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Distorted fish arches in PA
http://www.hobie.com/forums/viewtopic.php?f=78&t=28353		 Page 1 of 1
Author:  Jim_MI [ Sun Jul 25, 2010 8:23 pm ]
Post subject:  Distorted fish arches in PA
Love my new PA, but not so sure about the fish finder. I have an Eagle Cuda 350-S Map combo FF + GPS unit - first I've ever used. I use this on Lake Michigan, out to 150+ ft depths. When trolling in close, in 15 - 45 ft of water, I see fish arches but they are shaped like / instead of the expected ^ shape. I have yet to see any "arches" in water deeper than 60'. Maybe that's just me fishing the wrong place & time, but the distored arches makes me wonder if this is a problem with orientation of the transducer in the hull. According to the FF manual, partial arches shaped like this indicate that the transducer is tipped too far forward. This was installed by the Hobie dealer using the "official" Hobie FF mounting kit. The transducer is located just forward of the center mast post, accessable from the front hatch, which I think is a pretty common location. If others have the same problem with partial arches I will assume it has something to do with the shape of the hull and leave well-enough alone. If not, then I may need to rip out the glue & remount. What's your experience with the shape of fish arches in this hull, and deep water fish detection.
Jim
Author:  Synovis [ Mon Jul 26, 2010 5:51 am ]
Post subject:  Re: Distorted fish arches in PA
I'd take a look at your "scroll" speed.
you are proabably getting the scrunched up arches because you have a really slow scroll speed.

another thing to look at is whether you have it set for salt or freshwater.

I am not familiar with that model, but I know it makes a diff on my humminbird.
Author:  CBull [ Mon Jul 26, 2010 11:18 am ]
Post subject:  Re: Distorted fish arches in PA
Jim,

I too play in Lake Michigan! Salmon should be getting shallower very soon.

See below from a previous post, the text comes from the Lowrance web site but is generic concerning sonar technology;

I found some decent text on the Lowrance web site (http://www.lowrance.com/en/Support/Tips ... -Tutorial/ ), explaining why fish show up as arches when the watercraft is moving, note that when the watercraft is stationary or you're ice fishing, you will NOT typically see fish arches and you would tend to see straight horizontal lines appear and dissappear on the FF display as fish swim into, stay a bit and leave the transducer's cone angle;

"The reason fish show as an arch is because of the relationship between the fish and the cone angle of the transducer as the boat passes over the fish. As the leading edge of the cone strikes the fish, a display pixel is turned on. As the boat passes over the fish, the distance to the fish decreases. This turns each pixel on at a shallower depth on the display. When the center of the cone is directly over the fish, the first half of the arch is formed. This is also the shortest distance to the fish. Since the fish is closer to the boat, the signal is stronger and the arch is thicker. As the boat moves away from the fish, the distance increases and the pixels appear at progressively deeper depths until the cone passes the fish.

If the fish doesn't pass directly through the center of the cone, the arch won't be as well defined. Since the fish isn't in the cone very long, there aren't as many echoes to display, and the ones that do show are weaker. This is one of the reasons it's difficult to show fish arches in shallow water. The cone angle is too narrow for the signal to arch.

Remember, there must be movement between the boat and the fish to develop an arch. Usually, this means trolling at a slow speed with the main engine. If you are anchored or stopped, fish signals won't arch. Instead, they'll show as horizontal lines as they swim in and out of the cone."

