# Installing depth-finder in-hull



## Capslock118 (Feb 15, 2011)

Hi There,

So we are near purchasing a 25' sail for the first time.

I've always had a depth finder on motor boats prior, generally more for the fish finder part though I can never catch fish :laugher 

some depth finders allow the sonar to be installed inside the hull.

I am playing with the idea of installing a depth finder for the sail boat, again mainly for fishing and inside the hull rather than drilling through the hull to mount outside. But maybe this is a useless item and my meager budget would be better spent elsewhere.

what are your thoughts? do you own a depth/fish finder? if so, have you installed inside the hull? any problems come up doing that?

also, if installed in the hull, if I reversed the sonar so the part that is suppose to face toward the bow is now facing the stern, would I end up with readings ahead of the boat or is that not how it works?

it would be good to see depth ahead if that's how it works.


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## QuickMick (Oct 15, 2009)

I hooked up my hummingbird this way, works like a charm. i put it in a bag of water(which im not sure if you need to do) and moved it around till i liked reading, removed from bag and epoxyed it to the fiberglass (after a decent cleaning), when i pushed it down into the epoxy, i kinda used a twisting motion to get rid of any bubbles. there is another method with mineral spirits filled pvc but i didnt go that route. what your boat is made of is a consideration, as well as _not forgetting to offset the unit to keel depth_.

mine is in the engine compartment, but i use that unit primarily for fishing from the cockpit, and do have redundancy via thru hull unit. if you are worried about grounding id put the transducer toward the bow to 'see' what is in front of you.

good luck

edit: while i maybe off base, i believe the number of 'beams' affects size of area you can view-spinning the transducer wouldnt affect the range, persay just make everything on the scrolling display be backwards... i think~that last bit was giving me a headache... lol.


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## copacabana (Oct 1, 2007)

Rather than glue it with epoxy, you can use silicone. The advantage of silicone is that you can always remove the transducer at a later date or move it to another place. It has to point down to read the bottom (ie. in the same orientation it would be in if attached to the outside of the hull). It won't read ahead of the hull. You can try installing it ahead of the keel, but the millisecond warning it will give you before you run aground won't do you much good. It's best to find a place that has no turbulence and won't be out of the water when heeled over. I installed two in my boat and both work fine. Don't drill holes in your hull if you can avoid it!


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## Faster (Sep 13, 2005)

Forward looking depth sounders are available, but are pricey, use bulky transducers and are not 'the norm'.

Generally your depth indication is going to be whatever's directly below the transducer, wherever that is. Most are mounted ahead of the keel to give you a (very little bit of) warning, and only if you're going very slowly. Mounting ahead of the keel also minimizes interference from vortices and disturbances to the water that can confuse the sounder (as an example watch your sounder readings anytime you're reversing at some revs, or if you're backing down hard... the number is often all over the place)

Epoxy, silicone, or liquid cavity mounts are ways to mount inside the skin of the hull. I think it works better in single skin hulls vs cored hulls, and you will lose some range but it does avoid the issues with adding a through hull fitting.

A transducer intended as a through hull will probably need to be mounted in the lid of a liquid cavity style installation.


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## mitiempo (Sep 19, 2008)

They will not work in a cored hull, but even cored hulls are usually solid fiberglass along the keel. Any epoxied transducer can be mounted inside. On my current boat I installed a Raymarine ST-40 and it comes with an Airmar transducer designed to be mounted in a drilled hole. I just removed the nut and gasket and epoxied it in. I installed it under the aft most part of the V-berth, on the centerline. You will get less disturbance in this location. As posted above place the transducer in a baggie of water and check that your chosen location works first.


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## Joesaila (May 19, 2007)

I gonna do this! I want to connect it w my garmin chartplotter w/o drilling another hole in the hull. The original digital unit is only about 90% correct. It seems to blink off at the most inconvenient times. THANKS FOR THE GOOD POINTERS!


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## mitiempo (Sep 19, 2008)

YOU"RE WELCOME


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## sailor50 (Aug 26, 2009)

This months SAIL MAGAZINE addresses this issue right to the point.


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## mitiempo (Sep 19, 2008)

It is also addressed here: WEST SYSTEM | Modifying and Customizing Boats - Depth sounder installation

And as they state most manufacturers of depth sounders recommend epoxy for installing the transducer. Less chance of bubbles.


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## Faster (Sep 13, 2005)

In a somewhat related question:

We've recently upgraded to a compact plotter/depthfinder and we already have an aging but functional Signet DS. I'd like the expanded features of the newer/smarter depthsounder capabilities, and the internal install has appeal for obvious reasons.

