I need to replace my depth and speed instruments (original from 1979). What is the performance difference between a thru-hull and in-hull depth transducer? Is my 1970's fiberglass too thick to use an in-hull?
I used a shoot-thru-the-hull depth transducer for years with our 1973 Grampian 34. The hull on this boat is thick; I'd guess about ~1.5". Worked fine. Never a problem. As long as you can get a good angle, and good connection to the hull, and as long as there are no voids in the fibreglass, it should work fine.
I don't know about speed in-hull transducer. I don't know how they would work (digital calculation?). We used GPS as a speedo on that boat.
Shoot-through should work fine, as long as it isn't one of the Lowrance HDS ones. You might lose a couple of hundred feet of overall depth ability, but I don't think it really matters if you know you are in 1000 or 800ft of water. Solid glass hull? Won't work if there is a core or a void so you might need to experiment with the location.
Our CS-36 was delivered 35 years ago with a Radarsonics transducer that was glassed to the inside of the hull with some thickened polyester resin.
It has survived approx 7 different depth displays, a lightning strike (one of the only items that survived, the ST-60 it was connected to was fried), and is still going strong with our current ST-60 Depth.
I also have an Airmar P-79 installed and run to the ST-60, but not connected. Just curious to see how long our antique Radarsonics transducer lasts. Oh and the location where it shoots through is just forward of the keel and the hull is approx 1.25" thick.... 35 years & approx 70k nm.
The Airmar P-79's are pretty reliable and easy to install..
To find a good spot for a shoot through, hold the ducer against the hull with a baggie full of water in between. (Boat has to be in the water). Once you find a good spot where it works, glue it down with a big blob of clear silicone, being careful not to get any bubbles. The instructions always say to use epoxy, but then your stuck with the location and can't move the ducer without breaking it.
You don't need to buy a special shoot through ducer for this, any through hull or transom mount unit will work as long as it doesn't have a paddle wheel or anything attached.
The big advantage of a transducer that shoots through the hull is that there is one less hole in your boat. I do find, however, that my 2 transducers are a little bit slower to read the bottom. I potted mine in silicone (as Jgwinks suggests) making it easy to move them at a later date.
Tried putting in an in-hull transducer at the centerline forward where there was some V. Transducer fit into a oil filled tube. At the centerline the face was maybe 1/2 inch from the center of the V. Didn't work. That was some years ago maybe there's some transducers that work better than others when there's some gap.
Shooting through the hull will reduce the range of any transducer, but if you are just looking for shallow-water numbers, that won't matter to you.
I was used to sailing with no depthfinder and then ignoring the numbers (damned numbers, always in Arabic never proper Roman numbers, either ?!) until one year when I couldn't figure out where we were. Moonless night, too much background clutter, and then I realized "_ _ _ " didn't mean a dead depth sounder, it meant "my brains run out at 199" I was over the main channel in over 200' of water and that was easy to spot on the charts.
So using a through-hull, or a sounder powerful enough to reach depths you may not normally be interested in, may have some advantage at times.
The downside of course is hauling and drilling and sealing.
And how come no one makes a depthsounder, or any sailing instruments, that use proper Roman Numerals on the display, for the westerners among us?
I put one inside on my fishing boat, siliconed a plastic tube, filled it with anti-freeze, it worked fine. As it was somewhat in the way, I finally mounted the transducer on the transom, couldn't really notice much difference, if any.
The in-hull retains accuracy, and can be placed anywhere on the hull (closest to measure dtk as possible) and the through hull can really only be mounted on the bow section of the hull along with your speed impellor.
When I purchased my UFO 34 (built in 1983) it had a in hull depth transducer that was in some kind of oil bath. In less than 3m depth it would cut in and out for 10-30 sec. I replaced it with a standard Raymarine thru hull transducer and it has worked fine ever since. The hull was around 3/4 of an inch thick and the in hull transducer was probably installed 20 years ago.