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markpj23 15-05-2006 07:35

Depth Sounder Thru-Hull Alternative?
I have a Datamarine unit where you remove the 2" through-hull plug and insert the depth sounder head just prior to getting underway. I always approach this task with sweaty palms and elevated heart rate... and on my best day I still manage to get salt water everywhere. The whole thing just gives me the heebie-jeebies.... :eek:

I recall an alternative where a water-filled cofferdam of sorts is built against the hull and the transducer is fitted into it. So the depth sounder head / xmitter is not exposed to external sea water but rather sits in a water or silicon-liquid filled box. Somehow the instrument compensates for the different density of the hull vs water and calculates the true depth. I'm sure that steel or aluminum hulls would radiate a true sound wave, but am not so certain that fiberglass, due to its layered construction, would be able to do so.

I guess my options are:
1) live with the mess and get over my anxiety
2) send the mate to the bow with a lead line (not likely she says)
3) install a diving alarm :D

Can anyone here recommend an alternative system?

CaptainK 15-05-2006 08:15


The version I seen was instead of silicon in the box. I seen mineral oil placed inside the box.

Then the transducer is placed inisde the box. The wires run through the back of the box to the interior of the bottom of the boat.

jemsea 15-05-2006 08:18

Hi Mark,

On a Cal 22 that I knew I was going to sell soon and didn't want to put in a thru-hull I mounted a Hummingbird fish finder to the inside of the hull using Lifecaulk. I was careful to have no air bubbles between the sender and the hull.

It worked great. I was never in water over 60 feet deep using the boat on lakes but it found fish at that depth plus gave bottom contours.

From my experience as long as your hull is solid fibreglass with no air pockets it will work.

I've also heard of, but not tried, mounting a piece of pvc pipe of the correct size to the hull, then partial filling with non freezing liquid in which the sender is submerged and then sealing the pipe over the top of it.

I know this is not exactly what you are asking about because you already have a hole in the hull but hope it helps you find a solution.


GordMay 15-05-2006 08:42

Why do you ”... you remove the 2" through-hull plug and insert the depth sounder head just prior to getting underway ...” instead of just leaving the transducer (more or less) permanently installed?

Alan Wheeler 15-05-2006 12:46

Gord is spot on. You don't need to remove the Transducer. It is designed to permananetly stay in place. The only unit that normally comes out is the paddle wheel transducer.
There are two ways to do a "none through hull" fitting. Both require the face of the Transducer to be in contact with the hull. The easiest is to simply glue the unit to the hull using epoxy. You have to ensure a good clean hull surface. This technique means the transducer is there for keeps.
The second is to use a box filled with a mineral oil. This essentially does exactly the same as the epoxy glue, but allows removal if needed. The oil ensures no air bubble between the transducer face and the hull.
Both techniques equate to reduced performance, than if the unit was attached through hull.

markpj23 15-05-2006 12:52

This is a paddle-wheel type transducer and leaving it in all the time would foul it with marine growth. I've also seen and been told that painting these with anti-fouling paint can lead to problems, but perhaps that is the answer? Not sure how I'd keep the paddle wheel mechanism free of growth however....

hellosailor 15-05-2006 15:49

Mark, there is no way to mount a paddle-wheel inside the hull. Paddle wheels are speed sensors, not depth sensors. (I suppose you could have a combined unit?) And you are right, a paddle wheel in most waters should be pulled every time you leave the boat or it will become packed with critters. You *can* paint many of them with bottom paint, or the evil but effective tin-based bottom paint (sold in spray cans for use on aluminum outdrives) and that might even let you leave it in the water.

As long as the paint doesn't make the wheel bind (and doesn't cover a depth sounder if there IS a combined transducer in there) you shouldn't have a problem. I know some yards that get happy with the paint and jam up the paddle wheel, but a few minutes with a knife and toothbrush solve that.

But there really should be no reason to fear sinking the boat. If you get practiced at "pull with this hand, plug with that hand" you shouldn't take more than a pint to a quart of water in that gushing moment.

Its like the first time a boat is heeled hard over--you may get anxious but after a while you get used to it and accept it as normal.

Starbuck 15-05-2006 16:43

"The Boat is Sink— oh, okay, the plug is in. Whew!"
Yeah, Mark, this is the speed log. The switch becomes reasonably mundane with time.

I remember the first boat I owned that had one of these. I was pretty panicy when I saw that two inch flood of water coming into the boat.

