Maybe silly question ... Will an echo sounder work through a small alloy boat hull m?

[Refueler]

I have an ex Soviet era mobo of ~5m ... its actually called a "Progress 4". Designed for 4 people, folding tent top and up to 30HP.

The Hull of a motor boat "Progress 4" is made of duralumin alloy, which makes its technical characteristics relevant for good utilization. Design of riveted and has outstanding planing lines. .....

The plating Thickness of the bottom equal to about 2 mm, and the Board on the deck is 1.5 mm. Greatest length in this case is of 4.69 m, width up to 1,72 m Height of "Progress 4" is the size of 0.69 m,

I rescued it before sold of for the scrap aluminium value .. stuck a decent Mariner 20 on the back, old car seats ..... and now she is a treat to use.

Pal and I mostly use for fishing and it would be nice to have just a basic sounder giving us depth while on the move. No need for fish finding as I have a hand held fish-finder that I can put over when we stop.

I would not think the hull bottom is that thick as you can feel the bottom flex as she hits waves etc. and specs mention 2mm.

I could drill a hole and mount thru - but I regularly run this baby up rough banks etc. and don't think my Nasa head could stand that too often. I also want to mount the paddle wheel speed as well ... but same problem.

I have considered pivoted arms on the transom ... the units fixed to them and pivoted down into water, held by spring clips. If boat hits anything - arm flips up similar to RIB mounted transom wheels ....

TBH - speed is not so important as I have my HH GPS and phone ... but depth would be nice.

 

[jamie N]

I reckon that it'd be OK, and reckon that it'd be easy to test using a similar thickness 'tin', and hang it over the side as a mock up.

 

[Bilgediver]

I suspect it is just possible however you would need to put a tube inside the boat at a suitable location in which the transducer is submerged in oil. Be a lot easier to fit a transom mounted transducer of which many are available.

 

[Refueler]

I'm searching through transom mounts .... but 'blimey' talk about Marine Hype pricing !

Think I will create own 'kick-up' mount ........

 

[Boathook]

Post some pictures of it here once constructed, etc.

 

[ylop]

I'm pretty sure it will work - easy to test by using some plasticine or blutack to make a temporary "tube" that you can fill with a suitable oil. Although the general advice is you need to be in oil - many people have reasonable success with a good bed of silicone provided you can get all the air bubbles out of it - which is less messy if the oil reservoir is anywhere that might get knocked/spilled.

 

[Refueler]

Reading online ..... some reports say that it can cause transducer to overheat ..... seems a bit strange, but it goes on to talk about density's and vibration etc.

The river at deepest is 18m and that's a small area up river - most of river is about 6m. So I am not looking for extreme performance.

I've been trying to find something I can 3D print ... but so far nothing yet.

 

[Alex_Blackwood]

Reading online ..... some reports say that it can cause transducer to overheat ..... seems a bit strange, but it goes on to talk about density's and vibration etc.

The river at deepest is 18m and that's a small area up river - most of river is about 6m. So I am not looking for extreme performance.

I've been trying to find something I can 3D print ... but so far nothing yet.

Despite previous answers I would say No. The Ali hull will just act as a screen, even if you get a reading I would think that the accuracy would be very doubtful.

 

[Refueler]

Despite previous answers I would say No. The Ali hull will just act as a screen, even if you get a reading I would think that the accuracy would be very doubtful.

That agrees with many comments I found online elsewhere.

My present thinking is

1. Plastic tube clipped to the stern with transducer fixed to the end .... if it touches anything - it unclips and pivots up. It would need to pivot at top of tube to void it falling into prop if only double clipped.
or
2. Simple pad fixed to transom and then either a clip or U section with pivot pin. Transducer mounted to plate that pivots on the pin.

Still searching 3D archives ...........