Anybody think there would be cross-talk between two simultaneously operating systems? The new one has selectable Hz settings - should I make them different? I'm thinking of mounting them some distance from each other.. Any thoughts?


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## Omatako (Sep 14, 2003)

mitiempo said:


> They will not work in a cored hull, but even cored hulls are usually solid fiberglass along the keel. Any epoxied transducer can be mounted inside. On my current boat I installed a Raymarine ST-40 and it comes with an Airmar transducer designed to be mounted in a drilled hole. I just removed the nut and gasket and epoxied it in. I installed it under the aft most part of the V-berth, on the centerline. You will get less disturbance in this location. As posted above place the transducer in a baggie of water and check that your chosen location works first.


Does it matter how thick the hull is when the transducer is cemented in place? I recently put new transducers in for new instruments and had to enlarge the hole for the speed transducer. The hull at that spot is nearly 50mm thick!! Will the sonar go through this? The hull is solid GRP, not cored.

I have depth already but I'm such a poor fisherman that I'm sure a fishfinder will work wonders for my success rate.


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## mitiempo (Sep 19, 2008)

If it is that thick it may affect performance, especially at greater depths I would guess. But the only way to find out is to check with the baggie of water until you get a reading. Then place the transducer over the side to about the same depth as the place you tried inside the hull and confirm the depth is the same. Make sure there is a flat consistent bottom, not rocks with varying depth of course.


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## Omatako (Sep 14, 2003)

mitiempo said:


> If it is that thick it may affect performance, especially at greater depths I would guess. But the only way to find out is to check with the baggie of water until you get a reading. Then place the transducer over the side to about the same depth as the place you tried inside the hull and confirm the depth is the same. Make sure there is a flat consistent bottom, not rocks with varying depth of course.


Ah, but the problem is I don't have one and I'm reluctant to buy one to find that it isn't going to work. The only way then is drilling another hole and I know I'm not up for that.

Thanks for the thought anyway.


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## mitiempo (Sep 19, 2008)

Don't give up so easily.

Buy one from a store with a return policy and hook it up temporarily to try it.


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## copacabana (Oct 1, 2007)

André, I don't see why it won't work. My 2 transducers are also installed around the keel/turn of the bilge (thick hull) and I get a good reading. The only defect I have found so far is that it sometimes takes 2 or 3 minutes to get an initial reading when you turn on the unit. Once on, no problem. 

When you look for a good spot for the transducer installation you can try the ziplock bag with water test, but I found it didn't work for me. I think the bag doesn't sit flush on the bottom of the hull to get a proper reading. Remember, you can't have any air between the transducer and the water. Rather, get some modelling clay (kid's stuff) and make a temporary well on the hull to pour a little water. Then lay the transducer in this water and check the reading. Finally, I can't recommend using epoxy to glue your transducer in place. It may work, but what if you get an air bubble during installation? You'll probably need a chisel to remove it. Try common silicone from a tube. It's easy to install (lay a thick layer of clear silicone on the hull and gently push the transducer into it, careful to get any possible bubbles out). If you screw up, change your mind about the location of the transducer or decide to remove it, it's easy enough to remove and clean. Try that with epoxy! 

Cheers!


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## Rangernewell (Oct 23, 2010)

Hello friends...I was reading this thread and I dont want to pull it off subject, but I'm under the gun here..Boat show tomorrow  . The seal on my depth gauge in the cockpit area must be comprimised due to the fact that on cool nights and in damp weather there is enough moisture on the lense that it makes it very hard to read. As the day warms up ..no problem.This spring I was going to take it out and see it there is something I can do with it. Being the boat is stored I'm not able the check the make and model out. In general are these unitspermanetly sealed or made to come apart, can I remove it and reseal it or should look for a deal at the boat show and replace it.


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## Ilenart (Jul 23, 2007)

Omatako said:


> Does it matter how thick the hull is when the transducer is cemented in place? I recently put new transducers in for new instruments and had to enlarge the hole for the speed transducer. The hull at that spot is nearly 50mm thick!! Will the sonar go through this? The hull is solid GRP, not cored.


On my last boat I installed a thru hull transducer and it worked like a charm. However my new yacht has come with the transducer mounted inside and in shallow water (depth less than 2m) the depthsounder keeps losing the signal. Not sure how thick the hull is at that point, however something is not working right.

I have a spare thru hull not being used so I was planning on using this to mount the depth transducer through the hull.