And when I think about pulling the plug out and letting my current boat play submarine ("Dive, Dive!") :eek: momentarily, yes, it still gives me pause. But it's only a couple of seconds, and I'm sure you have the sender/plug ready to go each time. So even if you fumble it and it takes a whopping five seconds, you can't let a whole gallon into the bilge. As has been said, on a good day, it's under a pint

I do wish there were a completely dry way to do it: partial pull, close a valve, pull it out completely, take your time with the plug, slide it in partially, open the valve, and finish the insertion. But people would probably get lazy and depend on the valve instead of the plug.

I usually leave the sender in for a couple of weeks at a time, and switch it for the plug momentarily to clean, spin the paddle, and replace, or when I know I'm not going to be around the boat for a while. Reduces the number of times the exchange has to be done, and this is never a problem for me.

Confidence will grow with experience.

BC Mike 15-05-2006 19:54

If it is a depth sounder then epoxy the transducer to the inside of the hull, if the hull is solid. You can do a test using silicone, stick it on and try it in deep water. If the readings are okay then stick it more peranently in place. If it is a paddle wheel for a knot meter, the new Raymarine units have a couple of flaps in the tube that slow down the water intake, but your hand over the hole also works until you are ready with the plug, or paddle wheel. The new Raymarine unit also advices to use antifouling paint on the paddle wheel and through hull fitting.

Richhh 15-05-2006 22:06

Consider the next time when you remove the paddle wheel to apply a heavy dose of medicated baby diaper cream to the wheel and its support structure ... but not on the spindle area - apply thick and let dry. Desitin® is the USA brand that I use to good effect. The antibiotic in the 'waterproof' cream will help keep the growth to a minimum.

If you apply thin bottom paint consider to use oxytetracycline (from a veterinarian). Just pulverize the tablets to a very fine powder and mix it into the bottom paint ... apply the paint thinly. Oxytetracycline is an antibiotic will also keep the growth to a minimum.

Alan Wheeler 15-05-2006 22:08

I haven't seen a trough hull sounder trany with a paddle wheel as well. But OK. I keep my paddle wheel in the hull permanent. I can pull it, but I decided to try and see how long it would work. I coated a liberal coating of Lanocote over the thing and it works fine. It has been in the hull for 1yr now and I still have it work every time I go out. I don't get a lot of weed growth where I am, but even if it meant having to pull and recoat with lanocote every 4 or 5 months, It has to be better than what you are doing now. My through hull is 4-5ft below the water line, so when I pulled it for the first time, the blast of water that hit me in the face, fair soaked me, the inside of the boat and I got such a shock, I struggled to get the bung back in the hole. I don't intend doing that to often.

markpj23 16-05-2006 08:28

Thanks guys... so I can use some tricks to keep the growth off of the paddle wheel OR I can suck it up and sound the diving alarm...

Anyone know how to calculate how much water can move through a 2 inch pipe in 2 seconds? Hull depth at the through-hull is 3 feet. So the equation would probably be:

2" x 3ft x I-factor (how impaired I am from the grog) = Holy @$##$%!! :D

Seriously - anyone know how to calculate that? How about it Gord? I know it has to be much more than a pint or two.... plus that salt water gets to wash over my iron mast step on its way overboard... not pretty.

hellosailor 16-05-2006 10:10

Mark, don't even try to do the numbers. No matter what they are, it can't sink the boat!<G>

I don't like the idea of throwing antibiotics in the water. That breeds antibiotic resistant critters "in the wild" and that means the next time you cut yourself, there's a better chance you'll be infected by something that antibiotics no longer kill. A very, very, bad practice.

GordMay 16-05-2006 10:39

Approximate Flooding Rates, for smooth holes at selected depths, are calculated using the formula:
Flooding Rate in gpm = 20 x d x square-root of h
d = diameter of hole in inches
h = depth of hole underwater in feet (head)

Hence, for a 2" dia hole at 3 Ft depth:
Q = 20 x 2" x root 3 = 20 x 2 x 1.73 = 69.2 gpm (4,152 Gal per Hour)
69.2 / 60 = 1.15 gal per second, or 2.3 gal in two seconds

The actual flow rate ('Q') will be less than calculated above, due to frictional & turbulance losses at the aperture.

ssullivan 16-05-2006 10:54

I do the same as Wheels. I remove the knot log (paddle wheel) once a season. I also coat it with bottom paint very carefully, with a Q-tip. You can load it right up with paint, but watch out to be sure the paint doesn't create any surface on the wheel which can drag on the housing for the knot log.

I get at least a season out of a knot log this way. If it ever does stop working you can always pull it out like you are doing now. But why do it so many times when you don't need to? Just becomes a nuisance. :)

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