How Stupid Am I?

[Poida]

I have just rewired my boat. Ran a new 7 core cable down to the stern and rewired the bilge pump, the bilge blower and the rear nav light.

To make it easier to remove all the old wiring I cut it up into 3 sections and pulled it all out except for one wire that appeared to be stuck to something.

Since I had run new cables to power everything nothing should be attached to the old cables.

Oops, it was the echo sounder, yes I had successfully cut the echo sounder co-ax into 3 pieces.

Soldered them back together and checked for accuracy, yep 16.5 metre depth and what appeared to be 2 large Groper swimming under the boat.

Yep looks fairly accurate to me, that is if you ignore the fact it's parked on a trailer on my front lawn.

However all kidding aside (I lied about the Groper) I would like to test it, does anyone know the speed of sound through salt water. My search only found the speed through air and fresh water. Salt water should be a tad faster.

[SailDesign]

Got to be simpler to wait until you drop it in the water next time, for an accurate test. No point "testing" it in less-than-ideal consitions, and then ripping the bottom out on a nice rock you didn't "see".
Steve "practical"

[ron17571]

stuff happens,i had a drill that had died and thought id cut the cord off to save for something else,so of course i picked up my new makita and cut its cord off instead!ive gone to cordless since then,18 volt drill is just amazing.

[PAR]

You can splice those cables, though the trick to getting good response is "shielding", which is the tin foil or mesh looking wrapping just beneath the outer insulation jacket. Insure you have good connections and insulate them. Then use some tin foil (aluminum foil) to wrap the whole wire in this area, a few inches past the break in both directions. insulate this and she'll be fine. You can buy this shielding stuff, of the mesh or foil type, but why bother when you wife likely has enough to do hundreds of yards of cable in one of her kitchen drawers. You can insulate with tape (it will unravel) paint on liquid tape (messy) or heat shrink (that's the stuff to use). Insulate each wire, then the bundle, then the shielding.

[MikeJohns]

By Wilson's empirical formula.
Around 1500m/s for 15 deg C sea water. Will vary with temp and salinity.

Adding to what PAR said the idea is also to try and keep the impedance in the cable constant, at each join you have an impedance mismatch that will give you an echo. Joining the coax should be done with shield insulatation and core in as close a pysical match to the original as possible.

If it is just a single core coax the best result will be to add a BNC fitting to each end and a barrel connector for the join. Any computer network person has the crimpers for the BNC fittings, they are about 50c each and $1 for the barrel connectors.

[Poida]

Thanks Mike

I'll give her a test.
Joining the cables has already been done. One of my past jobs invoved a lot of cabling including co-ax. Even then I have been prone to cutting the wrong cable. I'm quite experienced at it.

[safewalrus]

Lead line - if you cut that up you resplice and measure again using your tape measure and if that missing the length between your finger tips is one fathom more or less, so give it a bit of slack! Also your longest boathook should be painted in foot thick/wide stripes (you got one I take it) for shallow work! Then you approaches new places slowly! clever these old buggers ain't they!!

[Frosty]

It may not be the case in all situations but I once extended a cheapo Hummingbird--probably the cheapest with normal ac houshold electrical twin side by side wire. I just bared the two wires in the depth sounder wire and twisted it together on the 6-7 meter extension houshold wire.( cant remember just how long was the extension). Taped it up with a bit of tape and it worked fine with no noticable loss in performance. But that was with me im not saying it could work in all cases.

I can only assume that this wire MUST not be cut so they can sell you one of their extensions???

[Poida]

The length of cable is important Jack as there is a certain impedence in the cable length which has to be maintained.

You can get away without using a proper joiner, as I did, without too much of a problem as I have done in the past, but not on an echo sounder, so the amount of inaccuracy is yet to be determined.

Also kinking it, standing on it or using cable clips that are too tight can stuff up the signal.

With the info Mike has provided I will give it a test.

[Frosty]

Yup I know thats what it says in all the instructions --But it worked for me.

[MikeJohns]

The two core light flex may well work but the interference to your radio equipment will likely be severe.

Originally Posted by safewalrus

...... Also your longest boathook should be painted in foot thick/wide stripes (you got one I take it) for shallow work! ....

Mike
We have a red ring around the 9' boathook at the boat draft. For getting into secluded inshore spots it's the most usefull tool aboard we send the survey team ahead in the dinghy .
cheers

[Frosty]

The hummingbird is the cheapest depth sounder ( fish finder) on the market. I think I am right in saying that it is a mere 50watts compaired to navman for instance that is over 100 watts. I run the same manufacturesrs equipment now today but on another boat. I use hummingbird for the bridge and Navman for the main below station. They sit side by side in the bilges in seperate oil baths .
I was told this wouldnt work as they would interfere with each other. The senders are inches apart. Hummingbird bieng American and Navman being Kiwi means that they have different frequencies and work fine.

I had no interference what so ever from the extended Hummingbird!!!!!
It is quite easy to feel the clicks of the transduceer when held against the hand -about 2 clicks per second,--if you hold it to your stomach you can get a reading.

I dont think it is dangerous because i tried it on the cat first.--- Is that a cat scan?