 

[andsarkit]

Just try it and see what happens . It will only take 5mins to put it in a plastic bag of water in the bilge. You will then be the go to forum expert on sounders in aluminium boats.
It will not overheat. The pulse is quite high power but short duration which is why the old whirly echo sounders run for many hours on a PP9 battery as the average power is very low.
I think that if the pulse can travel through air, water and GRP it should also pass through metal. It is a high frequency (150-200kHz) sonar pulse and not electromagnetic radiation. It is possible that the metal may resonate and distort the pulse but a quick test is the only way to find out.
A transom mount will require careful fairing to avoid bubbles and turbulence when travelling at speed.

 

[srm]

When doing inshore surveys we had to calibrate the echosounder before and after each days work by lowering a reflector at known depths below the transducer. I used an alloy sheet about 1mm thick and half a metre square as a reflector. It gave very good returns down to 20 metres, admittedly with a more powerful transducer than the typical yacht instrument.

Based on that experience I would be inclined to say no, but mounting the transducer face close in oil may be different. Try a mock up first. If it does work it will probably give a narrower beam, as the sound pulse will be re-transmitted from a larger area that the original transducer face. The problem is that the return pulse has to vibrate the hull plate then retransmit through the oil to the transducer. If you are over a hard bed such as rock or sand you will get a relatively strong return, but a muddy bottom will absorb the sound pulse and little will reflect back.

 

[AngusMcDoon]

I think that if the pulse can travel through air, water and GRP it should also pass through metal. It is a high frequency (150-200kHz) sonar pulse and not electromagnetic radiation. It is possible that the metal may resonate and distort the pulse but a quick test is the only way to find out.

P waves propagate from one material to another well when the densities of the materials are similar but reflect when they are different (it's not strictly difference in densities, but there's an approximate relationship with density). That's why sonar waves will propagate through an oil bath, a solid GRP hull and into the water, because oil and GRP have densities similar to water, but they then reflect off the seabed, because sand/mud/rock densities are significantly higher. It also explains why they will not propagate through a cored GRP or steel hull. Aluminium is between GRP and steel in density, so it may work somewhat, but probably not very well. When I had a aluminium hulled boat I mounted the sensor through the hull, not inside, for this reason. A good everyday example of this phenomenon is that you can't hear much of what's going on above the water when you are swimming below the surface, but you can her shrimps giggling with each other half a mile away.

 

[KevinV]

2mm ali is easy to work, five minutes with a hammer and you could recess a through hull sensor out of harm's way

 

[Ian_Edwards]

To know whether sound will travel easily between different material, you need to know what the acoustic impedance are. If they are similar you'll get good transmission, if they are different then you get a lot of reflection.
Echo sounders are designed to work in water, which has an acoustic impedance of around 1.5 MRayls after Lord Rayleigh, who worked out most of the physics of sound in the mid 1800's.

If you look at the acoustic impedance of:
Sea Water 1.5 MRayls
Mineral oil 1.19
Olive Oil 1,32
Aluminum 17.33
GRP ..... a real mixed bag, depending on resin, resin glass rations, it's around 2 to 3 and it needs to be void free.
All these figure are in Mega (10^6) Ralys
Air is very much lower at around 420 Ralys

You can see from these figures that an echo sounder will not work very well in air, so you need to put something between the transducer and the hull that will couple it, provide an air free transition, oil works well, because it's Impedance is close to that of water. But oil or water in a poly bag is unlikely to work well, because it's difficult to get all the air out of the interface between the poly bag and the hull.
GRP has an acoustic impedance quite close to that of water, so an echo sounder coupled with oil will work reasonably well, but you loose a far amount of acoustic energy.
Aluminum has a significantly higher acoustic impedance, so there will be much more reflection and less acoustic energy will get through. That doesn't mean it won't work, especially if you are only looking for 10m range, but you are unlikely to get a 100m range through Aluminum.

I'd give it a go, as said above, some temporary molding to hold the oil and form a reservoir in contact with the hull , put the transducer in the oil and try.

