SI Surface Blind Spot?

[trouts]

I have my Humminbird Helix 10 G3N SI transducer mounted on the bottom of my Hobie kayak. It is below the hull so that there is no hull obstruction to the left and right.

I was in a narrow canal (80ft width, 4-5ft depth) the other day with more than 50 good sized mullet (12" or so) swimming at the surface (the very top). I could visibly see them with my eyes but they weren't showing up on the screen. The dock pilings and shoreline were showing up perfectly clear.

I had the range set to 40' each side, and the sensitivity I changed from between 10-20 and I adjusted the contrast after each sensitivity change so that the dock pilings in the canal weren't washed out. I tried both 455 and Mega+ and no matter what still couldn't see the mullet on the screen.

So my question: is side imaging not able to see anything "above" the depth that the transducer is at? I know the angle is supposed to be 86 degrees, but even if it was 90 the mullet would still not show up right? A blind spot basically for surface water out to the side and up.

The side imaging worked fine in both 455 and Mega+ for finding schools of fish on the bottom out in the channel.

If you Google Image Search "Hobie Guardian" you can see an example of my transducer install.

[Wayne P.]

I have my Humminbird Helix 10 G3N SI transducer mounted on the bottom of my Hobie kayak. It is below the hull so that there is no hull obstruction to the left and right.

I was in a narrow canal (80ft width, 4-5ft depth) the other day with more than 50 good sized mullet (12" or so) swimming at the surface (the very top). I could visibly see them with my eyes but they weren't showing up on the screen. The dock pilings and shoreline were showing up perfectly clear.

I had the range set to 40' each side, and the sensitivity I changed from between 10-20 and I adjusted the contrast after each sensitivity change so that the dock pilings in the canal weren't washed out. I tried both 455 and Mega+ and no matter what still couldn't see the mullet on the screen.

So my question: is side imaging not able to see anything "above" the depth that the transducer is at? I know the angle is supposed to be 86 degrees, but even if it was 90 the mullet would still not show up right? A blind spot basically for surface water out to the side and up.

The side imaging worked fine in both 455 and Mega+ for finding schools of fish on the bottom out in the channel.

If you Google Image Search "Hobie Guardian" you can see an example of my transducer install.

Humminbird's Side Imaging "sees" from the water's surface to past vertical under the transducer on both sides. That is why an outboard mount or jackplate will cause a line of echoes to be recorded near the SI centerline.

No blind spot if the hull is not blocking the SI pulses.

[trouts]

So is it possible that my transducer being mounted under the kayak hull the pulses are being blocked to cause a blind spot by the surface close to the kayak? (so after a certain distance it would then begin to see the surface)

Also I've seen you say that 455kHz is the only true 90 degree coverage and 800 and MEGA are a lesser angle. Is that still true for the G3N models with new transducer? Because the guide says they all have a 86 degree angle.

Would it be more productive to just scan with 455 while going by docks for true 90 degree coverage?

[Wayne P.]

So is it possible that my transducer being mounted under the kayak hull the pulses are being blocked to cause a blind spot by the surface close to the kayak? (so after a certain distance it would then begin to see the surface)

Also I've seen you say that 455kHz is the only true 90 degree coverage and 800 and MEGA are a lesser angle. Is that still true for the G3N models with new transducer? Because the guide says they all have a 86 degree angle.

Would it be more productive to just scan with 455 while going by docks for true 90 degree coverage?

The specs state the coverage angle where the sound level loss is -10 db. The actual functioning coverage is a lot more. 455 kHz is the best for more horizontal coverage.

MEGA SI+ still has less horizontal coverage than 455 kHz.

[91reddog]

I have my Humminbird Helix 10 G3N SI transducer mounted on the bottom of my Hobie kayak. It is below the hull so that there is no hull obstruction to the left and right.

I was in a narrow canal (80ft width, 4-5ft depth) the other day with more than 50 good sized mullet (12" or so) swimming at the surface (the very top). I could visibly see them with my eyes but they weren't showing up on the screen. The dock pilings and shoreline were showing up perfectly clear.

I had the range set to 40' each side, and the sensitivity I changed from between 10-20 and I adjusted the contrast after each sensitivity change so that the dock pilings in the canal weren't washed out. I tried both 455 and Mega+ and no matter what still couldn't see the mullet on the screen.

So my question: is side imaging not able to see anything "above" the depth that the transducer is at? I know the angle is supposed to be 86 degrees, but even if it was 90 the mullet would still not show up right? A blind spot basically for surface water out to the side and up.

The side imaging worked fine in both 455 and Mega+ for finding schools of fish on the bottom out in the channel.

If you Google Image Search "Hobie Guardian" you can see an example of my transducer install.

Some fish throw a better image of the actual fish (white) than others. In your situation, it could be that the mullet dont throw much of a actual return (white) and at that level, the shadow would be projected way further than you were able to see.

[bobcoy]

There is a line of buoys connected by a line in front of the dam where I live, SI shows not only the buoys but the line connecting them. I have gone by at many different ranges and I can attest to solid objects showing up on the surface. No mullets here( except on a few heads) so I can't say if mullet is not very reflective or not but SI definitely shows stuff at the surface. Bob
NOTE: A transducer that is not level(side to side) could cause you to not see surface object. Maybe put that cooler and 12 pack on the other side of the boat.

[trouts]

I did some testing earlier and couldn't see a floating 3-4ft branch or my sandals on 455 or Mega+ 30-40' range. I went right next to them and from further away.

I think it's because of the hull blocking the signal that goes to the surface. I've got a transducer arm on the way and I'll test with that next week.

I see every dock piling though very well.

With fish that don't show much of a white return with SI, does turning the sensitivity to max then messing with the contrast so it's not washed out help or hurt?

[trouts]

So I moved the transducer to the back of the kayak on a RAM transducer arm so that there is no hull above it, very shallow maybe 2" from the surface. I can now see the mullet, the bottom of floating boat hulls, and floating docks that I couldn't see before. Tested with 455 and MEGA+, all 3 looked better with MEGA+.

On the way back to the ramp I put the transducer way deeper about equivalent to how deep it was under the hull (but still on the back of the kayak on the arm with nothing above it). I could still see the bottom of floating boat hulls and floating docks that I couldn't see before. Wasn't any mullet around anymore to verify that I could still see them.

It seems like when the transducer is mounted under a boat/kayak that the hull blocks part of the upwards SI beam even if there is full 180 degree clearance.

[MichAngler]

It seems like when the transducer is mounted under a boat/kayak that the hull blocks part of the upwards SI beam even if there is full 180 degree clearance.

You hit the nail on the head

its a matter of "Line Of Sight"

[BassTracker175]

Only 455kHz can reach 180?

[bobcoy]

If you don't see the surface you have something blocking the signal. It can't see around corners and if it is not close to level side to side you might also have that situation. The frequency does not make a difference other than how far out the signal has to go. If it is level and unblocked you will see the surface to at least 75' or so. Bob