Jim; The partial arch as you describe may be that fish are simply not directly below your transducer, but I would think that you would occasionally see a decent arch. Do you ever see a full or 3/4 arch? if you never see a full arch it may be that your transducer is tipped a bit. Actually I would think that the t-ducer's face or bottom surface must end up being parallel to the surface of the water (in all directions) and not necessarilly just parallel with the bottom of the yak, although I'm not certain just how fussy one needs to be about being parallel.
Author:  Jim_MI [ Mon Jul 26, 2010 12:05 pm ]
Post subject:  Re: Distorted fish arches in PA
Thanks for the replies. I have not messed with the scroll speed at all, so it is probably at the factory default, but I can do so next time I am out. I undestand the cone geometry that leads to formation of the arch. With a 20-degree cone you would have to have the thing tipped 10-degrees to NEVER see the back side of an arch. I see the horizontal lines, occasional vertical lines which I assume are noise, and the /-shaped half arches which indicate that the fish is always "closest" to the transducer when it is exiting the sonar cone. I am seeing these while moving at 1.5 - 2.0 mph relative to ground. The FF has an option to display hits as fish icons, and it is recognizing these partials as fish when I flip to that mode. Live-n-learn or remount? Anybody else seeing bad arches all the time?
Jim
Author:  PA-KETTLE [ Tue Jul 27, 2010 4:05 pm ]
Post subject:  Re: Distorted fish arches in PA
Hi, i don't know anything about fishfinders. But i did have a dealer expain a few things. He mentioned that some fish finders have duel depth sonar ranges. I'm thinking it was 50 & 100 feet ranges. If it's on the wrong setting you'll end up with flattened fish arches. Which models have the duel ranges i could'nt tell you. Just repeating what i heard from him, so i may not be accurate. I'm also in Michigan, just had my PA on a small lake once so far. I wonder what it's like on Lake Michigan? Are you using the Mirage drive?
Author:  Jim_MI [ Tue Jul 27, 2010 8:35 pm ]
Post subject:  Re: Distorted fish arches in PA
Pa-K: Thanks for the follow-up. My Eagle Cuda does not have the dual frequency transducer that you refer to, so I'm still not sure what is causing this. I did get out today and changed a number of settings, including chart speed, sensitivity, surface clarity, noise reduction, ping speed, etc. None of these settings gave the expected ^-shaped arches. I checked the transducer install, and it looks flush to the hull... who knows?
As far as fishing Lake MI, it is excitng. I am using the Mirage Drive and find it quite efficient I have a 17-ft touring kayak that I paddle on Lake MI, and feel equally secure in both. Yesterday I was 1-2 miles offshore in 3-4 ft swells and 15 mph wind and found the wind more problematic than the waves. For a low profile boat, the PA almost handles like a sailboat when the wind kicks up. I am trying to learn deep water precision trolling techniques, targeting salmon & lake trout, but having only limited success so far. I'd like to think this is all my fish finders fault, but know otherwise... I have caught some trout and steelhead but not on my deep water rigs.
Author:  CBull [ Fri Nov 04, 2011 11:08 am ]
Post subject:  Re: Distorted fish arches in PA
Jim,

It's been a while, so I thought I'd ask if you figured your issue out or are still having difficulty showing well defined fish arches in deep water.

I'm curious if there may be air bubbles under the transducer within the adhesive used to secure the transducer to the hull. I hear of several guys finding tiny bubbles and having to carefully re-do their t-ducer mounting.

As far as transducer location goes, you're using the same location I and many others use, so if the transducer isn't obviously tipped one way or another I'd suggest removing the transucer and carefully re-mounting it or borrowing a 2nd transducer and hang it over the side of the PA while over some deep water fish.
This may be done more easily with a friend in another boat first putting you over fish with his own FF and then you can try switching between the over the side transducer and the on board questionable transducer. This will let you know whether your transducer needs to be re-mounted or not.
Author:  Thinwater skinner [ Fri Nov 04, 2011 1:20 pm ]
Post subject:  Re: Distorted fish arches in PA
Synovis wrote:
another thing to look at is whether you have it set for salt or freshwater.


How do I find where the setting for saltwater and freshwater is located.. I have not seen this option on the Humminbird 385 ci... Thanks for any help you might be able to give me.
Author:  turtle2 [ Fri Nov 04, 2011 4:08 pm ]
Post subject:  Re: Distorted fish arches in PA
On the Humminbird 385ci, you must first select the "advanced" mode before you will see the saltwater option in the sonar menu. It is listed as "water type".
Author:  Jim_MI [ Fri Nov 04, 2011 7:51 pm ]
Post subject:  Re: Distorted fish arches in PA
Hi CBull -
I ended up pulling the transducer to use it for ice fishing last winter. I discovered hundreds of tiny bubbles had formed in the previously clear Marine Goop that was used to fasten the transducer to the hull. I am sure that this was my problem, since the arches-artefact got progressively worse with time. Instead of remounting in Goop this past spring, I Gooped the foam collar to the inner hull and added some plastic tube fittings so that I could deliver water into the foam collar from the cockpit hatch. The transducer is placed in the foam collar before launching. Then I use a hypodermic syringe to deliver a small "puddle" of water into the space between the transducer and the foam collar and I am good to go for the day. I get good fish arches now and see better detail and resoltion at lower sensitivity settings. I can mark small baitfish 50' down now. Worked great for salmon season too - finally landed a 22-lb chinook on my last day of vacation this fall.
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