Alternatively any one have other ideas?

Ilenart


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## Faster (Sep 13, 2005)

Rangernewell said:


> Hello friends...I was reading this thread and I dont want to pull it off subject, but I'm under the gun here..Boat show tomorrow  . The seal on my depth gauge in the cockpit area must be comprimised due to the fact that on cool nights and in damp weather there is enough moisture on the lense that it makes it very hard to read. As the day warms up ..no problem.This spring I was going to take it out and see it there is something I can do with it. Being the boat is stored I'm not able the check the make and model out. In general are these unitspermanetly sealed or made to come apart, can I remove it and reseal it or should look for a deal at the boat show and replace it.


Depends on the brand and instrument style, I reckon. If you can get it apart and determine that its a failed seal then maybe see if one of those desiccant packs can be left inside it to help absorb some of the moisture inside it once you've repaired the seal.


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## P35juniper (Feb 11, 2010)

I'm using a Raymarine !-50D, it came with a transom mount transduser, I took a toilet bowl ring of wax made a ball pressed the transduser to the hull where I wanted it, still going after a year. Even if it lonly asts for a year I'll be happy then get another $2.50 wax ring. And if I want to move it all I need to do is just wipe it off.


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## tomperanteau (Jun 4, 2009)

I have a Garmin GPS/depth finder that works great through the hull. The problem I had was that the original transducer had a temperature sensor on it, too. When I connected it to the through-hull, I of course lost the temperature function. Not a big deal, but the wife absolutely loves to fish and it was helpful to know the water temperature.


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## QuickMick (Oct 15, 2009)

tomperanteau said:


> I have a Garmin GPS/depth finder that works great through the hull. The problem I had was that the original transducer had a temperature sensor on it, too. When I connected it to the through-hull, I of course lost the temperature function. Not a big deal, but the wife absolutely loves to fish and it was helpful to know the water temperature.


im confused... two transducers? one shooting 'through the hull' (inside mount) and one 'through-hull?' hooked the old to the new unit?

if so maybe there is a compatiblility isssue between transd and the visual unit, as my 'shoot through' hull mounted hummingbird gets temps just fine?


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## mitiempo (Sep 19, 2008)

Quinn
It will measure the temp against the inside hull, not always the water temp.


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## QuickMick (Oct 15, 2009)

sheesh Brian, you are always giving me projects. Now i wonder if the differential is consistent, or more exponential.... as i would think that when its really hot the diff in the reading and the actual would be greater? where is my thermometer?!?! well now i at least i have a good excuse if they aren't biting. _Well that darn temp guage must REALLY BE OFF TODAY!!_


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## tomperanteau (Jun 4, 2009)

QuickMick said:


> im confused... two transducers? one shooting 'through the hull' (inside mount) and one 'through-hull?' hooked the old to the new unit?
> 
> if so maybe there is a compatiblility isssue between transd and the visual unit, as my 'shoot through' hull mounted hummingbird gets temps just fine?


No, not two. I cut and spliced in a through-hull. That is why the temperature sensor is no longer available.


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## ahab211 (Jan 6, 2008)

I suction cupped a transducer from my fishfinder when I pinched my sender cable on my permanent unit. It worked fine and I replaced the cable with a removable unit and it's a good thing as I used 3M 5200 to install the housing on the hull. I'll need a chisel to remove if I ever have to. Whether the sender inhull, throughhull, or on the hull they'll all work if fastened correctly.


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## chris_gee (May 31, 2006)

I installed one of the new Lowrance colour gps fishfinders with a Airmar P79 in hull transponder. That has an oil bath and is adjustable to hull slope to get the transponder horizontal within a few degrees.
As mentioned it requires a non composite GRP hull.
The general idea of epoxying in a transponder is ok if the hull is flat. If the transom is in the water you can use a transom mount.
On a yacht you have to avoid the keel area, any turbulence etc. This is more for fishfinding than depth because you want the cone to miss the keel.
Probably the best place would be forward of the keel. The catch is that the hull there is too v shaped. Pretty well everywhere else has sharply rising sides too. At best there was only a small area where it could go and that required a somewhat built up level of sealant on the lower side.
The results are somewhat less than hoped for. Despite adjustments the top couple of metres are a wipeout which doesn't matter but the sensitivity is a bit questionable. I think you lose 10% in theory. There also seems to be a difference in results on different frequencies. 200 is ok 50 seems hopeless.
The colour looks nice and a bigger chartplotter on a swing arm is good.
However if I was doing it again I would use a throughhull for better placement and results. All up it was not cheap and the fees for haul out if combined with some other job are worth it for better results. 
Other boat smay have flatter aft sections though.
Overall maybe I have some better idea of fish dispersion but so far anyway I caught far more fish without one.
I suspect they are overrated especially in relatively shallow water like 15 m or so where the cone shape actually means you cover a quite small area of water.


