?!! Or shall I just leave it till the first time I launch her?
Anyway, fantastic forum guys, I'm really loking forward to this year.
Regards
Gareth

 

[Jonrob]

You are correct in your assumption that the arm lifts to avoid damage,but I would not suggest you blow down the hole in the arm,you will find that quite a lot of pressure is required, and , if on your own, you probably will not be able to see the dial while blowing. Wait till you get the boat in the water!
Good luck, Have fun.

 

[AquapartPhil]

All great replies,

however if you need to replace the Pitot tube bit (not uncommon) they are often called a "speedo fish" by many stockists

Cheers,
Phil

 

[neale]

The techie term is a Pitot Tube. Works on pressure drop as previous note. Quite common on small mobo's, but not as accurate as the paddle wheel log.

They are pretty accurate at speed, but on my boat it doesn't register anything at all until about 10 knots so useless at slow speed.