Depth sounder question.

[steve brown]

:?: My boat dealer is finally sending me a depth sounder, having waited a good year for it. It is a Lowrance LST 3800. It is about 4" long and fits thru a 2 1/4" hole and records water depth and temperature. Included is skimmer type transducer that mounts on the transom. Initially, I had hoped to get one that used an oil bath type transducer that read through the hull. I have a 4" diameter PVC container attached and sealed to the hull in the starboard V-berth locker. Is the 3800 type of sounder better used on a motor boat? If used on a sailboat where on the transom does the transducer mount and is there a way to avoid penetrating the hull? If I go ahead with this installation I might install the guage in the cockpit below the companionway hatch. I seem to recall in our forum someone using a transom transducer. I do not wish to have a through-hull fitting. Than you again for all your fine advice. Steve

[mgoller]

Hi Steve,
My transducer came with instructions for mounting to the hull or transom.  The previous owner had attached a transducer to the transom so I used the same mounting bracket holes.  The holes are located near the turn of the transom to waterline.  The fiberglass is really thick here so the holes don't actually go through.  I used 1/2" ss screws and plenty of marine goop.  I don't have a problem and since the holes were there may as well give it a try.  I don't lose any sleep.
Now, if the transducer hadn't been there I would have inside mounted.  It seems to me that inside mounting forward of the keel would make the most sense.  Just make sure you are level with the water line and mount with a big gob of silicone.  Air bubbles in the silicone will ruin the installation.  Let the silicone flow out, then slowly sink the transducer in.  Tape and shim so it is level and doesn't move while it dries.
If you want in the future you can scrape it out by pulling a thin wire under it.  It will cut the silicone.
If you want the temperature feature it wont be as accurate in hull mounted.
My setup is a fish finder.  It gives me temp, speed, depth and bottom features.  As much as I have tried, the speed is not real accurate because of how slow a sail boat moves and the water turbulence at the transom on different angles of sail.  I had wanted the speedo for the log feature but without a reliable speed the log is meaningless.  GPS is better.  
The fish finder features are fun, but lead one to believe that there are dozens of fish just waiting for a baited hook whenever the boat is moving.  
I can pretty much tell if the water is warm or cold without a digital electronic.
To sum up, I would say the most important instrument on the sailboat would be a forward mounted depth transducer with a shallow water alarm.  I set mine at 3 feet because I don't have rocks.  You may want an alarm at 10 feet.  Depends.  You want to keep track of depth for safety and also as another means of navigation if you have charts.
The rest of the features are fun gadgets.

[steve brown]

Hi Marcus, My depth sounder came and it looks promising. The directions show how to mount it inside for a thru-hull reading. They illustrate removing the inner layer of the hull and the core so that the transducer mounts and reads directly through the outer hull layer. Is the hull on the 19 a solid single layer or a core assembly? I hesitate to cut into hull at all. I did check the unit connected to my battery against the compass and noticed no deviation of the card so it should mount on the bulkhead ok. Thanks again, Steve

[mgoller]

Hi Steve,
The hull is not cored.  It is solid fiberglass with a gelcoat.  I would ignore the instructions about grinding off layers of the hull and try the silicone first.  The concept is that if there are bubbles in the fiberglass or if there was a core then the transducer would not work.  The sonar pulses get lost in the air bubbles.  They have to pass through solids or liquids and bounce back to the receiver with a clean signal.  Compac uses a core on the deck of a material something like solid bondo.  It is superior to balsa because it doesn't rot.  This is not used in the hull, just deck.
I really think it will work.  
I had real mis-givings about cutting any holes in my boat anywhere and I am glad I didn't.  The instruments below the campanionway step look good there but I worry with a crew they will be obscured by feet and toes.
You are thinking about it which is good,  when you are done you wont have any second thoughts.

[mgoller]

By the way, my depth finder is an Eagle Fish Finder, $149, works really well, speedo, temp, depth and all the gadgets.