 

[Refueler]

Just try it and see what happens . It will only take 5mins to put it in a plastic bag of water in the bilge. You will then be the go to forum expert on sounders in aluminium boats.
It will not overheat. The pulse is quite high power but short duration which is why the old whirly echo sounders run for many hours on a PP9 battery as the average power is very low.
I think that if the pulse can travel through air, water and GRP it should also pass through metal. It is a high frequency (150-200kHz) sonar pulse and not electromagnetic radiation. It is possible that the metal may resonate and distort the pulse but a quick test is the only way to find out.
A transom mount will require careful fairing to avoid bubbles and turbulence when travelling at speed.

The boat tends to skip water anyway at speed, as most speedboats do .... so bubbles / turbulence are going to be wherever mounted. But because river has usually quite a few people in dinghys along it fishing .... speed is limited to avoid wash etc. Plus I can only carry so much fuel - we tend to stay at moderate throttle.

I can easily check depths as I have the handheld Fish Finder that gives depth ... that can be thrown over side and with its adjustable float - sensor set to same level as the inhull transducer.

TBH - the more I think on it ... the transom mount comes out on top ...

My thoughts are short plastic pipe pivoted at top with removeable pin. Transducer fixed to bottom of pipe and pipe hinged down into a standard plastic pipe clip. Transducer then just below hull bottom. If it hits anything - it unclips and pivots up.
Removeable pin at top - so I can stow away when not in use.

The other matter of course is the upgrade of battery onboard. At present I have a 7A/hr SLA (House Alarm battery) served by 5W solar panel to power my auto-bilge pump. I think I need to increase or add a larger battery so I can power GPS, Depth and Speed ..... I have a small 20A/hr Lawn Tractor battery I ressurected ... and a spare trickle charge Solar Panel ....
Will look at best way with those ...

 

[oldharry]

Not unusual for cheaper fishfinder type Sounders to be supplied with transom mounted transducers as standard anyway. A bit annoying to have to pay extra for a through hull transducer, though it's usually possible to stick them on in the bilge of a grp boat without significant sensitivity loss.

 

[DownWest]

I like the idea of the open plastic tube with the transducer dangling inside at the bottom. If you screw two of the standard clips to the transom, the tube will pop out if it hits anything. Prob 40mm PVC drainpipe would do it.

Edit: just checked the dia of an old seafarer transducer. 40mm. So 50 mm pipe needed.

 

[Refueler]

Not unusual for cheaper fishfinder type Sounders to be supplied with transom mounted transducers as standard anyway. A bit annoying to have to pay extra for a through hull transducer, though it's usually possible to stick them on in the bilge of a grp boat without significant sensitivity loss.

My HH Fishfinder has a transom bracket - but it has ratchet steps in it locking the transducer to angle. I can see the bracket or lugs on the transducer being broken ...

I like the idea of the open plastic tube with the transducer dangling inside at the bottom. If you screw two of the standard clips to the transom, the tube will pop out if it hits anything. Prob 40mm PVC drainpipe would do it.

Edit: just checked the dia of an old seafarer transducer. 40mm. So 50 mm pipe needed.

I was thinking more in way of setting transducer screw part into pipe - not the larger head. That way a smaller diameter tube can be used.

The Nasa Sounder has a screw part of 16.4mm diameter and is a loose fit into 3/4" water pipe. That's easily sorted with a ring to steady it. Cable passes up through pipe and into engine well etc. Such water pipe has specific clips easily obtained in shop ... all being plastic - no corrosion to worry about.

The speed log though is a different ball game as it has the integral screw ring and larger diameter body. My thinking here is maybe to find large C clips that can hold such body .... possibly creating a mount with arms that the speed log 'arms' can locate into providing pivot function ?

 

[DownWest]

Yes, but I was thinking of the head being in the pipe as better protected if you bash things often?

 

[Refueler]

Yes, but I was thinking of the head being in the pipe as better protected if you bash things often?

I never considered burying the head in the pipe TBH ... so fair comment. In all fairness - most run-ups the bank leave stern floating as the banks here are quite steep... even the swim areas have enough incline for stern to be OK.
The only time I really need to worry is when getting boat out for winter ..... I have to remove O/bd due to my ramp incline and the stern does touch as it starts to 'pitch up' as trailer drags out. But then I should have remembered to remove the gear ! (Murphys Law of course).