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## mitiempo (Sep 19, 2008)

I installed my Airmar transducer dead center under the V-berth as far aft as possible. Yes, it is not flat but a dam of butyl and a pour of epoxy creates a level surface. Next day I epoxied the transducer to the flat spot.
I have not noticed any problem with readings. If you lose a bit of sensitivity I think that would affect the deep readings more than the shallow ones.


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## chris_gee (May 31, 2006)

Airmar say the maximum thickness of the fibreglass is 1". Not possible to do the above on my hull. I note that they also recommend against epoxying in a transducer not specifically designed as an in hull as the power requirement is higher.


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## dacap06 (Feb 2, 2008)

*A Few Thoughts*



Faster said:


> In a somewhat related question:
> 
> We've recently upgraded to a compact plotter/depthfinder and we already have an aging but functional Signet DS. I'd like the expanded features of the newer/smarter depthsounder capabilities, and the internal install has appeal for obvious reasons.
> 
> Anybody think there would be cross-talk between two simultaneously operating systems? The new one has selectable Hz settings - should I make them different? I'm thinking of mounting them some distance from each other.. Any thoughts?


I agree with mounting the sensors well separated physically. Make sure the operating frequencies are well separated too, and also choose operating frequencies such that the harmonic frequencies don't converge for at least the first 4 harmonics (harmonics are integer multiples of the fundamental operating frequency).

Place the two units on different D.C. circuits to maximize electrical isolation. The only thing they should have in common is the battery lead to your D.C. power panel. If you think you may need more isolation, you can check with an oscilloscope. Look at your power / ground readings and add some simple power filters if the power looks dirty.

Hope it all goes well!

Tom


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## Faster (Sep 13, 2005)

Hey Tom, thanks for that.. I thought my question had been lost in the page change...

New question: Anyone had any luck using Airmar's new 'smart' transducers with NMEA capability?


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## Newsailer42 (Apr 6, 2020)

mitiempo said:


> They will not work in a cored hull, but even cored hulls are usually solid fiberglass along the keel. Any epoxied transducer can be mounted inside. On my current boat I installed a Raymarine ST-40 and it comes with an Airmar transducer designed to be mounted in a drilled hole. I just removed the nut and gasket and epoxied it in. I installed it under the aft most part of the V-berth, on the centerline. You will get less disturbance in this location. As posted above place the transducer in a baggie of water and check that your chosen location works first.


I realize this is an old comment, bit I bought an in-hull depth finder and discovered that my c&c 24 has a moulded v-birth over the the fiberglass hull. Do you think it will work thru the mould also?


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## mitiempo (Sep 19, 2008)

Newsailer42 said:


> I realize this is an old comment, bit I bought an in-hull depth finder and discovered that my c&c 24 has a moulded v-birth over the the fiberglass hull. Do you think it will work thru the mould also?


No it probably will not work because there is likely an air gap between the liner and the hull. Where in your boat do you have access to the inside of the hull below the waterline?


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## Newsailer42 (Apr 6, 2020)

It would have to be on the side in main cabin, toward the v-berth. Will look next time I go to boat. Thank you so much for your response.
Newsailer42


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## mitiempo (Sep 19, 2008)

Newsailer42 said:


> It would have to be on the side in main cabin, toward the v-berth. Will look next time I go to boat. Thank you so much for your response.
> Newsailer42


Try and keep it close to the centerline. Vertical as well, not angled with the hull bottom.


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## Newsailer42 (Apr 6, 2020)

Newsailer42 said:


> It would have to be on the side in main cabin, toward the v-berth. Will look next time I go to boat. Thank you so much for your response.
> Newsailer42


Thank you so much for taking the time to explain this to me.if u ever make it to Hampton, VA, or Chesapeake Bay, look me up


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## overbored (Oct 8, 2010)

here is the Airmar p79 manual it has a lot of good information that pertains to most in hull transducers. the do's and don't 's


http://www.airmar.com/uploads/InstallGuide/17-217-01.pdf


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## Newsailer42 (Apr 6, 2020)

Newsailer42 said:


> Thank you so much for taking the time to explain this to me.if u ever make it to Hampton, VA, or Chesapeake Bay, look me up


Thanks again


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