Sounder / Fishfinder Transducer on Planing Hull

diver 1 · 24 April 2012, 12:14

I have a Lowrance sonar fishfinder unit with a transom mount transducer on the transom of my rib.

The unit works fine at low speed 0 - 8 knots, but any quicker and I only get a faint seabed line and often an inaccurate fixed depth deading, typically 20.8 metres.

I presume that this is because the angle of the boats hull changes as the rib starts to plane so the transducer now is angled forwards (in line with the hull) rather than parallel to the seabed, therefore the echo is pointing forwards into open ocean rather than downwards towards the bed?

Ideally I'd like to see when I'm passing over shoals of mackerel etc as I'm heading out to fishing areas without spending ages searching around at low speed.

Is this achievable or do these fish finders only work on displacement vessels?

willk · 24 April 2012, 12:25

You might be better mounting it in the bilge well or bilges. That way it works through the hull and won't be affected by the bubbles and turbulence. If it's fitted properly (glued in) it will work at all speeds, except airborne...

jyasaki · 24 April 2012, 16:51

Quote:

Originally Posted by diver 1

I presume that this is because the angle of the boats hull changes as the rib starts to plane

Doubt it. The 'ducer's cone is something around 20 degrees if I remember right, so unless you're going 15 degrees nose up between rest and plane, it's likely not the transducer angle.

Try playing with how deep the transducer sits in the water (generally, flat to the line of the hull, with the face or bottom just slightly lower than the level of the hull. Then move up and down in small increments and gauge performance.) Took a bit of fiddling with mine until I found a reasonably solid reading at most speeds. Note that at high speed, you will get less of an echo, as the pings are spaced further apart on the seafloor.

Quote:

Is this achievable or do these fish finders only work on displacement vessels?

They work fine on a lot of boats. Including mine.

jky

captnjack · 24 April 2012, 19:24

99% chance its too high on the transom. Or its behind a chine, in which case move it to the side until its not. In either case, bubbly water is getting entrained under the hull and carried aft under the transducer. Move the transducer down 2-3mm at a time until its running in clean water and giving you a good signal at speed. Too low and it will throw up a rooster tail.

Peter_C · 24 April 2012, 20:35

Is the transducer mounted on the port or starboard side? Photos?

It should be mounted on the starboard side and sometimes a simple angle change is all it takes.

diver 1 · 27 April 2012, 01:21

Starboard side of single 90 engine. The tornado has a weird shaped transom, see attached pictures.

By what everyone is saying have I mounted the transducer too low then?

Will K - I'm not familiar with how the bilge area at the stern of the rib is constructed, I reckon I'd have to cut an access hatch out of the deck to achieve this and I'm reluctant in doing that.

diver 1 · 27 April 2012, 01:26

A wider photo of the whole transom... shows the full width

telcowilliam · 27 April 2012, 01:29

I have a similar problem on my tornado, I have had a little play about with mine but looking at your picture it looks like yours is way too low below the bottom of the hull, you could do with moving it up at least 60mm see if that changes the readings

wavelength · 27 April 2012, 06:07

stick it in the drain well with silicon. Ours is in there, been there years, no maintenance, not subject to damage, always works with no issues even at full speed.

diver 1 · 27 April 2012, 08:52

So just to clarify I have attached a photo of the drain well. The bottom of the drain well is considerably higher than the underside of the hull, and I was under the impression that there would be a large void between the hull and the drain well..?

If so surely the transducer wouldn't work?

Also should I use silkaflex or some type of hard epoxy to glue it on and is it ok to glue a transom mount transducer as for some reason I thought a hull bonded transducer was a different type?

Sorry for all the questions!

willk · 27 April 2012, 09:12

IF there is a void, then it WON'T work. Any bubble free adhesive medium can be used to stick it to the hull. The transom transducer is similar in operation to a through-hull - it's just the mounting that differs. If you have a paddle wheel on the transducer, you'll obviously loose the speed reading and the temp will be the temp of your bilge well!

You can TEST the set up without bonding it in - so long as the water is over the transducer and it's facing down, it should work.

diver 1 · 27 April 2012, 13:42

Will K, thanks for the idea. Of course it would be very simple to slacken the bolts, launch the baot and try moving the transducer different directions to try and improve it then failing that try with the transducer in the pool of water always in the well at the back of my boat.

Surely testing in the pool of water the echo woudl just bounce back off the top of the base of the well and effectively tell me the water depth is only a few mm deep, or am I being stupid?

Are tornado really stupid or is there some clever reason why the elephants trunk is located way about the base of the well. There is always 50kg or so of water there, is that deliberate?!

captnjack · 27 April 2012, 16:55

Quote:

Originally Posted by diver 1

Surely testing in the pool of water the echo woudl just bounce back off the top of the base of the well and effectively tell me the water depth is only a few mm deep, or am I being stupid?

The signal goes through the layer of water in your bilge, through the GRP, through the water and reflects off the actual bottom to return. The assuming the hull is not cored with something else (balsa, plywood, metal, or air voids).

wavelength · 27 April 2012, 17:07

it won't work through a void.

Quote:

and effectively tell me the water depth is only a few mm

no,simply put it works fine
Doesnt need to be a special in hull transducer-in practise a skimmer will also work fine. Ordinary clear silicon is what we use and works fine with the advantage it can be removed again without a drama. But unless you can find that single skinned bit of hull you're perhaps stuffed on this line of attack

captnjack · 27 April 2012, 17:26

Quote:

Originally Posted by diver 1

A wider photo of the whole transom... shows the full width

When underway does the boat sit in the water at that angle? Seems like underway the bow might be higher and the transducer will be shooting "forward" too much. Its hard to tell the angle for the back. It certainly seems low enough.